Technical Note:
Solar Simulator Definitions of Characteristics

This technical note briefly defines the terms and units most frequently used in technical discussions around solar simulation.

Actinic dose: quantity obtained by weighing spectrally the radiation dose using the action spectrum.

Actinic (radiation): the radiation that produces a specified effect.

Action spectrum (actinic): efficiency of monochromatic radiations for producing a specified actinic phenomenon in a specified system.

Air mass (relative optical): ratio of the slant optical thickness to the vertical optical thickness of the standard atmosphere.

Albedo: reflectance of solar radiation by the surroundings. This applies to the full integrated spectrum; the reflectance may depend strongly on the spectral region (definition limited to solar radiation).

Blackbody: a body that absorbs all radiant energy incident on it.

Collimation and angle of terrestrial solar irradiation: the terrestrial irradiance from the sun is composed of a direct beam with a collimation angle of approximately 0.5° and a diffuse component; the spectra and magnitude of each component changes throughout the day. Measurement of direct radiation requires limiting the field of view (FOV). (The recommended aperturing system limits the input to a slope half angle of 0.5°, an opening half angle of 2.65°, and a limit half angle of 4.65°. Measuring the total radiation requires an instrument with a 180° FOV.)

Daylight: visible part of global solar radiation.

Diffuse sky radiation: the part of solar radiation that reaches the earth as a result of being scattered by air molecules, aerosol particles, cloud particles, or other particles.

Direct solar radiation: the part of extraterrestrial solar radiation which, as a collimated beam, reaches the earth's surface after selective attenuation by the atmosphere.

Dobson unit (D.U.): measure of columnar density of ozone. 1 D.U. is one milliatmosphere centimeter of ozone at STP. Typical values range from 200 - 600 D.U. with values of 110 in the Antarctic "ozone hole."

Dose: term used in photochemistry, phototherapy, and photobiology for the quantity radiant exposure (of optical radiation of a specified spectral distribution). Unit, J m-2.

Dose Rate: term used in photochemistry, phototherapy, and photobiology for the quantity irradiance. Unit, W m-2.

Effective dose: that part of the dose that actually produces the actinic effect considered.

Effective exposure rate: the integrated product of the spectral irradiance and action spectra.

Erythema (actinic): Reddening of the skin, with or without inflammation, caused by the actinic effect of solar radiation or artificial optical radiation.

Erythemal radiation: optical radiation effective in causing actinic erythema.

Extraterrestrial solar radiation: solar radiation incident on the outer limit of the earth's atmosphere.

Global illuminance (Eg): illuminance produced by daylight on a horizontal surface of the earth.

Global solar radiation: combined direct solar radiation and diffuse sky radiation.

Infrared radiation: optical radiation for which the wavelengths are longer than those for visible radiation, 700 nm to 1000 µm.

Irradiance: describes the flux, radiative power density, and incidence on a surface. Units, W m-2 or W cm-2. The surface must be specified for the irradiance to have meaning. (Laboratory surfaces are not usually as large as a square meter; this happens to be the appropriate SI unit of area).

Langley: 1 calorie cm-2 = 2.39 x 105 J m-2 

Minimum Erythema Dose (MED): the actinic dose that produces a just noticeable erythema on normal, non-exposed, "white" skin. This quantity corresponds to a radiant exposure of monochromatic radiation at the maximum spectral efficiency (λ = 295 nm) of roughly 100 J m-2.

Ozone (O3): what is produced when molecular oxygen in the stratosphere absorbs shortwave ultraviolet (up to 242.2 nm), and photodissociates. Ozone can be a health hazard in concentrated amounts. (Our solar simulators use ozone free lamps.)

Solar constant (ISC): irradiance produced by the extraterrestrial solar radiation on a surface perpendicular to the sun's rays at a mean sun-earth distance (ISC = (1367 ±7 W m-2).

Spectral irradiance E(λ): the irradiance per unit wavelength interval at a specified wavelength.

Spectral irradiance units, W m-2 nm-1

To convert into W m-2 µm-1, multiply by 1000 (1000 E)

To convert into W cm-2 nm-1, multiply by 10-4 (10-4 E)

To convert into W cm-2 µm-1, multiply by 0.1 (0.1 E)

Standard solar radiation: spectra that have been developed to provide a basis for theoretical evaluation of the effects of solar radiation, and as a basis for simulator design. In this catalog, we refer to the ASTM E490, E891 and E892 standards, which define AM 0, AM 1.5 D and 1.5 G, respectively. We also refer to the CIE Pub. 85 and 904-3 standards, which define AM 1 and AM 1.5 G, respectively.

Sunlight: the visible part of direct solar radiation.

Sunshine duration: the sum of time intervals within a given time period during which the irradiance from direct solar radiation on a plane normal to the sun direction is equal to or greater than 200 W m-2.

Terrestrial spectra: the spectrum of the solar radiation at the earth's surface.

Ultraviolet radiation: optical radiation for which the wavelengths are shorter than those for visible radiation, <400 nm.

Note: For ultraviolet radiation, the range below 400 nm is commonly subdivided into:

UVA 320 - 400 nm

UVB 280 - 320 nm

UVC <280 nm

Uniformity: a measure of how the irradiance varies over a selected (or defined) area. Usually expressed as non-uniformity, the maximum and minimum % differences from the mean irradiance.

Visible radiation: any optical radiation capable of causing a visual sensation directly, 400 - 700 nm.