Like a White Flat in a 124mm reflector. Insane.
I had the same question not too long ago in the White Flat thread. The “spill light” only accounts for 25% of all light emitted from the LED. I didn’t think that sounded right at first until I did the math. Although the light in spill is more intense than the light hitting the reflector, the reflector represents 4 times the surface area of the front aperture of the reflector - which would be the spill light.
The explanation for the increased candela is that when the dome is removed, yes the lumens go down, but the effective surface area of the light is reduced almost fourfold. Because it is flat, not spherical. So the light intensity increases. Meaning - the LES gets closet to a point light source. Also, there is a bit of a TIR effect with a dome where light rays would bounce back into the phosphor - so you get more lumens, but over an effective larger surface area = meaning light intensity decreased.
Or Enderman’s Black Flat in a 300mm reflector recoil setup…
In a traditional reflector light, the spill accounts for about 50% of all the light, not 25%. This is why recoil style lights can be such great throwers. You have so much more light being focused instead of wasted. (If you consider spill light a wasted light)
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If I read the diagram right, that is true for a 45 degree spill angle, it seems to me that in the usual flashlight the spill angle is less than that, closer to 30 degrees.
One of the few sliced dome comparisons I did (a xhp70).
The smaller “die to reflector size” ratio shrinks the hot spot. By changing the ratio you can get bigger or smaller hot spots. This is why tiny dies plus big reflectors make such good throwers.
Also the amount of light per square millimeter, the intensity or luminance, of the led also plays a big factor. Short arc lights have tremendous luminance. Put them in a big reflector and you got a crazy thrower. 
On most LED reflectors I’ve seen, the half-angle is around 30 degrees. I would probably agree that with a 45 degree half angle, it’d be more 50/50, but I don’t see that as the case with most all LED specific reflectors. Even with a recoil light with an LED, it’s of most benefit when the focal point lies on the same plane as the opening of the reflector; since the LED will emit light in almost 180 degrees, that ensures virtually all the light is captured and reflected back.
Your talking about a very deep reflector? Sure, the angles vary based on the depth of the reflector. A shallow reflector tends to have more spill light and a deep reflector has less spill light.
Also keep in mind the spatial distribution of light from the led. The majority of the light is going out at a 90° angle, not so much to the sides. Having the hemispherical dome actually puts a bit more light out to the sides, into the reflector, while a domeless led puts it more out the front. So no dome means a bit less light hits the reflector. (I used to think removing the dome allowed more light to shine from the sides of the led, but nope)
It just goes to show how inefficient the traditional reflector style flashlight is. It’s still my favorite design, though.
(See, I’m learning and trying to be more scientific. )
Most all common reflectors we see in LED flashlights have a diameter very close to equal to the length from the edge to the center of the LED. That’s just about 30 degree half-angle. The spill is actually a bit wider than the half-angle suggests because the LED is not a point source. It is not the majority of light leaving through the front. I erroneously assumed that as well. The luminous intensity for 0 to 30 deg. is highest, but that represents a very small area of the total emitted light. While the light from 30 to 90 degrees is less intense, the area is at least fourfold greater than that emitted in the first 30 degrees. In other words, if you consider a hemisphere around an LED, 0-30 degrees represents 1/5 the surface area of the hemisphere, and 30-90 degrees represents the other 4/5. When you weigh the relative luminous intensity, it works out to around 25% from 0-30 degrees and around 75% from 30-90 degrees.
Also, part of the spill light does become part of the hot spot light, since they’re in the same space.
Of course, the reflector is not 100% efficient. But, in general, the angle between the axis of the LES and edge of the reflector will determine the ratio of spill to hot spot light.
Let me try to add to the confusion. In a perfect world, the emitter would be the theoretical point source at the focus of the parabola (reflector). If so, in a vacuum, all of the lumens would exit the reflector in a perfectly parallel beam, with the lux evenly distributed across the beam. AKA, all hot spot, no spill. Add an atmosphere and there will be some scattering, so a slight spill will occur. Now, we all know that our current emitters are not point sources, so if the center of the emitter is at the focus, that light will still be collimated into a parallel beam. but any lumens emitted away from the center will be reflected as shown by that wonderful diagram, thus spill and hot spot.
Now here is where it gets fuzzy. In a domed LED, if the center of the DOME is at the focus, will that improve the collimation and reduce the spill? If so, then de-doming the LED will move the light source away from the focus, with results I can’t quite visualize. I would really like to find some simulation software that I could play with to work this out better.
At this point it is getting late, and the water is starting to run into the tops of my waders, so good night all.
The rays of light would radiate out in all directions, and since the reflector must have an opening, there will be light that won’t hit it, thus spill.
I think I see what your saying. Your looking at the spatial distribution in 3 dimensions.
If we look at this picture, the light within circle 1 is the spill light missing the reflector. The light in the outer rings do hit the reflector. That’s probably the 1/5 and 4/5 your talking about.
The problem is I actually don’t know how Cree is measuring there spatial distribution. On one hand it looks like it might be a two dimensional slice down the middle, but it could be three dimensional. I need to do some more research to find out how exactly they’re measuring it. This is where someone like Maukka would come in handy.
Consider a hemisphere surrounding the LED with a radius “r” of 1 unit, a unit being the distance from the edge of the reflector to the center. In other words, the circle defining the edge of the reflector sits on said hemisphere. Now if we consider that that radius is about the same as the diameter of the reflector’s opening, that basically is a 30 degree half angle. So the surface area of a hemisphere is 2 * pi * r^2. The area of the spherical section or cap that we describe as “spill” is 2 * pi * r*2 * h, which is the height of said cap. The height is simply r - r*cos30(deg). Since r = 1 unit we can disregard it here. Thus the surface area of the entire hemisphere is 2 * pi units or 6.2832 and the surface area of the cap is 2 * pi * (1 - cos30) or .8418. Subtract those two and you get 5.4414. So that cap only accounts for 13.4% of the entire area of the hemisphere, while the rest accounts for 86.6. To “weigh” each section based on the luminous intensity at each angle would be tough, but even just guesstimating by coming up with an average luminous intensity for each section, will give you about 25 of the lumens coming off the cap, and 75% coming off the rest. Meaning, in a reflector that has a 30deg. opening, it captures 75% of the lumens and reflects it forward, while 25% accounts for most of the spill, with some percentage actually being part of the “spot.” But since the reflector is not 100% efficient, not all 75% of that light is collimated.
I think it’s funny how all those equations assume a perfect parabolic reflector for the LED. Anyone actually HAVE a perfect LED parabola reflector? Anyone know for 100% sure what the perfect parabolic equation is? Interestingly, most flashlight manufacturer’s seem to have missed…
I don’t think they assume that.
The reflector angles are always a compromise due to the led being so big. You need a pinpoint source of light for all the angles to be perfect. You get closer to this by either making the reflector diameter larger or making the source of light smaller.
Everything when engineering a light (or anything for that matter) is a matter of compromise. That’s because nothing is 100% perfect, and the closer you get to 100% the price goes up exponentially. For example, an electroformed reflector, with vacuum deposited aluminum coating, would be great, but it would cost far more than what we typically pay for the entire light. An argument I’ve heard is that the reflectors in our higher end consumer lights are machined from castings, and machined is not as good as electorformed. Well, the mandrels used for electroformed reflectors are machined as well - it just so happens they are machined to incredible tolerances and ground to almost perfection. One could turn a reflector in an ultra precision CNC turning center with PCD tooling and get an almost mirror-like surface, before polishing or coating. But that would send the cost of our lights through the roof!
Of course our LEDs are not perfect either. They don’t emit light perfectly evenly, they are not a perfect point source, they don’t even have a round LES (unless maybe you’re using a CBT-140). And the larger the LES in relation to reflector, the wider the beam.
I think these manufacturers know more than you think when it comes to reflector design. If you notice a lot of halogen or xenon lights, seem to have a wider than 30 degree opening. I think because the light is more of a point source, they purposely engineer more “spill” to the light to give it more utility. I think for LEDs they must have figured that 30 degrees gives a good balance of throw and spill, though I would guess that for throwers using larger LEDs, going a little less than 30 degrees would help tighten the beam up a little.
I’m sure the equation for a paraboloid that are used for the CNC machines that make reflectors are correct (no rocket science, it is a variant of y=x2), and a pretty good parabolic shape comes out of the machine. What makes them imperfect is the roughness of the machine marks, the quality of the coating, and sometimes manufacturers have the focussing of the led wrong.
If it is a given that there is a perfect formula then all of our reflectors should have the same internal shape. I have 230+ lights, they do not follow a set design where reflector shape is concerned.
My point is, to put it to a degree of measure, is that while the GT was close to this perfect formula it was still lacking. If the reflector cups in the GT4 are refined, the formula tweaked, then the GT4 should be a pretty amazing light indeed…
Virtually all flashlight reflectors are parabolic in nature. The reason is simply because light emitting from the focal point to any point on the surface will go forward parallel to the axis of the reflector. That is actually the definition of a parabola.
Technically a parabola is 2D, a surface created by the revolution of a parabola around an axis is a paraboloid.
Incandescent Mag lights use a spherical reflector and there are are millions upon millions of them out there. An unraveling of the virtual paraboloid theory.
The parabola is a diagonal sice taken from a cone. There are formulas as to what creates the perfect beam, the best for throw coming to a near square ratio of height vs depth. This is not necessarily my own theory but the work of some brilliant minds using 3D design in Autocad. Changing the height to width (dia at opening) ratio directly impacts the shape of the beam and the effective throw. Focal point at the base also directly impacts efficiency and can distort the beam profile to impact throw.
I’ve been on more than a few design teams where far greater minds than my own were employed… and built lights on the prototype results. 