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Digital Image Capture and Colour Management

National Gallery Company Online Picture Library files are derived from fully colour-calibrated digital image files created by The National Gallery[1] Read this article. Each image is consistent with any other, meaning that informed comparisons about colour, tone and brightness can be made by the users of the images to ensure consistent production to print.

In cases where we cannot re-photograph the painting digitally we have created scans from transparency.

Each image has been individually calibrated using the Gretag Macbeth 24-patch colour rendition chart.[2] This enables any inconsistencies in lighting to be corrected for each painting and ensures that the quality of the last image captured is as high as the first.

Each image has been individually checked at capture to ensure consistency. The quality assurance process at capture for each image had a mean ΔE unit < 4 for the 24 patches of the Gretag Macbeth chart, where a colour difference of one ΔE represents a just visible difference [3]. This means the images of the National Gallery collection are captured to the very highest colour and tonal standards and represent the paintings in the collection as closely as possible.

The tonal quality is ensured by using the Kodak Greyscale within the image plane. Using a Gretag Macbeth Spectroscan T-spectrophotometer the A, M and B steps of the Kodak greyscale have been measured with lightness (L) values of 97, 50 and 15 in the CIE Lab colour space. You will find that these steps in the images will match these lightness values within the images (using the 5x5 average picker in Adobe PhotoShop) [4].

File Formats and Colour Spaces

The Image files supplied are supplied as maximum quality jpeg (Joint Photographic Expert Group) in the Adobe (1998) RGB Colour Space [5]. The jpeg file format has been used to maximise quality while reducing file size. The Adobe (1998) RGB colour space has been adopted because it is now widely used and understood by photographers, designers and printers. The Adobe (1998) RGB colour space allows for end users to make the most of the colour information available, as it encapsulates the smaller sRGB IEC61966-2.1 (for use on monitors) and CMYK (Cyan, Magenta,Yellow and Key or Black, for printing) colour spaces.

Below is a graphical representation of Adobe (1998) RGB and one of the most common CMYK colour spaces, U.S. web Coated (SWOP) v2. Adobe (1998) RGB is represented by the larger triangular shaped border and U.S. Web Coated SWOP v2 is represented by the smaller area.

Colour Bars and Greyscale reference charts

We try to include Kodak Colour Control Patches and Kodak Greyscale reference charts within all images to aid correct reproduction of the paintings. However some small paintings may not show complete charts.

Why we don’t supply the images as CMYK

We do not supply the images in a CMYK colour space as all print presses have differing characteristics and the conversion to CMYK is best performed by the end user of the image. Please consult your printer who will be best able to advise you about conversion to CMYK.

Image Sizes available

The images are available at A3, A4 and A5 at 300dpi. Each file is prepared to a slightly larger size than the standard A3, A4 and A5 sizes to allow for an amount of cropping, as the whole of the painting surface is included within the image plane. The actual sizes that the files are prepared to are listed below

Paper Sizes Physical Size (mm) NGC Online Image sizes NGC Online Pixel Dimensions
A3 420 x 297 mm within 440 x 440 mm 5197 x 5197 pixels
A4 297 x 210 mm within 310 x 310 mm 3661 x 3661 pixels
A5 210 x 148 mm within 230 x 230 mm 2717 x 2717 pixels

Some smaller paintings may be smaller than the sizes quoted as in some cases they are not large enough to include a Kodak greyscale reference chart

Monitor Calibration

To ensure you can best judge the images you see on your screen, regular monitor calibration is required. There are some relatively simple ways to do this if you have the right machine or software.

If you are using the Mac OSX operating system you can use the Display Calibrator Assistant. www.apple.com/pro/training/colorsync/segment102867.html

If you are using a PC with Microsoft Windows and you have Adobe PhotoShop software you can use Adobe Gamma that comes as part of the Photoshop package.

www.adobe.com/support/

Alternatively there are a number of vendors who can supply monitor calibration systems.

Glossary of Terms

Spectrophotometer – an instrument for the measurement of spectral reflectance or transmittance

CIE Lab – A uniform colour space from the Commission Internationale de l’Éclairage (CIE) with three dimensional colour co-ordinates represented by lightness (L*), redness/greenness (a*) and yellowness/blueness (b*).

Colorimeter – an instrument for measuring reflected or transmitted light through red, green and blue filters.

Dpi – dots per inch. The resolution of a digital file. This term is always used in conjunction with the physical size of the files.


[1] See D. Saunders, J. Cupitt, C. White, S. Holt, ‘The MARCII Camera and the Scanning Initiative at The National Gallery’, The National Gallery Technical Bulletin, Volume 23 (2002), pp. 76–82

[2]The accuracy of image acquisition is judged by the average colour error for the 24 patches on a Macbeth Colourchecker chart; see C.S. McCamy, H. Marcus and J.G. Davidson, ‘A Color-Rendition Chart’, Journal of Applied Photographic Engineering, 2, no. 3 (1976), pp. 95–9.

[3] See R. McDonald and K.J. Smith,’CIE94 – a new colour difference formula’, Journal of the Society of Dyes and Colourists, III (1995), pp. 376–9.

[4]Some paintings with exceptionally bright whites brighter than the standard paper white have had their A lightness value adapted to ensure no loss of highlight information.

[5]The jpeg file format is the standardised ISO/IEC IS 10918-1 | ITU-T Recommendation T.81 by the Joint Photographic Expert Group