20
Luminous flux is the time rate of flow of light energy that is characteristic
of radiant energy which produces visual sensation. The unit of luminous
flux is the lumen, which is the flux emitted in units per solid angle by a
uniform point of source of one candela. Such a source produces a total
luminous flux of 4
π
lumens.
A radiant source may be evaluated in terms of luminous flux if the radiant
energy distribution of the source is known. If W(
λ
) is the total radiant power
in watts per unit wavelength, total radiant power over all wavelengths is
W
d
( )
λ λ
0
∞
∫
and the total luminous flux L in lumens can be expressed as
L
W
y
d
=
[
][
]
∞
∫
680
0
( )
( )
λ
λ λ
where y(
λ
) represents the luminosity coefficient as a function of wavelength
and d
λ
is a differential of wavelength.
Illuminance is the density of luminous flux incident on a surface. A
common unit of illuminance is the lux, which is the illumination produced
by one lumen uniformly distributed over an area of one square meter. It
follows that a source of one candela produces an illuminance of one lux at a
distance of one meter. A footcandle is one candela at a distance of one foot.
Spectral Response
The spectral response of a typical LI-COR LI-210SA Photometric Sensor
compared to the C.I.E. standard observer curve is presented in Figure 6. In
1976, LI-COR had sensor calibration data verified by the National Research
Council of Canada (NRC), one of the major standards laboratories in the
world. Information concerning these tests is available from LI-COR.
Calibration
The LI-210SA Photometric Sensor has been calibrated against a standard
lamp. The uncertainty of the calibration is
±
5%.
