Osvaldo da Pos, University of Padua, Italy
Appears in ISCC News, No. 429 (Sept-Oct 2007)
There are two basic convictions about colors: (1) they carry information about the objects they belong to; and (2) their appearance depends on the context of the light, the spatial disposition of nearby objects, and their temporal changes. The two features sometimes strongly conflict. Constancy, whereby the color of an object tends to appear unchanged although environmental factors vary, vies with dependence on context. It is not rare for contextual effects to be considered illusory, although their occurrence obeys established rules.
The figures here exemplify how a visual illusion can be analyzed. Why does the central square, always the same, appear different when the surroundings are varied?
Striking changes occur when the lateral squares start in contact with the central one, and then a small misalignment or gap between them produces a completely different appearance. When there is no gap or misalignment, a cross is seen, composed of one strip transparent over the other: Two superimposed colors are seen in the grey square at the same time and in the same direction of sight, one in front and the other behind and through the first.
The two colors seen in the central grey square depend on the colors of the two adjacent squares. Already Helmholtz  tried to explain why those specific two colors were seen: it depended on the knowledge of the laws governing additive color mixtures, which, in the case of complementary colors, give an achromatic result. Therefore the colors of the adjacent squares are perceived in the central grey because their fusion precisely gives rise to that particular grey. Hering gave a radically different explanation involving no cognitive activity, but only physiological interactions. The two adjacent colors induce their complements in the grey area, so both the colors are visible in that grey square, although in different parts. Nevertheless those two weak colors can spread inside the square and completely characterize it, as only the sides can limit their spreading. Accordingly the central square appears transparent because in it both the colors of the back strip and of the front one are simultaneously perceivable (this would ultimately be the basic definition of transparency/translucency).
Although cognitive science still follows Helmholtz, the current understanding of the colored transparency (or translucency) effect does not resort to higher level "knowledge," but rather to physiological processes (based on cone excitation ratios , physical principles (based on Kubelka-Munk rules ; or on spectral filtration ), psycho-physical models (based on color convergence , ), phenomenological models (based on color similarity , , X junctions ,).
Even today we do not share a unique explanation of why the central grey square appears so different in different situations, so most people still speak of perceived translucency as an illusion, implicitly assuming that when a good explanation is achieved no illusion will exist anymore. A reasonable objection would be that, even when we reach a convincing explanation, still we would remain amused in seeing that the same grey square appears so different in different contextual conditions; the illusory aspect would remain intact, despite the scientific explanation.
 Helmholtz H. von, 1866 Handbuch der Physiologischen Optik, Leipzig: Voss
 Ripamonti C., Westland S., da Pos O. 2004 Conditions for perceptual transparency. Journal of Electronic Imaging. 13, pp. 29-35
 Brill M. H. 1976 Physical foundation of the perception of achromatic translucency. MIT Research Laboratory of Electronics Progress Reports No. 117 January, pp. 315-320
 Beck J. 1978 Additive and subtractive color mixture in color transparency, Perception & Psychophysics, 23, 256 - 267.
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 Chen, V. J., & D’Zmura, M. 1998 Test of a convergence model for color transparency perception. Perception, 27, 595-608.
 Hering E., Über die Theorie des simultanen Contrastes von Helmholtz. Vierte Mitteilung. Die subjective "Trennung der Lichtes in zwei complementare Portionen" In: Wissenschafliche Abhandlungen; hrsg. von der Sachsischen Akademie der Wissenschaften zu Leipzig, Leipzig : G. Thieme, n. , pp. 1- 11)
 Da Pos O. 1989-1991 Trasparenze. Transparency. Icone: Milano
 Watanabe T., Cavanagh P. 1993 Transparent surfaces defined by implicit X- junctions. Vision Research 33, 2339-2346
 Masin S.C. 2006 Test of models of achromatic transparency. Perception 35(12), 1611-1624.