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F-11.1

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).

F-11.2

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.

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