# Gloss (material appearance)

(Redirected from Glossy)

Gloss is an optical property describing the ability of a surface to reflect light into the specular direction. The factors that affect gloss are the refractive index of the material, the angle of incident light and the surface topography.

Gloss is one of the factors that describe the visual appearance of an object. Materials with smooth surfaces appear glossy. Very rough surfaces such as chalk reflect no specular light and appear dull. Gloss is also expressed as lustre in mineralogy, or sheen in certain fields of application.

The appearance of gloss depends on a number of parameters which include the illumination angle, surface condition and observer characteristics.

Primarily light is reflected from a surface in one of two ways. In specular reflection, the angle of the light reflected from the surface is equal and opposite to the angle of the incident light. A diffuse reflection scatters the incident light over a range of directions.

Specular and diffuse reflection

Variations in surface texture influence specular reflectance levels. Objects with a fine surface texture, i.e. highly polished and smooth, allow a high percentage of light to be reflected from their surfaces making them appear shiny to the eye. This is due to a greater amount of incident light striking the surface being reflected directly back to the observer; the majority of which being reflected in the specular direction.

Conversely objects with rough surfaces cause the light to be deflected at different angles according to the surface profile resulting in a scattering of light away from the angle of reflection. This causes the object to appear dull or matte. The image forming qualities are much lower making any reflection appear blurred. The higher the degree of surface roughness, the greater the scattering of light resulting in a lower gloss level.

Equally the type of substrate material has an important effect on the amount of specular reflection from its surface. Nonmetallic materials, i.e. plastics / coatings, 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 than nonmetals.

## Theory

Surface gloss is considered to be the amount of incident light that is reflected at the specular reflectance angle of the mean of that surface. So, specular gloss is proportional to the reflectance of the surface.

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

Figure1:Specular reflection of light from a rough surface

Surface roughness in micrometer range influences the specular reflectance levels. The diagram on the right depicts the reflection at an angle $i$ on a rough surface with a characteristic roughness height $h$. The path difference between rays reflected from the top and bottom of the surface bumps is:

$\Delta r = 2h \cos i \;$

When the wavelength of the light is $\lambda$, the phase difference will be:

$\Delta \phi = \frac{4\pi h \cos i}{\lambda} \;$

If $\Delta \phi \;$ is small, the two beams (see Figure 1) are nearly in phase and therefore the specimen surface can be considered smooth. But when $\Delta \phi = \pi \;$, then beams are not in phase and through 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 an arbitrary criterion for smooth surface is $\Delta \phi < \frac{\pi}{2}$, then substitution into the equation above will produce:

$h < \frac {\lambda}{8 \cos i} \;$

This smooth surface condition is known as the Rayleigh criterion.