Help me understand optics...

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cainn
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Help me understand optics...

When you do this:


how is it that you still get this:

?

I want to be able to visualize how it is that you still get a hotspot in the exact same place (albeit slightly less bright) even though the light is only escaping from the outer edges.

I’m hoping that some hardcore experts will chime in, and that a fascinating and informative discussion will ensue.

Thanks in advance!

Confusius
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I’m no expert by any means, but I take it to be like this: your emitter has a certain angle in which it emits light, usually something between 90 and 120 degrees. Light that goes straight forward through the lens is not affected in any way, and creates most of the spill. Light that goes to the sides hits the reflector, which is shaped in a way that all the light hitting it is thrown to a focal point. What you are doing with this piece of paper is blocking most of the light that is not reflected (and some of the reflected light too). But the light that comes through on the sides is still directed to the focal point and thus creates the hotspot.

Maybe this picture helps a bit.

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cainn
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OK, awesome. Thanks Confusius. I kinda feel like an idiot now because it seems pretty obvious now you’ve explained it. For my own benefit, and perhaps that of some others, I quickly put together my own diagram based on yours (ignoring light that escapes directly):

We seem to be looking at a diagram of a car headlight here, and in this particular instance, we’d have a dimmer area in the center before and after the focal point, right? In fact even with an LED sitting right at the base of a reflector, this would also be the case would it not? I’ve noticed on some of my lights, at certain distances, that the center of the hotspot is dimmer than a ring of light that surrounds it. But I can’t say that I’ve noticed it on all my lights, even ones that use the same emitter and a similar sized reflector. This would probably be due to the reflectors having different focal points, and possibly the result of the positioning of the LED with respect to the opening, wouldn’t it?

Confusius
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cainn wrote:
We seem to be looking at a diagram of a car headlight here, and in this particular instance, we’d have a dimmer area in the center before and after the focal point, right? In fact even with an LED sitting right at the base of a reflector, this would also be the case would it not? I’ve noticed on some of my lights, at certain distances, that the center of the hotspot is dimmer than a ring of light that surrounds it. But I can’t say that I’ve noticed it on all my lights, even ones that use the same emitter and a similar sized reflector. This would probably be due to the reflectors having different focal points, and possibly the result of the positioning of the LED with respect to the opening, wouldn’t it?

Ideally, the reflector used in a flashlight should be designed for the used LED, so it sits at the focal point… but in budget lights, who knows? It would be interesting to see how the position of the emitter changes the beam, but none of my lights allow for such an adjustment, I think.
I’ve just tested it on my EDC lights, on very short distances I am able to see a dark center in the hot spot, but not on high distances. I think this is because they are OP reflectors, don’t have any lights with SMO reflectors on me right now.
But thinking back to the (horrible) beam of my old incan 2D Mag, there should be another donut on the other side of the focal point too Smile
Doughnut

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