Led's are usually measured in mcd, micro candela, the measurement of luminious intensity.
Lux - 5/6 lux street lighting, 750 lux drawing office.
What do you want to illuminate?
How bright?
What is the surrounding area like, shiney, matt, colour, etc...
Its a complex subject, 3 months min on a BTEC certificate, and intense.
As an example. I've used 4mm oval amber leds, with 60x120 deg viewing angle, 590-1300 mcd, untinted clear lens, to illuminate boiler, vacuum and steam chest pressure gauges on a full size steam locomotive. These work very well wth good mtbf.
On the other hand, 5mm round led's, 15 deg viewing angle to illuminate a speedometer (back lit).
More info....
If your not sure, play with them, they are cheap.
Mark