Influence of color space on color rendering evaluation

The color rendering index CRI that we commonly use is also obtained by calculating the distance between each standard swatch in the color space under the illumination of the source to be tested and the reference source. If the distance of the color of a standard swatch between the source under test and the reference source is greater in the color space, then the color fidelity of the source is worse.

In addition to the most commonly used CRI, other indices describing the color quality of the source, such as GAI, FCI, CQS, etc., are calculated by the distribution of colors in the color space. For example, GAI is calculated by calculating the area of ​​the wall surrounded by the CIE1976 u'v' color space under the illumination of the eight swatches used in the CRI.

In order to use color space to describe color stimuli more accurately, we want the color space to be as uniform as possible. That is to say, we hope that the difference in color coordinates in the color space is proportional to the color difference seen by our human eyes (ie, the larger the color coordinate distance is, the larger the color difference is), but not all color spaces have such uniformity.

In all color spaces, the worst uniformity is the CIE 1931 xy color space (also known as the xy chromaticity diagram), which is revealed by MacAdam Ellipses.

The CIE 1976 u'v' used by GAI also has unevenness. The color of 1296 Munsell swatches under CIE Illuminant C illumination is identified in Figure 1(a).

Figure 1 Distribution of Munsell swatches in CIE1976 u'v'

The 1296 swatches were used because the experiment found that the visual chromatic aberration between adjacent swatches was the same under Illuminant C. That is to say, if the color space is uniform, the distance between the swatch and the swatch should be the same, that is, one circle can be enclosed around the city. However, in Figure 1, these swatches are enclosed in one ellipse (for a clearer display, we have selected some swatches with moderate brightness in Figure 1(b)).

Numerous studies have focused on improving the uniformity of color space to better help us describe color and chromatic aberration. Currently, the best color space is CIECAM02-UCS. We also identified the colors of the 1296 Munsell swatches under Illuminant C in this CIECAM02-UCS space, as shown in Figure 2. We can see that the distribution of the swatches in Figure 2 is much more uniform than in Figure 1.

Figure 2 Distribution of Munsell swatches in CIECAM02-UCS

If two different light sources, one can increase the saturation of the color of some areas, and the other can only increase the saturation of the other color area. In the uneven color space, the improvement of color saturation of these two kinds of light sources cannot be reasonably evaluated by the color gamut index.

The latest index IES Rf also uses the most uniform CIECAM02-UCS, and CQS also attempts to adjust the CIELab space to CIECAM02-UCS (Note: CIELab has a significant improvement over CIE 1976 u'v').

In addition to color space uniformity, we should also consider the stability of the color gamut index for color temperature. If the color coordinates of the swatch in the color space change greatly with the change of the color temperature, this undoubtedly exaggerates the problem of color space non-uniformity. The GAI is greatly affected by the color temperature.

Figure 3 shows the distribution of eight swatches used to calculate GAI in CIE 1976 u'v' under 3000K blackbody and 6500K daylight models;

Figure 4 shows the distribution in CIECAM02-UCS.

It is very obvious that the areas covered by the two polygons in Figure 3 are significantly different, while the two polygons in Figure 4 are similar.

Figure 3 Distribution of eight swatches used to calculate GAI in CIE1976 u'v' under illumination from two sources

Figure 4 Distribution of 8 swatches used to calculate GAI in CIECAM02-UCS under illumination from two sources