The effect of gloss
We have indicated that objects appear coloured because they absorb some wavelengths of light. In fact all illuminated object, although coloured, will reflect some light from their uppermost layer. Light from this upper surface is unaffected by the object's colour, and will reflect some light from their uppermost layer.Light from this upper surface is unaffected by the object's colour, and will remain the same colour as the illuminate, which is normally white. Light from the surface reflection mixes with the coloured light from the body of the object and the combined effect is to reduce the colourfulness or saturation. The manner and degree of surface reflection can therefore significantly influence the object's colour.
Light from an object's surface may be reflected in a mirror-like or specular way, as might be the case with a very glossy print, or it may be a more diffuse reflection, as is the case with a print on uncoated paper. Between these extremes a combination of specular and diffuse reflections can occur, in varying proportions with different materials. The illumination itself may be directional or diffuse, and both of these factors can also affect the colour appearance.
If we have an illuminated smooth surface, some of the illumination is also directional, the object's appearance will be greatly affected by the angle of illumination relative to the angle of viewing (Figure 1.3). Where the angle of viewing (a), is the same as the angle of light incident on the sample, the light source itself will be seen in this situation the object will appear to have no colour, as the viewing angle is dominated by the reflection of the light source. By altering the viewing angle to position B, it is possible to view the surface without specular reflection. When viewed at this angle the colour appears very saturated as there is only a small amount of white light visible to desaturate it. This is why glossy prints appear particularly saturated and colourful, when viewed whit directional lighting, so long as the viewing angle aviods specular reflections.
When a smooth surface is diffusely illuminated, it is impossible to avoid viewing without seeing some surface reflections, as both the incident light and surface reflected light are coming from many angles simultaneously. Once again, the surface reflections are not coloured and take on the same colour as the light source. In most cases the illuminant will be white; therefore, surface reflections add white. Which serves to desaturate the colour. This is why coloured glossy surfaces look less saturated in diffuse illumination, as opposed to directional illumination.
Matt or rought objects also reflect a proportion of the incident light from their surface, but here the reflections are scattered at many different angles, even though the lighting may be directional. In this situation it is impossible to avoid viewing without surface reflections entering the eye and reducing saturation. Hence, matt surfaces always appear desaturated by these top surface reflections. This is one reason why matt prints appear less saturated than glossy prints.