[Laser] Fresnel lens

[Angel Vilaseca]

Hello

Why not just try both ways?

I tried some time ago and i recall the image at the focal point was
sharper with the lens the *right* way around. unfortunately, i cannot
remember which way it was...

73 

Angel HB9SLV

G0MRF@aol.com wrote:
> 
> 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.
> 
> My instinct says keep the grooves inside the box, then they won't fill up
> with dirt etc. But I'm sure the manufacturer says the focal length is around
> 212mm so the grooves will be exposed to the elements.
> 
> Any thoughts?  Does it matter?
> 
> The 3 groups will be meeting this week. I'll add some pictures of the various
> creations to my web site in about 10 days time.
> 
> 73
> 
> David   G0MRF     www.g0mrf.freeserve.co.uk/laser.htm
> 
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