BASIC CONCEPTS OF RADIATION - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 21

Table A1 . Useful Radiometric Units 1,2

Term Symbol Definition Unit and abbreviation

Radiant Energy Q Energy emitted, transferred,

or received in the form of
radiation

joule (J)

watt (W) defined as Jǜs -1

Radiant Power ĭ Radiant Energy per unit time

Radiant Exposure H Energy per unit area incident

upon a given surface

joules per square centimeter
(Jǜcm -2 )

(Dose in Photobiology)

Irradiance or Radiant Flux
Density (Dose Rate in
Photobiology)

E Power per unit area incident

upon a given surface

watts per square centimeter
(Wǜcm -2 )

joules per steradian (Jǜsr -1 ) Integrated Radiant Intensity IP Radiant Energy emitted by a

source per unit solid angle

watts per steradian (Wǜsr -1 ) Radiant Intensity I Radiant Power emitted by a

source per unit solid angle

Integrated Radiance LP Radiant Energy emitted by a

source per unit solid angle
per source area

joules per steradian per square
centimeter (Jǜsr -1 ǜcm -2 )

Radiance 3 L Radiant Power emitted by a

source per unit solid angle
per source area

watts per steradian per square
centimeter (Wǜsr -1 ǜcm -2 )

Optical Density OD A logarithmic expression for

the attenuation produced by a
medium
OD = -log 10 () O /) L )
) O is the incident power;
) L is the transmitted power

unitless

1 The units may be altered to refer to narrow spectral bands in which the term is preceded by the word spectral
and the unit is then per wavelength interval and the symbol has a subscript Ȝ. For example, spectral
irradiance E Ȝ has units of Wǜm -2 ǜm -1 or more often, Wǜcm -2 ǜnm -1 .
2 While the meter is the preferred unit of length, the centimeter is still the most commonly used unit of length
for many of the terms below and the nm or µm are most commonly used to express wavelength.
3 At the source L = d I/( d AǜcosT) and at a receptor L = d E/( d :ǜcosT).