USL PCA-100 - Manual (Page 9)

PCA I NSTRUCTION M ANUAL
Ultra Stereo Labs, Inc.
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Luminance is a term used to describe the amount of light reflected
from a surface. An often used synonym is brightness. The unit
of luminance is the foot-Lambert in English units, or the candela
per square meter in metric units. One foot-Lambert equals 3.462
cd/sq.meter.
The PCA color meter measures luminance and displays the
results on the laptop display.
Illuminance is the light flux per unit area falling on a surface. The
English unit of illuminance is the footcandle, (lumens per square
foot), the metric unit is the lux, (lumens per square meter). One
footcandle equals 10.76 lux. The PCA can measure illuminance
with the aid of a “Reflectance Standard”. The reflectance standard
is placed on the surface and the color meter is focused on the
reflectance standard. The luminance reading in foot-Lamberts
then translates directly to footcandles. Luminance readings in
candelas per sq. meter must be multiplied by 3.14 to obtain the
illuminance in lux.
This is the unit of luminous flux weighted by the photopic amplitude
vs wavelength function.
The term "color temperature" refers to the concept of an ideal
black body. The color of the radiation from the black body varies
directly with the temperature. As the temperature goes up, the
radiation changes from red through orange and yellow to white.
The absolute temperature in degrees Kelvin is referred to as the
color temperature. The locus of the black body color temperature
is shown on the chromaticity diagram in figure 5.
Since “color temperature” is based upon an ideal black body
concept, it cannot be measured directly. It is possible, however,
to derive a correlated color temperature (cct), value from the x
and y coordinates. A cct temperature value is calculated and
displayed for any set of x,y values on or near the black body
locus (figure 5).
1. Luminance
2. Illuminance
3. Lumen
4. Color Temperature
TERMINOLOGY
5. Correlated Color Temperature