It is still a matter of debate what is the critical variable for perceived transparency (Anderson, Singh, & O'Vari,
2008). Until recently, different systems of reference for setting the contrasts between figures were used in perceptual transparency research: reflectance (Metelli,
1985), luminance (Masin,
2006), Michelson contrast (Singh & Anderson,
2002), and lightness values (Beck & Ivry,
1988; Beck, Prazdy, & Ivry,
1984; Kitaoka,
2005). We chose to compute the contrast in terms of lightness values, which allowed us to provide perceptually scaled values of contrast regardless of the luminance levels in our stimuli. We used the following values of lightness: 3.0, 4.0, 4.5, 5.0, 6.0, and 7.0, which correspond to the following average luminance values: 6.97, 12.86, 17.00 21.14, 26.70, 32.33, and 45.23 cd/m
2 as measured through a photometer directly pointed to the figures on the screen. This correspondence fits the following conversion equation:
where W is the lightness value, and Y is the luminance level (Wyszecki,
1963).
Considering that the amount of contrast between regions varies according to the system of reference, the results will include a comparison between lightness, luminance, and Michelson contrast values. We will also calculate the best fit to the corresponding psychometric function for each system.