T3D Re: (Mostly OT) Projector lamp substitutions/modifcations]

[Tom Hubin <thubin@xxxxxxxxx>]

Rob wrote:
> 
>      I am in search of some information in the seemingly slight
> differences in the vast number of different types of projector lamps.
> Usually, each manufacturer would assign a unique three-letter code to
> its own bulbs, to force consumers to use their bulbs for replacement,
> and any information on substitution was kept in the dark.  The situation
> is completely different from that of vacuum tubes used in radios and TV
> sets, where a tube substitution manual was an important tool in every
> serviceman's tool box.
>      From what I understand, it is usually (always?) OK to substitute a
> bulb of the same filament location, base wiring and base orientation if
> the substitution is equal or "downward"  (e.g. - a 500 watt CZX is fine
> for the TDC 716 - designed for the 750 watt DDB, a 400 watt DAT is not
> only compatible for a 500 watt DAK - 400 watt DAT bulbs are now marked
> "DAT/DAK").

<snip>

One other feature besides filament location needs to be matched. The
filament should not be larger than the original. No harm if it is larger
but light is likely to be lost. The filament is usually imaged by the
condenser optics into the projection lens. Usually the image of the
filament just fills the projection lens aperture. If you substitute a
larger filament then the image of the filament will overfill the
projection lens and light will be lost.

This is the case with replacing the TDC716's 750watt DDB lamp with a
quartz halogen 750watt BTP lamp. The DDB lamp has a 10mm square
filament. The BTP lamp has a 10mm x 14mm filament. The result is dimmer
than if you used the 500watt CZX lamp with the 8mm square filament.

By the way, filament dimensions are seldom listed along with the lamp
characteristics such as power and base and color temperature and
filament centering. No idea why this is ignored but I usually have to
call an applications engineer to get that data or get a stock clerk to
measure one for me. Having a non engineer measure for you is usually
erroneous. I prefer to find one in stock locally and measure it myself.
For that measurement it does not even have to be a working lamp.

For more detail on lamp substitution, light losses, and heat problems
with TDC modifications see 
http://www.clark.net/pub/thubin/aosd/photo3d/ir_study.txt .

If you want to see the image of your filament at the projector lens,
unplug the projector and illuminate the filament with a flashlight.
Shine the light through the filament from the rear toward the projector
lens. Put a piece of white paper where the projector lens would be and
you will see the image of the filament as it enters the projection lens.

You cannot just turn on the projector because the bright image at the
projection lens is practically blinding.

Another method is to add an array of small apertures within the
condenser optics somewhere. This way only a small amount of light will
make it through the condenser optics. Make a nonflammable plate of tin
from a disposable pie plate or fold several layers of tinfoil. Cut the
same shape as any one of the condenser lenses. Drill or poke a dozen
small (1mm) holes randomly spaced. Place this over the IR absorbing
glass or any lens in the condenser optics.  Turn on the projector and
use a piece of white paper where the light would enter the projectoion
lens. You will see the image of the filament there.
 
Tom Hubin
thubin@xxxxxxxxx
AO Systems Design