[Laser] Large TV Projection Lens

[bernies at netaxs.com]

Clint, where did you obtain this lens?  I purchased one about a decade  
ago that sounds much like what you have from a mail-order electronics  
surplus company.  Did it arrive rolled up with a thin sheet of foam to  
prevent scratching?

I have also found this lens to be unwieldy, but made a frame from PVC  
pipe and elbows and small bungee cord loops with clamps attached.  The  
lens is "pulled" by the elastic-corded clamps to be centered withing  
its frame, but it's still an unwieldy sail.  But it can melt asphalt  
on a sunny day.

-Ed



Quoting Clint Turner <turner at ussc.com>:

> 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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