Alliance for Lighting Information

Uniformity Ratios in Ordinances

by David M. Keith

One of the ways that lighting ordinances restrict outdoor lighting is by mandating the uniformity of the lighting over some of or all of the site. Uniformity ratios are an important part of a complete set of lighting criteria and can have a positive effect on the quality of lighting installations. The level of uniformity over a site however is not the same as the uniformity over part of the site, such as walkways or parking lots. This and other basic aspects of uniformity ratios are obviously not understood in some ordinances and are sometimes grossly misapplied. This reappearance of associated problems due to technical ignorance is much more frequent and significant than necessary.

The first issue is that uniformity ratios can be defined in many ways and over any area. Therefore any ordinance 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. The most common uniformity ratio in ordinances is the ratio of the maximum-to-minimum illuminance at grade - initial - over the entire site.

The possible range of uniformity ratios can make it difficult to understand the implications of uniformity ratios in particular. A ratio of 3:1 for average to minimum is roughly the same as a ratio of 10:1 (to 12:1) for maximum to minimum. Meeting 3:1 and 10:1 ratios as criteria will be around 50-100% more expensive than meeting criteria of 6:1 and 20:1 (to 24:1). As requirements for uniformity increase, the almost all of the costs of lighting systems also increase, as do any pollutions associated with installing and operating the lighting system.

Furthermore the spacing of the calculation points effects the resulting uniformity ratio values. For example, a uniformity ratio of 10.0 for maximum-to-minimum might be easy to meet with spacing of say 20x20 and much more dificult to meet with spacing of 4x4. The same lighting system that produces 8.0 max-to-min on a 10x10 spacing might easily produce 10.0 or higher with 5x5 spacing, especially if the mounting height of the luminaire is under 25 feet. Any specification of uniformity ratios should include consideration of the spacing of calculations points. One reasonable solution is to space the calculation points based on the height of the luminaires, say no more than one-half or one-quarter of the mounting height. This can obviously become micro-management of the lighting desing and submittal, increasing the costs of plan review, and therefore one of the (often hidden) costs of applying uniformity criteria in ordinances.

All of these example uniformity ratios are to be applied to the relevant part of the site, not over the entire site. Working with these distinct areas means the entry road, parking lot and walkways can each be adequately and uniformly illuminated without having to be explicitly related to each other - or worse, limited by the value at some other point that could be all the way across the site. Applying uniformity ratios to an entire site can be effective, but the values and application need to be reasonable or they become technically wrong and practically useless. This is especially true for any ordinance that also includes restrictions for lighting at the property line, because this combination of restrictions can create severe limitations for lighting systems - sometimes making it impossible to meet the published IESNA recommendations.

Finally, applying uniformity ratios to entire sites can require lighting areas that otherwise need not be illuminated at all, or to lower levels. For example, if a uniformity ratio is required to be no more than 10 for maximum-to-minimum, then if the minimum illuminance at the edge of the property is 0.1 fc, the maximum allowed at any point on the site is the 1.0 fc. So the edge of a site may need to be "over-illuminated" to 0.2 fc so that the maximum for some other location on the site can be as high as 2.0 fc. This seems to contradict the goals of the lighting ordiances by increasing illumination at the edge of property , as well as requiring unnecessay lighting along with a corresponding increase in energy use and light pollution.

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