Question: I gotta ask about your “only good night lights” slogan. What’s wrong with light at night? Seems to me it makes life a lot easier and safer. — TE, Menlo Park, NJ
Answer: Lights at night can be a good thing, say, if you run a ball park or auto dealership. They provide security, safety, and product visibility. The problem with many of these lights is they send too many of their photons up into the night sky. This causes what astronomers refer to as light pollution, a diffuse skyglow that limits what you can see.
Would you water your lawn with a fixture that sprayed H2O on your driveway and street? Not likely. Water is precious and costly. So why would you similarly waste photons of light? Incredibly, the amount of light wasted each year in the US costs $1.5 billion. To generate the electricity to make that light we burn 24 million barrels of oil.
The solution is simple. Note how an unshielded fixture (left) has its bulb protruding. Light streams off the entire surface of that bulb, much of it escaping upward where it is wasted. By contrast, a fully shielded fixture (right) directs all its photons downward. The bulb is recessed. Less wattage is needed. This is good.
Every photon that escapes upward contributes to light pollution, reducing the contrast of the night sky. Next time you buy an outside light fixture, ask for one that is fully shielded.