Fig. 11.6. The density required to achieve corresponding colour reproduction for whites, greys, and blacks, is shown for three different surround conditions: average surround, as is common for reflec- tion prints; dim surrounds, for television and for the viewing of sheet-films on illuminated light- boxes; and dark surrounds, for pictures projected in dark rooms.
The way in which the above definition enables allowance to be made for the effect of the sur- round on the reproduction of white, grey, and black colours is illustrated in Fig. 11.6 (repro- duced from Chapter 6). The reproduction density required in order to achieve corresponding colour reproduction is plotted as ordinate, and the logarithm of the exposure of the original scene relative to white (Bartleson and Breneman, 1967b) is plotted as abscissa (Hunt, 1969). The requirements for three different surround conditions are shown: average surround, such as occurs with reflection prints; dim surround, as for television viewing or viewing cut-sheet transparencies on illuminated light-boxes; and dark surround, as for films projected in a dark room. The gamma has to be raised to about 1.25 and 1.5, respectively, for the latter two cases because the dim and dark surrounds reduce the apparent contrast. (Ambient lighting in television viewing situations is usually variable, but it has been found that, if the television display has a gamma of about 1.5 in dark surround conditions, then, as the ambient lighting is increased to typical levels, the amount of viewing flare added usually reduces the gamma to about 1.25 as required (Novick, 1969).) (See Section 19.13.)
When the gamma of a reproduction is increased, a beneficial increase in the purity of its colours occurs, because the higher gamma results in greater ratios between the tristimulus values (MacAdam, 1938); and, although a dark or dim surround may result in some reduction in saturation, a net increase normally occurs (see Section 7.9).
To allow for the effects of a change in illuminant colour (Evans, 1943; Burnham, Evans and Newhall, 1957) a chromatic adaptation transform (CAT) can be used to provide the tristimulus values of corresponding colours; these are colours that have the same appearance in illumin- ants of two different colours (see Chapter 34). These transforms need to allow for the fact that the colour of the illuminant does not always correspond to the subjective white point. For instance, when transparencies are being projected in a dark room with tungsten light, the colour of the light on the screen appears slightly yellowish, even when the observer is fully adapted to it, and hence the film has to be slightly bluish to produce an apparent white (Hunt, 1965b; see Section 5.7); but adaptation may be complete for medium greys and be in excess for dark greys (Helson, 1938; Judd, 1940). This is discussed further in Chapters 34, 35, and 36.
The concept of corresponding colour reproduction is probably the most appropriate to use generally in colour reproduction problems. It has the same advantage over equivalent colour reproduction as colorimetric colour reproduction has over exact colour reproduction: by relat- ing the colours both in the original and in the reproduction to a reference white, allowance is made for the fact that observers tend to perceive not in isolation but with reference to a frame- work provided by the environment. For example, when a sunlit scene is projected by tungsten light with a dark surround equivalent colour reproduction would call for colours of very high purity in order to produce the sensations of high colourfulness experienced in bright sunlight; but the observer knows that, in the somewhat dimmer conditions provided by the projector, all colour sensations are lower in colourfulness, and the picture will look more natural if this is taken into account. More research is required to quantify these effects, and it is important not to forget that (just as with the concept of colorimetric colour reproduction) the picture will tend to look more like the original if the illumination level is adjusted to be closer to that for the orig- inal scene. Thus, it is well known that raising the screen luminance of a projected colour transparency raises the quality of a picture of a sunlit scene (Bartleson, 1965); and reflection prints of brightly-lit scenes are usually much improved if viewed under strong lighting. Similar effects also occur in television: when the green sulphide phosphor was introduced (see Sec- tion 21.12), there was an appreciable reduction in colour purity, and colorimetric colour reproduction deteriorated; but, because the green sulphide phosphor enables pictures of higher absolute luminance to be produced, the loss of purity was offset by a gain in the appar- ent colourfulness of the colours, and the final effect was that the pictures were improved (Matthews, 1963): in this case corresponding colour reproduction was made worse, but equi- valent colour reproduction was made better.
Fig. 11.7. Chromaticities of preferred colour reproductions (D) for green grass, blue sky, and Caucasian skin colours in reflection prints, together with the chromaticities of typical real samples of these objects (0), and typical reproductions given by a negative-positive photographic system (), all for Standard Illuminant C (SC). Also shown are the chromaticities for the preferred reproduc- tion of Caucasian skin on television viewed in dim ambient tungsten light (•), together with the associated subjective neutral point (•). The broken lines indicate areas of chromaticity giving acceptable colour reproduction. The percentage relative luminances for the preferred colours were 27, 30, and 39 for grass, sky, and skin, respectively; typical figures for real grass, and skin are about 13 and 35, respectively.
11.10 PREFERRED COLOUR REPRODUCTION
There is a considerable body of evidence that for Caucasian skin colour the above concepts must be supplemented to allow for the fact that a sun-tanned appearance is generally pre- ferred to average real skin colour (MacAdam, 1951; Bartleson and Bray, 1962). There may also be other colours where similar considerations apply: for instance, blue sky and blue water are usually preferred in real life to grey sky and grey water; colour films can have some sensitivity to ultra-violet radiation and hence tend to increase the blueness of sky and water relative to the saturation of the other reproduced colours, but such a tendency, if not overdone, may well be preferred to a more consistent reproduction. It may also be desirable to introduce other distortions of colour rendering to create mood or atmosphere in a picture. These factors may be very important in practice, but it is felt that the concepts of spectral, colorimetric, exact, equivalent, and corresponding colour reproduction, provide a framework that is a necessary preliminary to any discussion of deliberate distortions of colour reproduction. In this context, preferred colour reproduction is defined as reproduction in which the colours depart from equality of appearance to those in the original, either absolutely or relative to white, in order to give a more pleasing result to the viewer.
Fig. 11.8. Chromaticities of preferred colour reproduction (•) for green grass, blue sky, and Caucasian skin colours, in transparencies projected with tungsten light, together with the chro- maticities of typical reproductions given by two reversal films (and ). The chromaticity of the open-gate light from the projector (I) has a correlated colour temperature of about 3400 K; the subjective neutral point, marked ‘Grey’, had a correlated colour temperature of about 3700 K. Also shown is the chromaticity of a typical real sample of Caucasian skin illuminated by the light of the projector (0). The broken lines indicate areas of chromaticity giving acceptable colour reproduction. The percentage relative luminances for the preferred colours (expressed relative to normal open- gate luminance) were 6, 16, and 34 for grass, sky, and skin, respectively.
In Figs. 11.7 and 11.8, chromaticities are shown for preferred colour reproduction of blue sky, green grass, and Caucasian skin colours (Hunt, Pitt, and Winter, 1974). These results were obtained by making colour photographs of outdoor scenes containing well-defined areas of one of these test colours, and then varying the colours of those areas only. This was achieved by using pairs of opaque masks to obscure either the test part of the picture or the rest of it: by making two successive exposures in register in an enlarger, series of reflection prints for each scene were made in which the colour of the area of sky, grass, or skin was var- ied, but the colour of the rest of the picture was kept constant; and, by using the masks with pairs of slides projected on a screen, a similar result was obtained for transparencies. The colour of the sky, grass, or skin area was varied by covering that area with uniform pale colour-filters when enlarging or while projecting; in this way, these areas retained their inher- ent variety of tones and colours, and only the overall average colour was altered; neutral filters were used to control the luminances of the two parts of the picture.
Each reflection print and projected picture thus obtained was judged by a panel of observers for the quality of the colour reproduction of its blue sky, green grass, or Caucasian skin. The average chromaticity and relative luminance of each of these colours was then measured and correlated with the observers’ judgments to obtain the results shown in Figs. 11.7 and 11.8. For the reflection prints, the judgments were made in typical indoor daylight, and the
colorimetry was evaluated for Standard Illuminant C. For the projected transparencies, the colorimetry was evaluated for the actual projector illuminant. The results of a similar invest- igation on the prefer