Old 04-25-2014, 02:35 PM #17
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Default Re: Spot size at distance vs. negative focal length

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Originally Posted by Laser Project View Post
Not to illuminate a target, but for someone at a distance to be able to see it. I'd like to TTL modulate the signal at some point in time and send data across it.
This is what you are going to do. Pretty cool.
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Old 05-15-2014, 04:20 PM #18
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Default Re: Spot size at distance vs. negative focal length

Laser Project, I know it has been a couple of weeks. Have you had any success? Just so you know, you can make your beam expander with either a negative or positive "front" lens with hardly any difference to the overall length since you are designing for such a large expansion ratio. (the difference in length between the two options would be in the ballpark of 1 inch). In response to your initial question though, you do need to know your initial beam diameter. If you want to take a 2mm beam and expand it to 200mm (a little less than 8 inches - since the usable part of the lens is in the center - you seem to have misunderstood this based on an earlier post about the "flat" part in the center of the lens. Avoid the outer 10-20% to minimize aberrations) then you would want a focal length ratio of 100, and your smaller lens would need a focal length of 0.24 inches or about 6 mm (or -6 mm for concave). If your small lens is also a convex type, it would be placed at 24.24 inches in front of the big lens, or if it is a concave type, it would be placed 23.76 inches in front of the big lens. This assumes a relatively parallel or collimated incoming beam (a bit of divergence is fine, but just "focused" to infinity). I would be very interested to see your results, so keep us posted. I am currently unable to do much on the hardware side of things, so must keep myself occupied with design and thought projects until I can break my tools out of storage.
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