BASIC CONCEPTS OF RADIATION - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 9

Fluence and fluence rate
In theory, fluence and fluence rate are the most important and useful radiometric
concepts in photochemistry and photobiology, since they actually best describe the dose
or dose-rate, respectively. However, these terms are frequently misused, and fluence is
frequently mistaken for radiant exposure H because the same units of J/m 2 . However,
the fundamental definition of fluence differs from that of radiant exposure. Fluence is
the radiant energy passing through a small unit area, but it does not include the cosine
response and fluence includes backscatter. This unit was virtually designed for use in
radiation biology and biochemistry. It is particularly useful in photochemistry and
photobiology to describe the microscopic exposure of a molecule in a scattering
medium. Likewise, fluence rate is also mistaken for irradiance E, since it has the same
units of W/m 2 , but it includes backscatter and ignores a cosine response. Thus, it is also
useful in photochemistry and photobiology in a scattering medium.
Figure 3. Schematic comparison of irradiance and fluence rate, both expressed as power per unit
area; and radiant exposure and fluence, both expressed as energy per unit area.
4. Action spectra
The CIE visual response functions: scotopic & photopic
Examples of action spectra are the CIE visual response functions. The scotopic
function describes the eye's response to low light levels experienced at night and this
action spectrum peaks around 505 nm. The photopic luminous efficacy function
describes the relative spectral response of the CIE “standard observer” (an average of a
number of individuals) to daylight; this response peaks around 555 nm. 1,2,7 The entire
science of photometry revolves around these CIE luminous efficacy functions as shown