Optical Properties - Glossary of Terms
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Absorption
That light which when incident upon on a material is neither reflected or transmitted. Absorption is the logarithm (base 10) of the reciprocal of the transmittance. For example, if the transmittance of a material is 10%, its absorbance is 1.0, while an absorbance of 0.3 corresponds to a transmittance of 0.5 (50%). See also: Beer's Law.
Achromatic
Describing the property of having no colour (or hue). For example a neutral white, grey or black colour.
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 relfectance coatings: Spectraflect; Duraflect; and 6080.
Beer's Law
Also known as the Beer-Lambert law. Describes the relationship between the absorption and the optical path length that light experiences upon transmission through a material. Absorption is the logarithm (base 10) of the reciprocal of the transmittance. Beer's Law describes how the absorption of light through a material varies linearly with the optical path length.
Black
The (near) complete absorption of light as a result of no (low) reflectance. In colorimetry, describes a colour of low saturation and low luminance.
BRDF
Acronym. Stands 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.
BSDF
Acronym. Stands for Bi-Directional Scatter Distribution function. BSDF is the generic name used to describe the scattering performance of a material, either in reflectance or transmittance. See also: BRDF; BTDF.
BTDF
Acronym. Stands 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 (diffuse) 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.
Colour
Can be described as being one aspect of an object’s appearance. In colorimetry, colour is a visual perception and is defined in respect of its hue, saturation and lightness.
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.
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.
Isotropic
A property of a light source when it radiates with equal intensity (flux per unit solid angle) in all directions. Not to be confused with Lambertian.
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.
Optical Density (OD)
Describes the level of attenuation of light as it passes through an optical filter. The higher the OD value, the higher the attenuation (and the higher the protection). OD increases with increasing thickness of absorptive filters. Optical density is the logarithm (to the base ten) of the reciprocal of the transmittance. For example, an OD of 1 means 10%, an OD of 2 means 1% and an OD of 3 means 0.1% transmittance (and so on)
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
The ratio of the directionally reflected flux to that reflected in the same direction by a perfect reflecting diffuser identically irradiated or illuminated.
Scatterometer
An instrument that measures the surface roughness and BDRF of a material.
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. See also: Diffuse.
SPF
Acronym. Stands for Sun Protection Factor. A measure of the protection afforded by a sun screen against UVB radiation. Determined in-vivo by measuring the delay in the onset of erythema on protected skin versus unprotected skin when illuminated by a solar simulator. Determined in-vitro by measuring the spectral transmittance of a thin layer of sunscreen. SPF does not address sunscreen protection in the UVA band for which alternative metrics are used.
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.
