[Laser] Fresnel lens

[Karel Kulhavy]

> Hello Group.
> 
> Much enthused by 3 groups of people building laser equipment in southern 
> England, I thought I would finally build a frame for my 284 x 284mm Fresnel 
> lens.
> 
> Then last night a club member said. "which way round does the lens go?" and I 
> had to say I didn't know.
> 
> I've done a quick test this morning and found that the focal length appears 
> different depending on which way round the lens is.
> 
> Etched grooves towards the detector  Focal length = 230mm
> Etched grooves facing away from the detector Focal length = 212mm.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

This is the right position.

-- 
Karel 'Clock' Kulhavy
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