I haven't verified it, but I remember hearing that the IR Sensor operates at 100 Hz, so I would guess that you would need to oscillate your IR LED at at least 200 Hz. Check out some info on Nyquest Frequency.

Hope that helps.

hi,

well, since he was asking about battery lifetime and not led aging this is only partly true.
driving the leds via pwm will definitely increase battery lifetime.

not sure if this affects the signal strength of the camera chip, tho. basically, "analogue" photosensive materials will show a difference. digital chips however work different and only simulate certain behaviour of analog sensors (including photosensitive chemicals as found in old cameras).

in theory, the influence should IMO be relatively small since pwm means 100% current but only e.g. 50% duration and the digital conversion is based on fixed timings. if the image sensors convert the peak signal, pwm driven led sources should show no difference. but thats just a guess, i'll hook up some ir leds to a uC later at different frequencies (same current tho) and check the results on a digital camera and a webcam for comparison...