Quote:
Originally Posted by dwells
You're on the right track here, but you're not entirely correct. Which is to be expected, really - outside of electrical engineers and serious hobbyists, this type of thing is hardly common knowledge.
LEDs come specified from the manufacturer with a handful of specifications. There are three important ones to this conversation: light output, voltage, wattage. Light output is an actual measurement of how much light the diode can emit, and is rated in units of either lumens or mcd (millicandelas). The bigger the number, the brighter the LED. Voltage is simply the voltage the LED needs to be driven at to function properly and meet its light output rating. This is generally about 3.2v for high-output white LEDs (CREE brand and the like). Watts are used to measure overall power consumption. Wattage is simply voltage multiplied by amperage, which is the current that the diode draws. So if an LED is 1W at 3.2v, that means about .3 amps, or 300 mA (milliamps).
So, for those of you still following, the really meaningful figure is the lumen or mca rating. This is due to the fact that not all LEDs are created equally, that is to say they aren't all equally efficient. One LED may be brighter at the same wattage as another, while simultaneously emitting less heat. That being said, while LEDs are constantly becoming more efficient, modern high-output emitters tend to be fairly close in efficiency. So chances are that a somewhat less efficient 2W CREE LED is still putting out more light than a newer 1W CREE LED, just not necessarily double.
Regarding voltage and brightness as mentioned above, it is true that running LEDs at a lower voltage will reduce their output. It also has the nasty effect of affecting the color temperature dramatically, so it is very rare to see LEDs dimmed via this method. LEDs will most likely be driven at their manufacturer rated voltage, generally by a mix of the driver, inline resistors, and being wired in series. It's technically possible to drive an LED higher than its rated voltage and get a bit more brightness out of it, but the result will be heat and a quickly dead LED.
As for the mentioning of frequency and pulsing, this is how LEDs tend to be dimmed. A technique called pulse width modulation (PWM) pulses power to the LED. Higher frequency is closer to a "constant stream" or power, so closer to fuller brightness. Lower frequency means lower brightness. Done properly, PWM is invisible to the human eye. However, this pulsing tends not to play nicely with cameras that have rolling shutters (a whole other lecture), and that flicker becomes perceivable.
So there you go, for those of you who read through this, you now have a much better understanding of LEDs and how they work!
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I was trying to explain the whole bit getting a little less technical, but I'm glad someone else did.