Collection Care blog

26 posts categorized "Imaging Science"

25 June 2015

A CT Scan of the St Cuthbert Gospel

A CT scan of the St Cuthbert Gospel – the earliest intact European book dating to the early eight century - has been published in a ground-breaking new book launched this week: The St Cuthbert Gospel: Studies on the Insular Manuscript of the Gospel of John, edited by Claire Breay, Head of Ancient, Medieval and Early Modern Manuscripts at the British Library, and Bernard Meehan, Head of Research Collections and Keeper of Manuscripts at Trinity College, Dublin. Colleagues from Collection Care and Medieval Manuscripts took the pocket gospel to the Natural History Museum for CT analysis to understand the structure of the ancient gospel, which was found inside the coffin of St Cuthbert in 1104.

On the right, three BL staff members stand. On the left is a computer, and in the centre is the scanner.
The British Library project team at the Natural History Museum. From left to right: Claire Breay, Flavio Marzo and Christina Duffy.

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X-ray computed tomography (CT) is a non-destructive technique which creates 2-D cross-sectional images from 3-D structures. The St Cuthbert Gospel was scanned using a Metris X-Tek HMX ST 225 CT scanner with an operating voltage of 225 kV at the Natural History Museum.

To protect the gospel during scanning it was placed inside a custom-made phase box and then secured upright in a bespoke piece of polyethylene foam.

Two images stitched together. Left: someone places the volume into the box. Right: The closed phase box stands upright surrounded by a piece of grey foam.
The St Cuthbert Gospel was placed in a phase box which was secured in a piece of foam.

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A facsimile of the gospel produced by Jim Bloxam and Kristine Rose was generously made available to the team during the CT scan. This enabled a direct comparison of materials known to be used in the facsimile with those unknown in the original St Cuthbert Gospel. Both volumes were placed inside the CT chamber on a precision rotation stage between an X-ray source and a detector.

Two images stitched together. Left: The actual volume and its facsimile are placed side by side and held together with a cord. Right: The two volumes enter the scanner.
The two copies were placed side-by-side in the CT chamber.

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As the volumes rotated on the stage through 360⁰ a conical beam of X-rays took digital projections in 0.5⁰ increments. The CT image pixels are displayed in terms of their relative radiodensity allowing us to scroll through the image slices revealing the materials underneath the leather binding.

Two images stitched together. Left: Four people sit in office chairs surrounding a desktop computer, looking at the results on the monitor. Right: An image of the computer monitor showing a couple black and white images--results from the scanning.
The results were poured over in the lab. From left to right; Christina Duffy, Claire Breay, Nicholas Pickwoad and Dan Sykes.

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The results were initially examined by the British Library team and Professor Nicholas Pickwoad, whose chapter in the new publication draws on the CT scan results and discusses how the central motif on the binding appears to have been made using a clay-like material, rather than gesso or cord as previously thought.

Two images stitched together. Left: The cover of the St Cuthbert Gospel, in a dark red leather with a raised floral motif in the center surrounded by a frame of Irish designs. Right: A magnified view of the raised floral motif.
The St Cuthbert Gospel with raised plant-motif decoration examined under high magnification.

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The scan of the heads of the actual Gospel and the facsimile. Scans of the raised floral motif of both the original and the facsimile. The material in the facsimile which creates the raised area is a starker white than the material of the facsimile. Scans of the original volume's boards and leather covering.

CT datasets contain vast amounts of information and samples can be visualised in many ways using various software tools. Drishti, which stands for vision or insight in Sanskrit, is an open source volume exploration and presentation tool. It allows volumetric data sets to be both explored and used for presentation of results.

The image of the Gospel in the software in an ivory tone.
A screen shot of the St Cuthbert Gospel as visualised in Drishti.

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CT scanning can provide tremendous amounts of information on the condition and construction of books and their bindings. This level of detail is unavailable through visual examination and can often lead to speculation. More information about the project can be found over on the Medieval Manuscripts blog. The new publication, The St Cuthbert Gospel: Studies on the Insular Manuscript of the Gospel of John, can be bought in the British Library shop or ordered online.

Christina Duffy (@DuffyChristina)

07 May 2015

Public event - Magna Carta: Under the Microscope

We’re delighted to announce that the conservation team behind the work done on the British Library collections in our latest exhibition Magna Carta: Law, Liberty, Legacy will be speaking at a public event on Friday 26 June 2015 18:30 - 20:30 to share their findings. Speaking on the night in the British Library Centre for Conservation will be Head of Conservation Cordelia Rogerson, conservator Gavin Moorhead, conservation scientist Paul Garside and imaging scientist Christina Duffy. Book your place here.

A set of four images. Top left: A conservation scientist cuts white foam on a green cutting board. Top right: A customer inspects the frame which lays on a table. Bottom left: A conservator uses a knife to prise open two layers of a mount board with the Magna Carta inside. Bottom right: An imaging scientist inspects the Magna Carta under magnification. The Magna Carta rests on a flat surface with a microscope above it; the magnified image appears on a computer screen.
Join our project team of conservators and scientists on 26 June 2015.

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The project spanned over three years in preparation for this year’s 800th anniversary of the 1215 Magna Carta and involved the reframing and scientific analysis of all of the Magna Carta charters held in our collections, including the two 1215 original versions.

The item rests on a soft surface while Gavin inspects it. The charter is house in cream mount board.
Conservator Gavin Moorhead works on the 1215 Articles of the Barons (Additional MS 4838).

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The team undertook an initial examination of the original frames to determine their structure and composition. At the event you’ll hear how probes were manually inserted into the frames to take samples of the air inside in order to determine what kind of micro-environment the charters were living in! The stability and compatibility of new materials, which would be used for mounting in the new frames, was ensured using infrared spectroscopy, pH tests, and lignin tests.

A pile of folded red and blue textiles rests on a table.
Mounting materials were tested before incorporation into the new frames. Join us to find out what the blue and red colours indicate.

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With the frames removed the team had a rare opportunity to investigate the condition of the manuscripts using near-infrared spectroscopy and high resolution digital microscopy. Unpublished images of the ink and parchment at up to 200 times magnification will be shared with the audience.

A up-close shot of the Magna Carta under a magnifier. Part of the charter is visible in the image along the wax seal.
What does 800-year-old ink look like at 200 times magnification? 

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You will also delve deep into the exciting world of multispectral imaging and see versions of the charters and their seals under ultraviolet and infrared light. The incredible results of the text recovery project on the damaged 1215 Canterbury Magna Carta, from which much of the ink was lost, will be shared.

Once our tests were complete it was time to rehouse the charters – you’ll hear from our conservator Gavin Moorhead about the challenges and decisions required to mount for display one of the most recognised manuscripts in the world which would feature as the dramatic finale to the exhibition.

The Magna Carta in its frame sits in a displace case in the exhibition.
The British Library's London Magna Carta at our exhibition, Magna Carta: Law, Liberty, Legacy.

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Don’t miss out on this great event and book your place now! We look forward to meeting you!

Christina Duffy

12 February 2015

Photographed by the Hand of a Sinner

Senior Imaging Technician Kristin A. Phelps takes us behind the scenes of the British Library’s Imaging Services where there are several ongoing digital projects at any given time. 

Click here for an Arabic translation of this article, as translated by the Thesaurus Islamicus Foundation and Dar Al Kutub Manuscript Project.

In the foreground is an open volume, cut off about halfway by the camera, with an illumination on the left page and text on the right. In the background is a computer screen showing the digitised illuminated page.This page shows a seated figure in blue and red robes writing on a piece of paper.

Prior to the 14th Century, Byzantine artists who painted icons preferred to shun hubris and leave their works unsigned. Their work would be placed in churches to be seen and revered by thousands of the faithful over the centuries. Occasionally, an artist would sign their work with the phrase “Painted by the hand of a sinner.” This allowed the sacred value of the icon to remain unfettered by human presence.

Fast-forward to the modern world and a secular context: millions of digital images are accessed every day on websites of museums, libraries, archives and other collections. These images are taken by unseen photographers and are unsigned. The anonymity of the process allows for ‘pure’ and non-distracted understanding of the object by a viewer. But, who are these modern day artists who make invaluable works of art, faith and history accessible to all of us? How does their particular art form impact what we are able to view on our computer screens, tablets and smart phones?

To answer these questions we are going behind the scenes of the British Library’s Imaging Services where there are several ongoing digital projects at any given time. One project in particular, the Greek Manuscript Digitisation Project (GMDP), is working to digitise centuries old Greek manuscripts, some of which include illuminated portraits of the Evangelists executed by anonymous hands. The British Library’s third phase of the GMDP began in April 2014 and is scheduled to be completed March 2015 with a target of digitizing over 300 manuscripts, which is roughly equivalent to 120,000 images. The project has been funded by the Stavros Niarchos Foundation, the A.G. Leventis Foundation, Sam Goff, the Sylvia Iannou Foundation, the Thriplow Charitable Trust and the Friends of the British Library, among others.

Three images in a row. On the left is a camera mounted to a photo stand. The centre image is a wider shot of the photo stand showing the camera mounted above a book which is being photographed. On the right there is another book being photographed. The books are held at a 90 degree angle, so the page being imaged is flat, and the other side of the book is strapped to a soft cradle.

A series of 8 photos in two columns showing Imaging Services staff. Many of them look into a camera's viewfinder or at the set up of the photo stand.

The British Library’s Imaging Services currently employs eight full time photographers, or Senior Imaging Technicians, who represent approximately 110 years combined of photographic experience at the Library. While two of these photographers have been tasked with working on the GMDP, all of the photographers will work on the project at one point or another. 

The eight photographers come from a variety of backgrounds including more traditional photographic backgrounds as well as artists, a former school teacher, a former 3D graphic designer with a specialty in computer gaming and a former archaeologist.

Once a book is delivered to the Imaging Studio, the physical digitisation process can begin. Every manuscript is unique and its physical condition can vary widely. For this reason, a conservation assessment is being performed for each manuscript to be imaged for any of the digitisation projects. This written report guides the photographer responsible for the book to ensure that manuscript is returned in the same condition it was received.

 Once the assessment has been read and understood, the manuscript is set up for capture on a cradle. Many manuscripts can be photographed using an L-shaped cradle, designed by the Conservation Department, to allow photography without damage to the material. When the manuscript has been appropriately set up, it can be photographed in RAW format page by page utilising Phase One cameras with digital backs.

A book rests strapped into its cradle. Above this the camera has caught a human hand in motion--it looks as if someone has perhaps just finished strapping the book in and you get a blur of their hand moving away. Senior Imaging Technician Neil McCowlen looks at his computer monitor.Once the images are captured, they are reviewed and edited in Capture One (minor adjustments only including cropping, straightening and exposure adjustments).  

Finally, the RAW files are processed into both Tiff and Jpeg files before being passed back to the various digital project teams for online publication.

Does this process sound simple and straightforward? It rarely is. Often times, a manuscript needs to be carefully propped up to become level, or has a page which is not flat. The photographer is then responsible for manipulating the manuscript with a very gentle and cautious manner to make the resulting image provide the best view of a page. In addition, items may be housed in glass which cannot be removed for imaging. Senior Imaging Technician Tony Grant gets a book set up in its cradle prior to digitisation.Or, objects may be large and unwieldy or extremely small. Lighting conditions may need to be changed if the manuscript contains significant amounts of gold leaf decoration. And, of course, there are always physical adjustments of the camera position and settings as well as employing a variety of lenses. Throughout the process the photographers have to use their judgment and experience in order to “do no harm” and yield images that represent faithfully the original material. After observing everything which must be considered to photograph a manuscript, the question arises: are these professionals artists or technicians? The Library photographers themselves are split when it comes to answering this question. Half of them consider their particular type of photography an art form whilst the rest view it as form of scientific imaging.

No matter what the answer to this question is, one thing is certain. These photographers deliver an impactful and important volume of work to the digital masses. Scholars from across the world have advanced their research without the need to physically visit the British Library. Thousands of people are able to connect with global cultural and religious heritage with a click of a button.

Of course, the GMDP project is just one example of a common wider trend of museums, libraries and archives digitising their holdings for online publication. In Europe alone as of 2014, 87% of cultural heritage institutions had digital collections. ENUMERATE’s 2014 survey found that the most important perceived reason for digital collections was academic research, which points to the growing field of Digital Humanities. With all the new material available online, a visual revolution of the democratisation of knowledge is happening. Now a scholar is no longer hindered by the inability to travel afar to libraries and museums to see objects; instant access to manuscripts and 3D objects is only a click away. Scholarship is becoming more diverse because open access to online collections allows those who wish to see something to be able to do just that. In fact, in its still young life, the third phase of the GMDP has already been the focus of scholarly research as well as being used and shared by a number of New Testament and Patristic blogs.

Of course, none of this would be possible without the diligent and specialised work of the photographers at institutions like the British Library. But if you asked one of the British Library Imaging Services’ photographers about their role in the process you receive humble responses, not dissimilar to what you would have expected from the original “sinners.”

Kristin A. Phelps, Senior Imaging Technician

A black and white image of Senior Imaging Technician Laurence Pordes standing before the photography set up.

21 October 2014

Paper cuts: small but mighty!

Hardly noticeable and barely bleeding, paper cuts are the mother of all library injuries. Anyone who deals with paper on a daily basis will have at some point suffered such an affliction. Paper cuts cause a seemingly out of proportion amount of pain due to the anatomy of our skin and the structure of paper. When very thin and held in place, a sheet of paper becomes inflexible and can exert very high levels of pressure – enough to slice through flesh! Yikes! Let’s go under the microscope to see what's happening...

An edge of a single sheet of paper, with the rest of the piece of paper blurred out and against a black background.
A single sheet of paper at x30.

 

A finger with a paper cut against a black background.
Paper cut (image source).


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Most paper cuts result from new sheets of paper held strongly in place. A rogue sheet may come loose from the pack but remain held in position by the rest of the tightly-knit sheets. In paper, more resistance is felt when a force is applied parallel to a sheet of paper. This has to do with the paper’s tensile strength. Tensile strength measures the ability of a material to resist rupture when force is applied to one of its sides under certain conditions. Held in place, the sheet of paper becomes extremely resistant to buckling, stiffens, and acts as a razor.

The edges of a stack of paper with one sheet, about halfway down, sticking out from the stack.
A sheet of paper that strays from the pack can cause serious paper cuts!

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A paper’s edge may appear to be smooth and flat, but on a microscopic scale paper edges are jagged. Paper cuts leave a wound more like one from a saw than a knife (a miniature papery saw).

The edges of pages of a book, looking quite sharp.
Pages from a copy book at x30 magnification. Fibres at the surface give paper a serrated edge. The black lines are page lines.

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Paper cuts are remarkably painful. They usually occur in the fingertips, which have a greater concentration of nerve cells (neurons) than the rest of the body – an evolutionary trait to protect us during the exploration of our environment. Neurons send chemical and electrical signals to our brain, and some of them, called nociceptors, detect potential harm. Paper cuts stimulate a large number of nociceptors in a very small area of the skin. Shallow paper cuts don’t bleed very much so pain receptors are left open to the air resulting in continuous pain as the wound cannot clot and seal. As we continue to use our hands, the wound flexes open, continually distressing these neurons.

Not only do paper cuts part the flesh with a micro-serrated paper edge, but they also damage skin either side of the wound due to the composition of the paper. Pain receptors are continuously irritated by the combination of cellulosic wood pulp, rags, grasses, chemically-coated fibres, and bacteria that make up paper. Paper may also include other additives such as chalk or china clay to make the paper easier to write on. Sizing gives us a great variety of papers to suit the specific type of ink we wish to apply, but involves mixing many additives into the pulp to determine the correct surface absorbency.

Paper cuts from envelopes can be particularly stingy due to the layer of glue along the sealing tab. The glue is made from gum arabic, which although edible to humans, can pack a punch if embedded inside a wound. Gum arabic is the product of hardened sap taken from two species of acacia trees, and is also used as a binder for watercolour painting, and in traditional lithography.

Magnified, adhesive on an envelope looks shiny and jelly-like.
Envelope adhesive x30
A close up of an envelope tab with adhesive.
Gum arabic glue at x30 and x200 magnification coats the paper tab on an envelope. When the gum is moistened it forms a seal with the adjacent paper.

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When skin closes around the paper cut these foreign particles become trapped inside causing a great deal of pain. This is why a cut from a razor blade is usually less painful than that from a paper cut: razor blades make clean incisions without leaving behind any foreign particles. It hurts initially, but the pain soon ebbs away. Bleeding caused by a razor cut helps to wash away any infection-causing particles, while paper cuts bleed very little (this also reduces your chances of getting any sympathy!)

Three blades of a razor are shown at close up, showing sharp edges.
Razor at x50
The sharp and smooth edge of the razor is even more apparent at high magnification.
Razor blade at x200. The razor’s edge is smooth allowing a clean incision without introducing foreign bodies.

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It might seem strange that sometimes needles for a flu jab require quite a bit of force to pierce the skin, yet paper (PAPER!) can slice through. This is due to the random orientation of collagen fibres in our skin allowing us to withstand pinpoint forces.

The tip of a finger is being poked by a needle.
Human skin feeling the pressure under a sharp pin, x20.

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Our skin does not have a comparable strength against shearing forces such as those exerted by paper, and so, we are susceptible to the small but mighty paper cut. Libraries can be dangerous places. Be careful out there!

Christina Duffy (@DuffyChristina)

 

Further learning:

Paper May Be the Unkindest Cut, Scientific American, Volume 306, Issue 3 , Mar 1, 2012 |By Steve Mirsky 

Why Do Paper Cuts Hurt So Much? Scientific American - Instant Egghead #25

 

07 October 2014

800 year old Magna Carta manuscript reveals its secrets

Ground-breaking multispectral imaging work of the British Library’s burnt copy of the 1215 Magna Carta has recovered text which has not been read in 250 years.

This work has been completed by British Library conservators and scientists in preparation for next year’s 800th anniversary of the sealing of the Magna Carta. The so-called ‘burnt’ copy of the Magna Carta is one of four original manuscripts from 1215 which survive. In February 2015, the four manuscripts will be brought together for the first time in history for a special 3-day event, which will allow further academic study of them side by side, as well as the once-in-a-lifetime opportunity for 1,215 people to view them together – you can enter our ballot to win your chance to be part of that event here.

Multispectral camera Colour image

Processed image

CC by Top left: The “Burnt Magna Carta” ready for multispectral imaging. Top right: A real colour image of a section of the charter. Bottom: A processed image of the charter enhanced to reveal text thought to be lost.

The British Library owns two of the original 1215 Magna Carta manuscripts (the other two are held at Lincoln and Salisbury Cathedrals). The story of the ‘Burnt Magna Carta’ (Cotton charter xiii 31a) held in our collections is a truly remarkable one of survival against all the odds. In 1731 it was damaged in the Cotton Library fire, and subsequently staff at the British Museum Library used 19th century techniques to try to flatten and mount it, which has contributed to its current condition today rendering the text very difficult to see.

The multispectral imaging of the burnt Magna Carta was conducted as part of a major project involving the reframing and scientific analysis of all the Magna Carta charters held in our collections ahead of the 2015 anniversary. The Collection Care team provided an initial examination of the original frames to determine their structure and composition. All original mounting materials in contact with the charters were tested using infrared spectroscopy, pH tests, and lignin tests to determine their stability and compatibility with new materials. Once the charters were removed from the frames, near-infrared spectroscopy and high resolution digital microscopy was used to investigate the condition of the ink and parchment as part of the overall condition assessment.

With the frames and glass removed there was a rare opportunity to employ the cutting-edge technique of multispectral imaging enabling us to virtually peel away the layers of damage currently affecting the manuscript.

Conservator Kumiko Matsuoka Conservation Scientist Dr Paul Garside

Conservator Gavin Moorhead Imaging Scientist Dr Christina Duffy
CC by Clockwise from top: Temporary housing is prepared to store the charter when removed from the frame; the original wooden frames are removed to enable access to the charter; the charter is released from the mounting; once the charter is free from the frame it can undergo condition assessment.

Burnt Magna Carta  Ink loss

CC by Left: The “burnt” copy of the Magna Carta, Cotton charter xiii 31a, is one of the four original manuscripts from 1215 which survive. Right: Much of the ink has been lost with only a few remaining initials (shown here at 50x magnification).

Multispectral imaging is a non-destructive, non-invasive imaging technique using different colour lights, including ultra-violet and infrared, to recover faded and lost text. A high-resolution camera is securely mounted directly over the charter, which is then illuminated with LED lights ranging from the ultraviolet at a wavelength of 365 nm, through the visible region, and right up to a wavelength of 1050 nm in the infrared region. The chemical composition of the material in the charter is varied (ink, parchment, etc.), and so reacts differently to the lights. We are able to see, and capture, additional information undetectable by the human eye.

CC by  An animated gif comparing the original colour and processed images. 

Ultra-violet colour image

CC by A colour UV image reveals regions of text which are completely faded to the naked eye.

Using this technology and expertise available to us in the 21st century, we are able to preserve the Magna Carta for the next 800 years and present these iconic documents in the best possible condition for visitors who come to see them during the anniversary year.

Multispectral data is still being processed and will be published along with other scientific data collected after the British Library’s exhibition ‘Magna Carta: Law, Liberty, Legacy’ which runs from 13 March – 1 September 2015.

You can see this Magna Carta manuscript alongside the 3 other surviving copies at the British Library on 3 February 2015 – find out more details and enter for your chance to win a place on www.bl.uk/magna-carta

Christina Duffy (@DuffyChristina), Imaging Scientist

27 July 2014

Fleas, mould and plant cells: under a 17th century microscope with Robert Hooke

This week we celebrate the 379th birthday of Robert Hooke, a Fellow of the Royal Society and key figure of early modern natural history and natural philosophy, born on 28 July 1635. Many of Hooke's innovations paved the way for a more rigorous scientific analysis of materials, for which we in Collection Care are very grateful. To mark the occasion we are thrilled to host a guest post from Puck Fletcher who has just completed a doctorate on space, spatiality, and epistemology in Hooke, Boyle, Newton, and Milton at the University of Sussex:

Hooke’s most famous work is the Micrographia: or Some Physiological Descriptions of Minute Bodies Made by Magnifying Glasses with Observations and Inquiries thereupon, published in 1665 by the Royal Society. It is a descriptive work detailing sixty observations of specimens at magnification, starting with the point of a needle, ranging through silk, glass drops, hair, and various plants, seeds, and tiny insects, all viewed through a microscope. It closes with observations of the fixed stars and the moon as seen through a telescope. 

The top cross section is a circle. The  negative space is black, and there are two 'patches' of texture: cross section B on the left and A on the right. A is amorphous in shape, and somewhat giraffe-like in texture--it is made up of various dots which are not perfect circles. B shows a more rectangular texture and comes to somewhat of a point at the bottom. Below this is a branch.
Two cross sections of cork and a ‘sensible’ plant. In his description of cork, Hooke coined the term ‘cell’ for biological contexts. Image source.

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The project was a collaborative one started by Christopher Wren who, in 1661, so impressed Charles II with his drawings of magnified fleas and lice (possibly the ones on which the corresponding Micrographia engravings were based), that the King requested more. Wren persuaded Hooke to undertake the bulk of this work and over the next few years, Hooke amassed his collection of observations, regularly bringing new drawings to the meetings of the Royal Society for approval by the other members.  

An illustration of a flea in profile. The fleas face is to the right.
Among the drawings and observations in Micrographia is this famous and extraordinarily detailed large-scale illustration of a flea. BL Shelfmark: 435.e.19, XXXIV. Image copyright The British Library Board. Read more.

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The impressive folio volume contains thirty-eight highly detailed engravings, which turned the book into an instant bestseller and secured its reputation as the most beautiful and lavish work of early European microscopy. The sense of magnified scale is staggering. A head or body louse, for example, is just a few millimetres long, but the engraved image is 52 cm long, roughly two hundred times actual size, a level of exaggeration that is emphasized by the fact that, large as the volume is, the reader must still unfold the oversized plate to view it.

Head or body louse from below.
Engraved image of a head or body louse, roughly two hundred times actual size.

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For his readers, Hooke’s illustrations brought a whole new world into view. Hooke captures this excitement in his preface, describing how, by means of instruments like the microscope, ‘the Earth it self, which lyes so neer us, under our feet, shews quite a new thing to us, and in every little particle of its matter; we now behold almost as great a variety of Creatures, as we were able before to reckon up in the whole Universe it self.’ Pepys was famously so enamoured of the book that the day after he brought home his copy, he stayed up until two in the morning reading it, describing Micrographia in his diary as, ‘the most ingenious book that I ever read in my life’. 

When looking at the large-scale, clear engravings in Micrographia, it is easy to imagine that this was the view Hooke had in his lens and that his task was simply that of looking and then recording what he saw. However, the practice was much more difficult and required considerable skill and experience – when Pepys looked through his microscope, he was disappointed to find that at first he couldn’t see anything at all! The lens making technology of the time meant that impediments to clear vision such as chromatic aberration or artefacts in the glass were not uncommon, and the view through a microscope was often blurred, distorted, and dark. It was difficult to make out true colours or to tell whether a shadow was a depression or protuberance, and the field of vision was quite small.1

Part of Hooke’s contribution in Micrographia was his skill as an instrument maker and technician. Although, as he reports, he had difficulties in seeing through his microscope, Hooke made his own adaptations to the commercially manufactured instrument, in particular devising an improved light source, which he called his ‘scotoscope’.  

A drawing of the 'Scotoscope'. The eyepiece one would look down is similar to what we use today, if a bit more 'pretty' with floral engravings. To the left of that is a flame providing light.
The microscope, featuring an improved light source.

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Hooke also worked diligently and looked very carefully, making multiple observations from multiple angles, of multiple specimens, created with various preparation techniques, to gather enough visual information to be able to produce a single image of what the whole object looked like, as near as he could make out. For Hooke, the act of looking through the microscope and recording what he saw was an interpretive one.

Hooke’s observations have been praised by modern scientists for their accuracy, and Howard Gest even credits him with the first accurate description and depiction of a microorganism, the microfungus Mucor, described by Hooke as ‘blue mould’.2 

In this illustration, blue mould looks flower like, with stems coming from the surface, some of which end in circular balls and others which end in petal-like shapes.
The microfungus Mucor (‘blue mould’).

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In his preface to Micrographia, Hooke heralds ‘artificial Instruments’ such as the microscope and telescope, and the methods of the new science based on observation and the careful and rational scrutiny of results, as at least partial correctives for the failings of fallen man and his limited sensory faculties. He also looks forward to the technology of the future, which he believes will enable man to see even more clearly.

‘’Tis not unlikely, but that there may be yet invented several other helps for the eye, at much exceeding those already found, as those do the bare eye, such as by which we may perhaps be able to discover living Creatures in the Moon, or other Planets, the figures of the compounding Particles of matter, and the particular Schematisms and Textures of Bodies.

Puck Fletcher

1Brian Ford’s wonderful book, Images of Science: A History of Scientific Illustration (The British Library, 1992), pp. 182–83, contains a photograph of the partial and distorted view through the sort of lens used by Hooke.  

2Gest, Howard, ‘The Remarkable Vision of Robert Hooke (1635–1703): First Observer of the Microbial World, Perspectives in Biology and Medicine, 48.2 (2005), 266–72 (p. 267).

19 July 2014

Secret underdrawings & cover-ups in the Mewar Ramayana

The Ramayana – “Rama’s journey” – is one of India’s oldest stories having first being written some two and a half thousand years ago. It follows the hero Rama from his birth and childhood in Ayodhya to his exile in the forest where his wife Sita is kidnapped by the wicked (and ten-headed!) demon king Ravana. With his valorous brother Lakshmana and helped by an army of monkeys and bears he leads the search for Sita, finally rescuing her from Ravana’s stronghold in Lanka. It is an epic story embodying the Hindu idea of dharma (duty). There are several thousand known surviving manuscripts and many different versions of the story across Asia. The Mewar Ramayana is one of the finest copies of the work, lavishly illustrated with over 450 paintings in large format. Recent digitisation by the British Library in partnership with leading Indian institutions has reunited the long-separated text and it can be viewed online featuring an introduction including links to contextual documents and high resolution images in ‘Turning the Pages’ with descriptive text and audio.

Multispectral imaging

We recently examined two paintings from the Mewar Ramayana using multispectral imaging to investigate the methods and workflow of the artist. Images are captured over fourteen spectral bands from the ultraviolet (UV: 365 nm) to the infrared (IR: 1050 nm) revealing information about underdrawings and techniques that can’t be seen under normal light. The two full page paintings are illustrations from Book 6 (Yuddhakanda, Book of war) of the Mewar Ramayana manuscript.

Book 6 fol. 27r (Add. MS 15297(1), f.27r)


Book 6 fol. 27r depicting the siege of Lanka in colour, ultraviolet, infrared, and blue light with an orange filter. Rama’s army of monkeys takes control of the four gates of the city as the ten-headed Ravana leads the defence after consulting his ministers.

Book 6 fol. 27r: Rama’s army of monkeys and bears hurl stones at their enemies. White pigment, possibly added as a later touch up, is observed under ultraviolet light on the elbows, arms and tails of the attacking monkeys.

Book 6 fol. 27r: Colour, ultraviolet, infrared sequence. In front of the gates to Lanka, a man struggles with a monkey. Under ultraviolet light the rough application of paint is evident on the man's hand where no attempt is made to stay within the lines. In the infrared image, the guidelines used to initially draw the figure (chest, back, elbow) are observed.

Book 6 fol. 27r: An archer fends off the monkey army. Incorporating high levels of detail in these paintings often led to a change in design layout. In the painting the archer is shown to be sitting cross-legged on the cart, but in the infrared image he is standing. The late addition of the cart is evident by the over painting of the wheel in order to indicate its attachment to the main frame of the cart. Other alterations were made such as the size of the soldier's orange foot in the top left, and the painting over of an isolated monkey tail on the horse's body in the bottom left.

Book 6 fol. 142r (Add. MS 15297(1), f.142r)

As the battle escalates Rama’s brother Lakshmana is seriously wounded by a spear. Hanuman the monkeys’ army general is sent to the Himalayas to pick up medicinal herbs.

Book 6 fol. 142r: Colour, ultraviolet, infrared, and blue light with an orange filter sequence of the painting.

Book 6 fol. 142r: Colour, ultraviolet, infrared sequence. Rama’s loyal brother Lakshmana is seriously wounded by a spear. In the ultraviolet image we can see touch-ups on the hands, arms and legs of the two monkeys trying to take the spear off Lakshmana. Under infrared light we can see underdrawings of the far left monkey who was originally positioned higher up.

Book 6 fol. 142r: Colour, ultraviolet, infrared sequence. In the ultraviolet image alterations to Rama’s clothing and the direction of arrows is observed. Under infrared light, the boat at the top of the painting with the three figures is shown to have been altered. It may have started out as a representation of deities in the sky similar to those seen in Mughal Mahabharata (Razmnamah) Or. 12076 f.76r. Other arrow positions have also changed.

Book 6 fol. 142r: In the infrared image, a different position for the ten-headed Ravana is shown to the right, where a single face in profile is revealed adjacent to a vertical line. This is completely obscured by the green pigment which we now see.

Multispectral imaging has proven a wonderful technology in allowing us to study collection items in new and exciting ways. These are just some of the observations made and we hope to share more in the future.

Christina Duffy (Imaging Scientist) and Pasquale Manzo (Curator Sanskrit)

20 May 2014

Discovery of a watermark on the St Cuthbert Gospel

A watermark of a post horn surrounded by a shield was recently discovered on the rear pastedown of the St Cuthbert Gospel (Add. MS 89000). The finding has just been published in the Electronic British Library Journal. The St Cuthbert Gospel is a late seventh century parchment volume and is the oldest intact European book. This Anglo-Saxon pocket gospel belonged to St Cuthbert of Lindisfarne (c. 635–687) and was discovered in 1104 in his tomb.

The pastedown, which is the endpaper attached to the inside cover board of a book, records the donation of the St Cuthbert Gospel (then known as the Stonyhurst Gospel) to the British Province of the Society of Jesus from the Reverend Thomas Philips, S. J. in 1769.

 

The front board of a book bound in dark brown leather. A rectangular decorated panel is in the middle of the board - it features a raised pattern in the shape of an interlacing vine, above and below which are rectangular panels of interlacing knot designs. The leather has deposits of dirt and small areas on the right-hand side of the board have become abraded.
St Cuthbert Gospel
A piece of cream paper pasted inside the back board of the book, covered with ten lines of Latin writing in dark brown ink. The paper is torn and folded in on the edges and at the corners. The number 91 is written in red crayon at the top of the piece of paper, above the first line of writing. The paper has some old dirt ingrained into its surface, which show the contours of the board and folds of the covering leather beneath.
Rear pastedown
A white representation of the piece of paper on the back board, showing only the outline of the watermark. The watermark is in the bottom right-hand corner. It consists of a cloud, inside which is an item in the shape of a hunting horn. A small tentacle or vine is coming out of the top of the cloud. Next to the cloud is a small design in the shape of a shoe with a high heel.
Watermark location


CC by Left: Front cover of the St Cuthbert Gospel. Centre: The rear pastedown showing a record of the donation: ‘Hunc Evangelii Codicem dono accepit ab Henrico Comite de Litchfield, et dono dedit Patribus Societatis Iesu, Collegii Anglicani, Leodii, Anno 1769; rectore eiusdem Collegii Ioanne Howard: Thomas Phillips Sac. Can. Ton.’ which translates to: ‘This Gospel Book was received as a gift from Henry, Earl of Litchfield, and given to the Fathers of the Society of Jesus, of the English College, Liège, in the year 1769, the rector of the college, John Howard, Thomas Phillips Canon of Tongres.’ Right: The watermark is located in the lower right hand corner

What are watermarks?

Watermarks are created by manipulating a piece of wire into a recognisable shape and fixing it to a paper mould such as the Japanese papermaking bamboo screen below.  Here the bamboo has been cut into strips and arranged into parallel lines called laid lines. The bamboo strips are held together by sewing thread at one inch intervals, which form the chain lines on a sheet of paper. Chain lines, laid lines and watermarks are visible when held up to a light source. Light can pass through watermarks easily because the paper thickness is reduced where wire is present in the mould.

The papermaking screen consists of two dark wood rectangular frames one on top of the other with a screen between them made from parallel strips of bamboo. A thin dark wood dowl bisects the top frame horizontally. The bamboo screen has lines of white stitching crossing it horizontally at regular intervals.
Paper mould

CC zero A Japanese papermaking bamboo screen

We typically notice watermarks on paper when they are held up to the light revealing a motif, initials or a date relating to the original paper mill. Watermarks are therefore useful in determining the provenance of paper and can help to identify its intended function. Watermarks can be difficult to image because they are often obscured by print on the page, or are located in the gutter (the space between the printed area and the binding).

An example of a partial watermark from a woodblock reprint dating to about 1476 is shown below. The reprint is of the Astronomical Calendar first published by Johann Müller (Regiomontanus) in Nuremberg in 1474 (British Library shelfmark IA.7). The watermark is found in the gutter with the other half located several folios later due to the ordering and cutting of the folios.

The gutter area of a book is shown horizontally. Grids of numbers are printed in black ink on both pages, and are coloured with yellow and red pigment on the top page. The watermark is in the margin of the top page, below the printed grid. It consists of a clover shape with three circular leaves. Two more circles like those of the leaves are positioned on each side of the central clover motif. Some of the watermark is obscured by the lower page.
Partial watermark in book gutter

CC by Watermark in the gutter of BL IA.7 when viewed through a light sheet 

When the page is adhered to a board on one side, such as the rear pastedown of the St Cuthbert Gospel, it is impossible for light to transmit and watermarks can remain undetected. A pastedown conceals the raw edges of the covering material and forms a hinge between the board and the text block.

A book bound in green leather with the front board open. The first page, the endpaper, is made from a single folded piece of cream paper, double the size of one of the book’s pages, the left hand side of which (the pastedown) is stuck to the inside of the board.
Pastedown example

CC zero An example of a front pastedown where one side of the endpaper is adhered to the front cover. Since the endpaper is fixed to the board it is difficult for light to penetrate and illuminate potential watermarks

The St Cuthbert Gospel watermark

A high resolution digital image of the pastedown was processed using ImageJ, an open source image processing software package. An image is comprised of a variety of layers or textures which can be separated. This allows pixels of interest to be isolated which may include faded writing, obscured text or watermarks. The watermark was revealed by converting the image from the standard RGB (red green blue) colour space into another space where tiny contrast differences were enhanced. The process of colour space analysis is fully explained in the publication: The Discovery of a Watermark on the St Cuthbert Gospel using Colour Space Analysis

The bottom right hand section of the rear pastedown, showing parts of the bottom six lines of writing in dark brown ink on cream paper.
No watermark observed
The same image, but with the colours reversed. The cream paper is now grey and black and the dark brown writing is shown in white. The watermark is shown in faint black lines.
Watermark visible


CC by Left: An image of the rear pastedown of the St Cuthbert Gospel where no watermark is observed. Right: The same image reveals a watermark in the lower right hand corner of the pastedown when processed into another colour space

Non-destructive science

Colour space analysis is being used at the British Library to enhance faded designs on binding covers, disclose watermarks and hidden inscriptions and to reveal text which has been chemically treated or erased. In many cases applying colour space analysis to certain multispectral images has proven successful.

Digitisation projects generate large amounts of high-resolution images which can be manipulated to discover hidden information without the need to access the item. This has significant implications for the long-term study and preservation of cultural heritage collection items. The rear pastedown in the St Cuthbert Gospel was formerly numbered f. 91 and is available to view on the internet as part of the Digitised Manuscripts website.

Christina Duffy (@DuffyChristina)

08 May 2014

Microscopy of the Lindisfarne Gospels, folio 3r

The Lindisfarne Gospels is one of the most magnificent manuscripts of the early Middle Ages. It was written and decorated at the end of the 7th century by a monk named Eadfrith who would go on to become Bishop of Lindisfarne and serve from 698 until his death in 721. An Old English gloss between the lines translates the Latin text of the Gospel and is the earliest surviving example of the Gospel text in any form of the English language. This translation was a late (mid-10th century) addition by Aldred, Provost of Chester-le-Street.

As one of the Treasures of the British Library the Lindisfarne Gospels undergoes strict condition assessments to ensure it is kept at ideal environmental conditions. Part of this assessment involves using microscopy to take a detailed look at the pigment behaviour. We posted some images in a previous post: Under the microscope with the Lindisfarne Gospels, and here we share some of the exceptional exuberance found on folio 3r.

A parchment page of illuminated text. The initial, in the upper left corner of the page, is the largest letter on the page, and extends down the left margin almost to the bottom of the page. It is decorated with swirling motifs, the heads of leopard-like animals, and interlocking birds. The main colours used for decoration are purple, light green, yellow, blue and black. There are two more letters on the first row, which are smaller than the initial but decorated in the same style. The rest of the text on the page, consisting of another five lines, is simpler and written in capitals in black ink. Some of the enclosed areas inside letters, such as A and B, are filled in with yellow, green and purple pigment. All these letters are surrounded by an outline of small red dots. A line of smaller, undecorated, red text runs along the top of each line, and an even smaller line of black text runs along the top of this
Folio 3r

CC zero Folio 3r of the Lindisfarne Gospels, Cotton MS Nero D IV. Examine in full detail here

The abstracted decoration found throughout the Lindisfarne Gospels is a spectacular example of Anglo-Saxon art. There are five major decorated openings in the manuscript, the first of which is found on ff. 2v – 3 and introduces the letter which St Jerome addressed to Pope Damasus. It was Pope Damasus who requested a revision of the Latin Bible text during the late 4th century. Folio 2v consists of an elaborate cross-carpet page and faces Jerome’s letter to Damasus in Latin with the opening Novum opus (New work). The intricate detail on this page has been interpreted as an act of personal spirituality and devotion. A few examples are shown below. Enjoy!

Top of folio 3r

A close-up of the second and third letters on the first line, showing their decoration of swirling knots and bird heads. The photo also shows the lines of red and black text running above them. There is a brown stain, caused by liquid, discolouring the parchment above the first letter.
Top of folio 3r
A close-up of two of the red letters (“p” and “I”) at 50x magnification. Under magnification the pigment appears yellow rather than red and both letters are covered with small cracks. The parchment background is grey-white and has a rough texture.
Folio 3r letter 50x
A close-up of a section of a red letter at 150x magnification. The cracks in the pigment are much larger in this photo and appear black in colour. The pigment again appears yellow rather than red.
Folio 3r 150x
A close-up of a spiral motif at 50x magnification. The spiral is drawn in black and its centre is coloured yellow. The yellow pigment is covered in cracks. There are red dots around the top and left sides of the spiral. The parchment background is grey-white and has a rough texture.
Folio 3r detail 50x

CC by Top: Upper section of folio 3r. Centre: Crackled pigment of lettering reading incipit prologus at 50x and 150x magnification. Bottom: Celtic-influenced spiral motif at 50x magnification

Centre of folio 3r

Tiny drops of red lead are also observed in early Irish manuscripts which heavily influenced the Lindisfarne Gospels. The Germanic zoomorphic style is evident with interlacing animal and bird patterns.

A close-up of the right side of the initial showing decorative animal heads which are purple with yellow noses and orange bodies. Sections of black text from the second and third rows are also shown. The black letters of the second row have areas filled in with purple and yellow pigment and are decorated with intertwining bird heads and necks. Around the initial decorative red dots are arranged in a diamond pattern; around the black letters red dots are arranged in straight lines.
Centre of folio 3r
A close-up of one of decorative animals on the initial at 20x magnification. The animal is drawn with strong black lines, and the orange pigment of its body is cracked. The parchment background is grey-white and has a rough texture.
Folio 3r 20x
The same area at 50x magnification. Cracks now show on an area of yellow pigment as well as on the orange. The purple pigment has an uneven, mottled texture.
Folio 3r pigment 50x
A close-up of the diamond pattern of red dots at 20x magnification. A single dot sits within each diamond. Under magnification the pigment appears yellow rather than red.
Folio 3r red lead 20x

 CC by Top: Central section of folio 3r from the British Library Catalogue of Illuminated Manuscripts with creature detail at 20x and 50x magnification. Bottom: Drops of red lead in a geometric pattern at 20x magnification

Bottom of folio 3r

Decorated initials exhibit yellow pigment (orpiment) bordered with drops of red lead. Craquelure is a network of tiny cracks caused by pigment shrinking due to age. When the disruption consists of perpendicular lines it is referred to as crackling.

An area of the fourth and fifth lines of text, which are written in black while the enclosed areas inside the letters are filled with blue, purple, green and yellow pigment. There are areas of brown liquid staining on both lines of text.
Bottom of folio 3r
An area of an illuminated letter at 30x magnification. The left side of the photo shows yellow pigment covered in dark cracks and decorated with red spots. A line of black ink runs vertically down the centre of the photo; the ink has a rough texture and is more thickly applied in some areas than others. To the right of the black line are three vertical rows of red spots. These are not perfectly circular but rather splodgy and uneven.
Folio 3r 30x
An area of the same letter at 100x magnification. Small pieces of yellow pigment are detaching where the cracks in it intersect with each other. The black ink sits unevenly on the parchment surface, and areas of it are shiny where they catch the light. At this level of magnification the parchment is not a uniform shade and there are dark flecks in the grey-white surface, which is very rough and uneven in texture.
Folio 3r 100x
A close-up at 20x of a letter “B” written in black ink with an outline of red spots. The surface of the black ink is very uneven, with a gritty texture. The enclosed areas of the “B” are coloured with light green pigment. A large flake of the green pigment is missing, and purple-coloured lettering from the reverse of the page shows through the parchment underneath. The parchment surrounding the letter is grey and discoloured.
Folio 3r pigment loss 20x
An area of the same letter “B” at 50x magnification. The light green pigment is mottled, and there are cracks around the area of loss. The letters showing through from the reverse of the page are clearly defined and overlaid with white cracks on the surface of the parchment. A line of black ink runs down the left side of the photo. It has an rough, uneven texture.
Folio 3r 50x

CC by Top: Lower section of folio 3r. Detail of decorated initals at 30x and 100x magnification. Bottom: evidence of loss of green pigment (verdigris or vergaut) from a decorated initial at 20x and 50x magnification. Text from the reverse (f. 3v) is shown through the parchment

For more details on the pigments used in the Lindisfarne Gospels see our previous post. The entire manuscript is digitised and available online here.

Christina Duffy (Twitter: @DuffyChristina)

28 April 2014

As white as a...colour calibration target

The Macbeth ColorChecker(R) is often observed in digitised images adjacent to the subject being imaged. It is a colour calibration target used widely by photographers to achieve consistent colour within a studio environment. Good colour management allows the photographer to have continuity to achieve the same result with any camera. The rectangular cardboard target consists of a grid of 24 squares of colour samples, each with a measureable spectral reflectance. Reflectance refers to the fraction of incident light reflected at an interface. The spectral reflectance of these patches does not change under different lighting conditions in the visible spectrum (this is not the case in the ultra-violet and infra-red – see footnote* below), so are reliable to track colour changes in this range.

A book lies open on a black background. The left-hand page is shown and is filled with a hand-drawn picture of four medieval people in a turret with crenelated walls. Three are kneeling down, on the right side of the picture. Two of these, a man in a blue gown and a lady in a green dress and white headdress, are holding their hands together as though praying. The third kneeling figure is wearing a red tunic and blue hood. He is holding a stick and looking over his shoulder towards the left side of the page. On the left side of the picture the fourth figure, a man, is standing over the other three. He is wearing a red and green gown and a red hat or turban. Above the volume is the calibration target, which is a black piece of cardboard covered with 24 brightly-coloured squares in different colours. The squares are laid out horizontally in four rows of six. Along the bottom of the target is a measurement scale in centimetres and millimetres.
Cotton Nero Ax f86v

CC by Calibration target shown over f.86v of Cotton Nero A.x. during imaging of Sir Gawain & the Green Knight. The target is set to be on the same focal plane as the folio. Targets are often cropped out of final processed images

The idea of a colour chart came about in a 1976 paper in the Journal of Applied Photographic Engineering by C. S McCamy, H. Marcus and J.G. Davidson entitled A Color-Rendition Chart. The abstract states “A color chart has been developed to facilitate quantitative or visual evaluations of color reproduction processes employed in photography, television, and printing.” Their paper has been cited over 350 times to date. The original chart consisted of a 4 x 6 array of patches, each 5 cm square.

A person holds a colour chart in front of their chest. The chart fills the photo except for their hands and some surrounding areas of their black and white shirt. The chart is a black piece of card covered with 24 squares of bright colours, laid out horizontally in four rows of six. It is roughly 30cm x 20cm wide
Original size target

CC zero The original colour chart consisted of square patches of side 5 cm. The same size chart is still available and used today

There are still 24 patches on modern colour calibration targets but smaller versions are now available with patches measuring 1 cm wide. The X-Rite ColorChecker(R) Classic target used in our lab is shown below with a scale, focusing target, and reference number that we attached.

A rectangular piece of black card with 24 squares cut out of it is laid over 24 squares of bright colour. The squares are laid out horizontally in four rows of six. A measurement scale in centimetres and millimetres runs along the bottom edge of the black card, to the right of the word “MegaVision” printed in white, and a small white rectangle filled with a combination of lines and numbers. The serial number 130901 runs vertically up the left edge of the card.
ColorChecker(R) Classic

CC by The ColorChecker(R) Classic target has 24 colours in a 6 x 4 grid. The colours are painted in matte on smooth paper and surrounded by a black cardboard border. 

The colours are roughly divided into four kinds. The top row is composed of colours which approximate natural objects such as human skin (dark and light), blue sky, the green colour of a leaf, and a blue chicory flower. The second row is made of miscellaneous colours encompassing a good range of test colours. The third row is comprised of the primary (blue, green, red) and secondary (yellow, magenta, cyan) colours, and the fourth row represents a uniform gray lightness scale ranging from brilliant white to black.

A chart with four columns. In each column are six coloured squares, with the name of the colour written to the right of the square. The text is in black on a white background.
Colours labelled


CC by Colours in the calibration target. These colours are precisely measured and can be described in terms of the Munsell color system (a colour space describing colours in terms of their hue, lightness and chroma)

Larger colour calibration targets do exist such as the ColorChecker(R) Digital SG which boasts a gamut of 140 colours.

A dark grey rectangle with rounded corners, covered with 140 brightly-coloured squares laid out horizontally in ten rows of fourteen. A 6cm measurement scale is in the bottom right corner of the rectangle. In the bottom left corner of the rectangle the word “gretagmacbeth” is printed in light grey. Along the top of the rectangle the words “Digital ColorChecker SG” are printed in light grey.
ColorChecker(R) Digital SG


ColourChecker(R) Digital SG boasts the widest colour gamut available. Its design is based on the original ColorChecker(R) target but is enhanced for digital photography. Image copyright X-Rite, from X-Rite website

For consistent colour, photographers can take a shot of the calibration target with the camera set to capture raw files. Shooting raw is the only way the camera chip can capture all of the information available in the scene. The image is opened in image processing software such as Photoshop, and a script is run on the image which opens it multiple times with different settings. Results are measured and a status is generated with values which can be used to fine-tune the camera’s colour calibration and get processed colour to match the original scene (or alternatively to distort the colour for special effects!).

While colour calibration targets are on the whole produced in the same way using the same materials, on average, every colour target is ever-so-slightly different. The colour difference may be very small and only measureable using other scientific methods. Colour difference is a metric of interest in colour science - the standard metric being Delta E (ΔE). This definition allows colour difference to be quantified in a way which is more reliable than just using adjectives, a practise which is detrimental to anyone whose work is colour critical!

Our multispectral imaging system captures images in Lab colour space, where L is lightness and a and b are colour-opponent dimensions. Lab colour space approximates human vision and is device independent. It includes all perceivable colours with RGB and CMYK spaces (see our previous post What the CMYK? Colour spaces and printing) sitting within its larger gamut, so file sizes are generally much larger. Values for L, a, and b can be tracked once the image has been white balanced using the white colour patch on our calibration target as a reference. However, Lab files don’t open in all software packages so quite often it is necessary to transform images into other spaces such as RGB, but the original Lab file is always stored.

Colour Science is a fascinating and growing area of research. For fun you can try out this Color IQ test from the X-Rite website to learn more about how you see colour, and to find out where you can get your own targets.

Christina Duffy (@DuffyChristina)

Footnote:

*While the Macbeth ColorChecker(R) provides 24 colours with consistent spectral reflectance under typical lighting conditions in the visible spectrum, it does not behave similarly in the ultra violet or infrared parts of the Electromagnetic Spectrum. Another material such as Spectralon is required for imaging outside of the visible range. The property which defines a diffusely reflecting surface (i.e. an ideal “matte”) is called Lambertian reflectance and Spectralon exhibits highly Lambertian behaviour with a spectral reflectance of >99% from 400-1500nm and >95% from 250-2500 nm. Spectralon is a fluoropolymer - others include PVF, PVDF and PTFE (Teflon). Spectralon has the highest diffuse reflectance of any known material over IR (infra-red), VIS (visible) and NIR (near-infrared) regions of the spectrum, and is therefore very expensive, but necessary to track colour difference during multispectral imaging.

Reference

McCamy, C.S.; Marcus, H.; Davidson, J.G A, 1976, A Color-Rendition Chart, Journal of Applied Photographic Engineering Volume 2, Number 3, pp 95-99

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