Last week I wrote about an experiment I did using a 100 watt white LED COB for a strobe light. This weekend my 100 watt blue LED COB arrived and I was eager to see if it improved the freeze effect of my strobe experiment. The short answer: no.
I started to have my doubts about the strobe duration I read about. 1/1000th of a second, or 1 ms, was what I was using. While I don’t remember exactly where I saw the number I had been searching for number used for photograph strobe lighting. So I went in search of other numbers. One product I saw was an LED strobe used for high speed photography. They listed light pulse duration between 20-1000 µs. My setup can do timings this fast so I decided to give it a try. The results: 100 µs light pulses stop a spinning fan blade.
The problem is, even with a 100 watts of LED light, a 100 µs light pulses isn’t much overall light. The product I was reading about uses 2,000 watts of LED light—20 times the light output I have available.
Time for a little math. I was using 1 ms light pulses at 100 watts which is 0.1 joules for energy. By switching to 100 µs pulses, I have 0.01 joules. I would need to dump in 10 times the power—1000 watts—to get the same amount of light energy. I could do a bank of 10x100 watt LED COBs, but there is one other thing to consider.
LEDs output is limited by their temperature. The temperature increases because there is a voltage drop across the diode in where the energy is turned into heat. Thus the current that goes through an LED must be low enough to allow the heat to escape. A 100 watt LED with the correct heat sink is rated for continuous usage. We are only pulsing the device for a 100 µs at a time, and at most once every 100 ms. This gives a duty cycle of 0.1%. I’ve read “Most high-brightness LEDs have an effective upper limit of 6X to 8X over current capabilities, based on a typical 150 microsecond (µs) pulse width and 0.15% duty cycle.” That means my single 100 watt LED should be able to handle between 600 and 800 watts for 100 µs. I would only need 2x100 watt LED COBs in order to produce 0.1 J light pulses.