philosopher Johann Heinrich Lambert (1728–
1777), exhibit this property; thus, it is appro-
priate to measure exitance only for Lam-
bertian surfaces.
Calculations for illuminance (footcandle
values) overlook the aesthetic, psychologi-
cal, and physiological variables of the human
visual process. When a convenient measure
of perceived luminance becomes available,
it will be more useful to calculate perceived
surface luminance values, which account for
these factors.
The perception of surface luminance is
based largely on the eye’s ability toadapt.
The irisdilates(opens) when illuminance is
low and contracts (closes) when illumi-
nance is high. It takes the eye longer to
adapt from light to dark than from dark to
light. When you enter a dark theatre on a
sunny day, it takes twenty to thirty minutes
for the eyes to adapt completely to the
lower illuminance. When you leave the
theatre and return to daylight, it sometimes
takes only seconds for the eyes to adapt to
the higher luminance.
In a more subtle way, the eyes are
continually adapting as you move in and
through variously lighted spaces or look
around at objects of varying illuminances.
Even measured luminance does not indi-
cate theapparent brightness because of
the eye’s ability to adapt. Measured lumi-
nance, then, yields a poor indication of per-
ceived brightness, which is modified by the
surrounding conditions and adaptation of
the eyes.
It is thebalanceof these relative lumi-
nances, not the quantity of illuminance
received on a surface, that determines suc-
cessful lighting design. Therefore, the
illuminance measurements that follow are
notto be used as the starting point for a
design. They are to be used only for lamp
and luminaire selection or to evaluate a
lighting design.
Luminous Intensity Distribution
Curve
The intensity distribution curve represents
the amount of luminous intensity (cd) gener-
ated in each direction by a light source in a
plane through the center of the source. Con-
sequently, the luminous intensity curve gives
a picture of the total light pattern produced
by a source.
Luminous intensity distribution curves
are available from luminaire manufacturers
and are often found on the back of the man-
ufacturer’s product data sheet. A polar
graphis used to represent the distributional
intensity of a luminaire, and arectilinearor
Cartesian graphto represent the distribu-
tional intensity of a directional lamp.
In the polar graph (figure 10.1), the
luminaire is located at the center of the radi-
ating lines. The radiating lines represent spe-
cific degrees of angular rotation from the 0°
axis of the luminaire (nadir). The concentric
circles represent graduating intensity
expressed in candelas, with values entered
along the vertical scale.
To determine the luminous intensity of
this luminaire at 30°, find the appropriate
angled line drawn from the center of the
luminaire. Follow the line until it meets the
polar curve, then follow the circular line origi-
nating at that point and read the luminous
intensity (in candelas) on the vertical scale.
In figure 10.1, the luminaire produces 1,850
cd at 30° from nadir.
For luminaires with symmetrical light
distributions, a single curve fully describes
the luminaire’s distribution. Often only one
side of the polar graph is shown, since the
other side is an identical, mirror image.
A luminaire with an asymmetrical distri-
bution, such as a linear fluorescent down-
light, requires curves in a number of planes
to adequately represent its distribution.
Typically, one curve is parallel to the
luminaire and another is perpendicular toPHOTOMETRICS