| MadSci Network: Engineering |
The question: "How long does it take a light bulb to light up? My question is something I have become interested in because of the new photo redlight cameras that governments are mounting at intersections. Driver's get a photograph of them in an intersection and a number box that says the light has been red for x seconds(some as low as .4 seconds), so you ran the redlight. The device that measures this starts when the electricity switches on for the redlight. My question is once the electricity hits the bulb how long will it take for the element to get hot enough for the bulb to transmit light at its brightest?"
I did a Google search on "incandescent turn on time" and found, among others, these useful Web pages:
From these sites, and others, it is obvious that incandescent traffic lights require a couple hundred millliseconds to turn on. However, municipalities are increasing the use of LED traffic lights, which can turn on practically instantaneously. I use the word can because there appears to be a delay built in to at least two of the brands of LED traffic lights, probably for the purpose of mimicking the older incandescent lights. There is a technical paper by the Traffic Operations Lab at the University of Illinois at Urbana-Champaign which reports the differences between standard incandescent traffic lights and two different brands of LED traffic lights. The reason that I say that the LED traffic lights must have built-in delays is that LEDs light practically instantaneoulsy, generally in the range of nanoseconds or microseconds, and the results of the TOL study indicate that the LED traffic lights operate nearly the same as the incandescent traffic lights. At schubert one can see that the rise and fall times of LEDs are typically in the nanosecond range (billionths of a second!).
So, if your assumption is correct that the "photo redlight camera...starts when the electricity switches on for the redlight", there will be 0.1 to 0.3 seconds delay, it seems, before the redlight is fully on. But I was not able to find out whether your statement is actually true! Since, apparently, the manufacturers of the LED stoplights have built in a several hundred millisecond delay in the turnon of the LEDs, it would also be possible that the manufacturers of the redlight camera devices build in a suitable delay for the turnon of the camera! I suggest, if you are actually that interested in this, that you contact the manufacturers of the redlight cameras to find out if a delay is indeed built in.
John Link, MadSci Physicist
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