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Old 04-17-2013, 08:39 AM #1
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Default wondering about lenses and 800-850nm...

i am no photography buff, but, my hobby project "night vision rifle scope" has me reading on stuff...

I know from photography sites and similar, that "ir" focuses differently than visible light. That actually for photography, the different colors of visible each focus slightly differently, and that some lenses cause "chromatic" abberations.

multiple lenses and special lenses/coatings correct for this...

a demonstration with a security cam, running in low ambient light, with a fairly strong IR light video... showed how you can have to re-focus some "zoom" lenses, as how it focuses ambient (visible) light, is a different "focus" from the strong IR light source.

basically, i gathered that different wvelengths of light, bend (refract) a different percentage from their original path, going thru the same lens. It makes sense that IR focuses behind/in front of the film/sensor... when i think of it as another "color".

in a PRACTICAL sense, this seems to be just a focus adjustment... i was wondering if there are any bizarre focusing things a NEWB like me would be expected to know about IR light coming from a laser diode...

i mean, if i can focus VISIBLE light a certain way thru a certain lens... are we just talking a slight focal length change to do the same thing with IR source of light? I'm not talking collimated/focused laser light... i'm thinking more along the lines of using it more like a flashlight for my application.

i realize laser output is different from incandescent and ambient light... but, i have been using a LED EMITTER flashlight to rough out the illuminator, and that is a fairly narrow bandwidth of output... am i stupid in not knowing that, for instance, that 800-850nm laser illumination needs special lenses and coatings that the almighty grinds and polishes and coats? or, can i just count on a few mm's of focal point difference?

I mean, remember i am not trying to focus and collimate the output... if 5 or 10 percent f the light is refracted off, or shifts wavelength or something, or reflects, it doesnt really concern me.

I just wanna make sure a "regular" optical lens isnt going to be like a BRICK WALL or anything...


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Old 04-17-2013, 06:28 PM #2
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Default Re: wondering about lenses and 800-850nm...

It's even less than a few mm focal length difference. The difference between focusing red and IR is literally less than the difference between focusing red and violet. The difference is minimal, and you'll only see it when both are present like in photography. Treat it like you would any other laser diode.
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Old 04-17-2013, 09:56 PM #3
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Default Re: wondering about lenses and 800-850nm...

So, my nuts and bolts fiddling with visible light from a p60 LED flashlight, and focusing the beam in a PVC focusing tube is valid, for lack of a better word? That's comforting. I was laboring under the layman's misconception that laser light was somehow special or different.

as near as i have it figured out, the production of the light is different. Yet, once the light is already produced, its just stronger. In my mind, the only measurable difference between standing somewhere and shining a small "spot" of IR light on a given area, and illuminating the same spot from a long distance, would be in how the IR light was produced.

Since i dont care about coherency nearly as much as most applications do, I can just treat the raw diode illumination output, rather like a expensive light bulb or LED emitter. The output just has a goofy beam shape, to speak of it like a flashaholic.
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