CurtPalme.com Home Theater Forum

JustGreg · Posted: Wed Jul 25, 2007 6:12 pm Post subject: First lens distance to tube face

Hey Barry...I was just reading about 144/145's fitted to Barco 808's in another thread:

http://www.curtpalme.com/forum/viewtopic.php?p=66248#66248

and got to wondering how close the lens should be to the tube face. The stock flapping distance is around .075". Not having taken on installing mine yet (damn landscaping!) I don't have any good measurements as to where the face of the first lens will end up in relation to the tube face after mounting. I realize the closer the better.
(I probably worded this all wrong but hopefully you know what I'm asking)

Thanks!

Greg

**Joust** · Posted: Wed Jul 25, 2007 6:22 pm Post subject:

the short easy answer is, Close enough to focus. Mr. Green

All Smartassness aside. I'll measure tonight if i get a chance.
I'm not entirely sure. I did All my measuring of the stock HD8 and made teh adapters and spacers to emulate those. Testing proved it all worked.
It will be good to have some measurements. We probably will end up with a better idea of min and max throw and screen size.

might have to dig out my bench projector now. My real one is mounted on the ceiling and I'm not about to move it.

JustGreg · Posted: Wed Jul 25, 2007 6:57 pm Post subject:

**Joust** · Posted: Wed Jul 25, 2007 7:39 pm Post subject:

hellifiknow.
I'll measure where it is now. both throw and width and raster width on the tube face.

JustGreg · Posted: Wed Jul 25, 2007 10:09 pm Post subject:

zGman · Joined: 22 May 2006 Posts: 598

Hi,

Measuring for a HD-8 is counterproductive...

Measurement of a NEC housing made for the 144 lens
showed that they would allow the lens to almost touch
the tube face (less than 0.010 inches (~ 0.2mm) clearance)

The larger the screen, the closer the focus point to the tube face.

HD 145's will focus a few mm further out for a given screen size.

HD-144 have a considerably smaller input opening than the 145's,
consequently they are best suited for smaller screens (consider
what happens to the entrance angle as the distance to the tube
face decreases - less light can be collected and corners will be dim)

Cheers,
Galen

JustGreg · Posted: Tue Jul 31, 2007 4:32 pm Post subject: Re: HD-144 lens

zGman · Joined: 22 May 2006 Posts: 598

Hi Greg,

I am not quite certain if my information is really answering your
question, and this issue seems to come up pretty often. I
will try a little more and maybe make some sense. (?)

For a given throw distance, there is one point in the adjustment
range where the lens will focus. The focus adjustment moves
the entire lens body relative to the tube face. As the throw distance
increases, the screen size increases, and the lens will focus at a point
closer to the tube face. Other than setting the throw distance,
you have no control over this focus point. If the lens runs out
of travel before reaching focus, then the throw is too long.
Whether or not the adaptor or housing can be modded
is another story, depending on the range of adjustment.

The construction of the lens adaptors basically sets the range of travel
for the main(center) focus. If the lens cannot travel close enough to
the tube face it will not focus. This should not be a problem for HD-145
lenses. However, for HD-144's, the mounting of the lens in the adaptor
is more critical, but only for a for a larger screen (over 87" wide)

However, the size of your raster will determine the maximum
screen width for a given throw distance. For any lens, there is a
drop-off in output as you move away from optical centerline -
this can be as much as 60% less light at the corners for 16:9.
and even less for a 4:3 set-up. So, your raster size is a trade off
on any CRT, with any lens. Good reasons not to go too small
of course, and vice versa.....

The issue of the reduced input opening on the 144's is separate
from focusing - but it does relate to the use of larger screens.
Again, because the lens needs to be closer to the tube at longer
throw distance. If the lens is closer to the tube the entrance angle
is steeper. therefore of the cone of light emitted by a point on
the phosphor, beyond the radius of the lens opening, a smaller
percentage can make it into the lens.

Personally I would not recommend a 144 beyond 87", but have been
pleased with 145's at 96" wide. That being said - no doubt there
are people happy with their 144's at 100" wide...hohoho!!!!

Hope that helps,
Galen