# 901.11 Light Emitting Diode (LED) Luminaire Performance and Computation of Roadway Illumination

## 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. Each I.E.S. type of illumination has a different pattern. The types of luminaires are: LED-A is mounted at 30 ft. and is mainly used for basic lighting; LED-B is mounted at 45 ft. and is used for continuous and basic lighting; LED-C is mounted at 45 ft. and is used for continuous and basic lighting. Refer to MoDOT’s standard plans, specifications and manufacturers data and utilization curves to assist in lighting design.

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 LED-A luminaires. Basic lighting of 45 ft. mounting height normally uses LED-B luminaires. In some cases to obtain required intensities on island noses or in intersections, the LED-C 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 901.1.2, 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 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. = $\frac{Lamp Lumens \times Utiliz. Coef. \times Maint. Factor}{Spacing \times Width}$

Spacing = $\frac{Lamp Lumens \times Utiliz. Coef. \times Maint. Factor}{Avg. \times Width}$

Where:

Avg. = Average footcandles (lumens/ft2) or Average lux (lumens/m2)

Lamp Lumens = Initial lamp lumen rating

Utiliz. Coef. = Coefficient of Utilization

Maint. Factor = 0.90 (for LED)

Spacing = either ft. or m

Width = Roadway width, either ft. or m