Max Lux at centre of hotspot for bare emitters - SSC P4 and XR-E

Does anybody know how bright the centre of the beam is for each of these LEDs? What I'm really interested in is what would be described in radio circles as the gain figure - ie Candelas/Lumen at the peak intensity point. The datasheets give a radiation pattern, however it is relative to 100% at the centre, without any info on how bright it is there.

The reason for this is that I'm trying to compare some optics for SSC P4 and XR-E LEDs. However the specs all seem to be a factor of how many times brighter the centre of the hotspot is with the optic compared to the bare emitter. Given that the radiation pattern is different, it is hard to compare without knowing the relative gains - it seems that the XR-E with it's narrower beam should be naturally brighter at the centre, but I don't know by how much.

chris_m said:

...
it seems that the XR-E with it's narrower beam should be naturally brighter at the centre, but I don't know by how much.

Click to expand...

Nope.

The hotspot of a reflector is formed entirely by the light emitted horizontally by the emitter.

Think about it.

Toshi

I'm not at all interested in the output with a reflector - only in the output of a bare emitter, so that's irrelevant.

chris_m said:

The datasheets give a radiation pattern, however it is relative to 100% at the centre, without any info on how bright it is there.

Click to expand...

If you know how the radiation pattern changes, you can compute the average radiation in a steradian. Then calculate the candela using the total lumens and the value of the steradian, then inverse the average to center.

Something like this,
Let x = intensity in center (peak)
Based on datasheets you calculate (numbers bellow are made-up) and get that the led emits in 1/4 steradian at average inetensity about 60% of the intensity in center.
So the intesity per steradian is 60% x / (1/4) = 4*0.6x = 2.4 x candela

But you know the total output of the led is 160 lumens =>
160 lumens = 2.4 x candela
and 1 limen is 4Pi candela
=> 160*4*Pi = 2.4 x
=> x = 16*6*Pi candela

Now you only need to find the correct values for a=1/4 and b=60%, c = 160 lumens
x = c*4*Pi*a/b

No idea if you can calculate a & b from datasheets...

PS: I know this was not the answer you were looking for... sry

XR-E P4 bin is approx. 38-40 cd at the centre @ 350mA (=38-40 lux @ 1m)

Thanks - that's just the sort of info I was after. Anybody have similar info for a SSC P4?

I don't think you got chris_m's point. his question is about the LED alone, not behind a reflector.

Second, the hotspot is not entirely formed by what you describe. It still has some light directly from the emitter.

The proportion of what is directly from the led and what is from the reflector depends on the LED radiation pattern.

chris_m's question is perfectly valid.

TMorita said:

Nope.

The hotspot of a reflector is formed entirely by the light emitted horizontally by the emitter.

Think about it.

Toshi

Click to expand...