22 August 2006
Average illuminance: This criterion is typically used to give an approximate
light level. Typically this is used for areas where the lighting may not be
continuous and therefore give a better value than maximum and minimum.
Maximum to minimum uniformity: This is often used in applications where
lighting will be continuous or cover large areas such as roadways and
parking lots. It is less useful to define lighting in areas such as entries
where lighting may be designed to highlight a particular point. Table 7-1
lists recommended ratios for various applications.
Average to minimum uniformity: This criteria is also used to ensure
adequate uniformity. This is an easy value to calculate with typical lighting
software, but more difficult to measure and verify in final installations.
Table 7-1 lists recommended ratios for various applications.
Table 7-1. Recommended Illuminance Uniformity Ratios for Exterior Applications.
Average / Minimum Ratio
Maximum / Minimum Ratio
Expressways and Major
Parking Facilities (primarily
20 : 1
Parking Facilities (night-use)
15 : 1
Pedestrian Walkways and
Point Calculations for Flood and Spot Lighting. Point calculations are a
calculation procedure that can be performed by hand or in simple, spreadsheet
formulas. They determine the illumination at a point in either the horizontal or the
vertical plane, and are reliable only for single luminaires. Manufacturers often provide
photometric data in "iso-footcandle" form, which permits rapid assessment of the
performance of a single luminaire.
Automated Calculations for Exteriors. Commercially available computer
programs perform point-by-point calculations. These programs permit multiple
luminaires and can take buildings and other obstacles into account. Most programs
generate CAD-compatible site isolux plots and analytical statistics related to illuminance
and uniformity. Luminance, veiling luminance, and small target visibility should also be
calculated for roadway applications.