Specular and diffuse components

Most surface colours present some degree of gloss and this gloss surface will reflect at some angle the mirror image of the light which is falling upon it. With a high gloss surface the colour of the light reflected at this mirror image angle is that of the light source, little affected by the wavelength selective absorption of the colour. It is generally then, a thing to avoid. It is called the specular component of the reflected light which is reflected at all other angles besides that of the equal and opposite angle to the incident light which is called the diffuse component.

If one is measuring a smooth flat sample with very little surface irregularity, such as a sprayed paint or a flat plastic moulding there is little doubt that exclusion of the specular component is desirable. However, with some samples such as textured vinyls and some textiles it is not always possible to exclude all the specular component so that it must be included to obtain repeatable measurements.