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Pop quiz time, everyone!

You're leaving the room, there's no one else in it and you won't be back for 10 minutes. Your hand is reaching for the light switch, but just as you're about to give it a flick you notice that the bulb is a fluorescent one.From an energy conservation point of view, is it better to:

Leave the light on until you get back?

Turn the light off as you go out the door?

Andy Rudin, director of the Interfaith Coalition on Energy, specializes in answering just these kinds of questions. His usual audience is religious congregations, but he's happy to share his information with everyone.

Says Rudin, "If you keep today's fluorescent light bulbs off for more than a couple of seconds, it's worth turning them off."

If this answer is surprising to you, you're in good company. According to Rudin, the misconception that it's inefficient to turn fluorescent bulbs on and off for short periods is a widely held.

The reason it's so widely held is that in the past it was true.

When fluorescent bulbs first became popular in the 1940s, the engineering wasn't as efficient as it is today and turning one of these bulbs on and off frequently would severely reduce its life span by wearing out the filament. Further, it took a lot of energy to start the bulb.

Throughout the 1950s, people routinely left lights in office buildings on 24 hours a day. Engineers calculated that the wear and tear of turning them on and off exceeded the cost of the cheap electricity that powered them.

Even in the 1960s, experts calculated that if you were going to turn fluorescent lights back on within half an hour of turning them off, that you'd come out ahead by not turning them off.

All that has changed. Today's fluorescent bulbs are better-engineered than ever before. According to Rudin, the initial current required for turning the bulb on lasts for only 1/120th of a second and even at it's peak that current is no more than five times larger than the normal energy used to power the bulb.

"The in-rush current," points out Rudin, "does not use a significant amount of energy since it lasts for such a short time. Fluorescent lights only have to be turned off for one second to save the amount of energy that will be used when turning the lights back on again."

But what, you might ask, about the wear and tear on the filament?

Well, that wear and tear on the filament has an impact on the life span of a fluorescent bulb, but he'd add that the filament is also wearing out during normal operation.