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ArcticMyst Security by Avery

Spot size at distance vs. negative focal length






Joined
May 3, 2014
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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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