Photometrics
It is impossible to see a footcandle. What is seen is luminance, which is a
function of the amount of light falling on a surface and the reflectance of that
surface, modified by the surrounding conditions and adaptation of the eyes.
Measurement of Light
Photometry is the science that measures
light. Five terms are commonly used to
quantify light: intensity, flux, illuminance,
exitance, and luminance.
1.Intensityis the light emitted in a specific
direction by a source. Properly calledlumi-
nous intensityand defined asflux per solid
angle in a given direction, it is measured in
candelas (cd). Intensity in a succession of
directions is plotted on a distribution curve
or polar graph (figure 10.1).
2.Fluxis the light emitted in all directions
by a source. Properly calledluminous flux
and defined astime rate flow of light,itis
measured in lumens (lm).
3.Illuminanceis the density of light at any
given point on a surface. Properly defined
asdensity of flux incident on a surface
measured perpendicular to the surface,it
is measured in footcandles (fc).
4.Exitanceis the total quantity of light
emitted, reflected, or transmitted in all
directions from a surface. Properly
defined asdensity of flux leaving a sur-
face, it is measured in lumens per square
foot (lm/ft^2 ).5.Luminanceis the accepted term for light
that is reflected from a surface in a given
direction (back toward the eyes). Properly
defined asintensity of flux leaving a sur-
face in a given direction, it is measured in
candelas per square foot (cd/ft^2 ).Measurement Limitations
Illuminance is frequently used to measure
the quantity of light in architectural space
because it is the easiest and least expensive
unit to measure. Yet it is impossible toseea
footcandle!
What is seen isluminance, which is a
function of the amount of light falling on a
surface and the reflectance of that surface
(its ability to reflect light). We see an object
or surface only when light is reflected from
that object or surface back toward the eyes,
or when it is emitting light itself (self-lumi-
nous).