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
A
Absorption
That light which when incident on a material is neither reflected or transmitted.
Achromatic
Described as having no colour (or hue), for example a neutral white, grey or black colour.
Additive Primary Colour
The additive primary colours are Red, Green & Blue. When equal amounts of light of the three additive primary colours is combined, white light is generated. Combining two additive primaries in equal amounts creates a subtractive primary colour. See also: Subtractive Primary Colour.
Angstrom
A unit of wavelength of light. One Angstrøm (Å) is 0.1nm (or 10-10 m).
Average (Luminous) Intensity
Applies to LEDs. The near-field luminous intensity for a non point source. Defined in CIE document 127 as being the intensity (luminous flux per unit solid angle) measured by a photodetector with an input area of 100 mm2 positioned at either 316mm (condition A) or 100mm (condition B) from the tip of the LED source, measured on the mechanical axis of the LED. Average luminous intensity has units of candela (cd).
B
Bandwidth
See Linewidth.
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; WRC-680; and WRC-6080.
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.
Blackbody Radiation
Radiation that is full or complete, containing all wavelengths. The spectral power distribution of light emitted from a blackbody is a function of its temperature only and is described by Planck’s law. See also: Colour Temperature.
Blackbody Radiator
A source that emits blackbody radiation.
Brightness
That attribute of visual perception that describes the degree to which an object emits or reflects light. In colorimetry, brightness is used in the HSB colour model – Hue, Saturation and Brightness. Describes the lightness of the colour, on a scale ranging from black to white. Can be confused with saturation hence its use is discouraged. Can also be used instead of luminance – again its use is not recommended.
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.
C
Candela
The SI base unit used in photometry. The candela is the unit of luminous intensity. The candela is one of the seven base units of the SI system. Since 1979, the candela has been defined as: “the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 Hertz and has a radiant intensity in that direction of 1/683 Watts per steradian” (where the steradian is the unit of solid angle).
Candle Power
The luminous intensity of a light source having units of candelas.
CCD
Acronym. Stands for Charge Coupled Device. A monolithic, two-dimensional semiconductor (silicon-based) detector array. When illuminated by optical radiation in the range between approximately 300 and 1100nm, produces a photocurrent, the magnitude of which is proportional to the level of light received. Each detector in the array is called a pixel.
CFL
Acronym. Stands for Compact Fluorescent.
Chroma
The lightness of a colour, denotes the degree of lightness or darkness. See also: Lightness.
Chromatic
Described as having colour (or hue) – not white, grey or black.
Chromaticity
In colorimetry, describes the intensity or level of saturation of a colour (hue), defined as the distance in the particular colour space of a colour from the neutral grey colour with the same value.
Chromaticity Coordinates (CIE)
A numeric descriptor of colour. Defined as the ratio of the three tristimulus values XYZ in relation to the sum of the three, designated by xyz respectively. It is normal to assume that the chromaticity coordinates refer to the CIE 2° observer (1931). For reflected colour, it is assumed to be calculated for standard illuminant C unless specified differently. See also: x, y; u’, v’.
Chromaticity Diagram
The two-dimensional graph which plots the chromaticity coordinates. For the CIE 2° observer (1931), x is plotted as the abscissa, y as the ordinate. The colour space plots the spectrum locus of monochromatic radiation in the spectral range 380-770nm.
CIE 1976 L*u*v* Colour Space
A uniform colour space adopted by CIE in 1976 that is used when measuring colour as a result of additive mixing from emitters of light.
CIELAB
A uniform colour space adopted by CIE in 1976 that is used in the measurement of small (normally reflected or transmitted) colour differences.
CIE 1976 L*u*v* Colour Space
A uniform colour space adopted by CIE in 1976 that is used when measuring colour as a result of additive mixing from emitters of light.
CIELAB
The colour space within which L*a*b* colour coordinates are plotted using a Cartesian coordinate system. This colour space plots equal colour differences at approximately equal distances. The L* value denotes the lightness, a* represents the red/green axis, while b* represents the yellow/blue axis. CIELAB is that colour space used in relation to the measurement of reflected or transmitted colour.
CIE Luminosity Function (Y)
See: Photopic Response.
CIE Standard Illuminant
Standard light source spectral power distributions as defined by CIE for several types of light source. See Illuminant A, B, C, D, E & F. Acronym. Stands for Cyan, Magenta, Yellow & blacK. See also: Subtractive Primary Colour. An instrument that measures the colour of the light it receives. Applies to instruments that measure reflected (transmitted) light as well as to those that analyse the light emitted by a source. Applies to instruments that use RGB optical filters to mimic as close as possible the tristimulus colour response of the human eye. See also: Tristimulus.
CIE Standard Observer Function
See: Photopic Response.
CMYK
Acronym. Stands for Cyan, Magenta, Yellow & blacK. See also: Subtractive Primary Colour.
Colorimeter
An instrument that measures the colour of the light it receives. Applies to instruments that measure reflected (transmitted) light as well as to those that analyse the light emitted by a source. Applies to instruments that use RGB optical filters to mimic as close as possible the tristimulus colour response of the human eye. See also: Tristimulus.
Colorimetry
The science of measuring the colour of light (emitted, reflected or transmitted) from an object with a spectral response matching that of the human eye. See also: Tristimulus.
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.
Colour Difference
Being the size of the difference between two colours in a defined colour space.
Colour Rendering Index (CRI)
Colour rendering Index (CRI). A parameter which defines how well colours are rendered by different sources of illumination compared to a defined standard illuminant. There are fourteen special colour rendering indices (Ri where i = 1-14) which define the colour rendering of the light source when used to illuminate standard colour tiles. The general colour rendering index (Ra) is the average of the first eight special colour rendering indices (which correspond to non-saturated colours).
Colour Space
The three-dimensional solid enclosing all possible colours. The dimensions of the colour space can be described in a number of different geometries, which leads to various spacings within the colour space.
Colour Specification
Being the tristimulus values, chromaticity coordinates and luminance value (or other colour scale) that are used to uniquely describe a colour numerically.
Colour Temperature
Colour temperature defines the colour of radiation emitted from a perfect blackbody radiator held at a particular temperature. Colour temperature is reported in units of Kelvin (K). The plot of the chromaticity coordinates of a blackbody radiator with temperatures from 1,000 to 20,000 Kelvin is called the Planckian locus. Colours on this locus in the range from 2,500 to 20,000 K are considered to be white, with 2,500 K being reddish white and 20,000 K being bluish white (warm to cool white). See also: Correlated Colour Temperature.
Cone (Receptor)
The visual receptor in the retina of the human eye responsible for daylight adapted, colour vision. There are three types of cones, each sensitive to red, green or blue wavelengths. See also: Retina; Rod.
Cornea
The transparent front part of the human eye. To use an engineering analogy, the cornea functions both as the "dust cap" to the eye and also performs crude focussing of the light rays into the eye.
Correlated Colour Temperature (CCT)
Correlated Colour Temperature (CCT) describes the colour temperature of those light sources whose colours don’t fall exactly on the Planckian locus (i.e. for non blackbody emitters). The CCT of a non-Planckian source is the blackbody colour temperature that the source resembles most closely. Correlated colour temperature is reported in units of Kelvin (K). See also: Colour Temperature.
Cosine Response
Describing the Lambertian spatial response of a perfect reflecting or transmitting diffuser whereby the intensity from or through the surface varies with the cosine of the angle subtended between the direction of view and the normal to the surface. Also applies to light meters (radiometers and photometers) designed for measuring irradiance or illuminance which should apply a cosine angular response to light rays received at angles other than normal incidence, typically achieved using a transmission diffuser or integrating sphere. See also: Lambertian.
CRT
Acronym. Stands for Cathode Ray Tube (i.e. a TV screen).
D
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, that is scattered light obeys Lambert’s Cosine Law.
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.
Diode Array
A detector that comprises a linear array of segmented photodiode detectors. See also: Photodetector; Photodiode; Spectroradiometer.
Dominant Wavelength
Dominant wavelength is a useful measure of the effective wavelength (in nm) of non-monochromatic light sources, in particular LEDs. It is defined in relation to a specified standard illuminant (usually Illuminant E, but this is arbitrary). It is a measure of the hue (or colour sensation) produced by the light source. Not to be confused with peak wavelength. Dominant wavelength is normally applied to coloured LEDs whereas correlated colour temperature is applied to white LEDs.
Duraflect™
Labsphere's proprietary, waterproof, diffuse white coating for the UV-VIS-NIR spectral region (350-1200nm).
E
F
Far Field
In photometry, that distance from a lamp (called the photometric distance) where the emitter is considered to be a point source, usually at a distance of between 5-10 times the source diameter (although this is not the case for LEDs). Intensity measurements are performed in the far field, in which the inverse squared law applies. See also: Near Field; Intensity; Average Intensity; Inverse Squared Law; Point Source; Photometric Distance.
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.
Fluorescent Lamp
A lamp comprising a glass tube filled with mercury gas and coated on its inner surface with a phosphor. When the gas is charged with an electrical current, ultraviolet radiation is produced. This in turn is absorbed in the phosphor, causing it to emit visible light by the pocess of fluorescence. See also: CFL; Fluorescence.
Foot-candle
The English unit of illuminance. One foot-candle (fc) equals one lumen per square foot, which equals 10.76 lux. See also: Illuminance; Lux.
Foot-Lambert
The English unit of luminance. One foot-Lambert (fL) equals 1/π candelas per square foot, which equals 3.426 candelas per sq. meter. See also: Luminance.
Fovia
That part of the eye’s retina that has the greatest density of cones. See also: Retina, Cone.
G
Goniometer
A goniometer is a mechanical positioning device which allows one to change the relative angle (φ, θ) at which a photometer (also a spectroradiometer or colorimeter) views a light source. This can be accomplished by either moving the detector around a stationary source or by tipping and tilting the source whilst the detector remains in a fixed position. See also: Goniophotometer.
Goniophotometer
An instrument which performs measurements of the luminous intensity of a light source as a function of viewing angle. Can also be used to measure the total luminous flux of a light source by measuring the directional luminous intensity and summing over 4p steradians. See also: Goniometer.
H
Hue
The perceived colour of a light source.
I
Illuminance
Illuminance (Ev) is the luminous flux received per unit area. Illuminance is measured in lux (lx) where 1 lux equals 1 lumen per square meter. See also: Foot-candle.
Illuminant A
Illuminant A (CIE) represents the colour temperature of an incandescent lamp (2856K). See also: Standard Illuminant; Colour Temperature.
Illuminant B
Illuminant B (CIE) represents the colour temperature of direct sunlight (4874K). See also: Standard Illuminant; Colour Temperature.
Illuminant C
Illuminant C (CIE) represents the colour temperature of an indirect sunlight (6774K). See also: Standard Illuminant; Colour Temperature.
Illuminant D
Illuminant D (CIE) represents the colour temperature of daylight. There are several standard illuminants in the D series including D50 representing bright incandescent light (5000K) and D65 which represents natural daylight (6504K). See also: Standard Illuminant; Colour Temperature.
Illuminant E
Illuminant E (CIE) is the colour temperature of an artificial, normalising source (5500K). Has chromaticity coordinates of x=0.3333 & y=0.3333. See also: Standard Illuminant; Colour Temperature.
Illuminant F
Illuminant F (CIE) represents the correlated colour temperature of light from a fluorescent lamp. There are several standard illuminants in the F series including F2 representing cool white light (4200K), F7 which represents broadband daylight (6500K) and F11 which represents a narrow band white lamp (4000K). See also: Standard Illuminant; Colour Temperature.
Imaging Photometer (Colorimeter)
A photometric instrument capable of spatially-resolved luminance, illuminance and colour measurements. Sometimes referred to as a video photometer. Employs a CCD detector.
Incandescence
Light that is emitted by thermal radiation at a temperature high enough to be visible.
Incandescent Lamp
A lamp which emits light as a result of resistive headting when an electrical current passes through a metal wire in a vacuum.
Infragold™
Labsphere's proprietary electro-plated gold coating for the infrared spectral region (>800nm).
Infrared
Describing that part of the electromagnetic spectrum comprising optical radiation having wavelengths longer than 780nm but shorter than 1mm. Infrared radiation is absorbed in the eye’s cornea and so is not visible to the naked eye.
Integrating Sphere
A hollow, spherical chamber with a diffuse, high reflectance interior coating. Used in photometry to measure the total luminous flux (power) of a light source.
Inverse Squared Law
Describes the relationship whereby the illuminance from a light source varies in inverse proportion to the square of the distance from the source. Applies to the case of a point source of light. See also: Illuminance; Point Source.
Iris
The variable limiting aperture found in the human eye.
Irradiance
Irradiance (Ee) is the radiometric analogue of illuminance, the radiant flux received per unit area. Irradiance is measured in
Isotropic
A light source should be called isotropic when we wish to imply that it radiates with equal intensity (flux per unit solid angle) in all directions. Not to be confused with Lambertian.
J
K
Kelvin
Unit of measurement of colour temperature (K). The Kelvin scale starts at absolute zero (-273 Celsius). See also: Colour Temperature.
L
Lambert's Cosine Law
See Lambertian.
Lambertian
A Lambertian surface is one that reflects with equal radiance or luminance in all directions. In other words, the reflected intensity from a Lambertian surface varies in proportion to the cosine of the angle subtended between the direction of view and the surface normal.
Laser
A light source typically characterised by a combination of narrow spectral linewidth, a collimated beam and a high radiant flux (power). From the acronym: Light Amplification by the Stimulated Emission of Radiation.
LCD
Acronym. Stands for Liquid Crystal Display.
LED
Acronym. Stands for Light Emitting Diode. LEDs are solid state light sources which generate light by electroluminescence when an electrical current is passed through a semiconductor p-n junction.
Light
Light is a term that we use to define that part of the electromagnetic radiation that humans can see (“visible light”). More accurately described as optical radiation, it forms part of the electromagnetic spectrum that encompasses radio waves, microwaves, x-rays and gamma rays. Optical radiation is that which obeys wave-like behaviour, as opposed to particle-like. Not all optical radiation is visible to the naked eye and falls into that part of the spectrum between microwaves and x-rays, having wavelengths between 10-7 ≤ l ≤ 10-4 meters.
Lightness
See Chroma.
Linewidth
Denotes the spectral purity of a light source, the spectral width of a light source is normally defined as the full width half max (FWHM) bandwidth. See also: Bandwidth.
Lumen
The SI unit of luminous flux (lm).
Luminaire
A light source together with its housing or reflector.
Luminance
Luminance (Lv) is the luminous flux emitted per unit solid angle and per unit area. In the SI system, luminance is measured in candelas per square meter (cd m-2) where 1 cd m-2 equals 1 lumen per steradian per square meter. The photometric analogue of radiance. See also: Foot-Lambert; Nits.
Luminescence
Luminescence can be taken to describe the process of fluorescence or phosphorescence. See: Fluorescence, Phosphorescence.
Luminous Efficacy
The efficiency of a light source, the quotient of luminous flux divided by the total radiant flux. Reported in units of lumens per Watt.
Luminous Exitance
Luminous exitance (Mv) is the luminous flux emitted from a surface per unit area, measured in units of lumens per square meter (lm m-2). Geometrically equivalent to illuminance, luminous exitance is not however reported in lux.
Luminous Flux
The luminous flux (Fv) or more correctly the total luminous flux of a light source is the total photometric power emitted in all directions. Luminous flux is measured in lumens (lm), which is the photometric analogue of the Watt.
Luminous Intensity
The luminous intensity (Iv) in a given direction (often abbreviated to intensity, sometimes referred to as beam candela) is the photometric power from a point source emitted per unit solid angle. Intensity is measured in the SI base unit of the candela (cd, or mcd where 1 mcd = 10-3 cd) where 1 candela equals 1 lumen per steradian. Luminous intensity is a far field measurement of a light source; luminance is the measurement made in the near field.
Lux
The SI unit of illuminance (lx).
M
Mean Spherical Candle Power (MSCP)
Mean Spherical Candle Power (M.S.C.P.) is the luminous flux of a light source divided by 4p steradians.
Mesopic
Photopic (cone) vision operates when the eye sees luminance levels of ≥ 3 candelas per square meter, whereas scotopic (night time) vision operates when the luminance falls below 0.01 candelas per square meter. For luminance levels between 0.01 and 3 cd m-2, both rods and cones contribute to the vision process. This in-between region is called mesopic, and research is ongoing into the development of a standard observer function.
Micrometer
A unit of wavelength of light, normally used to describe infrared light. One micrometer - or micron - (µm) is 10-6 meters.
Monochromatic
Light that is composed of a single wavelength or colour (hue), having 100% purity and a narrow linewidth.
N
Nanometer (nm)
A unit of wavelength of light, normally used to describe visible light. One nanometers (nm) is 10-9 meters.
Near-Field
In photometry, that region close to a lamp where the emitter is not considered to be a point source, usually at a distance of less than 5-10 times the source diameter (although this is not the case for LEDs). Intensity measurements are performed in the far field, in which the inverse squared law applies. See also: Far Field, Intensity, Average Intensity, Inverse Squared Law, Point Source.
Nit
A historically significant unit of luminance. Derives from the Latin "to illuminate". One nit equals 1 candela per square meter.
O
Optical Radiation
Describing that part of the electromagnetic spectrum comprising rays that exhibit a wave-like property (as opposed to particle-like). Includes ultraviolet, visible and infrared radiation.
P
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.
Photodetector
A generic name given to a device that detects optical radiation. See also: Photodiode, PMT, Diode Array, CCD.
Photodiode
A discrete semiconductor photodetector that produces a photocurrent the magnitude of which is proportional to the level of light received when illuminated by optical radiation. In photometers, the detector is based on silicon with a photopic filter; the combined detector plus filter is designed to provide a spectral responsivity scaled to match that of the photopic response, the CIE standard observer for photopic vision V(l).
Photometric Distance
That distance from a light source that defines the far field. See also: Far Field.
Photometer
An optical instrument that measures a defined geometric property of visible light with a spectral response matching that of the human eye (normally the daylight adjusted response). See also: Radiometer; Spectroradiometer.
Photometry
The science of measuring optical radiation with a spectral response matching that of the human eye. See also: Radiometry; Spectroradiometry.
Photomultiplier Tube (PMT)
A photomultiplier tube (PMT) is an optical detector that comprises a photocathode which is held in vacuum and emits electrons when exposed to light. This charge is accelerated by a high voltage field causing the electrons to hit a metal plate, whereupon more electrons are emitted (the multiplier effect). Not normally used in photometers, PMTs are commonly used as part of a spectroradiometer system.
Photopic (Response)
The daylight-adapted relative spectral response of the eye is called the spectral luminous efficiency function for photopic vision, V(l) (V-lambda). This is an empirical distribution, that was first adopted in 1924 by the International Commission on Illumination (Commission Internationale de l’Eclairage, CIE). Applies for luminance values above 3cd/m2 to the spectral range between 380 and 780nm having a peak of response at 555nm.
Pixel
The individual detector element in a two-dimensional CCD detector array.
Plankian Locus
The locus of points on the CIE chromaticity diagram that plots the chromaticity of blackbody radiators of differing colour temperatures.
Plankian Radiator
A synonym for blackbody radiator.
Point Source
A light source is considered to be a point source if it is observed at a position at or beyond the photometric distance, that is, the far field. See also: Intensity, Average Intensity, Far Field, Photometric Distance.
Polychromatic
Used to describe optical radiation that is comprised of multiple discrete wavelength or having a continuous spectrum.
Primary Colour
In colorimetry, primary (additive) colours are red, green and blue, and their subtractive opposites cyan, magenta and yellow. See also: Additive Primary Colours, Subtractive Primary Colours.
Purity
Purity is a measure of the degree of saturation of a light source, reported as a percentage. It is defined with respect to a specified standard illuminant (usually illuminant E, but this arbitrary). As most (coloured) LEDs have a narrow spectral emission (that is, near-monochromatic), they have a purity of close to 100% (in other words, the colour cannot be distinguished from a true monochromatic light source). See also: Saturation.
Q
R
Radiance
Radiance (Le) is the radiant flux emitted per unit solid angle and per unit area. Radiance is measured in Watts per steradian per square meter (W sr-1 m-2). The radiometric analogue of luminance.
Radiant Exitance
Radiant exitance (Me) is the radiant flux emitted from a surface per unit area, measured in units of Watts per square meter (W m-2). Geometrically equivalent to irradiance.
Radiant Flux
The radiant flux (Φe) - more correctly, total radiant flux - of a light source is the total power emitted in all directions. Radiant flux is measured in Watts (W), which is the radiometric analogue of the lumen.
Radiant Intensity
The radiant intensity (Ie) in a given direction (often abbreviated to intensity) is the radiant flux from a point source emitted per unit solid angle. Intensity is measured in units of Watts per steradian (W sr-1). The radiometric analogue of luminous intensity.
Radiometer
An optical instrument that measures the absolute amount of a defined geometric property of light.
Radiometry
The science of measuring the absolute amount of optical radiation.
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.
Retina
The light-sensitive area on the rear of the human eye containing photo-receptors. The area containing the highest concentration of photo-receptors is called the fovea. See also: Rod; Cone; Fovea.
RGB
Meaning Red, Green & Blue which are the additive primary colours. Used in relation to the human eye’s tristimulus colour response. See also: Additive Primary Colour.
Rod
The visual receptor in the retina of the human eye responsible for night-time, monochromatic vision. See also: Retina, Cone.
S
Saturation
The attribute of colour perception that describes the degree of departure of the colour from the neutral grey of the same lightness. See also: Purity.
Scotopic
The dark-adapted relative spectral response of the eye is called the spectral luminous efficiency function for scotopic vision, V’(l). The scotopic response was first adopted by CIE in 1951 and applies to the spectral region between 380 and 780nm, with a peak at 507nm.
SI
Systeme Internationale d’Unites, the international metric system of measurement units
Silicon Photodiode
See Photodiode.
Solid Angle
See Steradian.
Spectral Power Distribution (SPD)
The power distribution of a light source as a function of wavelength. Determines the colour rendering properties of a light source.
Spectraflect™
Labsphere's proprietary white reflectance coating. Barium sulphate based. The coating is hydroscopic. Available as a coating service and as a liquid paint 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.
Spectrometer
An instrument that separates polychromatic light into its constituent wavelengths. Employs a dispersive optical element, usually a diffraction grating. A spectroradiometer and a spectrophotometer comprise a spectrometer combined with a detector, normally a CCD array.
Spectrophotometer
A measuring instrument that measures the magnitude of light reflected from or transmitted through a material as a function of wavelength.
Spectroradiometer
An instrument for measuring the spectral radiant power from a light source as a function of wavelength. Combines a spectrometer with a photo detector. Can be used to accurately calculate the photometric and colorimetric properties of a light source.
Spectrum
The spectral arrangement of electromagnetic energy in order of wavelength. See also: Spectral Power Distribution (SPD).
Spectrum Locus
The plot of the chromaticity coordinates of monochromatic light in the CIE colour space diagram. See also: Chromaticity Diagram (CIE). Specular
Specular
Describing the mirror-like reflection or transmission from or through an object.
Standard Illuminant
See CIE Standard Illuminant.
Standard Observer
See: CIE Standard Observer Function; Photopic Response.
Steradian
The steradian is the (dimensionless) unit of solid angle. A sphere contains 4πsteradians (and by definition, a hemisphere comprises 2πsteradians). A steradian is that solid angle subtended from the centre of a sphere by a spherical surface area having an area equal to the square of the radius. In other words, the steradian solid angle of a beam of light is equal to the projected area divided by the square of the distance.
Subtractive Primary Colour
The subtractive primary colours are Cyan, Magenta & Yellow. When applied to white paper in equal amounts, these combine to produce black. Combining two subtractive primaries in equal amounts creates an additive primary colour. See also: CMYK, Additive Primary Colour.
T
Thermopile
A broadband detector (thermal rather than optical) that is commonly used in the radiometry of lasers. Comprises an array of thermocouple junctions.
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.
Tristimulus
Describing the RGB 2° colour matching functions, which CIE transformed into the XYZ colour matching functions. The XYZ functions define the CIE 1931 standard colorimetric observer. The Y channel (green) was chosen to match that of the photometric response, so that colour measurement instruments could double as photometers. For observer fields of view of greater than 2°, CIE published its 1964 supplementary standard colorimetric observer.
U
Ultraviolet (UV)
Describing that part of the electromagnetic spectrum comprising optical radiation having wavelengths between 100 and 400nm. Ultraviolet (UV) radiation is absorbed in the lens or cornea and is not visible to the naked human eye.
UVA
That part of the Ultraviolet spectral range with wavelengths between 315 and 400nm.
UVB
That part of the Ultraviolet spectral range with wavelengths between 280 and 315nm.
UVC
That part of the Ultraviolet spectral range with wavelengths between 100 and 280nm.
V
Visible (Light)
Describing that part of the electromagnetic spectrum comprising optical radiation having wavelengths between 380 and 780nm that can be detected by the retina in the human eye. Wavelengths in this range combine to create the sensation of colour.
V(λ) Function
See Photopic.
W
Watt
The SI unit of power and radiant flux (W).
Wavelength
The periodic peak-to-peak distance in an electromagnetic wave. For ultraviolet and visible light, defined in nanometers (nm); for infrared light, defined in micrometers (µm).
White Light
Light that appears to be white. Can contain a continuous spectrum of light (as from the sun) or comprise as few as two colours. White light comprising two colours exhibits poor colour rendering. Light on the Planckian locus (blackbody curve) on the CIE chromaticity diagram with colour temperatures between 2500 and 20000K is considered to be “white”. See also: CIE Standard Illuminant.