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By Tom Scott, Dean of Sciences at San Diego State University
broadcast  2002
(reprinted by permission of the author)

San Diego Lighting Part 1

In 1984, San Diegans worried that population growth could light our skies and blind the telescopes on Mounts Palomar and Laguna. We realized that dark skies and adequate lighting could both be achieved using low-pressure sodium streetlights, and mandated them under the "dark skies" policy.

Electric lighting comes in two forms: incandescent bulbs and fluorescent lamps. With incandescents, electricity heats a filament until it glows white hot. Most of the energy is given off as heat rather than light, so incandescents are wasteful.

With fluorescents, gas atoms are excited by electricity, causing them to glow in their characteristic color. Because they don’t generate heat, all fluorescents are more efficient than incandescents, yet there are differences among them.

Mercury vapor lamps give off a bluish light, but much of their energy is in the invisible untraviolet, so it’s wasted. They’re only about 2½ times as efficient as incandescents.

Sodium vapor is the gas of choice because the energy of excited sodium ions is almost all in the visible range. But there’s a debate about which type of sodium lamp to use. With high pressure, the tube is densely packed with sodium ions, which not only glow sodium yellow but also interact to produce a pink color. High-pressure sodium is six times as efficient as incandescents.

With low-pressure sodium, only 1 millionth as much sodium vapor is put in the tube, so the ions don’t interact. They give off only one color: sodium yellow. Our eyes are quite sensitive to this, so we get maximum visibility from the electricity we use. They’re nine times as efficient as incandescents.

The single color of low-pressure sodium presents an important advantage, and a problem. The advantage is to the astronomer, who can filter out only yellow, and still have nearly the entire unpolluted spectrum to view. The problem is that the rest of us can’t determine true colors under low-pressure sodium. We see colors by comparing them. If there’s only one, we can’t make that comparison, as you know if you’ve tried to identify your car under low-pressure sodium. Critics argue that eyewitness accounts of criminal activity may lose value.

Recently, San Diego City Council voted to shift from low to high-pressure sodium lamps. Next week we’ll look at the impact of that decision.

For KPBS, this is Tom Scott, Dean of Sciences at San Diego State University.

San Diego Lighting Part 2

San Diego City Council recently voted to convert from low-pressure sodium to the more natural light produced by high-pressure sodium lamps. The financial cost will be $3 million over a three-year period for the conversion, plus about $900,000 annually in increased electricity use. A Union-Tribune poll revealed that San Diegans opposed the change by 47% to 33%.

Why approve an action that’s both expensive and unpopular? The motivation was that high-pressure sodium emits a range of colors that give a more natural aura than low-pressure, which produces only yellow. The argument was that high-pressure sodium would give eyewitnesses the ability to determine colors more accurately, and so allow them to describe crime scenes with greater certainty.

The change is a blow to astronomers, who were able to filter the yellow light of low-pressure sodium, and view the rest of the spectrum unpolluted, but who will soon have to contend with light they can’t filter without also losing light from the stars they study.

It could have been worse. The original proposal was to switch to high-pressure sodium throughout the city, with a bias toward the use of unshielded, decorative acorn fixtures that send as much damaging light upward as they do useful light to the ground. Arguments made by Paul Etzel, Chair of Astronomy at SDSU, and others, led to a compromise whereby the fixtures will be capped to direct all light downward, where only 10% reflects back into space. Moreover, the low-pressure lights will be retained within a 30-mile radius of Mount Palomar to reduce the impact on the Hale telescope there. The other major observatory, at Mount Laguna, is 35 miles east of the city limit, and so the protective ban need not apply.

While our renowned astronomers may be most directly impacted by this change, the lives of many San Diegans will be marginally diminished. The only time I’ve been amused to be caught in a San Diego traffic jam was last November 18, between midnight and 4 a.m., when Interstate 8 was clogged with travelers bound for the deserts and mountains to view the Leonid meteor shower. This is a community that loves nature in its many manifestations, including its warm, brilliant days and its dark, revealing nights.

For KPBS, this is Tom Scott, Dean of Sciences at San Diego State University.