The Effects of Chromatic Conditions for Transparency

When two rectangles overlap crosswise like a letter X, we perceive them as transparent in certain conditions. Perception of transparency is constrained by the luminance and chromatic relations of the overlapping and non-overlapping areas and the background, but not necessarily by their physical transmission profiles. In this study, we investigated the relation between luminance and chromatic mechanisms for the percept. Using two crossed rectangles (2 deg × 8 deg), we measured the ranges of chromaticity and luminance of the overlapping area (2 deg × 2 deg) that allowed the percept of transparent layers.

We found that the ‘transparency’ luminance range was shifted by the intensity of the background. When the two rectangles were isochromatic and isoluminant (10 cd m⁻²) on a dark background, the overlapping area had to be around 0.3 log units brighter than the rectangles. When the background was D₆₅ white (60 cd m⁻²), the luminance range for transparency was lower than the luminance of the rectangles. When the two rectangles differed in colour, however, the ‘transparency’ range of the overlapping area varied around the luminance of the rectangles. Transparency was perceived best when the colour of the overlapping area was on the additive mixture chromaticity line of the two rectangles. We discuss how the background influences the appearance of transparency and how the chromatic mechanism is effective in transparency.