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Light Measurement - Glossary of Terms

Glossary

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


Achromatic

Describing the property of 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.

Angstrøm (Å)

A unit of wavelength of light. One Angstrøm (Å) is 0.1nm (or 1 x 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). Not to be confused with luminous intensity which is a far-field measurement of the luminous flux per unit solid angle from a light source. See also: LED; Luminous Intensity; Luminous Flux; CIE; Far-Field; Near-Field

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; and 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. See also: Luminance.

Candela (cd)

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). One candela equals one lumen per steradian. See also: Luminous Intensity; Steradian.

Candela per Square Meter (cd/m2)

The SI unit of luminance. One cd/m2 equals one lumen per square meter per steradian. See also: Luminance; Steradian.

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 Lamp.

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: (CIE) x, y; (CIE) 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

Acronym. From the French name "Commission Internationale de L'éclairage". In English this is the International Commission on Illumination. CIE sets the standards and provides a worldwide forum for the exchange of information on the science of light, lighting, colour and vision. Click here to visit the CIE's web site 

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 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.

CIE Standard Observer Function

See: Photopic Response; Scotopic Response; Tristmulus 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 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)

A metric 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 colours. 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: Black Body Radiation; 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 as a "dust cap" and also performs primary focussing of the light rays into the eye.

Correlated Colour Temperature (CCT)

Correlated Colour Temperature (CCT) describes the colour temperature of those white 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). CCT can be considered to be a simplified metric for describing the colour of a white light source. For coloured light sources, dominant wavelength can be used to express colour in a single number. See also: Colour Temperature; Dominant Wavelength.

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. See also: Cosine Response.

Cosine Response

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. 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: Cosine Diffuser; Lambertian.

CRT

Acronym. Stands for Cathode Ray Tube. The type of television display that was popular before the introduction of flat panel displays (LCD, OLED, RPTV & plasma).

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).

Electroluminescence

The emission of light as a result of the passage of an electrical current. Distinct from incandescence which is the emission of light as a result of heat. See also: LED.

Electromagentic Radiation

Electromagnetic radiation is generated by the motion of electrically charged particles. Optical radiation, light, X-rays, radio waves and microwaves are all examples of electromagnetic radiation. See: Light; Optical Radiation.

EULUMDAT (File)

A standardised data file which expresses the light output of a luminaire as luminous intensity versus angle. EULUMDAT files are assigned the file extension ".ldt". Used by lighting designers to model the illumination performance of a lighting scheme using commercial lighting design programmes. An EULUMDAT file is commonly referred to as "photometric data" for a luminaire. See also: Goniophotometer; IES (file).

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.

Flicker

Flicker is defined as the variation of light output over time and occurs in every light source, at varying degrees, usually as their power is drawn from an AC source (frequency of 50Hz in the UK). Modulation in light output can be caused by the light source itself. This is known as photometric flicker and varies with each technology, for example, an incandescent bulb does not appear to flicker due to its inefficient thermal persistence keeping the filament hot, masking the switch in direction of the current. In contrast to this, an LED responds instantaneously to changes in the power supply so any effect of photometric flicker is obscured by electrical flicker such as noise on the AC distribution lines or dimming an LED using pulse width modulation.

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 process of fluorescence. See also: CFL; Fluorescence.

Foot-candle (fc)

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 (fL)

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. 

Goniometer

A goniometer is an (electro) 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 (far-field) luminous intensity or illuminance or (near-field) luminance 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 integrating over 4π steradians. A goniophotometer is commonly used to measure so-called "photometric data" for luminaires. Photometric data in this context is the light output (luminous intensity) as a function of angle presented in industry-standard file formats. See also: EULUMDAT; Goniometer; IES.

Hue

The perceived colour of a light source. 

IES (File) 

A standardised data file which expresses the light output of a luminaire as luminous intensity versus angle. Derives from the acronym for the Illuminating Engineering Society of North America (IESNA). IES files are assigned the file extension ".ies". Used by lighting designers to model the illumination performance of a lighting scheme using commercial lighting design programmes. An IES file is commonly referred to as "photometric data" for a luminaire. See also: EULUMDAT (file); Goniophotometer.

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 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 which represents 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 which represents 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 heating 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 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.

Intensity

See: Luminous Intensity

Inverse Squared Law

Describes the relationship between the illuminance from a light source which varies in inverse proportion to the square of the distance from the source. Applies to the case of a point source of light in the photometric far-field. See also: Far-Field; 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 Watts per square meter.

Isotropic

A light source is isotropic when it radiates with equal intensity (flux per unit solid angle) in all directions. Not to be confused with Lambertian.

Kelvin (K)

The unit of measurement of colour temperature and correlated colour temperature. The Kelvin scale starts at absolute zero (-273°C). See also: Colour Temperature; Correlated Colour Temperature.

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.

Laser

From the acronym for Light Amplification by the Stimulated Emission of Radiation. A light source typically characterised by a combination of coherence, narrow spectral linewidth and a collimated beam. The term "laser light" refers to electromagnetic radiation with wavelengths between 150nm up to 11µm. A laser was first demonstrated in 1960 by Theodore H Maiman working at the Hughes Corporation, although the term ‘laser’ was first coined by Gordon Gould of Columbia University.

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. See also: Electroluminescence; OLED.

Light

Light is a term that we use to define that part of the electromagnetic radiation that humans can see (“visible light”). Visible light has a wavelength bteween 380 and 780nm. 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.

Lightness

See Chroma.

Linewidth

Denotes the spectral purity or width in wavelength of a light source. The spectral width of a light source is normally defined as the full width half max (FWHM) linewidth. The term bandwidth is often used interchangeably with linewidth.

LOR

Acronym. Stands for Light Output Ratio. Being the ratio of the luminous flux produced by a luminaire to that produced by the lamp (or lamps) fitted to the luminaire. A measure of the efficiency of a  luminaire. See also: Luminaire; Luminous Flux.

Lumen (lm)

The SI unit of luminous flux (lm).

Luminaire

A light fitting which generally comprises a lamp in a housing together with a reflector and/or lens. May include control gear (drive electronics).

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 where 1 cd/m2 equals 1 lumen per steradian per square meter. The photometric analogue of radiance. Luminance is a near-field measurement of a light source; luminous intensity is the measurement made in the photometric far-field. See also: Far-Field; Foot-Lambert; Luminous Intensity; Near-Field; Nit; Steradian.

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 the luminous flux emitted divided by the electrical power consumed. Reported in units of lumens per Watt (lm/W). 

Luminous Exitance

Luminous exitance (Mv) is the luminous flux emitted from a surface per unit area, measured in units of lumens per square meter. 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) 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 photometric near-field. See also: Far-Field; Luminance; Near-Field; Steradian.

Lux (lx)

The SI unit of illuminance (lx).

Mean Spherical Candle Power (MSCP)

Mean Spherical Candle Power (M.S.C.P.) is the luminous flux of a light source divided by 4π 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/m2, both rods and cones contribute to the vision process to varying degrees. This "in-between" region is called mesopic, and research is ongoing into the development of a standard observer function for mesopic vision. See also: Photopic; Scotopic.

Micrometer (µm)

A unit of wavelength, normally used to describe infrared radiation. Often abbreviated to 'micron'. One micrometer or micron (µm) equals 1 x 10-6 meters. One micrometer also equals 1000 nanometers (nm).

Monochromatic

Light that is composed of a single wavelength or colour (hue), having 100% purity and a narrow linewidth.

Monochromator

See Spectrometer.

Nanometer (nm)

A unit of wavelength of light, normally used to describe visible light. One nanometer (nm) equals 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. See also: Luminance.

OLED

Acronym. Stands for Organic Light Emitting Diode. A type of light emiting diode (LED) with an emissive electroluminescent layer made of organic compunds. Related devices are Light Emitting Polymers (LEPs). See also: Electroluminescence; LED.

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)

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 (light) and infrared radiation.

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.

Photometric Distance

That distance from a light source that defines the far field. In the photometric far-field, illuminance (irradiance) varies in inverse proportion to the square of the distance form the light source. See also: Inverse Squared Law; Far-Field; Near-Field. 

Photometer

An optical instrument that measures a defined geometric property of visible light with a photopic spectral response matching that of the human eye. See also: Photopic; Radiometer; Spectroradiometer.

Photometry

The science of measuring optical radiation with a photopic spectral response matching that of the human eye. See also: Photopic; Radiometry; Spectroradiometry.

Photomultiplier Tube (PMT)

A photomultiplier tube (PMT) is a photodetector 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 in traditional scanning spectroradiometer (monochromator) systems.

Photopic (Response)

The daylight-adapted relative spectral response of the eye is called the spectral luminous efficiency function for photopic vision, V(l).  This is an empirical distribution that was first adopted in 1924. Applies for luminance values above 3cd/m2 to the spectral range between 380 and 780nm having a peak response at 555nm. The photopic response explains why the human eye is more sensitive to green light than blue or red light. At luminance levels below 3cd/m2, the eye transitions through a mesopic response. At luminance levels at or below 0.01 cd/m2, the eye becomes dark adapted and possesses a scoptic response. See also: Mesopic; Scotopic.

Pixel

The individual detector element in a two-dimensional CCD detector array. See also: CCD. 

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.

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. 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. Geometrically equivalent to irradiance and the radiometric analogue of luminous exitance.

Radiant Flux

The radiant flux e) - more correctly the 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. The radiometric analogue of luminous intensity.

Radiometer

An optical instrument that measures the absolute amount of a defined geometric property of light. A laser power meter is a type of radiometer. See also: Photometer; Spectroradiometer.

Radiometry

The science of measuring the absolute amount of optical radiation. 

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 scotopic (night-time, monochromatic) vision. See also: Retina; Cone.

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. The Scotopic response applied to luminance levels of 0.01 cd/m2 of less. See also: Mesopic; Photopic.

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, colour temperature and colour rendering properties of a light source.

Spectraflect

Labsphere's proprietary white reflectance coating. The coating is barium sulphate based and 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.

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. A (scanning) monochromator is a form of spectrometer in which the diffraction grating rotates to transmit one wavelength at a time, whereas a CCD spectrometer analyses a wide range of wavelengths simultaneously.

Spectroradiometer

An instrument for measuring the radiant power from a light source as a function of wavelength. Normally combines a spectrometer with a photodetector or CCD. Can be used to more accurately calculate the photometric and colorimetric properties of a light source than a photometer or colorimeter by multiplying the spectral radiant power by standard photopic and tristimulus observer functions.

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).

Standard Illuminant

See CIE Standard Illuminant.

Standard Observer

See: CIE Standard Observer Function.

Steradian

The steradian is the (dimensionless) unit of solid angle. A sphere contains 4π steradians (and by definition, a hemisphere comprises 2π steradians). 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 inks of these colours are 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.

Thermopile

A broadband detector (thermal rather than optical) that is commonly used in the radiometry of lasers. Comprises an array of thermocouple junctions.

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 photopic 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.

(CIE) u' Chromaticity Coordinate

Together with the CIE v' chromaticity coordinate provides a numeric description of colour. The CIE u' chromaticity coordinate is plotted as the abscissa (horizontal axis) on the two-dimensional colour space graph defined by CIE to communicate the colour of a light source. The CIE v' chromaticity coordinate is plotted as the ordinate (vertical axis). Refers to the CIE 10° tristimulus observer function (1964) as plotted in the 1976 uniform colour space chromaticity diagram. The intersection of u' and v' within the CIE colour space diagram uniquely defines the colour of a light source. See also: Chromaticity Coordinates; Chromaticity Diagram; Tristimulus; (CIE) x; (CIE) y.

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.

(CIE) v' Chromaticity Coordinate

Together with the CIE u' chromaticity coordinate provides a numeric description of colour. The CIE v' chromaticity coordinate is plotted as the (vertical axis) on the two-dimensional colour space graph defined by CIE to communicate the colour of a light source. The CIE u' chromaticity coordinate is plotted as the abscissa (horizontal axis). Refers to the CIE 10° tristimulus observer function (1964) as plotted in the 1976 uniform colour space chromaticity diagram. The intersection of u' and v' within the CIE colour space diagram uniquely defines the colour of a light source. See also: Chromaticity Coordinates; Chromaticity Diagram; Tristimulus; (CIE) x; (CIE) y.

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.

Watt (W)

The SI unit of power and radiant flux.

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.

(CIE) x Chromaticity Coordinate

Together with the CIE y chromaticity coordinate provides a numeric description of colour. The CIE x chromaticity coordinate is plotted as the abscissa (horizontal axis) on the two-dimensional colour space graph defined by CIE to communicate the colour of a light source. The CIE y chromaticity coordinate is plotted as the ordinate (vertical axis). Refers to the CIE 2° tristimulus observer function (1931). The intersection of x and y within the CIE colour space diagram uniquely defines the colour of a light source. See also: Chromaticity Coordinates; Chromaticity Diagram; Tristimulus; (CIE) u'; (CIE) v'.

(CIE) y Chromaticity Coordinate

Together with the CIE x chromaticity coordinate provides a numeric description of colour. The CIE y chromaticity coordinate is plotted as the (vertical axis) on the two-dimensional colour space graph defined by CIE to communicate the colour of a light source. The CIE x chromaticity coordinate is plotted as the abscissa (horizontal axis). Refers to the CIE 2° tristimulus observer function (1931). The intersection of x and y within the CIE colour space diagram uniquely defines the colour of a light source. See also: Chromaticity Coordinates; Chromaticity Diagram; Tristimulus; (CIE) u'; (CIE) v'.

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