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Gloss (material appearance)
Encyclopedia
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.
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,
, for an unpolarized light of intensity
, at angle of incidence 
, giving the intensity of specularly reflected beam of intensity 
, while the refractive index of the surface specimen is 
.
The Fresnel equation is given as follows :
Surface roughness in micrometer range influences the specular reflectance levels. The diagram on the right depicts the reflection at an angle 
on a rough surface with a characteristic roughness height 
. The path difference between rays reflected from the top and bottom of the surface bumps is:
When the wavelength of the light is
, the phase difference will be:
If
is small, the two beams (see Figure 1) are nearly in phase and therefore the specimen surface can be considered smooth. But when 
, 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 
, then substitution into the equation above will produce:
This smooth surface condition is known as the Rayleigh criterion.
, 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.
Topography
Topography is the study of Earth's surface shape and features or those ofplanets, moons, and asteroids...
.
Gloss can be said as a view of material appearance
Visual appearance
The visual appearance of objects is given by the way in which they reflect and transmit light. The color of objects is determined by the parts of the spectrum of light that are reflected or transmitted without being absorbed...
. 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
Lustre (mineralogy)
Lustre is a description of the way light interacts with the surface of a crystal, rock, or mineral. The word lustre traces its origins back to the Latin word lux, meaning "light", and generally implies radiance, gloss, or brilliance....
in mineralogy
Mineralogy
Mineralogy is the study of chemistry, crystal structure, and physical properties of minerals. Specific studies within mineralogy include the processes of mineral origin and formation, classification of minerals, their geographical distribution, as well as their utilization.-History:Early writing...
, or sheen
Gloss (paint)
Paint and other finishes come in a variety of finish gloss levels, which correspond to different levels of specular reflection.Some common names for levels of gloss include: flat, matte, eggshell, satin, silk, semi-gloss, high gloss. These terms are not standardized, and not all manufacturers use...
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 reflectanceReflectivity
In optics and photometry, reflectivity is the fraction of incident radiation reflected by a surface. In general it must be treated as a directional property that is a function of the reflected direction, the incident direction, and the incident wavelength...
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,
, for an unpolarized light of intensityIntensity (physics)
In physics, intensity is a measure of the energy flux, averaged over the period of the wave. The word "intensity" here is not synonymous with "strength", "amplitude", or "level", as it sometimes is in colloquial speech...
, at angle of incidence , giving the intensity of specularly reflected beam of intensity , while the refractive index of the surface specimen is .The Fresnel equation is given as follows :
Surface roughness
on a rough surface with a characteristic roughness height . The path difference between rays reflected from the top and bottom of the surface bumps is:When the wavelength of the light is
, the phase difference will be:If
is small, the two beams (see Figure 1) are nearly in phase and therefore the specimen surface can be considered smooth. But when , 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 , then substitution into the equation above will produce:This smooth surface condition is known as the Rayleigh criterion.
Gloss measurement
Specular reflection is measured with a specular glossmeterGlossmeter
A glossmeter measures specular reflection. The light intensity is registered over a small range of the reflection angle. The intensity is dependent on the material and the angle of illumination. In case of non-metals the amount of reflected light increases with the increase of the illumination angle...
, and reported in gloss units. Unpolarised white light is concentrated by a condenser
Condenser (heat transfer)
In systems involving heat transfer, a condenser is a device or unit used to condense a substance from its gaseous to its liquid state, typically by cooling it. In so doing, the latent heat is given up by the substance, and will transfer to the condenser coolant...
lens onto a field aperture
Aperture
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...
, 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
Standards organization
A standards organization, standards body, standards developing organization , or standards setting organization is any organization whose primary activities are developing, coordinating, promulgating, revising, amending, reissuing, interpreting, or otherwise producing technical standards that are...
for gloss measurements are ASTM
ASTM International
ASTM International, known until 2001 as the American Society for Testing and Materials , is an international standards organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, and services...
D 2457, DIN EN ISO 2813 and DIN
Deutsches Institut für Normung
is the German national organization for standardization and is that country's ISO member body. DIN is a Registered German Association headquartered in Berlin...
67530.