UFC 3 -520-01
June 10, 2002
F-11.1.1 Illumination. The primary calculation is to determine illum ination at the task in
the horizontal plane. The default height of the task plane is 762 millimeters (30 inches)
above the floor indoors, and on the surface of the ground outdoors. Illumination is
measured in footcandles (lumens per square foot) or lux (lumens per square meter). A
secondary calculation involves determining the illumination occurring in the vertical
plane, also measured in footcandles or lux.
F-11.1.2 Exitance. Exitance calculations are a secondary calculation that can be
performed to determine the reflected light emitted by a surface. Because exitance
calculations presume Lambertian (diffuse) distribution, they should only be used to
assess surfaces with these characteristics.
F-11.1.3 Luminance. Luminance calculations are a second ary calculation type that can
be performed to assess the brightness of surfaces or sources, and in particular with
respect to each other. Luminance calculations are especially useful for determining
visibility and relative brightness for visual comfort.
F-11.1.4 Light Loss Factors. Ensure calculations account for light loss according to
IESNA recommendations. Assume average or longer-than-average maintenance
cycles for recoverable loss factors, including luminaire dirt depreciation (LDD) and lamp
lumen depreciation (LLD).
Calculations of Lighting for Interior Spaces. For each interior space,
perform a calculation employing one of the following methods to determine luminaire
quantity and layout.
F-11.2.1 Lumen Method. The lumen method is a calc ulation procedure that can be
performed by hand or by simple, spreadsheet formulas. It determines the average
illumination in a space, and is reliable only for spaces with a regular and uniform "grid"
of luminaires in which general lighting, providing task light levels everywhere, is
appropriate. The lumen method also can be used for determination of "ambient"
illumination in rooms in which localized "task lights" are used strictly for task light. Refer
to the IESNA Lighting Handbook for additional infor mation.
F-11.2.2 Point Calculations Using Flux Transfer Calculations. Commercially
available computer programs that assume Lambertian (matte or flat) room surfaces can
perform point calculations. These calculations indicate illumination at specific points
and are capable of exitance and luminance calculations as well. Some programs can
incorporate objects in space to assess the lighting in a non-empty room. Many
programs generate perspective views of illuminated rooms, although due to the lack of
specular reflectivity these rooms appear unnatural.