An Introduction to the Physical Basis for Deriving River Information by Optical Remote Sensing - Fluvial Remote Sensing for Science and Management - page 45

Table 3.1 Radiometric and reflectance quantities used to quantify light fields. Note that each of these
quantities can be expressed per unit wavelength to provide spectral information, in which case the units
would include a per nanometer (nm − 1 ) factor as well.

Quantity Symbol Units Unit Symbol

Radiant energy Q Joule J

Radiant flux Φ Watt = Joule/second W

Wm − 2 Radiant flux density: irradiance E Watt/square meter

Wm − 2 sr − 1 Radiance L Watt/square meter/steradian

Irradiance (volume) reflectance R dimensionless N/A

sr − 1 Bidirectional reflectance

distribution function (BRDF)

r BRDF per steradian

Building upon this concept of radiant energy, we can
define several other radiometric quantities relevant to
remote sensing, summarised in Table 3.1. First, consider
the rate at which the energy present in our light beam is
propagating. The amount of radiant energy transferred
per unit time is called the radiant flux or radiant power ,
assigned the symbol Φ , and expressed in units of J/s, or
Watts (W). A related quantity of fundamental interest is
the rate at which this radiant flux is delivered to a surface.

This radiant flux density is referred to as irradiance and
is denoted by E , with units of Wm − 2 . The irradiance thus
represents the amount of radiant energy received by a
surface per unit time and per unit area. As illustrated
in Figure 3.1, the actual amount of energy received also
depends on the angle at which the light beam strikes the
surface. This angle of incidence is given the symbol θ and
is measured relative to a line perpendicular to the surface;
θ = 0 ◦ indicates a beam striking the surface from directly

θ

E 0

E 0

= 0

θ

θ ) E = E 0 cos(

AIR
WATER

AIR
WATER

θ

(a) (b)

Figure 3.1 Projected area effects on the irradiance E incident upon a surface. (a) Surface is oriented perpendicular to the incident
radiation, and the irradiance is simply the radiant flux per unit area E 0 . (b) Surface is oriented at an angle relative to the incident
radiation. The angle of incidence is measured relative to a line drawn perpendicular to the surface. In this case, the irradiance is

reduced because the flux is projected, or spread, over a larger area and is given by E θ = E 0 cos( θ ). Figure adapted from Schott, J.R.
Remote Sensing: The Image Chain Approach . New York, Oxford University Press, 1997.

θ