Page 209 - Algae

192 Algae: Anatomy, Biochemistry, and Biotechnology

SPECTRAL RADIANT FLUX (SPECTRAL RADIANT POWER)

Spectral radiant flux at wavelength l is radiant flux per unit wavelength interval. It is defined as:
dF
F l ¼ (5:8)
dl
and is measured in watts per nanometer (W nm 21 ).

RADIANT FLUX DENSITY (IRRADIANCE AND RADIANT EXITANCE)
Radiant flux density is the radiant flux per unit area at a point on a surface. Radiant flux density is
22
measured in watts per square meter (W m ).
There are two possible conditions. The flux can be arriving at the surface in which case the
radiant flux density is referred to as irradiance. Irradiance is defined as:

dF
E ¼ (5:9)
d A
where F is the radiant flux arriving at the infinitesimal area dA.
As irradiance is the radiant flux per unit area, it can be expressed as mole of photons per
area per unit time (mmol m 22 sec 21 ). Modern instruments measure in situ irradiance directly in
these units.
The flux can also be leaving the surface due to emission or reflection. The radiant flux density is
then referred to as radiant exitance. As with irradiance, the flux can leave in any direction above the
surface. The definition of radiant exitance is:

dF
M ¼ (5:10)
d A
where F is the radiant flux leaving the infinitesimal area d A.

Typical Values for Irradiance (in W m 22 )
Sunlight 1000
Skylight 100
Overcast daylight 10
Moonlight 0.001
Starlight 0.0001

SPECTRAL RADIANT FLUX DENSITY
Spectral radiant flux density is radiant flux per unit wavelength interval at wavelength l. When the
radiant flux is arriving at the surface, it is called spectral irradiance and is defined as:

dE
E l ¼ (5:11)
dl
When the radiant flux is leaving the surface, it is called spectral radiant exitance, and is
defined as:

dM
M l ¼ (5:12)
dl