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Optical Coatings & Materials - Glossary of Terms

Glossary of Terms

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Barium Sulphate

A white pigment commonly used to coat the interior of integrating spheres and other reflecting chambers. Exhibits diffuse reflectance. Forms the basis of the following Labsphere diffuse reflectance coatings: Spectraflect; Duraflect; and 6080.

BRDF

BRDF is an acronym for Bi-Directional Reflective Distribution function. BRDF is the parameter normally used to define the degree to which a material approaches the ideal of Lambertian reflectance. BRDF is the ratio of incident irradiance to reflected radiance for a given direction of illumination and defined direction of viewing. BRDF has the units of inverse steradians (sr-1). The BRDF of a perfect diffuser is 1/π. See also: BTDF.

BTDF

BTDF is an acronym for Bi-Directional Transmissive Distribution function. BTDF is the parameter normally used to define the degree to which a material approaches the ideal of Lambertian transmittance. BTDF is the ratio of incident irradiance to transmitted radiance for a given direction of illumination and defined direction of viewing. BTDF has the units of inverse steradians (sr-1). The BTDF of a perfect diffuser is 1/π. See also: BRDF.

Cosine Response

Describes that property of a perfect diffuser for which the reflected (or transmitted) light intensity varies with the cosine of the angle subtended to the surface normal. See also: Diffuse; Diffuser; Lambertian.

Diffuse

Describing the scattering (non-specular reflection or transmission) from or through an object. An object is described as being a perfect diffuser if it exhibits Lambertian-like reflectance or transmittance.

Diffuser

Often referred to as a cosine diffuser, an object which exhibits Lambertian like reflectance or transmittance. Transmission diffusers are used in photometry to impart a cosine response at the input to a detector to correct for the effective illuminance of off-axis rays, or to more uniformly illuminate a multi-element detector. Thin sheets of ground glass (quartz), PTFE as well as the input port of an integrating sphere all function as effective transmission diffusers.  

Duraflect

Labsphere's proprietary, waterproof, diffuse white coating for the UV-VIS-NIR spectral region (350-1200nm).

Fluorescence

The emission of light at a longer wavelength as a result of absorption of light at a shorter wavelength in a fluorescent material. Applies for that case whereby the emission occurs at the same time as the absorption. See also: Luminescence; Phosphorescence.

Infragold

Labsphere's proprietary electro-plated gold coating for the infrared spectral region (>800nm).

Lambert's Cosine Law

See Lambertian.

Lambertian

A Lambertian surface is one that reflects (or transmits) with equal radiance or luminance in all directions. In other words, the reflected (transmitted) intensity (flux pew unit solid angle) from a Lambertian surface varies in proportion to the cosine of the angle subtended between the direction of view and the surface normal. See also: Cosine Response; Diffuser.

Luminescence

Luminescence can be taken to describe the process of fluorescence or phosphorescence. See: Fluorescence, Phosphorescence.

Phosphorescence

The emission of light at a longer wavelength as a result of absorption of light at a shorter wavelength in a phosphorescent material.  Applies for that case whereby the emission occurs after the absorption. See also: Fluorescence, Luminescence.

Reflectance

The percentage of light that reflects from an object. A spectrophotometer determines the reflectance as a function of wavelength. The reflectance from an object is generally categorised as being either specular or diffuse, or some combination of the two.

Reflectance Factor

Ratio of the directionally reflected flux to that reflected in the same direction by a perfect reflecting diffuser identically irradiated or illuminated.

Spectraflect

Labsphere's proprietary white reflectance coating. Barium sulphate based. The coating is hydroscopic. Available as a coating service and as a liquid paint (6080) for user application. Best applied by spray painting.

Spectralon

Labsphere's patented diffuse reflectance material. A solid thermoplastic which exhibits the highest diffuser reflectance of any material in the 250-2500nm range. Integrating spheres and reflectors are machined from a block of Spectralon. Spectralon is widely used as a white reflectance standard (99%). Can be doped with inorganic materials to create grey scale standards (2-80%), colour standards, wavelength standards and fluorescence standards.

Spectrophotometer

A measuring instrument that measures the magnitude of light reflected from or transmitted through a material as a function of wavelength.

Specular

Describing the mirror-like reflection or transmission from or through an object.

Transmittance

The percentage of light that transmits through an object. A spectrophotometer determines the transmittance as a function of wavelength. The transmittance from an object is of two types: specular and diffuse.

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