Re: Dissolves for 3-D projectors (fwd)

[Yiing Lin <ylin@xxxxxxxxxxxxxx>]

---------- Forwarded message ----------
Date: Mon, 27 Nov 95 19:44:12 PST
From: Michael Kersenbrock <michaelk@xxxxxxxxxxx>
To: photo-3d@xxxxxxxxxxxxxxx
Subject: Re: Dissolves for 3-D projectors

> >The fader just needs to be a "thyristor" (actually, a triac or related device)
> >with a control circuit that has a decay shape that takes into account the 
> >rapid brightness drop-off of a "nominal" projection lamp vs. effective RMS 
> >voltage.
> 
> It may also need to take into account the hysteresis of incandescent bulbs.
> At least with the wall switch dimmers, the brightness of the bulb goes like
> this:
> 
>                        *
>                      *
>                    *
>                  *
>               * *
>    (OFF)   *    *
>       *         * (ON)
> ****************

It's been at least 15~20 years since I designed something with triacs (etc)
but as I recall, that hysteresis was due to "cheap circuitry", not
the bulb.  Like not using a Diac in a triac's control lead or something
like that (don't remember the details would have to look it up).  There
also were quadracs with the diac function built-in, but that was a long timeago too,
but I'm sure that light dimmers for the house have every penny squeezed out.
Something I'd not be worried about for my own kitchen table one-of designs.  :-)
I'd first look up solid-state relay variations for a good one with little
of my time/labor needed to use and mount/heat-sink it.  I'm willing to pay more
to save my time and labor when I'm only doing one (or just a few).  If it were
for TDC to put in their design... I'm sure there would be forces asking to put the
hysteresis back in to cut the cost a whopping ten cents.  :-)

> >The projectors and their bulbs would probably need to have a basic match like
> >the two halves of a single projector -- at least to the extent of general 
> >brightness and bulb type to keep bulb color and brightness matched when 
> >switching from one to another without color bias changes.  
> 
> The color balance will change at different brightness levels.

Yes, that's what I meant.  We probably couldn't dim the bright ones to match the dim
ones (due to bulb age, bulb type, or because one projector type has darker polarizers
than the other) for that reason.

We'd want "naturally" matched projectors.  I don't think a color shift *during* a
fade-out would be too important unless it was really really slow for some reason.
But then we could "feature it" if we can't "fix it".  Sunset simulation as a bonus!!!!

> 
> Do commercial multi-projector fader systems turn the bulbs on and off?
> I'd think all that power cycling would reduce bulb life - maybe a shutter
> mechanism (before a diffuser) would work better.

Might -- however about twenty years ago I did a design that included control of
a projection lamp (don't ask... you wouldn't belive what it was doing) and it was cycled 
on a test jig to test things out.  It was claimed by an old time engineer then 
(I was a new engineer) that cycling, at least at the one minute-on/off sort of 
rate would not reduce the bulb life, and that the "hours" rating
would be maintained.  Testing verified his claim.  In the list's new dissolver project,
if a triac sort of arrangement is used, the lamp is softly turned off/on w/o any big
surges at the transitions, so anything surge induced there shouldn't hurt -- just whether
the thermal cycling will affect the filament structure.  I don't know the answer to
that.  Probably depends on the particular lamp and manufacturer too.  Anybody know or want 
to run some tests?  We were maintaining the fan motor, so the envelopes should be in
good shape. 

I wonder if there are any hidden gotcha's like maybe some projectors having
thermal cycle problems on their polarizers?  Mine has the polarizers toward-the-screen
from the stereo-slide, but I understand some have it toward the lamps.

Design bugs are *so* sneaky little devils!  :-)

> John R

Mike K.


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