Intensity-Modulated Illumination of Paintings and an Unexpected 3-Dimensional Effect

Leonardo, Vol. 11, p. 213. @ Pergamon Press Ltd. 1978. Printed in Great Britain OO2&094X 1781070I 4 2I 3W2.00/0 INTENSITY-M0DULATED ILLUMINATI0N 0F 3-DIMENSIONAL EFFECT PAINTINGS AND AN UNEXPECTED A. A. Mills* The whitest paints used by artists reflect about 90% of the light incident upon them, while black surfaces still reflect as much as 4 or 5%, so the maximum luminance scalethat can normally be realised upon an opaque base is of the order of 20:l. Most older paintings would be expected to show a shorter range than this, due to such factorsas sulphidedarkening, fading of pigments and the yellowing of oily media and varnish. In a real 3-dimensional scene, objects seldom receive the same illumination. Objects in shadow receive much less radiation than those in direct sunlight, sothat stillless light is reflected if they are inherently highly absorbing. The result is a much longer luminance scale. L. A. Jones and H. R. Condit gathered quantitative photometric data for a wide selection of scenes and found that an average range was 160:1,with values going up to 760:1 for scenes containing brightly sunlit white clouds and deep foreground shadows[l]. It is therefore apparent that the luminance range of a painting will rarely match that of the scene depicted, particularly for an outdoor or sunlit subject [2]. Some compression of the scale is inevitable, and no amount of ‘flat’ illumination directed upon it will correct this. This argument does not apply to pictures painted in transparent colours on a transparent support such as glass, which (likephotographic colour slides)may be illuminated and viewed by transmittedlight. However, only a few painters have worked with this medium, for example Gainsborough [3]. In order to extend the luminance scale of a painting upon an opaque support, light must be directed preferentially upon the lighter areas of the work with progressively lesser amounts upon the darker portions. Around 1908 R. W. Wood realised that intensity modulation could be obtained by illuminating a painting through a photographic diapositive (slide)of itself[4],but the techniques and apparatus available at that time rendered the procedure clumsyand impracticable, and his schemewas not published. I have investigated the effects of illuminationg high quality reproductions of works by Vermeerand by other artists in this manner,using modern 35-mm monochrome transparencies and associated equipment. I found that precise registration between the projected image and the original was most important. This was facilitated by adding a zoom lens plus vertical and horizontal screw adjustments to the slide projector. When exact alignment was achieved, it was observed that Vermeer’s works, in particular, were dramatically transformed: one appeared to be viewingthe actual scene through an open window rather than a flat representation *Planetary scientist and artist, Dept. of Astronomy and History of Science, University of Leicester. Leicester Road, Leicester LEI 7RH, England. (Received 22 Feb. 1978) of it. As well as the glowing brilliance of sunlit areas, resulting from the extended luminance scale (an effect shared by the paintings of other artists), an unexpected 3dimensional appearance was apparent to many independent viewers looking at Vermeer’s ‘View of Delft’, ‘Head of a Girl’ and ‘Lady and Gentleman Drinking Wine’. Normal binocular vision was effective in the darkened room. The reasons for this illusionare not entirelyclear to me, but they are no doubt related to other methods of achieving an apparently stereoscopic effect [51. The reasons appear to depend on the skill of the painter and on viewers’ experience of pictures: (1) The skill of Vermeer in constructing the perspective and modelling the shadows, colours and textures. His suggested use of some form of optical aid, such as the camera obscura [6], is significant. Presumably works by other artists that similarlyembody strong and geometricallycorrect depth information [5] would be equally effective. (2) A longer luminance scale is normally associated only with real scenes. Also the low visibility of the picture frames used renders it difficult to localise the surface of the pictures, which appears to be important in normal viewing [7]. Mixing the intensity-modulated illumination obtained with a slide with a certain amount of ordinary light modifies the effect and may improve the subjective impression...