Color My World

One key area of research involves the eye’s spectral response and which bulb types are best for nighttime vision. Amateur astronomers, more than most, appreciate that we see differently during daylight than we do in darkness. The cones, which function only if exposed to luminance levels of at least 3 candelas per square meter, provide photopic (daytime) vision. Although they discern color, cones are most sensitive to yellow light, near 555 nanometers in wavelength. The rods, on the other hand, kick in when the luminance falls below 0.01 cd/m², during scotopic (nighttime) vision. Rods detect brightness, not color, and are most sensitive to blue-green light, around 507 nm.

In deep twilight and under most outdoor-lighting situations, our eyes utilize a third range of vision, termed mesopic, for which the rods and cones work together to varying degrees. And it’s here, says researcher Ian Lewin (Lighting Sciences), that the situation gets complicated, because to the eye all lumens are not created equal. Various lamps emit light with distinctly different spectra, and not all are a good match to the scotopic sensitivity range. As the chart above shows, “white-light” sources such as metal-halide lamps produce most of their output in the scotopic range, whereas bulbs using high- and low-pressure sodium create yellowish spectra that the eye does not see as well in low-light situations.

For example, major roadways in North America typically are lit with an illuminance of no more than 1 lux, or 0.1 foot-candle — roughly 10 times that from the full Moon. To get the same level of scotopic response within the eye, HPS streetlights would need to be roughly one-third brighter — and LPS streetlights nearly twice as bright. A more critical benchmark may be a driver’s response time under varying streetlight sources. At the lighting levels typically found on side streets or in off-street surroundings, two to six times more HPS light is needed, compared to a metal-halide standard, to achieve equivalent response times.

New Directions in Streetlight Design

An estimated 35 to 50 percent of all light pollution comes from streetlights, which typically are on every night from dusk until dawn and vary widely in design. Most streetlights have a rounded lens of plastic or glass below the lamp housing, the shape of which dictates how the beam is distributed onto the ground. Some have advanced optics to spread their light in specific, well-controlled directions, while others cast their output more indiscriminately — including up into the sky.

Streetlights termed full cutoff, which typically have flat bottoms or lenses, emit no light above horizontal, whereas cutoff fixtures more leniently allow 2½ percent of the light to shine per degree of angle at or above horizontal. The semi-cutoff classification, which allows even more upward-directed light, is the one most commonly used. Semi-cutoff “cobra-head” fixtures are easily recognized during the day by their deeply rounded refractive lenses and at night by the strong glare they cast into the eyes of drivers.

Although there’s no question that full-cutoff streetlights produce the least amount of glare and minimize light trespass (spillover) onto adjacent property, they are not necessarily roadway engineers’ fixtures of choice. “Sag-lens” or “drop-dish” fixtures with cutoff optics are now widely used. Their proponents, particularly the National Electrical Manufacturers Association, contend that using cutoff fixtures results in fewer streetlights, and thus less total reflection off the ground and less light pollution, than going solely with the flat-lens alternative espoused by dark-sky advocates.

Put another way, full-cutoff installations designed to meet current IESNA targets for light levels, uniformity, and glare might, in theory, use more electricity than, say, those using cutoff fixtures that produce more glare and uplight but whose light distribution below horizontal is well controlled. “This is very powerful stuff,” says Paul Lutkevich, former head of the IESNA’s Roadway Lighting Committee, and it suggests that the current classification system could be improved upon. To that end, Lutkevich says, the IESNA has formed a committee to come up with a new method of classifying fixtures with regard to light trespass, skyglow, and glare.

New Standards Needed

Meanwhile, there’s talk that the current illumination standards themselves need revision. For example, by reducing glare, roadway designers may be able to lower the illuminance levels produced by their streetlights. Recent research has focused on what is termed “small-target visibility,” or STV — how well drivers can spot, and react to, distant objects on or along the roadway — and the presence of glare factors heavily in such assessments. Preliminary assessments suggest that pavement doesn’t need to be nearly as bright when STV-based criteria for visibility are used.

Notably, officials in Calgary, Alberta, recently found that retrofitting their semi-cutoff streetlights with full-cutoff fixtures greatly increased illumination levels on the ground below — so much so that they are halving the wattage of the bulbs, a move that will save $1.4 million annually in energy costs. City officials add that the electricity they’re saving will reduce emissions of carbon dioxide from coal-burning power plants by more than 300 kilograms per fixture each year.

A growing trend in many towns and cities is to renovate their main thoroughfares with decorative lighting that harks back to the gaslight days of the past. Unfortunately, “period” fixtures that look attractive by day may be harsh sources of light at night, all too often blinding pedestrians and drivers alike with glary bulbs. However, many lighting manufacturers now carry product lines that house the bulb inside the fixture’s cap or utilize internal louvers that direct light downward — thus creating an aesthetic, low-glare compromise that satisfies both municipal planners and light-pollution activists.