Re: [photo-3d] Halogen viewer bulbs

["John A. Rupkalvis" <stereoscope@xxxxxxxxxxxxx>]

----- Original Message -----
From: "John Goodman" <jgood@xxxxxxxx>
To: <photo-3d@xxxxxxxxxxxxxxx>
Sent: Sunday, March 11, 2001 5:26 PM
Subject: [photo-3d] Halogen viewer bulbs


> Wouldn't the bulb with the manufacturer's
> lower voltage rating provide a higher color temperature
> illumination and a shorter lifetime at the voltage delivered by
> two (alkaline) batteries? Is the difference really negligible?

Absolutely correct.  Also, the lower voltage lamp on the same batteries will
give greater light output (more lumens).  All bulbs with filaments operate
over a range of voltage.  The length of that range is from the point at
which we can visibly see the filament begin to glow to the point at which
the heat is so high that the filament melts.  At that point the filament,
which is usually coiled like a spring, pulls apart, and we say that the bulb
is burned out.  If it pulls apart slow enough, the electricity will arc
across the separation until it separates far enough so that it can no longer
jump the gap.  That makes it behave similar to a one-time flashbulb.  You
see a bright flash, and that is it.  Maybe.

A trick that old-time projectionists used to do with small projectors when
the bulb burned out (and was still too hot to grab to replace, or maybe they
didn't have a spare), was to just turn the projector upside down and gently
shake it with the power still turned on.  If they were lucky, the filament
would drop back on to the support stems, and arc once again, welding itself
back to the posts.  It would then work for a while longer, hopefully long
enough to finish the show.   I have successfully done this more than once
with both my TDC Stereo Vivid (two lamps) and my Stereomatic 500 (one lamp).

Anyway, back to the viewer.  The optimum voltage (usually, but not always
the voltage stated on the bulb or the box it came in) is a compromise of the
above three parameters that the manufacturer feels would best meet the
application.  Thus you may find different voltages correctly stated for the
same lamp, depending upon which is most important to the intended usage:
brightness, color temperature, and lamp life.  So-called long-life bulbs are
usually different from ordinary lamps only in the recommended voltage.  If
you operate a higher voltage (labeled) lamp on a fixed voltage, it is
usually going to last longer than a lower voltage lamp on the same voltage.

Both the optimum current draw and the optimum voltage are determined by the
filament size, the filament alloy (usually tungsten + one or two other
metals alloyed for the particular characteristics required of the design),
and a few other factors, such as the resistance across the total electrical
path (including posts, base, internal connections, and the operating
temperature.  Most of the energy that does not go into light, is wasted as
heat.  How much heat is determined not only by the size of the filament
(think power factor, or wattage), but by the bulb design itself.  Usually
the larger the envelope, the less heat.  Also, larger bases tend to "sink"
more heat.  Very few bulbs are total vacuum anymore; most have a gas inside,
the conductivity of which also affects the temperature.

The total design, and recommended operating characteristics, determine the
best voltage and the current draw (at that voltage) for which you should
operate a particular lamp for your application.  Usually the manufacturer of
a commercial viewer will specify their recommendation for the best lamp for
what they think your intended use will be.  This recommendation is based
upon those lamps widely available at the time of manufacture.   If they are
still in business when newer lamp designs come on the market, they may
update their recommendations (but, don't count on it).

If you plan on "upgrading" a viewer with a newer lamp, such as replacing a
conventional tungsten filament lamp with a halogen cycle type, physical size
is not the only consideration.  Heat may be the most important.  Halogen
lamps usually have smaller envelopes (the quartz cover), and therefore run
hotter than equivalent non-halogen of the same wattage (higher temperatures
are required for the halogen cycle to operate)  A safety factor is usually
built in.  Viewing time also may affect what you can get away with (heat
takes time to build up).

The first thing that you should do is measure the temperature with the
existing bulb.  An ordinary household thermometer can be used.  Find a
position inside the viewer, (but not touching the bulb) for the thermometer
that is within its range (so that it reads about half way between room
temperature and the maximum that it is calibrated for).  See how long it
takes to reach its maximum with the cover off of the viewer (its heat
balance point, after which it will no longer get any hotter).

After letting it cool down, install the proposed new lamp and repeat the
above.  If the temperature is more than 10% hotter after the same amount of
"on" time, you may exceed the safety factor.  Either you may redesign the
viewer (added heat sinks, more ventilation, more clearance around, and
especially above, the bulb, etc.), or you should seriously consider not
making the change.  A very reliable indication that the conversion is not a
good idea,  is when you smell smoke, and observe the plastic case sagging
and melting.  I prefer the thermometer test myself.

JR




 

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