LED Throw Mechanics

Last night I had my L4 outside, and I noticed that if I shined it down the street, I got quite a good light reflection from street signs, and other reflective material.
Well over a half a block. This brings up a couple of questions.

1- If the light is throwing that far, why doesn't it throw useable light the same distance?

2- Has anyone ever tried (or heard of) using mirrors to relect light back into a reflector to increase output or throw?

Much like a mirror refracted telescope that gathers light from great distances, I was wondering if it would be posible to reverse the mechanics of this.

The reason why those street signs and other reflective material "appears" so bright even at such distance is because they are design to direct the light back at you. Try shining your light at the sign, and walk about 5 meters towards the left or the right of the light, do you still see the sign being very brightly lite? I think the answer will be no. As said, they reflect light directly back to you, so you'll see it only near the position of there the light is from.

Okay some thoughts-

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Bravo25 said: 1- If the light is throwing that far, why doesn't it throw useable light the same distance?

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A number of factors are involved:

Scatter - this will happen regardless of how well collimated the beam is, and can be affected by atmospheric conditions, distance, properties of the materials reflecting the light back for your eyes to see. Remember, if you a illuminating an object one block away, the light must actually travel two blocks to get to your eyes

Material - what colours/frequencies of light are being reflected back?

Ambient light - could wash out the beam that you are projecting, or alter your low-light vision

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Bravo25 said: 2- Has anyone ever tried (or heard of) using mirrors to relect light back into a reflector to increase output or throw?

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Mirrors, lenses or prisms can be used to collimate the 'beam', the amount of light still depends on your light source. Prisms tend to shape and redirect beams with the least light loss, but there is loss. If you want perfect collimation (laser) would such narrow beam be considered useable light?

Hope this helps.

On telescopes the small mirrors are not used to make the stars brighter but to fold the light in order to make the telescope shorter. The focal length of the telescope stays the same, the physical length of the telescope is shorter.

Ever notice how we seem to talk about LED light as if it were a different form of light? (Not that this thread was headed that way) While a white LED's spectral distribution may be different than an incandescent bulb's, it's still just regular light. So, throw is determined by the total output of the source and how well collimated (focused) it is. The collimating is the hard part for many LEDs, since their very shape determines a large part of the LEDs beam properties, even before the addition of reflectors. That's why Luxeons often use lenses to reshape the beam.

The advantage of a bulb over the LED is that a bulb is a close-to-perfect 360-degree light source (minus the area for the base). This means that it is easy to throw a reflector on it to shape the beam any way that you want to. Any light which hits the reflector becomes part of the spot, and any light which misses the reflector becomes part of the flood.

An LED starts as a 180-degree source, and the plastic housing just makes it worse. So most of the light is already being thrown forward (for standard LED housings). The only way to tighten it up is to use a lens, but then you have no flood whatsoever, which is also problematic.

The McLux PR917 and PR-T917 (E2E bodies) are the furthest throwing very small 2x123w LED lights. They are the first in their class to out-throw some popular incandescent 2x123 lights, including the E2E/MN03.

Brightnorm

Re: LED Throw Mechanics - couple of items

First of all, if you really want throw, and a great beam, go for some serious optics. Try shining your flashlight through a telescope or your binoculars(into the eyepiece, out the objective). ;-) Adjust focus for best beam, etc. It works.

Second, some telescopes work only with mirrors, no glass required. The mirrors gather and focus the light, not just fold it.

I commonly will shine a light at reflective signs (sometimes while driving) and the angle doesn't matter. It lights up brightly no matter what the angle is. It's a real trip to do large overhead signs and blink quickly. Yeah, I know, it'll be some time before I grow up.

There is a new light(s) out now that has the luxeon facing backwards towards the flashlight, so that all of light produced is from being reflected back out by the reflector. I think it's by Pelican and it's called the Reflex technology.

Pelican's method of facing the LED backwards gives the reflector of all the light emitted by the LED. Seems like a sound approach.

Speaking of lighting up road signs. To keep myself awake while driving long distances at night, I'd bring along a small laser pointer and try to "shoot" road signs well ahead. It's amazing how brightly the red laser will show up, even over long distances. Stopped doing this because I figured it would end up with a run in with the law.

Also it seems the led's reflect brighter than incandecents on reflective thing like car liscne plates, signs, etc. I guess because of the more blue side of the light is florecing the white in the signs more... /ubbthreads/images/graemlins/icon3.gif