(Please refer to pages 78-82 of Electrical Contractor magazine for the graphs, which Ian Lewin refers to in his lighting column).
Since my last article, which ran in July 2000, the movement to control light trespass has gained momentum. Throughout the world, there is growing concern about pollution of the nighttime sky by unwanted light. Such light can be unaesthetic, irritating, and invasive.
It can also be dangerous, particularly when drivers experience distracting glare from badly placed or poorly focused luminaires. “Sky glow,” created from misdirected nighttime lighting, destroys our view of stars, planets, and constellations. Astronomers are becoming increasingly concerned about this, as sky glow obscures observations, which may be important to space research.
On the other hand, nighttime lighting is crucial for security, safety, and amenity. Not lighting an area where it is needed can be problematic and even hazardous. Consequently, demand is high for outdoor lighting that is properly designed, uses well-engineered fixtures, and is installed to minimize dangerous conditions.
Nighttime lighting should illuminate the intended area, while preventing it from falling outside that perimeter. Ideally, light should stop at the property line. However, light levels tend to reduce gradually and some spill light is inevitable. This makes controlling light trespass difficult.
By using some basic principles, however, light trespass can be minimized. These principles involve the distribution pattern of the light from a fixture, where it is placed and how it is aimed.
Candlepower is the intensity, or concentration, of light in a particular direction. For most fixtures, candlepower measurements vary because of the different directions light is emitted. A candlepower graph helps to show these patterns of emission.
In Figure 1, drawn in a plane perpendicular to the lamp, the length of the arrows represent the strength of the rays in different directions. Here, the maximum candlepower is measured from the downward vertical at a 60-degree angle. In Figure 2, the ends of the arrows have been joined to form a candlepower curve, which shows any pattern of light from a fixture, for any vertical plane through it.
A “forward throw” luminaire produces a candlepower distribution as shown in Figures 1 and 2. It can be used to light an area 60 feet wide from a 35-foot pole (see Figure 3). The maximum candlepower at 60 degrees is directed to the far edge of the area. This is desirable—the maximum should always be directed to the farthest point because the illuminance, or footcandle level, on the ground decreases rapidly with increasing distance. As shown with this luminaire, light emitted towards points near the pole should have low candlepower to minimize the “hot spot” at these short distances.
The fixture in Figure 3 produces a form of candlepower distribution called “full cut-off.” A well-designed reflector optically controls its light rays. Not only does it have strong reflection of light to 60 degrees; virtually no light is emitted at angles greater than this. On the candlepower curve, the light emission curve runs back to zero very sharply, above the 60 degrees maximum, indicating negligible emission above the beam.
Even light emitted directly from the lamp in a forward direction is shielded by the edge of the fixture at angles above 60 degrees. No light is cast horizontally or upwards. The full cut-off fixture is ideal for perimeter parking because it is not glaring and good visibility is created. Light trespass is well under control.
Figures 4 and 5 are similar to Figures 1 and 2, but show the light pattern produced by a conventional floodlight, which has its maximum candlepower projected perpendicular to the cover glass. To aim this maximum candlepower at the farthest point away from the area being lit (needed for adequate footcandles), the floodlight is tilted upwards at 60 degrees.
Figure 6 shows the conventional floodlight lighting the same area as the full cut-off fixture.
However, much light is cast at angles above 60 degrees. The candlepower distribution curve for the floodlight shows an obvious bulge in the curve above the angle of maximum candlepower created by these rays. It lacks the sharp run-back above the beam shown by the full cut-off luminaire. Because the floodlight must be aimed at 60 degrees to light the complete area, the higher-angle light rays spill outside this area, causing considerable light trespass.
Viewers seeing this light are subjected to very high levels of glare, causing reduced visibility. This is light trespass at its worst, yet this lighting technique is common today.
Note also that there is substantial light emitted by the floodlight above the horizontal light that travels directly into the atmosphere. This causes sky glow, or light pollution, of the night sky.
While photographs cannot fully capture glare effects, the presence of a high level of glare is apparent in the photograph of the floodlighting installation (at right). The visibility that the floodlight produces is poor. Lighting an area with the full cut-off unit (see page 78), however, shows a well-lighted area with good light control and little, if any, glare or objectionable spill light.
It is clear that full cut-off fixtures outperform floodlights. It is important to ask how efficient and energy-saving the two fixtures are.
Efficiency is percentage of the lumens of the lamp, which actually are emitted by the fixture. Both fixtures can be very efficient—some floodlights may have higher efficiency than some full cut-off units. However, if much of the light emitted by the floodlight goes to areas where it is not needed, the light is wasted. Wasted light equals wasted energy.
For example, consider a full cut-off luminaire equipped with a 320-watt metal halide lamp. With this luminaire, 50 percent of the lamp lumens fall on the target area, producing the desired footcandle level. However, with the floodlight, maybe 10 percent spills outside the area to be lighted and only 40 percent of the lamp lumens are available to light the area. To produce equal footcandles on the desired area to the full cutoff light, the floodlight would require a 400-watt lamp.
Not only does the full cut-off fixture provide low glare, little spill light, and zero uplight, it can also save energy. In the example above, the two fixtures produce an equal amount of light, but the floodlight uses 25 percent more energy.
There are many fixture types for a variety of installations. Some may hold a small advantage over the full cut-off in that they might be used at slightly greater spacings. However, all tend to produce more spill light and glare than the full cut-off luminaire. With the newest designs of full cut-off fixtures, however, spacing can be as great as with other fixture types. In many cases, greater spacing can be achieved because of superior optics, which place the light exactly where it is needed.
When selecting a fixture, it is critical for contractors to be aware of light trespass issues. Sensitivity to environmental concerns is increasing and complaints, particularly in residential neighborhoods, are also on the rise. Selecting the wrong luminaire can be a costly mistake to realize down the road. Installing the right luminaire the first time can benefit an area’s surroundings and save your customer valuable time and money.
LEWIN is the president of Lighting Sciences Inc., of Scottsdale, Ariz. Currently he is the Chairman of the IES Lamp Spectral Effects subcommittee.