Understanding reflectors beam shape

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YuvalS
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Understanding reflectors beam shape

When buying TIRs the description usually includes the beam angle but for reflectors the description is usually only diameter and height. 
How can I know what is the beam pattern of the reflector out of these numbers?

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bibihang
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It also depends on light source a.k.a the LED type. The output beam pattern is a result of the combination of both reflector + LED.

Own enough flashlights with different types of reflector and LED and you will get a grasp on this. Smile

The more economical way is to read up enough reviews of different lights with beamshots, that helps as well.

alpg88
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Very loosely, in general to get a tight beam from a reflector you need a deeper reflector, (for leds), but  the same size reflectors can be made as a flood or a spot. however in real world, wider beams do not need deep reflectors, so very few make deep flood reflectors, so large deep reflectors would most likely be tight spot. smaller, shallow reflectors tend to be a flood. Also surface matters, if reflectors looks like an orange peel, you will not get tight beam from it, tight reflectors are almost always smooth, like a mirror.

YuvalS
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Say I am using a XM-L2. What dimensions of reflector will give me ~10 degrees beam? 

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alpg88
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YuvalS wrote:

Say I am using a XM-L2. What dimensions of reflector will give me ~10 degrees beam? 

Ratio of depth to diameter should be 1 or more.  with xml, to make true 10* it has to be a large deep reflector.  size of Maglite , about 50mm, at very least, but i'd go even bigger. 

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To get a narrow beam the diameter needs to be smaller than the depth. like this one CPC “compound parabolic concentrator”
YuvalS
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So is it means that TIR is much better than a reflector for a narrow beam? 
I have a 10 deg TIR with diameter of 20 mm and height of less than 2 cm 

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Quadrupel
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To use in headlamps for sure… + TIR is lightweight.

Unheard
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> How can I know what is the beam pattern of the reflector out of these numbers?

A parabolic reflector is made for a point light source and the optical setup is made to have a 0° spot, so the spot size equals the opening of the reflector. The spot widens only because the die size is finit.

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In theory (ha!), spill is from light that comes straight out of the LED and doesn’t touch the reflector, and light that comes out the sides and hits the reflector gets reflected exactly forward to form the hotspot.

So again in theory, from a perfectly point-source emitter that’s exactly in the focus of the 3D parabola, there should be a 0° hotspot, and the shape of the hotspot should be a ring. Outer diameter would be the diameter of the reflector, inner diameter (“hole size”) would be determined by where the widest-angle light of the emitter would first hit the reflector.

Of course, LEDs aren’t point-sources, but big honkin’ squares. The corners are the farthest away from the focus, so “blur” the hotspot the most. That “blurring” is what makes the hotspot bigger/smaller and creates a fake beam-angle. Also why, say, a ’351 or XM-L2 or XHP70.2 gives a bigger hotspot than, say, an XP-E2 or white/black flat.

So in general, the bigger the diameter of the reflector and smaller the size of the emitter, the tighter the hotspot.

I got an Anekim that uses one of those CSI.NY or whatever chips, and has a nice tight hotspot. My C8 that has an XP-E2 is underpowered but still throws quite nicely because of the chip:reflector ratio.

Anyhoo, there might be some formula that could either be theoretically calculated or be empirically derived, but I haven’t seen it yet. LOL

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YuvalS wrote:
So is it means that TIR is much better than a reflector for a narrow beam? 
I have a 10 deg TIR with diameter of 20 mm and height of less than 2 cm

Tightest hotspot light I got is my Catapult Mini, which has a TIR lens.

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alpg88
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YuvalS wrote:

So is it means that TIR is much better than a reflector for a narrow beam? 
I have a 10 deg TIR with diameter of 20 mm and height of less than 2 cm 

 

no it does not mean that, most throwy light k75 is using  reflector.

no way that tir will make true 10* beam with xml2, maybe with xpe2, or another led with 1x1, or 2x2 die, but not for 5x5 die.  

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For the same reflector’s diameter and LED source, depth of the reflector doesn’t actually do much in throw. Deeper reflector will make your hotspot smaller but it won’t really help much in absolute throw. Maybe just a tad bit, but negligible anyway I would say.

Diameter of the reflector on the other hand has the most influence in throw.

Beam angle is only part of the equation of throw because it’s only about the reflector/optics. Another important factor for throw is the luminance of the LED, or known as “surface brightness” by some. It is basically lumens per square area, so the higher this value is the more intense of the light it is, which will result it better throw.

As a general rule of thumb, LED with smaller die size + domeless generally give better throw – but this is just a generalization and there are of course some exceptions to this, so if you want to be absolutely sure on this you have to always calculate the luminance.

Beside that, smooth reflector gives better throw than OP reflector given all else being equal.

So by knowing some of these basic principles, and owning a few lights with different LEDs and reflectors/optics you will be able to estimate how a flashlight’s beam pattern would be like given the type and size of the reflector/optics, and the type of the light source.