When light illuminates an object, it interacts with it in a number of ways: • Absorbed within it (largely responsible for colour) • Transmitted through it (dependent on the surface transparency and opacity) • Scattered from or within it (diffuse reflection, haze and transmission) • Specularly reflected from it (gloss) Variations in surface texture directly influence the 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 whilst rough surfaces reflect no specular light as the light is scattered in other directions and therefore appears dull. The image forming qualities of these surfaces are much lower making any reflections appear blurred and distorted. Substrate material type also influences the gloss of a surface. Non-metallic materials, i.e. plastics etc. produce a higher level of reflected light when illuminated at a greater illumination angle due to light being absorbed into the material or being diffusely scattered depending on the colour of the material. Metals do not suffer from this effect producing higher amounts of reflection at any angle. The Fresnel formula gives the specular reflectance, R_s , for an unpolarized light of
intensity I_0 , at angle of incidence i , giving the intensity of specularly reflected beam of intensity I_r , while the refractive index of the surface specimen is m . The
Fresnel equation is given as follows : R_s = \frac{I_r}{I_0} : R_s = \frac{1}{2} \left[\left(\frac{\cos i - \sqrt{m^2 - \sin^2 i}}{\cos i + \sqrt{m^2 - \sin^2 i}}\right)^2 + \left(\frac{m^2 \cos i - \sqrt{m^2 - \sin^2 i}}{m^2 \cos i + \sqrt{m^2 - \sin^2 i}}\right)^2\right]
Surface roughness Surface roughness influences the specular reflectance levels; in the visible frequencies, the
surface finish in the
micrometre range is most relevant. The diagram on the right depicts the reflection at an angle i on a rough surface with a characteristic roughness height variation \Delta h . The path difference between rays reflected from the top and bottom of the surface bumps is: :\Delta r = 2 \Delta h \cos i \; When the wavelength of the light is \lambda, the phase difference will be: :\Delta \phi = \frac{2\pi}{\lambda} \Delta r = \frac{4\pi \Delta h \cos i}{\lambda} \; If \Delta \phi \; is small, the two beams (see Figure 1) are nearly in phase, resulting in
constructive interference; therefore, the specimen surface can be considered smooth. But when \Delta \phi = \pi \;, then beams are not in phase and through
destructive interference, cancellation of each other will occur. Low intensity of specularly reflected light means the surface is rough and it scatters the light in other directions. If the middle phase value is taken as criterion for smooth surface, \Delta \phi , then substitution into the equation above will produce: :\Delta h This smooth surface condition is known as the
Rayleigh roughness criterion. == History ==