Collection Care blog

4 posts from April 2014

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 measurable 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.

Figure 1: 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.

Figure 2: 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.

Figure 3: 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.

Figure 4: 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.

Figure 5: 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

X-Rite website, or follow X-Rite on Twittter

25 April 2014

Time-lapse video of Kitaj Tapestry rehanging

We recently rehung the R.B. Kitaj Tapestry If not, not in the St Pancras Entrance Hall after it was removed for conservation cleaning. You can read about the process and all the gritty details (hoho!) here.

Working with a 6.75 metres high by 6.75 metres wide tapestry is no mean feat and required the help of many people, so we thought we would honour all those involved by immortalising them in a time-lapse video! (Best viewed with Chrome)

The rehanging required the erection of scaffolding several days before and the hoisting of the tapestry up to the top platform. For safety purposes the rehanging was undertaken under darkness when the library was closed to the public. It was just like Night at the Museum, but in a library...and without anything coming alive...but exciting nonetheless.

Collection Care staff were guided by tapestry experts from Textile Conservation Ltd. and we are really pleased with the result. Come visit the British Library and take a look for yourself!

Christina Duffy (@DuffyChristina)

23 April 2014

Boom! Pow! Wham! Conservation Unmasked

Our next exhibition; Comics Unmasked, Art and Anarchy in the UK, opening on
May 2nd, will be a surprising one to many. It promises to challenge myths, expectations and stereotypes; and to explore subjects such as politics, violence, gender, sexuality and breaking social conventions.

The relatively good condition of most of the comics from our collection certainly challenged our expectations. For a number of readers, myself included, comics are seen as a relatively modern and populist genre, with striking graphics and minimum text. Conservators often associate anything modern (i.e. 20th century) and mass produced with poor quality paper; similar to the paper used for newspapers that becomes friable and yellows quickly when exposed to light. A few such items, one example shown below, did come to the studio requiring minor edge and tear repairs, but contrary to our expectations the majority of the comics were printed on good quality, often glossy, paper.

A close up of a conservator's hands holding a sample of Japanese repair tissue next to the damaged edge of the paper object being treated. This is to better assess matching qualities of the repair paper to the object to best support and stabilise weak and damaged areas. Such characteristics conservators look for are matching colour, tone, thickness and surface texture.
Matching tissue paper to support the damaged edge.

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Conservation is usually very privileged in getting a preview on the items going into the exhibition, but not this time. Out of over 200 items prepared for display only a handful required conservation work. In the majority of cases the work was limited to flattening of folded pages followed by mounting or supporting a concertina style strip within a book.

Square and rectangular weights in various colours and sizes are placed around the edges on top of a stack of wooden boards.  The weights are small enough to fit in one outstretched hand. The stack of boards has a semi-transparent material sticking out called bondina, as well as a thick opaque paper called blotter.  When objects have wrinkles or creases, they can be lightly humidified and then put in between a sandwich of blotter and bondina.  The blotter will absorb and excess moisture from the humidification of the object, and the bondina acts a release if any adhesives are used for repair before pressing.
Pressing objects between wooden boards

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A close up of an opened book on a white table. The opening is of a black and white early 20th century comic. The left page is originally double the length and vertically folded in half. In this image the left page has been opened up to show the full comic, with a piece of mount board below to support the full length of the page opening.
Comic strip opened up and supported within a volume

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The first items to be prepared for the exhibition were not that recent either, one example being: ‘The Scourge of Monthly Expositor of Imposture and Folly’ from 1813 showing John Bull preparing for General Congress. This item also needed repair to the edges and mounting.

On open book with a concertina page fully extended out and held down with a glass weight in the top left corner. A concertina fold is continuous parallel folding, much like an accordion or a zig-zag pattern.  The image on the page is a comic depicting a ship in crashing waves on the left, in the middle of the comic the is a standing pig character in full colourful clothing in front of a tree, and to the right there is a lagoon with other colourful characters.
Concertina style comic strip before conservation



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With objects from our collection being in ship-shape condition and one third of all objects in this exhibition coming from other institutions, conservation had a lot more work to do on loans. All items loaned had to be condition checked, some in-situ and others in the studio. Most of those worked on in the studio were paper-based, but we also condition checked unusual objects, such as Judge Dredd’s Helmet and Ally Sloper's Ventriloquist Dummy!

A ventriloquist Dummy stands in a brown jacket buttoned up once by his chest, he has a white shirt on underneath, an off-white bowtie and sand coloured trousers. He has a big pink nose, no hair, and his mouth is open and looks. Like he is smiling a bit. He is standing between a bookcase full of books and a chair full of documents. This image would have been taken in the private owners’ home, as conservators sometimes have to condition check items loaned to the British Library prior to exhibition.
Ally Sloper's Ventriloquist Dummy

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Sadly, we didn’t get any visits from Superman, Spiderman or Catwoman in the studio; but we did get to meet a ‘super heroine’ when the new poster advertising the exhibition was unveiled in the St Pancras Entrance Hall in mid-March!

The Comics Unmasked exhibition will be the UK’s largest exhibition tracing the history of the British mainstream and underground comics. It is a must for all lovers of comics, while for others it will be a trip down memory lane re-visiting well known comic characters, and meeting those we heard about but never met!

Graphic image of a female superhero leaning against a brick wall at the beginning of an alleyway littered with trash and worn posters on the walls. Her face looks unimpressed, with bruised knees, and holding up a flask in her left hand, while her right arm rests across her abdomen, supporting her left arm. In the background there appears and old and knocked out super hero slumped up against a dark doorway leading off of the alleyway.
The exhibition poster showing a female posing after vanquishing a generic super hero

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Iwona Jurkiewicz

01 April 2014

Handle with Care: Using Collections

Poor handling is one of the main causes of damage to library and archive collections. The damage is accumulative occurring over many years, and is not always immediately apparent. Conservation work is costly and can mean that certain items are not accessible for long periods. All users have responsibility for the care of collections, and information and advice on how to handle collections should be available to all users. If you are unsure about how to handle an item here at the British Library, then don’t hesitate to ask our very experienced staff in the Reading Rooms.

There are guidelines published by the Preservation Advisory Centre outlining the differences in handling protocol for books and bound volumes; documents and letters; maps, rolls and charters; prints and drawings; photographs; papyrus; and even globes. There is such a variety of material available for consultation that it is well worth reading through carefully. Loose items should always be kept in order, seals should be supported with flat foam, the surface of photographs should never be touched, papyrus leaves mounted in paper should be turned by the supporting paper, and although gloves are not recommended for flat material, they are required for touching certain materials such as lead seals or the surface of globes.

Collection Care icons have been designed for use in the British Library reading rooms and we share them under a creative commons license. Heritage organisations are encouraged to use them freely in their own publications. Image files can be copied from the Collection Care icon page or higher resolution .eps versions are available on request (collectioncare@bl.uk).

Top left icon is a hand holding a card saying “Pass.” Top right icon shows aa coat, bag and umbrella crossed out, indicating no coats, bags or umbrellas are allowed. The bottom left icon shows two hands outstretched, indicating clean hands. The bottom right icon has a teacup, sandwich, and water bottle crossed out, indicating no food or drinks allowed.
Graphic icons illustrating do’s and don’ts of the British Library



The top left icon is a pencil, indicating to only use pencils in the designated area.  The top right icon shows a highlighter, and two pens crossed out, indicating no pens allowed. The bottom left icon shows s book opened, supported on the right and left sides, with snake weights on top of either page. This icon is a prompt for readers to use book supports when needed.  The bottom right icon shows a desk pilled with books on top and falling off, reminding readers to keep their area tidy.
Graphic icons illustrating do’s and don’ts of the British Library



The top left icon is a cell phone with a volume symbol crossed out beside it, indicating to turn sound off. The top right icon is a camera crossed out, indicating no cameras allowed. The bottom left icon shows a laptop with a volume icon above crossed out, reminding readers to turn off any sounds.  The bottom right icon is a camera with the flash going off crossed out, indicating no flash photography is allowed.
Graphic icons illustrating do’s and don’ts of the British Library
The top left icon is the profile of a face with a hand held up to the face with the index finger outstretched by the lips in a shushing action, indicating this is space is a quiet area.  The top right icon has a pair of opened scissors and a scalpel, indicating no sharp implements allowed.
Graphic icons illustrating do’s and don’ts of the British Library

CC by Collection Care icons by The Brtish Library Board is licensed under a Creative Commons Attribution 2.0 UK: England & Wales License

You can also find out more about handling collections in our British Library Collection Care videos.

Christina Duffy