BASIC CONCEPTS OF RADIATION - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 20

ANNEX - Summary of Radiometric Quantities
The following radiometric quantities may be used in photobiology,
photochemistry, photodermatology and illuminating science, and are briefly
summarized here:
Irradiance (surface dose rate) and radiant exposure (surface dose) are units specifying
power or energy incident upon a plane. These quantities are the most fundamental dose
quantities used in all of photobiology. The units most commonly used are W/cm 2 and
J/cm 2 , respectively. 1 W = 1 J/s.
Fluence rate and fluence are used in some very sophisticated studies, where the internal
surface dose with backscatter is included. These quantities are used correctly most often
in theoretical studies of dose distribution and where photochemistry at the molecular
level in tissue is enhanced as a result of multiple scattering events in tissue.
Unfortunately, these terms are frequently misused to mean irradiance and radiant
exposure because the units of W/cm 2 and J/cm 2 are the same.
Radiance (irradiance per solid angle) is an important quantity used by physicists in
specifying a source. This quantity limits the ability of lenses and reflective optics in
concentrating a light source. For, example, a xenon-arc lamp has a very high radiance
and its energy can be focused to produce a very high irradiance on a target tissue. By
contrast, a fluorescent lamp tube has a much lower radiance, and its energy cannot be
focused to a high concentration. The units are W/(cm 2 ·sr).
Radiant Intensity (power per solid angle) is used to indicate how collimated a light
source really is. Although useful for specifying searchlights, it normally has very
limited use in photobiology. The units are W/sr.
Spectral quantities (units per wavelength) are used for specifying the energy, power or
irradiance per wavelength interval. When calculating a photobiologically effective dose
the spectral quantity must be multiplied by the action spectrum. Examples: spectral
radiant power, spectral irradiance, spectral radiant exposure, etc. The units for each
quantity are modified by adding "per nanometer," e.g., W/cm 2 becomes W/(cm 2 ·nm).
Photon (Quantum) quantities (units of photons) are used primarily in theoretical
studies, and in photochemistry. In this case the radiant exposure is specified in
photons/cm 2 and irradiance is specified in photon/(cm 2 ·s).