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Gloss is an optical property which indicates how well a surface reflects light in a specular (mirror-like) direction. It is one of the important parameters that are used to describe the visual appearance of an object. The factors that affect gloss are the refractive index of the material, the angle of incident light and the surface topography.

Apparent gloss depends on the amount of specular reflection – light reflected from the surface in an equal amount and the symmetrical angle to the one of incoming light – in comparison with diffuse reflection – the amount of light scattered into other directions.

Specular and diffuse reflection

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, ${\displaystyle R_{s}}$, for an unpolarized light of intensity ${\displaystyle I_{0}}$, at angle of incidence ${\displaystyle i}$, giving the intensity of specularly reflected beam of intensity

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, ${\displaystyle R_{s}}$, for an unpolarized light of intensity

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 depend

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, ${\displaystyle R_{s}}$, for an unpolarized light of intensity ${\displaystyle I_{0}}$, at angle of incidence ${\displaystyle i}$, giving the intensity of specularly reflected beam of intensity ${\displaystyle I_{r}}$, while the refractive index of the surface specimen is ${\displaystyle m}$.

The Fresnel equation is given as follows : ${\displaystyle R_{s}={\frac {I_{r}}{I_{0}}}}$

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

${\displaystyle \Delta r=2h\cos i\;}$

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

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

If ${\displaystyle \Delta \phi \;}$ is small, the two beams (see Figure 1) are nearly in phase and therefore the specimen surface can be considered smooth. But when ${\displaystyle \Delta \phi =\pi \;}$, 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 ${\displaystyle \Delta \phi <{\frac {\pi }{2}}}$, then substitution into the equation above will produce:

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

This smooth surface condition is known as the Rayleigh roughness criterion.