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CAMS 93

6.3.1 CIECAM97s

In 1997 the CIE Technical Committee TC1-34 agreed to adopt a CAM known as
CIECAM97s (Luo and Hunt, 1998a). CIECAM97s comprises two parts: the first
part is a CAT that computes the corresponding tristimulus values; the second
part calculates a set of colour-appearance descriptors for the corresponding
tristimulus values. The particular CAT that is used in CIECAM97s is known as
CMCCAT97 (Luo and Hunt, 1998b) and this is described in Section 6.2.2.
CIECAM97s includes a forward and a reverse mode. The forward mode
transforms the tristimulus values of a sample under a non-daylight illuminant,
such as A, to those for the corresponding colour under a daylight illuminant (in
fact, the equal-energy illuminant) and then computes some terms that describe
the colour appearance of the sample under the daylight illuminant. The reverse
mode is used to predict the tristimulus values under a non-daylight illuminant
based upon the colour-appearance descriptors for a sample viewed in daylight.
The output of the forward mode is a set of attributes that predict colour
appearance: brightness, lightness, colourfulness, chroma, saturation and hue.
The predictions from the CIECAM97s model are in agreement with a number
of colour-appearance phenomena such as chromatic adaptation (CMCCAT97 is
included within CIECAM97s), Hunt’s effect, Stevens’ effect (Stevens and
Stevens, 1963) and the surround effect (Bartleson and Breneman, 1967).
However, there are certain well-known colour-appearance phenomena that
CIECAM97s cannot predict (Luo, 2002b).
The forward mode considers a sample viewed under a (non-white) test
illuminant against an achromatic background and computes the corresponding
colour-appearance attributes for the sample under a (white) reference illuminant.
The starting data required for the forward transform include the tristimulus
values of the sample in the test illuminant (X, Y, Z), the adopted white in the test
illuminant (X WT , Y WT , Z WT ), the reference white in the reference illuminant (X WR ,
Y WR , Z WR ), the Y tristimulus value of the achromatic background against which
the patch is viewed (Y ), and the luminance (in units of cd/m ) of the reference
2
B
white (L ) and the achromatic background (L ) against which the sample is
W
A
viewed.
The model requires that values are assigned to four parameters, F, c, F LL and
N . Different values are recommended for these parameters depending upon the
C
viewing conditions (Luo, 2002b). However, for large moderately illuminated
scenes the following values should be used: F ¼ 1.0, c ¼ 0.690, F ¼ 0.0 and
LL
N ¼ 1.0. The following steps describe in detail the computation of the forward
C
mode of CIECAM97s.
Step 1: Calculate the RGB values for the test sample (R, G, B) and for the
reference white under the test (R WT , G WT , B WT ) and reference fields (R WR , G WR ,
B WR ) using Equations (6.6). Note that this stage is equivalent to multiplying the