Page 208 - Algae Anatomy, Biochemistry, and Biotechnology

Working with Light 191

A surface can be described as a continuum of infinitesimal points, each occupying an infinite-
simal area dA,

d A
dv ¼ (5:5)
r 2
where dv is the differential solid angle of the elemental cone containing a ray of light that is
arriving at or leaving a infinitesimal surface dA. The symbol d stands for differential, the operator
that reduces the applied variable to an infinitesimal quantity.
Most radiometric measurements do not require an accurate calculation of the spherical surface
area. Flat area estimates can be substituted for spherical area when the solid angle is less than 0.03
steradians, resulting in an error of less than 1%. This roughly translates to a distance at least five
times greater than the largest dimension of the detector. When the light source is the Sun, flat
area estimates can be substituted for spherical area.

RADIANT ENERGY
Light is radiant energy. When light is absorbed by a physical object, its energy is converted into
some other form. Visible light causes an electric current to flow in a light detector when its
radiant energy is transferred to the electrons as kinetic energy. Radiant energy (denoted as Q)is
measured in joules (J).

SPECTRAL RADIANT ENERGY
A broadband source such as the Sun emits electromagnetic radiation throughout most of the
electromagnetic spectrum. However, most of its radiant energy is concentrated within the
PAR. A single-wavelength laser, on the other hand, is a monochromatic source; all of its radiant
energy is emitted at one specific wavelength. From this, we can define spectral radiant
energy, which is the amount of radiant energy per unit wavelength interval at wavelength l.It is
defined as:

dQ
Q l ¼ (5:6)
dl
Spectral radiant energy is measured in joules per nanometer (J nm 21 ).

RADIANT FLUX (RADIANT POWER)
Energy per unit time is power, which we measure in joules per second (J sec 21 ), or watts (W). Light
“flows” through space and so radiant power is more commonly referred to as the flow rate of radiant
energy with respect to time or radiant flux. It is defined as:

dQ
F ¼ (5:7)
dt
where Q is radiant energy and t is time.
In terms of a light detector measuring PAR, the instantaneous magnitude of the electric
current is directly proportional to the radiant flux. The total amount of current measured over a
period of time is directly proportional to the radiant energy absorbed by the light detector during
that time. For phycological purpose radiant flux is expressed also as micro moles of photons per
second.