Perceptual transparency

In our everyday life, we often experience the view of objects through transparent surfaces. Physically transparent surfaces allow the transmission of a certain amount of light rays through them. Sometimes nearly the totality of rays is transmitted across the surface without significant changes of direction or chromaticity, as in the case of air; sometimes only light at a certain wavelength is transmitted, as for coloured glass. Perceptually, the problem of transparency is much more challenging: both the light rays coming from the transparent surface and those coming from the object behind it do reach the same retinal location, triggering a single sensorial process. The system somehow maps this information onto a perceptual representation of two different objects. Physical transparency was shown to be neither a sufficient nor a necessary condition for perceptual transparency. Fuchs (1923) showed that when a small portion of a transparent surface is observed, neither the surface colour, nor the fusion colour is perceived, but only the colour resulting from the fusion of that of the transparent surface and that of the background. Tudor-Hart (1928) showed it is not possible to perceive transparency in a totally homogeneous field. Metzger (1975) showed that patterns of opaque paper can induce the illusion of transparency, in the absence of physical transparency. In order to distinguish perceptual from physical transparency, the former has often been addressed as transparency illusion.