Reflector width vs depth for throw?

I know a bigger deeper reflector will allow more throw from a given LED. But what’s the ratio or limitations of this?

As in, if you had a shallow 4” diameter reflector, vs a very deep (say 3” deep) narrow (1.25” width) reflector?

Is there an optimum configuration? Guessing it’s all about angles and light collection, but not sure how to figure it out exactly.

It’s pretty much all about the diameter rather than depth.


Sounds strange...

I mean, Crelant 7G5 has a large reflector (about 60) but very deep too (judging from outside about 55mm) and it throws a smaller hotspot than for example a HD2010 which has about same diameter but shallower reflector (about 45mm, did not bother to open and eyeballed from outside)

I think that depth plays a big role with throw. Something like you need a large reflector to catch all the side light from the emitter, but also need the steep reflector angle to throw it further.

A large dish reflector would not throw at all, in my understanding...

Thinking more about it, if we take a parabola, what changes is the distance of the focus point, e.g.the point wher you have to place the emitter. If you had a very flat dish parabola the focus would be far away outside in front of it, could well think a "recoil" setup for example (UF-007 and similars)

But waiting for more experienced user to share some light... er.. throw.. on the subject

I have one of those halogen spot lights, its abou 7 1/2” diameter reflector, but only about an inch deep. It throws like hell, so I’m not sure about the depth/diameter relationship. It probably also depends on the emitter itself - bulb vs led etc.

Ok, but in your example the halo bulb sits much higher in the reflector, whereas usually led emitters sits at the very bottom of reflector. The focus point height is different, it's the parabolic shape difference I was trying to explain above

Using the same reflector with leds would be not feasible, as you would need a led that emits light at almost 360degree, like bulbs do.

If a led emits at 125degree, for example, any portion of the reflector that is below the 125degree cone is just wasted space. To catch and redirect most of the side emitted light the reflector needs to be inside that 125degree angle, and that results in a reflector which must be increasingly deeper as long as it increases its diameter.

Well, not sure that my explanation makes sense... It does have sense in my mind but it's not easy to explain in english...

Here is some info on throw.

Depth indeed only has a minor influence on throw: Shallow reflectors have a bigger ‘dead hole’ (unused reflector area in the center; especially with XR-E) which reduces the effectively used apparent reflector area, and it is only that area which determines throw (supposing equal LED and losses and perfect reflector shape).

I have been thinking about this.

The parabolic reflector gives more or less an enlarged image of the led at infinity (not very different from what a lens does), so the stronger the curvature of the reflector (=shorter focal point) the the more enlarged the image gets, which is a large hotspot. There is two ways of getting more light into the hotspot: 1) concentrate the light to a smaller hotspot by making the focal point larger (less curved parabolic reflector), or 2) direct a larger portion of the light to the hotspot by making the reflector deeper.

So in case of two reflectors with the same diameter this is the trade off:

The shallow one will have less curvature -> longer focal point -> smaller concentrated hotspot. But a lot of light is 'lost' into the spil.

The deep reflector will have more curvature -> shorter focal point -> light is spread out to a larger hotspot. But the hotspot, although larger, does contain a larger portion of the total light output.

I am not sure which effect is larger in terms of lux-measurement in the center of the hotspot, but in a thrower the spill is distracting me from what I'm aiming at, so I am fan of the deep reflector.


This is what I meant, sorry for the lousy paint job, but that's all I could make in a few minutes

@ DrJones, thanks for the link! Time to study

It's hard to draw an image by hand, so I'm not saying you don't know this and didn't try to illustrate this, but the light must enter and leave the reflector at the same angle.

djozz: The parabolic mirror for LEDs does not have one single focal length; the reflector parts near the LED have a short focal length, those at the edge have a long focal length. Focal length = distance to LED. A deep reflector usually produces a smaller hotspot core, but with a wider corona. Not more throw though.

Rockspider: That's the idea - except that the angle is not 125°, but nearly 180°. Take an XM-L, apply a small current, and view it from the side: you'll get light even at nearly 90° to the side. Those 125° are the full angle for half the intensity, not the maximum angle.

You might now say that the intensity to the sides is lower, thus the light near, say, 85° doesn't contribute much, but that's wrong: The luminous intensity goes down, but not the luminance, and it's the luminance that is important for throw.

To get the most throw from a single point emitting light, (of course leds are not single point, still)

i think you need a fixed width to depth ratio and a fixed parabola shape.

Any deviation from this (i don’t know what it is but i’m sure it’s known/given or can be calculated) will result in less throw.

It will either increase the size of the hotspot or create flood around the hotspot or both. (depending on the new ratio and the parabola shape)
A deep reflector will have less spill and more ‘enlarged’ hotspot and flood but not more throw.

Example: assume the ‘proper’ width to depth ratio is 1 to 1 then a reflector with double the depth (1x2 vs 1x1) would not throw more. It will have less spill, more light in the wider hotspot area but not more throw.
(i like deep reflectors for bike lights)

If the reflector really is a parabole, it indeed has one focal length (f) that determines the magnification of the led, similar to the parabolic mirror of a telescope (but instead of imaging infinity into the focal point, the focal point -actually focal plane, where the led is- is imaged at infinity, so the other way around). If parts of the reflector have a different focal point, the reflector is not parabolic.

I may be missing something that is happening in the real world of flashlight reflectors (leds not precisely in the focal point, reflectors that have different than parabolic shapes, just a cut-off portion of the parabole is used because a led does not emit 360deg, which will have its influence on the image), but the above is what a true parabolic curve does as far as I know

Ok, I see that what I was trying to explain is referred as point (1a) 'dead hole' and my drawing above would have been an explanation of this, and the fact that a reflector for a halo would not be as good for a led.

Anyway DrJones it's hard to me to understand your last sentence, if the light emitted past the 125° and up to 180° is lesser, how comes it's so important for throw? Luminance is the surface brightness, right? So also the brightness is lesser when viewed from the side, isn't it?

Coming to practical examples, I've had some lights in which placing a washer between reflector and emitter it resulted in a smaller hotspot (with larger corona) which in my impression gave more throw because the hotspot looks brighter now, more concentrated. Thus by doing this washer shimming I lost some side light, as the light emitted at very large angles approaching 90° now hits the washer and not the reflector.

Still indoor and outdoor it looks to me it throws more now. Am I wrong?

Johm: There is no ideal depth to diameter ratio. Indeed having a very deep reflector won’t increase throw, but it won’t decrease it either.

djozz: Telescope mirrors and typical flashlight reflectors differ in one way: Telescopic mirrors have a quote long focal length compared to their diameter - for a reason. That way, all parts of the mirror have nearly the same distance to the focal point, thus nearly the same focal length. With those flashlight reflectors it’s different, they are sort of built around the focal point. Every part of the reflector has to have the same focal point of course, but since the different parts have different distances to that focal point, and the distance to the focal point is the focal length, they have different focal lengths and thus cast images of different sizes. This is what makes the corona around the hotspot core. I suggest an experiment: Take a light with a quit big reflector. Take a piece of cardboard and punch a hole into it and put it in front of your light. Move the hole around: When at the edge of the reflector, only light from an outer part of the reflector gets through, and you get a small image of the LED. Move the hole inwards, and the image gets bigger.

Rockspider: “Luminance is the surface brightness, right?” Yes. “So also the brightness is lesser when viewed from the side, isn’t it?” No: The apparent area of the LED seen from the side gets smaller, and that smaller area multiplied with the same luminance gives the lesser luminous intensity to the side. Throw however depends on luminance, not on that apparent area.

The eye is fooled quite easily; it especially is sensitive to contrast. Less corona gives a sharper shape to the spot and thus more contrast, so it looks more distinct and seems brighter. But actually you loose light that would otherwise be directed into the hotspot, too, so throw decreases.
It may only really have increased throw if the LED was misaligned before and you also shifted the LED relative to the reflector into a better spot.

Aha, that was new to me, thanks dr., every day something to learn!

So by adjusting the curvature of the reflector at the various distances from the focal point you can make a precisely calculated non-parabolic reflector that creates a better image of the led than a parabolic reflector can, and thus get more lux in the hotspot, right? If so, do flashlight manufacturers use them?

  • I love this kind of discussion. DrJones is doing a good job but it would be great if Bill Nye decided to stop here and weigh in :nerd_face:

Awesome thread, guyz :smiley: :beer: Thanks!
Always wanted to see a good comparison between reflectors - grab a current regulated driver with, lets say, XM-L emitter. And than put one reflector - white walling and lux meter, change reflector - white walling and lux meter… same driver and LED for all reflectors. It’s the best way to see the differences. Those who do DYI mods and stuff can do that pretty well :smiley:

Thanks for the explanations DrJones

For sure in one of the flashlights the original emitter position was not optimal, because when I shimmed the reflector the hotspot and corona become bright and tight, then I tried to add another washer to see any difference and the beam was bad again with fuzzy hotspot.

On other flashlights maybe it's more a matter of preferences... instead of a big hotspot with thin corona, sometimes I get a smaller concentrated hotspot but with a bigger corona... Some compromise has to be accepted I think.

I respectfully disagree with this comment.
If you take a reflector of good proportions for throw (i would say ideal ) most, if not all the surface of the reflector would direct the light hitting it towards a ‘small’ hotspot.

If you then stretch this reflector to a bigger depth, then the curvature of the reflector would not be able to direct light to the center but rather close to it.
The actual reflector area that sends light to the center in smaller in the deep reflector.

The curvature of the parabola that redirects from point to point (like throw with led) is determined by a mathematical equation, is it not?

BTW: I love your work.

I'm a layman, so I can't get into the theory, but I can tell you an example of two reflectors where the shallow one gives a tighter hot spot, than the deeper one.

The first one is 29.5mm deep and the second one is 40mm deep. The shorter one is better at throwing a small center spot, with less side spill. The deeper one is better at flooding an area, with a much less defined hot spot and a lot of spill. Both tested with the same light (Maglite) and emitter (XM-L T6).

It's just an example that depth does not make the difference.