Sensory and physical determinants of perceived achromatic transparency

What are the physical and sensory determinants of perceived transparency? To explore this question, we simulated pairs of physically different neutral density filters on a CRT and asked observers to match their perceived transparency. Matching was accomplished by adjusting one of two physically independent filter properties, reflectivity and inner transmittance. Results show that observers can make reliable matches through a linear trade-off of these two properties. In a separate experiment, observers matched the perceived contrast of the overlaid regions. The reflectivity and inner transmittance values for contrast matches are similar to those of perceived transparency matches, suggesting that perceived image contrast is the sensory determinant of perceived transparency. In variegated displays, neither Michelson contrast nor other standard contrast metrics predicts contrast appearance. When perceived transparency is plotted in terms of filter reflectance and filter transmittance, perceived transparency corresponds closely to filter transmittance.