The human visual system (HVS) is remarkably good at detecting subsurface light transport - we can easily tell the difference between a translucent glass and an opaque metal, translucent wax and opaque stone. The extrinsic factors include, but are not limited to, the illumination direction ( Fleming & Bülthoff, 2005 Xiao et al., 2014), object shape ( Fleming & Bülthoff, 2005 Gigilashvili et al., 2018b), and the color of the surface a translucent object is placed on ( Gigilashvili et al., 2020a). How this passage of light through a material relates to a visual sensation of translucency remains unclear to date. A photon can get absorbed or scattered, that is, redirected toward a different direction when there is a change in the index of refraction, either at the external surface of the object, or inside its volume ( Tavel, 1999). They define how the light propagates through the media. The intrinsic factors are the physical parameters found in the radiative transfer equation ( Chandrasekhar, 1960), such as the index of refraction, and the absorption and scattering coefficients, as well as the scattering phase function. Translucency is impacted by multiple intrinsic and extrinsic factors. A translucent appearance is usually the result of a visual stimulus incident onto a retina from the objects permitting some degree of the subsurface light transport. The etymology of the term is related to the Latin words “trans” (through) and “lux” (light) - implying light penetration inside the body of the material ( Kaltenbach, 2012). According to the ASTM (2017) translucency is “the property of a specimen by which it transmits light diffusely without permitting a clear view of objects beyond the specimen and not in contact with it.” Translucency is seen as a phenomenon “between the extremes of complete transparency and complete opacity” ( Eugène, 2008). Although the color information incident on the human retina encodes important information about the objects and materials, overall sensation also depends “on the appearance of that colour due to the relationship between the light transmitted, the light reflected, and the light scattered by the body of the object” ( Pointer, 2003). Translucency is among the most essential visual attributes of appearance, along with color, gloss, and texture ( Pointer, 2006 Eugène, 2008), remaining the least studied one among those ( Anderson, 2011). According to the International Commission on Illumination (the CIE - Commission Internationale de l'Eclairage) total appearance “points out the visual aspects of objects and scenes” ( Pointer, 2006). By their appearance, we can effortlessly identify materials within seconds ( Sharan et al., 2009 Wiebel et al., 2013). For instance, we use the visual appearance information to judge whether materials are fragile or elastic, whether food is spoiled or edible. How different objects and materials appear to human observers is important not only in commerce, where customer choice and satisfaction are often influenced by the visual look of the product, but also in trivial daily tasks performed by humans. Furthermore, this review summarizes current knowledge gaps, fundamental challenges and existing ambiguities with a goal to facilitate translucency perception research in the future. This article overviews the knowledge status about the visual perception of translucency and highlights the applications of the translucency perception research. However, little is known how the optical properties of a material relate to a perception evoked in humans. Accurate prediction of the appearance of the translucent objects can have a significant commercial impact in the fields such as three-dimensional printing. The knowledge about the visual mechanisms of the translucency perception remains limited. Translucency as an optical property of a material relates to the radiative transfer inside and through this medium, and translucency as a perceptual phenomenon describes the visual sensation experienced by humans when observing a given material under given conditions. Translucency is an optical and a perceptual phenomenon that characterizes subsurface light transport through objects and materials.
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