[Laser] Fresnel lens

[laser@mailman.qth.net]

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


--- StripMime Report -- processed MIME parts ---
multipart/alternative
  text/plain (text body -- kept)
  text/html
The reason this message is shown is because the post was in HTML
or had an attachment. Attachments are not allowed.
Please post in Plain-Text only.---