Environmental Engineering Reference
In-Depth Information
absorption coefficient is expressed as inverse units of length, e.g., cm
-1
and the
reciprocal of D is termed the penetration depth, where about 63% of the incident energy
is absorbed, and at twice that depth, only 13.5% of the incident flux remains, etc.
3. Radiometric quantities and units
Radiant energy
The radiant energy (CIE/ISO Symbol: Q) is the energy emitted, transferred or
received in the form of radiation. The SI unit: is the joule (J). The symbol Q is used to
identify radiant energy. If there is an e subscript (Q
e
) we know that we are using
radiometric units in joules, but if the Q has a subscript v (Q
v
), then we know that this is
luminous energy in lumens, and the energy is dependent upon the CIE action spectrum
for daylight vision (the eye's photopic response). The CIE visual response functions will
be discussed in a later section.
The fundamental quantity of energy is actually defined in physics in terms of
mechanical work; work is the product of a displacement d and the constant force F in
the direction of d, such that Work = Force • Displacement. The work done by the
constant force F on an object is equal to the change in energy of that object. As force is
expressed in the units of kg•m/s
2
, the fundamental unit of the joule is basically defined
as: 1 Joule = (1 kg • m/s
2
) • 1 m. In quantum mechanics, the energy of single photons
can also be described as noted in Equation [1]. The annex describes radiometric
quantities.
Radiant power
Radiant power (also termed in some circles, “radiant flux”) is a measurement of
the radiant energy passing a point in a time interval (seconds). The CIE/ISO standard
symbol may be either P (for “power”) or the Greek letter ĭ for “flux.” A watt is a unit
of power, such that: 1 watt = 1 joule/second (J/s). Remembering that energy is defined
in terms of work, it is easy to see that power is work/time. This energy can also be
converted to heat, and calorimeters are used to measure mechanical heat, or they can be
used to measure the heat energy produced by the absorption of radiant energy, thus the
fundamental quantities are related in optics, mechanics and thermal physics.
Radiant exposure
Radiant exposure is the radiant energy incident on a surface divided by the area
(projected to the normal) of the surface. The SI unit is Joules/m
2
. The CIE/ISO symbol
is H. In photobiology radiant exposure is also called the “exposure dose.”
Irradiance
The irradiance on a surface is the radiant power incident on a surface divided by
the area of the surface. Irradiance multiplied by the exposure duration, the time t in
seconds, is the radiant exposure. The CIE/ISO symbol is E. Irradiance is known as the
“exposure dose rate” in photobiology.
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