Roadway Lighting

Street/Pedestrian Lighting
The many benefits of a full cutoff design

Lighting distribution graphic of full cutoff and non cutoff lighting

This graphic indicates the lighting distribution of a full cutoff fixture and a globe style non-cutoff fixture. The diagonal line represents the "cutoff angle" above which glare and light transmission begin to diminish in an amount that increases at angles closer to the horizontal. No direct light would be emitted and no glare would be seen above the horizontal line. Full strength light and glare are emitted below the diagonal.

In the lower illustration the non-cutoff offers no directional control, except that the fixture base and pole block illumination from the area near the pole base.

Light diminishes exponentially over distance according to the inverse squares law. For example, increasing the distance light is emitted by a factor of four causes illumination to diminish by a factor of 4²= 16. Vision is enhanced by uniformity in illumination when average-to-minimum illumination ratios are kept to around a 4 to1. Consequently, illumination projected at too great a distance -- "reaching" too far -- is unproductive. Glare's effect , however, does not diminish to the same extent, and causes problems at far greater distances.

This is the basis for the "4 To 1 Rule" which says light should not be emitted at angles that project illumination at a distance exceeding four time the fixture's mounting height. See IDA Information Sheet No. 78 for a more detailed discussion. A downward angle of about 75 degrees (with 90 degrees representing the horizontal) will project illumination just under four times the fixture's mounting height.

So emitting light at a downward angle sufficient to direct it no further than four times the fixture's mounting height:

Saves energy by eliminating waste light emitted above about 15 degrees
below horizontal and all light emitted above horizontal.
Increases driver and pedestrian safety by greatly reducing glare in the horizontal plane
which forms our field of view as we drive and move about in vertical positions.
Provides directional control against light spillage onto other property.
Reduces urban skyglow.

Pedestrian visibility graphic in full cutoff and non-cutoff lighting

This graphic shows the difference in the illumination distribution from the two fixtures. In the top illustration the pedestrians are much more visible to the driver. Light is directed onto the target area on-property, only to an effective distance, promoting energy efficiency and minimizing light trespass and urban sky glow.

In the lower illustration the pedestrian is in a hazardous situation. The globe fixture projects glare into the eyes of the driver causing him to experience veiling luminance. Because the pedestrian is between the fixture and the driver, she appears as a silhouette near the exposed light source, which forms a bright background in the driver's field of view. To make matters worse, she's near the pole base where the illumination is comparatively low.

The globe also scatters the majority of its light above horizontal and onto other property, creating nuisance, waste, and urban sky glow. The poor visibility created by a non cutoff fixture, frequently used to create a historical look, is often addressed by placing such fixtures very close so that one fixture illuminates the pole base area of adjacent fixtures. This remedy carries an extreme monetary cost for the numerous added fixtures and additional installation and maintenance costs, extracts both a monetary and environmental cost due to added electricity requirements, creates yet more light trespass and urban sky glow, and creates more counterproductive glare.

The Indiana Council on Outdoor Lighting Education, ICOLE http://home.att.net/~icole

All material here is copyrighted by its authors as noted and can be used for non-commercial purposes to promote outdoor lighting education, providing proper credit is given.