Perceptual transparency

Perceptual transparency

Perceptual transparency is the phenonmenon of seeing one surface behind another. []

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 raysthrough them. Sometimes nearly the totality of rays is transmitted across the surface withoutsignificant changes of direction or chromaticity, as in the case of air; sometimes only light ata certain wavelength is transmitted, as for coloured glass.Perceptually, the problem of transparency is much more challenging: both the light rayscoming from the transparent surface and those coming from the object behind it do reach thesame retinal location, triggering a single sensorial process. The system somehow maps thisinformation onto a perceptual representation of two different objects.Physical transparency was shown to be neither a sufficient nor a necessary condition forperceptual 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 totallyhomogeneous field. Metzger (1975) showed that patterns of opaque paper can induce theillusion of transparency, in the absence of physical transparency. In order to distinguishperceptual from physical transparency, the former has often been addressed as transparencyillusion.

Paradoxically, however, two models developed within a physical context have longdominated the research in the field of perceptual transparency: the episcotister model byMetelli (1970; 1974) and the filter model by Beck et al. (1984).

Metelli’s episcotister model and the luminance conditions for transparency

Though he was not the first author to study the phenomenon of transparency illusion, theGestalt psychologist Metelli was probably the one who made the major contribution to theproblem. Like his forecomers, Metelli faces the problem from a phenomenical more than from aphysiological point of view. In other words, he did not investigate which are thephysiological algorithms or the brain networks underlying transparency perception, butstudied and classified the conditions under which a transparency illusion is generated. Indoing so, Metelli marks an approach to the problem that will be followed by many scientistsafter him.The model is based on the idea that the perceptual colour scission following transparencyis the opposite of colour fusion in a rotating episcotister, i.e. a rotating disk that alternatesopen and solid sectors.Metelli referred to colour fusion in a physical situation in which an episcotister rotates infront of an opaque background of reflectance A; the episcotister has an open sector of size t (aproportion of the total disk) and a solid sector of size (1-t) having reflectance r. Thereflectance of the solid sectors and that of background are fused by rotation to produce avirtual reflectance value p:

P=t*A +(1-t)*R

that is the weighted sum of background reflectance and episcotister solid sectorreflectance.

An episcotister is not a transparent object. Nevertheless, Beck et al. (1984) proposed analternative model, based on transparent filters, that has the characteristic of including theeffects of repetitive reflections between the transparent layer and the underlying surface.Both the episcotister model and the filter model, in their original formulation, were writtenin terms of reflectance values. A consequence is that their validity as physical models isdependent on the illumination conditions. However, both models can be rewritten in terms ofluminance, as shown by Gerbino et al. (1990).Though physically correct in a number of situations, the filter model did never gain asignificant role in the prediction of perceptual transparency, perhaps because in spite of beingmuch more complicated than the episcotister model, it doesn’t lead to significantimprovements in predictions about the occurrence of the illusion.

While Metelli’s episcotister model has long remained the preferred framework for thestudy of luminance conditions in transparency illusion, its validity as a theory of perceptionhas been challenged by different studies.Beck et al. (1984) showed that only constraints (i) and (ii) imposed by the episcotistermodel are necessary for illusion of transparency; when constraints (iii) and (iv) are notfulfilled, the illusion can still be experienced. They also argued that the degree of perceivedtransparency depends on lightness more than reflectance.Masin and Fukuda (1993) proposed as alternative conditions for transparency to (i) and (ii)the ordinal condition p Є (a, q) [or q Є (p, b)] , that was shown to agree better thanepiscotister model with transparency-judgements performed by naïve subjects in a yes-notask (Masin 1997).Metelli’s equations were extended to three dimensional colour space by D’Zmura et al.(1997). According to the model, transparency illusion would be generated by coherentconvergence and translation in colour space. However, also in colour space, evidence wasfound in which the perceptual appearance does not reflect the physical model. For instance,D’Zmura et al. (1997) showed that equiluminant convergence and translation in colour spacecan elicit an impression of transparency, even if no episcotister nor physical filter cangenerate this stimulus configuration. Chen and D’Zmura (1998) showed deviations from thepredictions of convergence model when the transparent regions have complementary hues.


*Beck, J., K. Prazdny, et al. (1984). "The perception of transparency with achromatic colors." Percept Psychophys 35(5): 407-22.
*Metelli, F. (1970). An Algebraic Development of the Theory of Perceptual Transparency. Contemporary Problems in Perception. London, Taylor and Francis.
*Metelli, F. (1974). "The perception of transparency." Scientific American 230: 91-98.
*Tudor-Hart, B. (1928). "Studies in Transparency, Form and Color." Psychologische Forschung X: 255-298.

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