[Laser] Large TV Projection Lens

Clint Turner turner at ussc.com
Tue Feb 1 21:36:16 EST 2011


I have one of those large projection lenses - let me describe it:

It's approximately 100cm x 80cm with a focal length of about 70cm and 
came rolled up, apparently being a rather thick vinyl.  (I'm not 
familiar with any acrylics or polycarbonates that can be rolled up so 
I'm assuming that it's vinyl.)

The one that I have does, in fact, appear to be in infinite-conjugate 
lens - equivalent to a Plano-Convex and designed to be oriented with the 
grooves outwards (toward infinity.)

I've not done more than play with it a bit for several reasons:

- I need to build a proper, latticed frame to hold the lens.  To work 
properly - and assuming that it was made accurately for infinite focus 
(and that's a BIG if!) - it needs to be kept planar to within millimeter 
flatness.  This simply cannot be done by holding it and the frame - to 
work best - should really be latticed, somewhat akin to a 
"divided-light" window pane.

- It is a sail.  If there is any wind where it is used at all, it will 
move/flutter and when you consider that the beamwidth will be a fraction 
of a degree, the need for high stability is understandable.  If there is 
more than "light air" (very gentle breeze) then its use outside becomes 
complicated.  Since our work has been usually done on/near the tops of 
mountains, we have to be concerned with its "wind load."

- It's awkward.  Since I don't have a vehicle that can carry such a 
large device while protecting it from damage (e.g. put in a case) I'd 
have to assemble the frame and install the lens on-site.

  -These lenses are typically of very fine pitch (that its, lots of 
grooves per inch) and as such, they have higher scattering losses - less 
of a problem with transmit, but more of a problem with off-axis light 
source when used for receive.  The one that I have is of *VERY* fine 
pitch:  I'd need a low-power microscope to determine its pitch to any 
accuracy.

- It's extremely fragile.  In its intended application (projection TV) 
it would have to be handled with linen gloves to prevent cosmetic 
damage, but as a collimator or collector of light, minor scratches 
(which will be inevitable) won't be a major problem.

Unfortunately, I've not had the opportunity to test such a lens to 
ascertain its quality as I have with other lenses on this web page:

http://modulatedlight.org/optical_comms/fresnel_lens_comparison.html



Since I haven't built a frame for mine, I also haven't tested its 
characteristics such as precise focal length, blur circle size, etc. but 
it's on my (rather long) list of things to do.  From my initial tests, 
it looks "Ok" in terms of image quality, but I can't tell more than that...

How well do I think that it would work?  'Dunno - probably fine, but it 
will be a bit awkward...

73,

Clint
KA7OEI



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