[photo-3d] Projector Brightness

[<donaldparks@xxxxxxxxxx>]

     I like to view stereo slides projected on a large screen where marginal
projector brightness can be a significant problem.  For years I owned a Compco 500
projector and about 5 years ago I acquired a Realist Model 81 and a TDC 116 from my
Dad when he and my Mom decided to move into a retirement center.  This last year I
have started using and comparing these projectors a lot more.  All three have the
same wattage projection bulbs, 500W, but there is a huge difference in their
brightness.

     More people use the TDC Projectors than the other two so I used my TDC 116 as
the reference for a comparison.  I adjusted the projector to screen distances to
produce comparable size realist size slide images and took light meter readings at
the screen.  The Compco was very dim (about -0.7 stop) in comparison to the TDC
116.  The Realist Model 81 was much brighter (about +0.7 stop) in comparison to the
TDC.  My Dad had both 500 W and 750 W bulbs to fit the TDC 116 so I did a
comparison using the higher wattage bulbs.  I was very surprised that there was
essentially no increase in brightness when using the 750 W bulbs.

    I decided to check the voltage supplied by the outlet in my living room.  I
knew from experience that this can vary between 110 and 120 volts depending on the
other loads in the neighborhood and in your house.  Also the voltage can be drawn
down significantly when you plug in a high wattage appliance, like 1000 watts worth
of projector.  My outlet with no load measured 114 volts and with the projector
turned on the voltage dropped to 107 volts.  This lowering of voltage will cause a
lighting appliance to draw less current and therefore produce less light.

    I borrowed a meter from the university where I teach that can measure the true
voltage, current and wattage drawn by a particular appliance from a particular
electrical outlet.  With the lowered (107 V) voltage my TDC 116 was only drawing
960 watts and the fan was about 80 of the 960.  So both 500 watt projection bulbs
(total 1000 watts) were only drawing about 880 watts total.  This reduction in
voltage makes a big difference in the potential brightness of the projector.

    To compensate for the lowered voltage I used a variable transformer.  This is a
standard laboratory device that costs about $200.  It plugs into the standard
electrical outlet and the projector plugs into its outlet.  If the input were 120
volts, the output from this transformer can be varied between zero and 140 volts.
I used the meter to monitor the output voltage to the projector and adjusted it
upward to the full rated 120 volts.  This increased the brightness of my projector
significantly (about +0.5 to 0.6 stops).  This would be a reasonable way to
increase the brightness of an older stereo projection.  The projection bulbs are
rated for 120 V.  Adjusting the voltage to that value should not shorten the rated
life of the bulbs although it will shorten the life compared to operating the bulbs
at a lower voltage.  It would be possible to increase the brightness even more by
increasing the voltage above 120 if you were willing to accept shorter bulb life.

     This type of information has probably been posted before I joined the list.  I
was excited by the results and thought I'd summarize my experiments for your
benefit.  Since George got his RBT projector I've been wondering how it's
brightness compares to an old TDC 116.  George, how much brighter is you RBT
projector?

Don Parks




 

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