Hi! I’m trying to pick an LED to use as an indicator in a power-constrained situation and trying to figure out what will be bright enough to see with the naked eye when holding the board. I understand that LEDs are rated for both brightness in mcd and light output in lumens, but I don’t have a good handle on how a given mcd output will be perceived.
I’m looking for a guide similar to the decibel scale, which maps a quantifiable value to some well-known sources of various loudness.
I know there’s some complications here with wavelength-dependent eye sensitivity, background light levels, viewing distance, but have any of y’all seen a general graph or guide like below that could be used to guide selection of an LED?
See also Twitter thread on this idea here:
Welcome to the forum.
The required brightness is heavily dependant on the ambient light brightness. A dark room is completely different from outdoors on a sunny day.
It’s also heavily dependent on the wavelength, some colors appear many times brighter than other colors to humans when at the same brightness.
Another factor is the angular width of the light emission, wider patterns often appear a bit brighter than narrow beams.
The only reliable way of figuring out the correct brightness that I’ve found is to build variations and try them out.
1 Like
Paul is correct, pupils adjusting for ambient light, an LED that is painful to look directly at in a dimly lit room is barely visible outside in sunlight.
This is a good recommendation for content however, I’m passing this along to some of our content creation team.
Forgot another huge cause of variation during use, ambient temperature. When running over a -20°C to 50°C ambient range the current in the LED needs to vary by a huge delta to keep the brightness constant.
IIRC, back in the 90s when I did this for a measuring device the current needed to vary from 2mA @ -20°C to a bit over 20mA @ 50°C with the LED we were using.
