Is it safe to exceed an LED max current when ON time is less than 100%?

For an LED that is rated at 20mA, is it safe to PWM it at 40mA half the time? Will that make it nearly as bright as if continuously supplied with 20mA?

Addendum:

Thanks Olin and tcrosley for your answers. I didn't want to distract from the question by adding the details of what I am doing, but here it is:

I am working with an LED strip from http://www.environmentallights.com/LED-Strip-Light-Double-Density-4-Wire-Red-Green-Blue-by-the-5-meter-reel_P3846.aspx. They do not have a datasheet per se. I think the LEDs are connected as shown in this image http://www.ladyada.net/wiki/_media/products/ledstrip/astripsch.png.

Based on what I read about LEDs in general, 20mA per color seemed safe. I tested it - using 12V, potentiometer and 3 inches long strip, that is 3 leds in series each color - for short periods, few minutes, and 20mA per color seems to have worked fine, good enough brightness and no overheating or color intensity variation.

I will be working with up to 2 feet length strips, that is 160mA per color and 480mA to get White. I may have up to 10 strands to control. That will add up to about 5Amps. I will be using the Propeller microcontroller from Parallax to flash the strips in various patterns and colors. Will use mosfets to drive the LEDs.

I was thinking of supplying about 200mA instead of the 160mA needed per color, but then I will PWM the leds such that only one color is on at a time; at a frequency of 1KHz. So to get white, each color will be on for one third the time, that is 0.3 msec. At any moment only one color is on, only 200mA is drawn. I imagine the leds will be less bright, hopefully not by much.

So, for 10 strands, I will deal with only 2A instead of 5A.

Please let me know if what I am thinking is off.

Thanks.

Answer

The datasheet will have this information. Some LEDs that can only handle 20mA continuous are spec'd for very brief pulses of a much higher current. You would have to specify the PWM frequency to be able to look up the answer to this question in a datasheet.

For one particular product, the CREE CLV1A RGB LED, you may drive red at 50 mA continuous or with a 200 mA pulse (pulse width ≤0.1 msec, duty ≤1/10), and blue and green at half that current. It's all in the datasheet. You can also see that there is a diminishing return as the relative luminuous intensity vs. forward current is not linear at high currents.

I think you should not worry about the brightness loss from driving your LEDs at a 50% duty cycle at the maximum continuously rated current. IIRC the eye has a non-linear response to brightness and they will appear somewhat brighter than half as bright, and you won't destroy your LEDs if you screw up the PWM.