Perceived transparency: Tradeoffs between reflectance and transmittance

What governs the degree of perceived transparency? Can it be related to physical properties of filters, such as reflectance from the front surface, or transmittance through the absorptive media? Achromatic backgrounds composed of random sized, overlapping ellipses were generated on a CRT. Two neutral density filters were simulated as moving on top of this surface. The standard filter was fixed in reflectance and transmittance, and observers adjusted the test filter so that the two appeared equally transparent (if possible). In one set of conditions, the test filter had a fixed reflectance and observers adjusted transmittance. In the other set, the test filter had a fixed transmittance and observers adjusted reflectance. Trials from both conditions were randomly interleaved and the observer was not informed whether reflectance or transmittance was adjustable. Results showed that: (1) When the fixed parameters of the two filters were set to be equal, observers adjusted the variable parameter to also be equal. (2) When the reflectances of the two filters were fixed at values differing up to 20%, observers were almost always able to adjust transmittance to make a match, and the transmittance of the test filter was set very close to that of the standard. There was a slight trend to reduce the transmittance when the test reflectance was higher than the standard. (3) When transmittance was fixed at different values between filters, observers found it difficult to match transparency. However, when the difference in transmittances was small, the reflectance of the test filter was increased to match filters with lower transmittance and decreased to match filters with higher transmittance. The findings indicate that transmittance may be more important than reflectance in perceived transparency. However, some tradeoff is possible between transmittance and reflectance, and observers may adjust brightness to equate contrast of overlaid regions.