When selecting or recommending a light source for low-light applications, such as roadways or parking lots, consider how people actually perceive light from different sources.

Today, the lighting options competing in the roadway lighting applications marketplace are metal halide and high-pressure sodium sources.

Photopic, scotopic, and mesopic conditions

A true assessment of a lamp's lumen output should be based on the eye's response to the visibility conditions. But standard lumen measures define a typical person's response to a light source only during the high light levels typical of daylight and interior lighting. The term for these high light levels is "photopic."

Standard measures also apply only to the central vision of the eye, not to the side or peripheral vision that is so important for rapidly identifying roadway hazards.
When the light level is very low, the viewing conditions are no longer photopic. Instead, they are referred to as "scotopic." Typical scoptopic conditions are starlight levels at night.

In between photopic and scotopic, "mesopic" conditions refer to twilight and typical street lighting levels.

Rods and cones

Two types of light receivers in the retina of the eye cause the change in the eye's spectral response. One receiver is called rods and the other, cones.

The rods are responsible for human vision at low or scotopic light levels and are located in the peripheral field of view, away from our direct line of sight. As the light level reduces, cones become less active; rods become more active.

Rod vision is very important for night driving. Rods see peripheral objects, such as a car approaching down a side. Cones only see objects that are viewed directly.

Eye color sensitivity and lumens

If a lamp's lumen output viewed under scotopic light levels is determined using photopic measurements, the resulting value does not accurately assess the true amount of light the lamp produces as perceived by the human eye. Under scotopic conditions, the eye's visual response changes dramatically. The eye's sensitivity to yellow and red light is greatly reduced, while its response to blue light greatly increases. In fact, as the light level diminishes, the effective lumens of yellow light sources are reduced and the effective lumens of blue/green sources increase.

High-pressure sodium (HPS) lamps produce primarily yellow light. Eye sensitivity to yellow under photopic conditions is very high. As a result, high-pressure sodium lamps have high lumen ratings as perceived by the eye in daylight. On the other hand, under scotopic conditions, the effective lumens are greatly reduced because HPS produces very little blue and green light.

Metal halide lamps

A typical metal halide lamp has strong light output in the blue, green, and yellow regions. The result is a high lumen output for this light source at all light levels. The blue light output of metal halide is in the high sensitivity region of the eye for low light levels. This means that the true lumens actually increase for a metal halide lamp as the light level reduces and the eye shifts to a blue/green peak sensitivity.

Research supports these conclusions. The ability to detect fine contrasts was found to be improve under metal halide as compared with sodium sources. In addition, experiments showed reaction time under high-pressure sodium lighting to be roughly 50 percent longer than it was for metal halide. These results demonstrate that a light source's color output significantly influences visibility.

Effectiveness of metal halide versus high-pressure sodium

Additional research studies conclude that a high-pressure sodium lighting system will require approximately six times the luminance level of an equivalent metal halide system. Consequently, a roadway lighting system using metal halide lamps will be equivalent to a high-pressure sodium system using lamps of six times the lumen output. The six-times -greater average luminance level of metal halide versus that of high-pressure sodium occurs where it is strongly needed, in side or peripheral viewing and dark areas, where hidden hazards may be present.

Visual effectiveness dramatically increases with metal halide's white light with high blue/green content under nighttime conditions. Moreover, metal halide is a highly energy-efficient light source. These facts, combined with powerful, independent data corroborated by several researchers, are worthy of serious consideration by electrical contractors specifying roadway lighting.

BARD is a product manager with Venture Lighting International, headquartered in Cleveland, Ohio. She has over 15 years' experience in the lighting industry in the areas of marketing, engineering, and operations.