[mythtv-users] fresnel lenses.. any experiences?

[match at ece.utah.edu]

LOL!! Yes I've viewd several of these in various forms, but not from this particular 
vender.

Do you guys realize that this is how Sam Runco got started? In (I think it was 1967) he 
placed an advert in Popular Mechanics magazine to sell kits for U.S. $19.95 very similar 
to what this guy is peddling. Basically used a Sony 13" TV in a box with a fresnel lens. 
He couldn't make up the kits fast enough!

I'll cut to the chase... it's a fun toy, but it's a toy. You can get a viewable picture in a 
completely darkened room on a highly reflective screen up to about 45 inches diagonal 
but it's still very dim (referenced to a 13" Sony of yore... YMMV.) Even at only 45 inches 
you have dots the size of the buttons on your shirt, so if that's what fires your rocket...

There was a company here in my town in the seventies that sold a bunch of these. For 
U.S. $150 you got a 50" silver parabolic screen and the box, but you had to provide your 
own Sony 13" TV. You had to push the little Sony so hard to get any brightness at all 
that the tube would last maybe 6 months before showing signs of serious  blooming and 
phosphor burn. But In the 70's, it was a kick while it lasted, as long as you could sit in 
that one seat in the house that was in the direct line of fire for the parabolic screen!

Read this guys eBay ad carefully. Those wonderful home theatre pictures he posted 
cannot possibly be images created by the device he's selling, and to his credit he 
admits this fact. And his claims of 150" displays are not realistic (In my opinion.) At least 
not an image you'd actually want to be in front of. Think of it like this... that fresnel lens 
can only focus the light that it can catch, and a little bitty (probably the size of a sheet of 
writing paper, I haven't seen this particular lens) is not going to capture much of the light 
from a 25" set. So, with a big TV its efficiency is going to be very low, because most of 
the light from the TV is wasted, going every which way, but not directly toward the lens. 
Now with a little 13" TV, there is less total lumens, but the lens is able to capture more 
of it, so the efficiency is higher. Either way, 45-50 inches in a totally darkened room. Tit 
for tat. On the other hand, put a 25" fresnel in front of a 25" TV and I'll believe the 150" 
claims, still in a seriously darkened room.

Still, 7 quid for two lenses and the plans... if you want to play, it sounds like an OK deal. 
Just don't expect to have a home theatre quality PJ when you're finished. Then again, 
maybe the technology has advanced some in the last 30 years... Nah, still can't fool the 
laws of physics...

On 24 May 2004 at 1:54, Craig Tinson wrote:

> I got an advert about this the other day:
> 
> http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=3089170698&category=11073&sspagename=STRK%3AMEBWA%3AIT&rd=1
> 
> anyone given it a try? for the 50quid or so it'd take to build.. was 
> thinking about giving it a shot (the home theater pic looks cool too! lol)
> 
> just thought someone might have come across this or similar..
> 
> cheers
> 
> Craig