I need a red and a yello LED indicator in my battery operated project. I am looking at specs on Digikey and they list brightness as mcd, viewing angle and test current. Viewing angle I know, but how do I figure how much current and the related mcd so they will be bright enough and still use as little current as possible?
Modern LED may be typically rated at
eg 30 cd at 15 degrees at 20 mA.
1 mA usually noticeable but not bright
5 mA nice and bright
20 mA eye wateringly bright. almost painful.
Larger viewing angles decrease brightness with square of angle ratios.
Trying it and seeing is liable to be useful.
lumen = candela x 2π x (1 - cos(A/2))
candela = lumen / (2π x (1 - cos(A/2) ) )
Where A is the full cone angle of the source.
A modern LCD screen is typically 300 lm/m^2 at the surface.
Have a look towards bottom of Wikipedia candela page - and ask questions.
Blockquote Viewing angle I know, but how do I figure how much current and the related mcd so they will be bright enough and still use as little current as possible?
The authoritative information is the manufacturer's datasheet.
For example Digikey's part number; 67-1714-2-ND's; datasheet lists the following:
which gives you in addition to your mcd and viewing angle, the necessary forward voltage and steady current. If you supply the forward voltage and steady current listed in the datasheet you can expect the mcd listed in the datasheet,
Note: Digikey give a workable but incorrect steady current for the LED. I would trust the datasheet and not the Digikey search page for the correct values.
Of course you can always reduce the forward voltage and steady current until you get the desired brightness.
I do not think you can count 50 mcd as any kind of "average" value given the varaiety of LEDs produced today.