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Performance Data Evaluating Performance
What do lamp isofootcandle diagrams tell us, and
how should they be used?
Overview
Isofootcandle Diagrams Versus Point-by-Point Calculations
An isofootcandle curve is a line plotted in an imaginary plane, perpendicular to
the axis of the light source (typically horizontal and/or vertical) and comprised of
points of equal illuminance. See diagram 1.
This curve can be used to illustrate the maximum distance from the source the
desired illuminance can be achieved and the region through which this level is
met or exceeded. A series of isofootcandle curves for a given light source is
known as an isofootcandle diagram. See diagram 2.
Each isofootcandle curve represents a unique illuminance value in the same imaginary
plane.
What is the most accurate way to evaluate emergency
lighting units to determine maximum center-to-center
spacing?
Lamp isofootcandle diagrams are commonly used to compare one light source to
another in terms of light distribution and intensity. For this reason, many lamp
manufacturers provide isocontour diagrams for the lighting professional.
However, lamp isofootcandle diagrams alone in no way offer a reasonable prediction
of the illuminance levels one might expect to obtain for a horizontal work
plane or path of egress.
The diagrams are simplistic and do not take into account any of the application
variables known to impact an indoor lighting layout including, but not limited to:
the surface reflectance of the ceiling, walls and floors; the room geometry; and
the true distance and angle at which the light strikes the path of egress and other
surfaces. Collectively, these variables have a major impact on the overall lighting
layout.
344 Emergency / PSG8 www.lithonia.com
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HORIZONTAL
VERTICAL
LTL10586
MR24 K0606
6-volts,5.4-wattskrypton
HORIZONTAL
VERTICAL
Diagram 1
Isofootcandle curve
10 20 Ft.
1 0.5 0.3
1 0.5 0.3
10 20 Ft.
Diagram 2