Newcomers to light measurement are often bewildered by the galaxy of arcane terms which surround it. To make matters worse, some of these terms (the worst offender is probably “intensity”) are common words that often carry different meanings in other, even closely related, fields.

Here is a list of the most common terms together with a brief description of each. Click on any word in underlined type within a definition to see an expanded definition in a separate small window.

At the end of the page is a list of suggested references on radiometry.

Black Body – a heated material that emits light as a result of being hot. The spectrum of a black body is determined by the temperature alone. An incandescent lamp and a hot electric stovetop have spectra that are good approximations to black body spectra.

Candela – the photopic unit of intensity, equivalent to lumens per steradian.

Chromaticity – a numerical representation of the color of a light source.

CIE – the International Commission on Illumination, the international body that establishes guidelines for light measurement.

Color rendering – a description of how well a light source displays surface colors.

Color rendering index (CRI) – a numerical representation of color rendering. The technique for computing CRI is described in CIE publication 13.3 (1995).

Color temperature - Any light source that has the same chromaticity coordinates as a black body can be described as having the color temperature of that black body. The terms color temperature and black body temperature are not synonymous. Color temperature is derived from colorimetric calculations. There are limitless different spectra that possess a particular color temperature and have little or no resemblance to the black body curve for that temperature.

Correlated Color Temperature - the color temperature of the the black body that is closest to the chromaticity coordinates of the light source.

Flux – optical power. The radiometric unit is the “Watt”; the photopic unit is the “lumen”.

Footcandle – a vanishing (but not fast enough) unit of illuminance equal to one lumen per square foot. 1 footcandle = 10.764 lux.

Footlambert – a vanishing (but not fast enough) unit of luminance equal to one candela per pi square feet or, equivalently, one lumen per pi square feet per steradian. 1 footlambert = 3.426 candela/square meter (nit).

Illuminance – photopic irradiance. The unit is “lumens per square meter”, or, equivalently, “lux”.

Intensity – flux per solid angle. The radiometric unit is “watts per steradian”; the photopic unit is the “candela”. Note – Intensity only has meaning when the light source is very small compared to the relevant distances involved AND the irradiance falls off proportionally with the square of the distance from the source.

Irradiance – flux per unit area impinging onto a surface. The radiometric unit is “watts per square meter; ” the photopic unit is “lumens per square meter” or, equivalently, “lux”. Note – the term “irradiance says nothing about the direction at which light strikes the surface.

Lumen – a quantitative measure of the human visual response to optical flux.

Luminance – photopic radiance. The unit is “lumens per square meter per steradian” or, equivalently, “candela per square meter”.

Luminance – photopic radiance. The unit is “lumens per square meter per steradian” or, equivalently, “candela per square meter”.

Luminous efficacy – the efficiency of a light source in producing visible light, expressed in lumens per Watt. Note – the Watts can be measured as a radiometric quantity or at the electrical source. The distinction is generally specified.

Lux – a unit of illuminance equal to one lumen per square meter.

Nit – a [seldom used] unit of luminance equal to one candela per square meter or, equivalently, one lumen per square meter per steradian.

Photopic – pertaining to the human visual system and its response to light.

Projected area – the area of a surface as projected on a plane perpendicular to the viewing direction. The projected area is equal to the actual area times the cosine of the angle between the normal to the surface and the viewing direction.

Projected solid angle – used in calculating radiative transfer when the directions between all parts of the source and all parts of the receiver are not essentially the same. The projected solid angle does not lend itself to a simple short definition. A good treatment can be found in the reference by Bartell at the end of this list.

Radiance – the amount of flux radiated by a projected area of surface per steradian of solid angle. The radiometric unit is “watts per square meter per steradian”;

Radiometric – pertaining to physical units of light measurement.

Solid angle – the two-dimensional space contained in a [symmetrical or irregularly shaped] cone radiating from a point. It is analogous to the one-dimensional space between two lines radiating from a point, which we call a plane angle. Solid angle is measured in steradians.

Spectra (n.) - Plural of spectrum

Spectral (adj.) – a description of light that separates it into its wavelength-by-wavelength (or frequency-by-frequency) components.

Spectrophotometer – an instrument for measuring the spectral transmitting or reflecting properties of materials. A spectrophotometer is not a device for measuring light, but rather for measuring the optical properties of materials.

Spectroradiometer (n.) – a device for measuring spectral flux, spectral radiance, or spectral irradiance.

Spectroradiometric (adj.) – a measurement of light that looks at its spectral components.

Spectrum (n.) – a description of light that separates it into its wavelength-by-wavelength (or frequency-by-frequency) components.

Steradian – a measurement of solid angle. A solid angle of one steradian from a point is formed by a [symmetrical or irregularly shaped] cone radiating from the point that intersects an area equal to the square of the radius on a sphere centered at the point . There are 4pi or approximately 12.6 steradians in a complete sphere.

Watt – a physical unit of power or the flow of energy.