# 901.11 High Pressure Sodium Luminaire Performance and Computation of Roadway Illumination

### From Engineering Policy Guide

## 901.11.1 Iso-Footcandle Diagram

An iso-footcandle (iso-lux) diagram is a means of showing the illumination on a roadway surface from one or more luminaires. Such a diagram is a graphical representation of points of equal illumination connected by a continuous line. Footcandle (lux) values on a horizontal plane from a single unit with a 30 ft. mounting height are available for 150W, 250W Type III, 400W and as shown on Figure 901.11.1. These curves are for a luminaire mounting height of 30 ft. Correction factors as listed are used for other mounting heights. Each I.E.S. type of illumination has a different pattern. The types of luminaires are: Type III medium distribution, semi-cutoff, with 150-watt clear lamp is mounted at 30 ft. and is mainly used for basic lighting; Type II medium distribution, semi-cutoff, with 250-watt clear lamp is mounted at 45 ft. and is used for continuous lighting; Type III medium distribution, semi-cutoff, with 250-watt clear lamp is mounted at 45 ft. and is used for continuous and basic lighting; the 400-watt Type III medium distribution, semi-cutoff is mounted at 45 ft. and is used for continuous and basic lighting.

Luminaires used are the internal ballast type where the type of distribution is determined by the socket position of the lamp. Different makes of luminaires produce a variety of light patterns. The diagrams shown are typical and can be used for design purposes. From these diagrams the initial level of illumination on the pavement from one or more luminaires can be determined. The average maintained intensity at any point on the pavement can be obtained by multiplying the sum of the initial footcandle (lux), from the various contributing light sources, by the luminaire maintenance factor of 0.70.

Computer programs are available to calculate lighting intensity and other information such as uniformity. These programs can be used for most lighting configurations. Several figures have been developed for many typical layouts to aid the designer in determining location and type of luminaires.

**901.11.1.1 Continuous Lighting.** The configuration, spacing and type of luminaires for continuous lighting depend on many factors, including the type of roadway, the adjacent roadway features, the roadway width, and the setback of the luminaires from the edge of the travelway. Maximum spacing guidance for luminaires to achieve the required intensity and uniformity for staggered configurations and one-sided configurations is available. The staggered configuration is preferred when possible since it provides better uniformity. Other configurations are possible and can be considered.

**901.11.1.2 Basic Lighting.** Basic lighting of 30 ft. mounting height uses 150-watt Type III medium distribution, semi-cutoff luminaires. Basic lighting of 45 ft. mounting height normally uses 250-watt Type III medium distribution, semi-cutoff luminaires. In some cases to obtain required intensities on island noses or in intersections, the 400-watt Type III medium distribution, semi-cutoff luminaires are used. Figures 901.7.1, 901.7.3, 901.7.4 and 901.7.5 provide information for the location and types of luminaires to provide average maintained intensities for typical layouts.

For intersections with raised islands, Figures 901.1.4, 901.7.1 and 901.7.4 are to be used in conjunction to determine locations and types of luminaires. It is important for luminaire bracket arms to be oriented at right angles to the projected edge of through lanes, as shown in Figure 901.7.1, to provide the proper light distribution.

## 901.11.2 Utilization Curves

Utilization curves provide a means of determining the average footcandle (lux) illumination over the pavement where lamp size, mounting height, width of pavement and spacing between luminaires is known or assumed. Conversely, the desired spacing or any other unknown factor may be determined if the other factors are given. The utilization factor varies to some extent with various makes of luminaires: however, the curves shown on Figures 901.11.1, 901.11.2, 901.11.3 and 901.11.4 are considered average and are used for design purposes. These curves are for a luminaire mounting height of 30 ft. Correction factors as listed are used for other mounting heights. The utilization curves show how much light falls on the pavement, but does not show how the light is distributed. They must be used with the iso-footcandle (iso-lux) diagrams for the same luminaire to evaluate uniformity and the ratio of minimum intensity to average intensity.

## 901.11.3 Formulas for Computations

Avg. =

Spacing =

Where:

Avg. = Average footcandles (lumens/ft^{2}) or Average lux (lumens/m^{2})

Lamp Lumens = Initial lamp lumen rating

Utiliz. Coef. = Coefficient of Utilization

Maint. Factor = 0.70

Spacing = either ft. or m

Width = Roadway width, either ft. or m