Larger emitters need larger reflectors to throw well?

LED Flashlights – General Info
1

I frequently read posts where people say larger emitters need larger reflectors to throw well as if it was a given that larger emitters don't throw well in small reflectors where XR-E does well. But the assumption seems to be it has something to do with the size of the emitter.

It's actually more correct to say that emitters that don't have as great a surface brightness as other emitters need large reflectors to compensate (dimmer lights need larger reflectors).

A large emitter that had the same surface brightness as a XR-E (if one existed) would do well in a smaller reflector. It may be more efficient as far as its lumen output in a larger reflector but it should throw as well as the XR-E in the same reflector.

I've never seen anyone point this out specifically and it's a common comment. However if I've missed something and am not entirely correct I'm sure Dr. Jones or someone else will point it out.

I don't think I'm incorrect though. An aspheric is looking at a very specific slice of the emitter and whether an emitter is big or not isn't going to affect that slice (for the purposes of throw).

2

I'm not sure which has the brighter surface brightness (don't remember the size of each and the max lumen output when driven to max spec). If you know the answer and if the XP-G has a greater surface brightness (which I suspect is the case) then yes it would do better in a smaller emitter as far is throw is concerned than a XM-L.

I suspect (and someone will correct me if I'm wrong) that a larger emitter is more efficient in a larger reflector (proportionately larger-proportionate to its larger size).

3

doesn't a larger or deeper (parabolic) reflector increase throw for all emitters regardless of die size?

4

When I modded RL-2088, bigger reflector gave me almost double lux reading.

5

I think the reason a larger reflector is required for a larger emitter is simply due to scale.

A reflector has a focal point where parallel rays of incoming light will all converge. Flashlights use this in reverse - rays of light starting from the focal point bounce off the reflector and out the front in parallel rays (throw).

The key word here is point. Light rays that don't start out at the focal point don't end up parallel to the others (flood). As the reflector gets larger, relative to the the area of the emitter (die or chip), the more the emitter appears as a point.

This is the way I make sense out of it, "Its my story and I'm sticking with it".

-Crux

6

Yes. That is the point rather than talking about a large emitter needing a large reflector a larger reflector helps all emitters. The large emitters we are generally talking about today are dimmer emitters. If they were small dimmer emitters they would still need larger reflectors.

Depth isn't what does it however. That reduces the spill but doesn't do much for throw. I think it basically increases the corona. Throw is about surface brightness and reflector diameter and that's about it.

As was pointed out in RA's thread on CPF a long time ago...think about it from the perspective of looking at the reflectors from a distance. You can see a smaller reflector if the light is very bright or you can see a dimmer light from the same distance if the reflector is larger. We can see large dim things and bright small things.

7

The viewing angle of the emitter also has alot to do with the reflector needed to focus the light. An XPG for example has alot more light coming of the sides of the dome than a XRE which is why in similar reflectors the XRE will have better thow even though over all lumen output is less.

8

I agree that the greater the diameter the further those outer rays are from the emitter and therefore the emitter looks more like a point.

That's also why I said that proportionately a larger die may need a larger reflector that is larger in proportion to the increase in size of the die. But mainly we are just describing how/why reflectors work but they work with small emitters in this way as well.

If the reflector or aspheric is only "seeing" in the focal point a small slice of the emitter if doesn't really matter for the purposes of throw how big the rest of the emitter is because that's not contributing to throw but rather to the corona or spill.

In any event when we describe large emitters (as they are made today) what we are really talking about is dimmer emitters so size isn't really the relevant factor.

It's kind of like describing incan lights in your house as 100 watts. When you tell me that your 100 watt light is bright I know what you mean but you are really just telling me how much energy it uses to produce illumination. If you told me how many lumens it was outputting that would be the relevant characteristic.

It's the same with emitters. The relevant part is surface brightness and not size. It just so happens that at the moment larger emitters have less surface brightness. Just like it so happens with incand that 100 watt lights all have the same lumen output so I know what you mean even though 100 watts as a description really tells me nothing about the lights output.

9

The relevant limits described:

http://en.wikipedia.org/wiki/Etendue#Maximum_concentration

According to the math in wiki at least, min collimation angle alpha is about inversely related to aperture area S (optic size) for smallish values of alpha.

10

gcbryan is right; a bigger diameter does not at all need a bigger reflector for throw per se.

Actually the SST-90, the biggest one, can compete with, say, the XP-E in 'surface brightness', and both would throw equally well with the same reflector or lens - the XP-E @ 1A, the SST-90 @ 9A, that is. The SST-90 would give a spot 9 times as big as the XP-E, containing 9 times as much light (luminous flux), but with the same intensity, thus with the same throw.

A larger reflector isn't even more efficient (or only a negligible bit), defining efficiency here as percentage of light going into the spot instead of the spill. Efficiency in that sense only depends on the aperture angle of the reflector or lens; it's about 75% for a typical reflector with ~60° full opening angle - for a reflector. A typical 60°-lens gets the forward 25%...

XM-L U2 has about 30% less emittance ('surface brightness') than the XR-E R2 EZ900, which is the king in that respect - those data are according to the specs, i.e. at their specific max current and at 25°C junction temperature...

The viewing angle does have some influence - but not the specified full-width-at-half-maximum (FWHM) viewing angle, but instead the maximum viewing angle, which is ~180° for all XP-*, XM-L and SST-*0 - but only about 120° for the XP-*, caused by those LEDs' collar. That causes a bigger 'dead hole', that is the center part of the reflector not hit by the LED light. That area does not contribute to throw then. With aspheric lenses the viewing angle has *no* direct influence on throw.

oldbobk, absorption in the lens usually is rather negligible unless you use really low quality lenses. The gain from diameter is much higher: 30% more diameter for example will yield ~70% more spot intensity!

agenthex: That equation only holds under the condition of a constant aperture angle; in that case the bigger aperture area S comes with a bigger focal length, and it is that focal length that reduces the beam divergence alpha. With a single (aspheric) lens, alpha=arctan(s/2f), or just s/2f for small angles, where s is the apparent LED diameter and f is the focal length. You can produce a *very* thin beam with, say, XP-G and an f=25cm lens of only 20mm diameter, f counts, not the diameter. *Much* losses though.

11

Why would the SSt-90 beam angle be wider? Would a bigger reflector be able to "tighten up" the beam? This would put 9 times as much light in the same spot as the XP-E.

I'm confused...

-Crux

12

The size of the hotspot is determined by the size of the emitter. The SST90 is 9 times the size of the XR-E and therefore its hotspot is 9 times larger.

13

Yes, a bigger LED casts a bigger image.

"9 times as big" referred to the area, should have said "3 times as big with 9 times the area"

A bigger reflector would indeed tighten the spot. Make it 3 times as big, and you get the same small spot as the XP-E, but with 9 times the intensity and thus 3 times the throw.

Then again, take the XP-E and put it into the bigger reflector, and it will have the same intensity and throw - just a smaller spot again.

Throw is mainly about reflector size. Actually one could say throw is directly proportional to reflector size, if everything else is equivalent.

As a rule of thumb, good XM-L lights for example have a NEMA-throw of about 8000 times their reflector diameter (less for OP).

Dedicated throwers (XR-E, smooth deep reflector) are more like 9000.

Typical small aspherics (XR-E) have values of 10000 to 13000 (at least those I measured; lens diameter without it's border).

14

So it's diminishing returns - once a reflector is big enough to make a nice tight beam (throw), making it bigger won't help much, except to keep the light from going through the washer in your pants pocket.

So, does this mean that a bigger emitter needs (or perhaps, can make good use of) a bigger reflector?

-Crux