The purpose of this study was to investigate visual depth perception of real physical colour stimuli with red-green and blue-yellow opponency. We modified the Howard-Dolman stereotest where subjects should determine the closer of two bars emitting the light of definite colour placed in front of a CRT monitor served as colour background. Two-alternative forced-choice paradigms were used to determine depth perception threshold values. Thresholds close to 5-10 arcsec for luminance and colour contrast stimuli were revealed both for red-green and blue-yellow stimuli. A term, stereosensitivity -a reciprocal magnitude of the stereothreshold -was introduced to separately analyse luminance and colour contrast contributions in total depth perception. Comparing stereo sensitivity values for different colour stimuli, stereo sensitivity in case of red-green is still better than comparing to blue-yellow colour contrast. The colour and luminance contrast in CIE L*a*b* units (at least in the restricted dynamic range) could be applicable as a psychophysical metric to characterise stereo sensitivity. We suppose the colour and luminance contrast guided stereo mechanisms as two additive contributions in total stereo sense. Stereosense thresholds are more sensitive to luminance contrast changes as compared with colour contrast changes (in L*a*b* metric).

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Perception of different color contrast stimuli was studied in the presence of light scattering in a fog chamber in Clermont-Ferrand and in laboratory conditions where light scattering of similar levels was obtained. Blue (shortest wavelength) light is scattered in fog to the greatest extent, causing deterioration of vision quality especially for the monochromatic blue stimuli. We have done spectral measurements of the light source in different density fog conditions and no spectral changes were found produced by the fog. Psychophysical measurements of the acuity in fog were done for two subjects with optotypes analyzed for red and blue stimuli.