Gloss (material appearance)

Gloss is an optical property, which is based on the interaction of light with physical characteristics of a surface. It is actually 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 can be said as a view of material appearance. Materials with smooth surfaces appear glossy, while very rough surfaces reflect no specular light and appear dull. Chalk is an example of a very rough surface that reflects little to no specular light. Gloss is also expressed as lustre in mineralogy, or sheen in certain fields of application.

Qualitative and quantitative view

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 I₀, 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 λ, 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.

Gloss measurement

Specular reflection is measured with a specular glossmeter, and reported in gloss units. Unpolarised white light is concentrated by a condenser lens onto a field aperture, which is located in the focal plane of the source lens. The reflected beam at the surface is later collected by the receptor lens. The intensity of the beam is then measured through a photodetector.

The common angles of incidence for gloss measurement are 20°, 60° and 85°. In some cases 45° and 75° geometry is used. Low gloss surfaces are recommended to be measured with 85° settings.

The typical standards for gloss measurements are ASTM D 2457, DIN EN ISO 2813 and DIN 67530.

See also

• List of optical topics
• Visual appearance
• Glossmeter

References

• Meeten, G.H. (1986). Optical Properties of Polymers. London: Elsevier Applied Science. pp. 326–329. ISBN 0-85334-434-5. 

External links

Instrumentation for measurement and evaluation of gloss characteristics is available from:

• Rhopoint Instruments