Re: increasing fresnel lens light concentration?

John Roentgen wrote:
>> 3. If that doesn't fix it, try getting some big sheets of paper and
covering parts of the lens. You'll probably find that different annular zones of the lens have different focal lengths. You may be able to figure out how to bow the lens enough to bring all the zones to a common focus. If you're really gung-ho, you could drill a hole at the centre, and cut the zones out with a scriber on a string, then arrange them on a frame to put all the foci together.

Not sure what you mean by this approach. I gather from what you're saying that it's possible to tighten the focal point more this way(?). If I drilled the hole and cut the pieces, would I cut them say into six large pieces like parts of a pie? And then how do I move them for a tighter focal point, in the same flat plane or ? Since the lens was acquired cheaply, I am considering this approach if I can understand it more fully.


If the lens has been designed to be a good condenser as opposed to a good image, the focal length may depend on the distance from the lens axis. The same may be true if it was designed for finite conjugates.

Think onion layers, not pieces of pie. You can measure the focal length as a function of radius by covering all but a narrow radial slice, and looking for the position of best focus. If the centre has a focal length half an inch longer than the edge (which would be roughly typical), you can make the foci coincide by bowing the lens outwards in the middle, or by cutting it into concentric circles and spacing them appropriately along the axis.


Cheers,

Phil Hobbs
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