11.11 DEGREE OF METAMERISM
In the case of colorimetric or exact colour reproduction, the degree of metamerism can be assessed by a direct comparison of the spectral reflectances (or relative power distributions) of the original and reproduction. But in the cases of equivalent, corresponding, and preferred colour reproduction, the colours in the reproduction must, in general, be physically different from those in the original: hence there must always be some metamerism. However, some reproduction colorants will tend to produce corresponding colours with greater degrees of metamerism than others, and some means of assessing this would be desirable. For this pur- pose it would probably be good enough to assess the degree of metamerism for an appropriate
colorimetric colour reproduction situation, and to regard the results as indicative of the degree of metamerism for the other cases. Thus, for a picture of a sun-lit outdoor scene projected with a dark surround by tungsten light, the degree of metamerism could be assessed by comparing the spectral reflectance curves of various original colours with those of dye-concentration combinations in the film that are metameric matches to them for tungsten light.
11.12 CONCLUSIONS
Spectral colour reproduction (equality of spectral reflectances or of relative spectral power dis- tributions), is a desirable objective in proofing systems and in the duplication of images. Although not attainable in most other imaging situations, it provides a useful basis for deter- mining the degree of metamerism of reproduction systems; this is an important feature in the mail-order catalogue business.
Colorimetric colour reproduction (equality of chromaticities and relative luminances) is a useful criterion when the original and reproduction have the same viewing conditions and use illuminants of the same colour, and is applicable to colour photocopying.
Exact colour reproduction (equality of chromaticities, relative luminances, and absolute luminances) ensures equality of appearance for original and reproduction if the viewing condi- tions are the same for both, and is applicable to virtual reality systems.
Equivalent colour reproduction (chromaticities, relative luminances, and absolute lumin- ances such as to ensure equality of appearance) can allow for all effects of viewing conditions, and is applicable to marketing on the web.
Corresponding colour reproduction (chromaticities and relative luminances such as to ensure equality of appearance when the original and reproduction luminance levels are the same) allows for all effects of viewing conditions except absolute luminance levels, and pro- vides a realistic criterion for general application.
However, for some objects whose colours are well-known, preferred colour reproduction may be required, wherein departures from equality of appearance (whether at equal or at dif- ferent absolute luminance levels) may be required in order to achieve a more pleasing result, and is applicable to portraiture and reflection prints for the consumer market.
For a discussion of Colour Reproduction Indices, see Chapter 37.
REFERENCES