Post 190 –by Gautam Shah
Objects are seen when illuminated. Light illuminates objects, but in the process affects it in several ways. Some light is absorbed within it which is mainly responsible for perception of colour. If the object is transparent to translucent, than part of the light gets transmitted though it. The transmitted light (through the object) may get re-reflected through the object. In the process the re-reflected light gets scattered. Light also gets specularly reflected from it the surface of the object provided it has reflective properties.
⊕ Specular reflection: The substantial quantum of light is reflected from the surface, in equal and opposite direction, to the angle of incident light.
⊕ Diffuse reflection: The light that gets scattered in many directions.
Gloss is one of the most important of parameters that describe the visual appearance of an object. It is an optical property of a surface to reflect light in a specular (mirror-like) direction. A specular reflection can be said to be a reflection in very small range of direction. This is distinctively different from diffuse reflection, where the incoming light is reflected in a broad range of directions. Factors that determine the gloss are the refractive index of the material, the angle of incident light and the surface topography.
Distinctive examples of specular and diffuse reflections are glossy and matte paints. ‘Matte paints have almost exclusively diffuse reflection, while glossy paints have both specular and diffuse reflections’. A surface of a non-absorbing material such as plaster, are nearly perfect diffuser, whereas polished metallic objects, glass, and some polymers are specularly reflective.
Texture of a surface is a key determinant of level of specular reflection. Objects with a smooth surface, i.e. highly polished or containing coatings with finely dispersed pigments, appear shiny to the eye due to a large amount of light being reflected in a specular direction. Rough surfaces have irregular defects or undulations which scatter the light in other directions, and so appear dull. Dull surfaces do not reflect the image (like a mirror), so the reflections seem blurred and distorted.
In case of transparent and translucent materials, not only there is surface level specular reflection but the back face coatings-treatments and closely placed materials (substrate materials) also reflect specularly. The reflecting material of mirrors is the highly polished surface of glass, and its capacity to reflect light from the back side coating of lead, aluminium or silver.
Gloss is often considered a subjective term (though scientific tests exist) for representing specular reflection of a surface. Commercially, in the surface finish world, gloss is recognized as Flat or matte, Egg shell, Semi-gloss, Full gloss. A Flat surface is considered free from sheen even when viewed at an oblique angle whereas at the other extreme Full gloss is a mirror like face at all angles of view. Satin (silk) finish is somewhere between Egg shell and semi gloss finish.
A rough guide for level of gloss of a surface is: Flat (0 to 10 % reflective), Low Sheen (10 to 25% reflective), whereas Glossy (70–90% reflective).
- The non-reflecting surface of a flat-matt finish helps to hide or de-emphasize the imperfections of the surface to be painted. Semi-gloss finish and high-gloss finishes highlight such imperfections. Flat-matt surfaces do not reflect light at any angle so are very useful as a coating-finishing over very extensive surfaces such as corridors, lobbies, halls, ceilings, and exterior faces, etc. Flat-matt finishes over the floors, also offer an assurance of their being no slippery surface. Flat-matt surfaces show off true colours at all angles, so are visually more recognizable. Flat-matt surfaces may look patchy if the substrate or the application techniques are improper. Automobile interior trims are intentionally designed to be of matt finish so as to minimize the irritating reflection while driving.
- Semi-gloss and full gloss finishes offer better wearing, cleaning and humidity resistance than flat paints. Flat paints may show polishing or burnishing marks when rubbed for cleaning. Semi gloss surfaces are slightly deceptive and may not show true colour or depth perception.
- High gloss surfaces get easily tarnished by oil, grease or waxy streaks. High gloss surfaces are first to show off substrate impurities and malfunctioning of plasticizing agents or entrapped moisture, etc. High gloss surfaces show of minute scratches or dents etc. in terms of defective specular reflection. High gloss surfaces are very deceptive in colour recognition.