Blanco County News
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What to Do About Those Evil 175W Mercury Vapor Security Lights
Wednesday, June 11, 2008 • Posted June 10, 2008

You’ve seen them. They’re everywhere. Hanging on poles in the neighbor’s barnyard; on signs and billboards around town; on the sides of self-storage buildings, offices and warehouses all over Blanco; even on city-owned utility poles illuminating Blanco’s streets. It’s the ubiquitous 175 watt mercury vapor “security light.” Evil outdoor lighting at its worst. Do you have one of these on your property? Does your neighbor? Bad, bad, bad for you…for him…and for us all.

People buy the 175 watt mercury vapor security light because it is cheap. It typically costs less than $30. Cheap security, right? Well, we know that light per se does not equal security but did you know that even a $30 security light isn’t cheap? That’s because it costs money to run the light and the 175 watt mercury vapor light is very inefficient. Because of its ballast, a typical 175 watt lamp uses about 200 watts of electricity per hour. If it burns “dusk to dawn” for an average of 10 hours per night it is on for approximately 3650 hours per year. 200 watts times 3650 hours equals 730,000 watts per year (730 KwH). The Pedernales Electric Coop currently charges $.09618 per kilowatt hour (resident rate) meaning that the 175W mercury vapor security light costs $70.21 per year to run, over twice the cost of the fixture itself!

A typical 175W mercury vapor light puts out about 7850 lumens of light (a lumen is a measure of the amount of light a given fixture produces). A comparable 55W low pressure sodium lighting fixture rated at 8000 lumens, on the other hand, will use only 292 KwH for the same 3650 hours of illumination time. Using the same electrical rate, the low pressure sodium light costs only $28.08 to run for the year. On a cost per lumen basis the difference is even greater when you consider that a mercury vapor light will degrade and get dimmer and dimmer the longer it is in place so the amount of lumens it produces is ever diminishing while the cost of running it remains the same. Low pressure sodium fixtures may cost more initially but the pay back comes quickly.

Oh, did I mention that the 175W mercury vapor security light wastes most of its light. Because of the round, unshielded plastic housing most of these lights use, much of the light shines sideways, directly outwards from the fixture for a long distance (creating annoying and sometimes dangerous glare for neighbors and passers-by) and upwards above the horizon (creating light pollution and sky glow that diminishes the visibility of our starry nights). In fact, any light that emits at an angle greater than 70 degrees from a line drawn straight down from the light to the ground is wasted light because by the time it hits the ground it isn’t really very bright. Properly shielded, one can use a lower wattage lamp and still put the same number of lumens on the ground in the area where the light is actually needed yet virtually eliminate light trespass and glare. I think we can all agree that glare is a bad thing. The 175W mercury vapor security light is the “King of Glare.”

People buy the 175 watt mercury vapor “security light” under the mistaken notion that the light provides, you know, “security.” It doesn’t. It merely lights up the property…and that of the neighbors’. Whether or not the light deters a would-be burglar is conjecture. For many criminals or vandals, it simply makes their task easier. If you want real security, consider a light equipped with a motion detector. The light is off until someone comes in range of the detector and then it comes on. The newer “pet immune” motion detectors use technology that divides the covered area into adjustable upper and lower coverage zones. Only when an object (such the home owner returning from an evening excursion or a burglar trying to break into the house) enters both sections will the light go on. This prevents things like the family dog or a stray cat from triggering a false alarm. Now think about it. If your neighbors know you’re not home and a security light that isn’t normally on comes on, wouldn’t that trigger their reaction more so than them just seeing a security light that is on all the time remain on? The same goes for businesses, public facilities, and even our Blanco schools where, obviously, dusk-to-dawn security lights have failed to prevent some major incidents of vandalism in the recent past.

The 175W mercury vapor security light is close to the worst of all possible lights. It produces glare, it often trespasses onto adjoining property, it contributes to light pollution, and it wastes energy.

So, what can you do if your neighbor has one of these energy wasting, night sky ruining light bombs? Well, some have resorted to the .22 solution. This is most definitely not recommended. A much better approach is to suggest to the neighbor that he either 1) shield the 175W mercury vapor light (a galvanized light shield costs about $30), or 2) put it on a “pet immune” motion detector (they cost about $100), or 3) change it out for more energy efficient, night sky friendly/neighbor friendly lighting fixture that eliminates light pollution, light trespass and glare while saving electricity. It might help (a lot), by the way, if you would offer to pay for whatever solution is most appropriate. It’d be worth it. You’d get rid of an annoying source of glare and the neighbor would get better illumination, better security and lower electric bills. I’d say that’s a win-win situation. Why not try it!

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