Saturday, February 04, 2012

Not all LEDs are created equal

As many of you are aware, I've been shooting quite a few high speed videos lately. The one real limitation I have is with light. These high speed cameras are just gobbling up light like it was going out of style. Here's an example. Take your camera - if it has manual controls, go ahead and dial in an exposure of 1/1000 sec, an f-stop of around 2.8 (although for decent depth of field, I like to get around f5.6 or more) and then try and take a picture of something indoors. For our videos, we're using a pair of 500W tungsten lights. They toss out plenty of light for speeds under 5,000 fps - but they get really, really hot. 200+°C hot. Plus, a 500W light on a 120-volt circuit draws about 4 amps of current. You don't have to add too many lights before you're really sucking up a lot of electricity (and converting way too much of that into heat instead of light).

So LEDs are a really attractive option. They don't necessarily draw as much electricity and they are much more efficient. Running much cooler. They are a bit more expensive - probably about two or three times as expensive as tungsten. There are cheaper LED light panels, but the "color" of their light isn't matched to that of tungsten or sunlight - so using them together sometimes yields weird colors in the subject. LED stands for "light emitting diode." A diode is a component of an electric circuit that only allows the electric current to travel in one direction. So for a DC (direct current) circuit, the flow of electricity is in the same direction and there's not much to see. But, the electricity that "comes out of the walls" to power our appliances, lights, and televisions is AC (alternating current). The current switches direction (alternates) 60 times a second. In once cycle, 1/60th of a second, the electricity flows in one direction, and then reverses direction again. So an LED would light up for half of that cycle, then go off, then go back on again for half of the following cycle, and so on...

Good LEDs are set up to deal with this brief interruption and remain lit. I had thought that even cheap LEDs would remain lit sufficiently to provide a source of light that doesn't flicker (at 60 times per second). Shooting at anything above 30 frames per second renders any normal AC light source useless. Imagine my dismay when I looked at some AC-powered LED lights. One was an LED "worklight" and the other was a single panel for mounting under a cabinet. (cost around $20-30).

The under-cabinet light is flickering, while the worklight remains steady. This means that finding an economic set of lights is going to take some additional research to make sure they don't flicker...


  1. The flickering is dependent on the power supply; cheap ones will flicker because they're only using half the AC wave; you can get supplies that put out steady DC so the LEDs don't flicker. This is not dependent on the LED itself, but the power supply. Very good explanation here:

  2. I hesitate to comment because it's more of a thought than a suggestion but.. most panels have circuits powered by each of the supplies 120v legs. If you could find an outlet from each leg and divided your lights between them, maybe that would help.