# Apperent LED brightness?

Discussion in 'The Projects Forum' started by MikeML, Oct 21, 2014.

1. ### MikeML Thread Starter AAC Fanatic!

Oct 2, 2009
5,451
1,071
I am planning to use a red LED as an On/Off indicator. The forward current will be half-wave rectified 60Hz Sine with the following values:

Peak=6.2mA
Ave= 1.97mA
rms= 3.1mA

What dc current would the same LED have to be driven with to get the same apparent brightness?

Last edited: Oct 21, 2014
2. ### wayneh Expert

Sep 9, 2010
13,612
4,412
My speculation: The curve of perceived intensity is not quite linear, but lays over a bit at higher current. Therefore the perceived intensity does not track exactly with RMS current, because the peaks in current don't give the proportional peak in brightness.

To calculate the RMS brightness, you'd need to integrate the function for the brightness versus time, not the current.

Or, just plan on using a DC current slightly less than the AC rms current, maybe 3.0 or 2.9mA.

3. ### DickCappels Moderator

Aug 21, 2008
3,918
1,084
If the LED is considered to have a constant voltage and the illuminance Vs current is pretty linear and the eye averages the incident luminance (these are all pretty good assumptions for 60 Hz, not quite as good for 50 Hz) then the apparent brightness will be proportional to the average current.

4. ### wayneh Expert

Sep 9, 2010
13,612
4,412
Forget my use of the term "rms", which always confuses me. Replace it with time-weighted average (i.e. the integral over time, which I think is what Dick is referring to?). I tend to think of these as equal, and so I sloppily and incorrectly interchange with "rms", but they are not always the same thing.

5. ### crutschow Expert

Mar 14, 2008
16,524
4,453
RMS current is only accurate to use with a linear, resistive type load where the voltage drop is proportional to the current. It is not meaningful for a load with a relatively constant (or non-linear) voltage drop like an LED.