Luminance of small normal LED green bulb

If you have the manufacturer's part number, you usually can find a PDF of their data sheet. "Simple LED green bulb" can mean jsust about anything.

First off, I think it will be specified by the current through it, rather than the voltage across it. It is a diode, after all ...

Isaac

Hello Arfa

Actually, the cd/m2 unit is not so meaningless - it's the measurement unit of luminance

You may have heared the unit "nit" being referred to (apparent brightness of CRT, LCD screens, EL- foils, or any other flat and rather regular "area light sources"), it's the same unit. 1 nt = 1 cd/m2. The integration over the visible wavelengths is weighted with the human eye perception sensitivity, so it measures how bright a luminous area appears to be (independent of the radiating area, distance, and viewing angle, for that part see the cos(beta) term in the definition).

In principle this measurement applies to any area that can emit light, the active die area of the LED chip including. I've got no real experience with actual values, but the German wiki page on luminance has some examples.

A white LED is listed as 50Mnt, a fluorescent tube as 11knt. The website for a Philips "Altilon" high-power LED mentions 60Mnt when driven at its rated current of 1000mA, but this looks like a thermal challenge. This LED integrates a copper heat spreader that has to be mounted with M3 screws to a sizeable heatsink. A typical "bulb of normal small size" is rather unlikely to drive the chip at a current density comparable to the above, so the perceived brightness per unit area should be less. Green LEDs however will have a noticeably higher nits rating than white LEDs of the same chip current density because of the peak in the human eye response to green wavelengths (and because the white LED prosphor is never 100% efficient).

It seems like typical LED datasheets mostly skip this parameter, and if any specify it, those tend to be LEDs that are intended to be mounted in secondary optics designed for a very high luminous intensity (headlights, searchlights and the like). There's some (partial) sense in it, since the luminance is a figure-of-merit parameter when it comes to integrating the LED into high intensity narrow beam optics, but is a little less useful otherwise. The other use case where this parameter gets important is the "sunlight visibility" - the LED visibility under bright sunlight. To be visible the LED chip needs to have a considerably higher luminance than the LED and its immediate surroundings would have by way of reflection when under direct sunlight (this is easier said than done as most LEDs don't achieve such high luminance values). Green LEDs help in this regard because of the eye sensitivity and because of the colour contrast, the sunlight not being green after all.

To the OP: There's a regular on the SE* newsgroups who knows a lot about light sources - Don Klipstein. He's got a page

and since he knows a lot more than I do about all sorts of light emitting things, he might know some more reliable typical values from experience.

Regards, Dimitrij