PR-680/680L User Manual
29
I
LLUMINANCE
/
I
RRADIANCE
Illuminance is defined as the density of luminous flux incident on a surface, or light falling on a surface
and is calculated from
Irradiance (watts/meter²)
. Three factors help determine illuminance: the
luminous flux of the source, the angle of incidence of the light falling on the plane and the distance from
the source to the measured plane. Illuminance is given by
lux (lumens per meter²)
and
footcandles
(lumens/foot²).
The relationship between footcandles and lux is:
1 footcandle = 10.76 lux.
In the
following example, the light falling on the measured plane is the illuminance.
FIGURE 73 - ILLUSTRATION OF ILLUMINANCE
The (optional) accessories used to measure illuminance are the CR-680, and the RS-3 or SRS-3. The CR-
680 is a diffuser that serves as the surface upon which the incident light falls. During calibration these
accessories are photometrically calibrated using a standard lamp whose candela rating has been
certified to NIST standards. By knowing the candelas of the lamp, the illuminance can be calculated by:
2
d
cd
I
=
EQUATION 5 - ILLUMINANCE FROM CANDELAS
Where:
I
= Illuminance in
lux
or
footcandles.
d =
Distance in feet (
footcandles
) or meters (
lux
).
Following a measurement, candelas can be calculated by:
2
*
d
I
cd
=
EQUATION 6 - CANDELAS FROM ILLUMINANCE
H
OW TO
M
AKE AN
I
LLUMINANCE
/
I
RRADIANCE
M
EASUREMENT
Illuminance measurements are made one of two ways with the PR-680/680L. The first method involves
the use of the CR-680 cosine receptor. The second method utilizes the use of a reflectance standard
such as the RS-3 or SRS-3.
Source
Measured
plane
Distance
Lumens