"Radiant emittance" is an old term for this quantity. This is the emitted component of radiosity. Radiant flux emitted by a surface per unit area. This is sometimes also confusingly called "spectral intensity". The latter is commonly measured in W⋅m −2⋅nm −1. Radiosity of a surface per unit frequency or wavelength. This is sometimes also confusingly called "intensity". Radiant flux leaving (emitted, reflected and transmitted by) a surface per unit area. Non-SI units of spectral flux density include jansky (1 Jy = 10 −26 W⋅m −2⋅Hz −1) and solar flux unit (1 sfu = 10 −22 W⋅m −2⋅Hz −1 = 10 4 Jy). Irradiance of a surface per unit frequency or wavelength. Radiant flux received by a surface per unit area. Watt per steradian per square metre, per metre The latter is commonly measured in W⋅sr −1⋅m −2⋅nm −1. Radiance of a surface per unit frequency or wavelength. Watt per steradian per square metre per hertz Radiant flux emitted, reflected, transmitted or received by a surface, per unit solid angle per unit projected area. The latter is commonly measured in W⋅sr −1⋅nm −1. Radiant intensity per unit frequency or wavelength. Radiant flux emitted, reflected, transmitted or received, per unit solid angle. The latter is commonly measured in W⋅nm −1. Radiant flux per unit frequency or wavelength.
This is sometimes also called "radiant power", and called luminosity in Astronomy. Radiant energy emitted, reflected, transmitted or received, per unit time.
Photographers more familiar with base-2 logarithmic scales (such as exposure values) can convert using log 2( H) ≈ 3.32 log 10( H). In sensitometric data, such as characteristic curves, the log exposure is conventionally expressed as log 10( H). The use of radiometric units is appropriate to characterize such sensitivity to invisible light. Many photographic materials are also sensitive to "invisible" light, which can be a nuisance (see UV filter and IR filter), or a benefit (see infrared photography and full-spectrum photography). Only in this appropriately weighted case does the H measure the effective amount of light falling on the film, such that the characteristic curve will be correct independent of the spectrum of the light. If the measurement is adjusted to account only for light that reacts with the photo-sensitive surface, that is, weighted by the appropriate spectral sensitivity, the exposure is still measured in radiometric units (joules per square meter), rather than photometric units (weighted by the nominal sensitivity of the human eye). t is the exposure duration, measured in s.E v is the illuminance of the surface, measured in lx.Definitions Radiant exposure ĭenoted H e ("e" for "energetic", to avoid confusion with photometric quantities) and measured in J/m 2, is given by H e = E e t,
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