Illumination Terms and Definitions
| Term | Definition |
| Action Spectra | An action spectrum describes the relative effectiveness of monochromatic optical radiation to elicit a given biological response. |
| Blackbody | The ideal thermal radiator that absorbs completely all incident radiation, whatever the wavelength, direction of incidence or the polarization. |
| Candela (cd) | Metric unit of luminous intensity defined as 1/60 the normal intensity of one square centimetre of a blackbody at the solidification temperature of platinum. |
| Chromaticity | Properties of light related to hue and saturation, but not luminance (brightness). |
| CIELab | An approximately uniform colour space produced by plotting in rectangular coordinates L* (lightness), a* (redness/greenness) and b* (yellowness/blueness). |
| CIE Standard Illuminant | An illuminant represents an aim spectral power distribution of a theoretical real light source. For example, CIE Illuminant A is a mathematical representation of tungsten halogen (incandescent) having a colour temperature of 2 856K, and CIE Illuminant D65 is a mathematical representation of a phase of daylight having a correlated colour temperature of 6 504 K, recommended by the CIE if daylight is of interest. |
| CIE Standard Illuminating and Viewing Conditions | Reflectance
and transmittance
measurements depend in part on the geometry of illumination and viewing.
Accordingly the CIE has specified four illuminating and viewing
conditions namely, 45/0, 0/45, d/0 and 0/d.
With the 45/0 geometry, the sample is illuminated at an angle of 45° from the normal to the sample surface and viewed normal to the sample surface or within 10° to the normal. The 0/45 geometry is the reverse of 45/0. The 0/d (normal/diffuse) geometry is one in which the sample is illuminated by a beam whose axis is at an angle not exceeding 10° from the normal to the sample surface. The reflected flux is collected by an integrating sphere and viewing is accomplished by a receiver being directed toward an inner wall of the sphere. In all geometries the viewing cones should not exceed 5°. The d/0 geometry is the reverse of 0/d. |
| CIE Standard Observer | The CIE 1931 (or 1964 Supplementary) Standard Colorimetric Observer represent an average observer of normal colour vision when viewing a stimulus at a 2° (or 10°) field of view. In practice, these two CIE Observers are referred to as the "2° observer" and "10° observer". |
| Colorimeter | An instrument measuring colorimetric quantities, such as the tristimulus values (X, Y and Z) or colour coordinates (e.g. CIELab L*a*b*) of a colour stimulus. |
| Colour Atlas | Collection of colour samples arranged and identified according to specific rules (e.g. Munsell colour system or Natural Colour System (NCS)). |
| Colour Space | A geometric representation of colours in space, usually in three dimensions. |
| Correlated Colour Temperature | Temperature of a blackbody radiator which produces the chromaticity nearest to that emitted by the light test source. |
| Erythema | A photochemical injury occurring after a critical radiant exposure (sunburn). |
| Footlambert (fL) | Unit of luminance equal to 3.426 cd.m-2. |
| Gamut | A colour gamut of a colour reproduction system is a graphical representation of the range of colours available in a device independent colour space. |
| Illuminance | Luminous flux incident per unit area of a surface. |
| Integrating Sphere | A sphere coated with a highly reflective, diffuse, spectrally neutral material on the inside, used to collect light reflected from or transmitted by the sample. |
| Irradiance | Radiant flux incident per unit area of a surface. |
| Luminance | Luminous intensity of reflected or transmitted light in a given direction per unit of projected area of the source as viewed from that direction. |
| Nanometer (nm) | Unit of length equal to 10-9 meter commonly used for identifying wavelengths of the visible portion of the electromagnetic spectrum. |
| Photometric | Pertaining to the measurement of the intensity of light. |
| Radiance (W.m-2.sr) | Radiant power per unit source area per unit solid angle. |
| Radiometric | Pertaining to the measurement of radiation (optical power). |
| Reflectance | Ratio of the intensity of reflected radiant energy to that reflected from a defined reference standard. |
| Spectrophotometer | An instrument measuring the ratio of two values (standard and sample) of a radiometric quantity (transmittance or reflectance) at the same wavelength. |
| Spectroradiometric | Pertaining to the measurement of the absolute amount of radiant flux at each wavelength of the spectrum. |
| Spectroradiometer | An instrument used to measure the radiant energy from a source at each wavelength throughout a pre-defined portion of the electromagnetic spectrum. |
| Transmittance | The amount of emitted light of a specific wavelength which is not reflected or absorbed, but passes through a substance. |
| Tristimulus values (CIE) | Percentage amounts of the three standard or matching stimuli (designated as X, Y and Z) necessary in a three additive mixture required for matching a colour. |
| Uniform Colour Space | Equal spatial distances in the colour space do not represent equal visual colour differences. |