Impact of background field extent and illuminance on chromatic adaptation

Ma Shining – KU Leuven

Chromatic adaptation is an important visual mechanism for adapting to the changes in the color (spectrum) of the illumination. CIE recommended the CAT02 chromatic adaptation transform (CAT) embedded in CIECAM02 to predict adaptive color shifts. The equation predicting the degree of adaptation (D), is only related to the luminance of adaptation field. This paper will report the influence of background size on D, with equal vertical eye illuminance.
Memory Color Matching (MCM) method has been used to collect corresponding color sets for 3 adapting field extents: 20°, 40°, 60°, illuminated by thirteen neutral and colored illuminations. In the experiments, the stimulus background was a white stage and the test stimulus was a 3D grey cube centrally positioned in the stage. The vertical illuminance measured at the eye position of observers was kept at 7 lux for all the adapting fields. The background luminance values for the three field extents were 180 cd/m2, 35 cd/m2 and 20 cd/m2, respectively.
During the experiments, the observers were asked to adjust the apparent color of the cube to neutral grey by a keyboard. Note that they needed to adapt for 45 seconds before starting matching. To minimize starting bias, each experiment was repeated 4 times with 4 different chromaticity starting points evenly distributed in hue. Eleven observers with normal color vision participated in the experiments.
Observer uncertainty and variability (intra- and inter-) were quantified by MCDM (Mean color difference from the mean) in terms of color difference in CIE 1976 u’v’ space. The mean inter- and intra-observer MCDM values calculated across all illumination conditions for 20°, 40° and 60° adapting field are respectively 0.0097, 0.0075, 0.0094 u’v’ units and 0.0095, 0.0105, 0.0081 u’v’ units.
For each field of view, 12 sets of corresponding color (CC) sets were derived from the 13 illuminations by selecting the illuminant EEW as reference. Firstly, the performance of the CAT02 model has been tested. The color differences between the visual result and the CAT02 predictions averaged over the 12 CC sets are 0.0348, 0.0232, 0.0201 for the 20°, 40° and 60° adapting field respectively. The prediction error decreases as the adaptation field becomes larger which indicated that D is less overestimated under larger background.
Secondly, to minimize the prediction error (color difference), the D value was optimized for each field of view and illumination condition. For almost all the illuminations, the optimized D value increases with larger adapting field even though the luminance is eight times lower for 60° adapting field than 20°. It can be concluded that the lower performance of the CAT02 model is due to an overestimated D, especially for small background sizes.
In conclusion, the extent of the adapting field should be taken into account to improve D formula in the future.