Tate has produced these representations of Frank Auerbach's work, in conjuction with The National Gallery and Hewlett-Packard Labs, using multiple images of each painting to digitally represent their surface structure.

When a light is shone, at a harsh angle, across the face of a painting, any unevenness in the surface structure will cast shadows. Photographing this effect, known as a 'Raking Light Photography', is a traditional way of documenting the surface structure of painting.

However this approach will only record the shadows cast by light from one angle at a time and it is difficult and very time consuming to exactly repeat any given light position during subsequent examinations.

Polynomial Texture Mapping, PTM, is a new approach being applied to the examination of paintings. This process involves capturing a special set of digital images of an painting lit from various different, but measured angles. These images and the associated light measurements are then combined together within a computer to produce a virtual light map of a painting's surface.

Now it is possible to achieve this with a digital camera and one light source, carefully moving and measuring the light position between pictures. However, this can take a long time and requires a lot of patient work.

In order to make the process easier The National Gallery have produced a prototype imaging dome to facilitate the image capture process and test the technique, which was loaned to Tate for this Special Imaging Treatment.

The dome is composed of a wooden framework with mounting points for 24 lights and a digital camera. The lights used in this prototype are defused 12V tungsten halogen spot lights, wired up to a control box. Each light is angled to point at the floor in the middle of the Dome and the position of each light is measured, relative to the center of the base of the dome. The colour of lights used in this prototype vary slightly from one to another, a white target is captured for each of the lights to facilitate image processing to compensate for this variation.

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Image showing the effects of white balancing on one of the images of Alexander Cozens's Scirocco Sunset. The original image is displayed on the left and the white balanced image is shown on the right. Once all of the lights have been calibrated in this way, the dome produces a more accurate representation of the artwork.

A painting is positioned, on the target below the dome, exposure measurements are taken and then a set of 24 images are captured, one image for each of the lights. A second set of 24 images of a white target is also captured; these are used in the caibration process as noted above.

[left] Composite image, showing the eight images captured using the bottom eight lights, of Frank Auerbach's Rimbaud.