Alliance for Lighting Information

Uniformity Ratios

by David M. Keith

Uniformity ratios can be defined in many ways and over any area. Therefore any definition that does not clearly state what values are being compared in the uniformity ratio is technically incomplete and practically useless. The definition for uniformity ratio can vary from average-to-minimum to maximum-to-minimum, and can be applied to vertical or horizontal (or even both) values for either illuminance or luminance - initial or maintained - at grade or above - over the entire site or part of it (see Table 1). The most common uniformity ratio in ordinances is the ratio of the maximum-to-minimum illuminance at grade - initial - over the entire site.

Table 1: Uniformity Ratio Options

Metric: illuminance or luminance or ?
System Life: initial or maintained or ?
Measurement Location: at grade, at ? height, facing ? direction
Area: site or site with setback or other defined sub-areas (such as parking lot or sidewalk)
Numerator: maximum or average
Denominator: minimum

Values for uniformity ratios can be shown in different ways, creating some confusion. The presentation as a ratio - such as 3:1 - can be misleading, and the appearance of a single number can also be confusing. The IESNA has adopted the practice of presenting uniformity ratios as single values (e.g. 6.0) and noting that these values are the maximum allowed. This means that unlike most other lighting criteria, the uniformity ratio criterion is the maximum value that may occur for the lighting system to meet the set of criteria.

The distinction between initial and maintained values is typically ignored, since the current lighting industry techniques for predicting maintained levels makes the (questionable) assumption that the distribution of the light does not change over the maintenance cycle of the lighting system.

The possible range of uniformity ratios can make it difficult to understand the implications. A ratio of 3:1 for average to minimum roughly corresponds to a ratio of 10:1 for maximum to minimum. Compared to systems meeting criteria of 6:1 and 20:1, systems meeting 3.5:1 and 10:1 ratios as criteria will have increases in lighting system costs and energy use of 30% or more. As requirements for uniformity become more stringent, the cost of lighting systems also increase, as do all pollutions associated with installing and operating the lighting systems.

For reference purposes, example uniformity ratios are cited from the relevant IESNA recommended practices, in all cases referring to maintained values:

Table 2:  Example Uniformity Ratios from IESNA Recommended Practices

Recommended Practice		Ratio Definition			Value(s)

Roadway Lighting (RP-8-00)		Avg:Min Illum. @grade		3.0 - 6.0
				Max:Min Lumin. @ grade		5.0 - 10.0

Parking Lot Lighting (RP-20-98)	Max:Min Illum. @grade		20.0 (15.0 for special security)

Walkways & Bikeways (DG-4-94)	Avg:Min Illum. @grade		4.0 - 10.0

All of these example uniformity ratios are to be applied only over the relevant part of the site, not the entire site. In the case of the roadway criteria, the ratios are to be applied to only the specified calculation points (defined in IESNA RP-8-00). These roadway calculation points are on a specified grid that (almost) always produces much lower uniformity ratios than would be produced by a regular (e.g. five foot by five foot) grid of points covering the same area. Therefore using these roadway uniformity ratio values for more typical area calculation grids - even on roadways - is requiring excessive uniformity, which increases the costs of a lighting system. By comparison, the walkway and parking lot uniformity ratios are for any grid of points and therefore suitable for general application over those distinct areas within a site.

All uniformity values are sensitive to the spacing between the calculation points, with smaller spacing between points producing higher ratios. For example, changing the grid from 5x5 to 6x6 can sometimes make the difference between failing and meeting a uniformity ratio criterion. Therefore such ratios should be associated with a reasonable spacing of calculation points.

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