Re: [photo-3d] Projector Brightness

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

----- Original Message -----
From: <donaldparks@xxxxxxxxxx>
To: <photo-3d@xxxxxxxxxxxxxxx>
Sent: Thursday, April 12, 2001 11:36 PM
Subject: [photo-3d] Projector Brightness


> I adjusted the projector to screen distances to
> produce comparable size realist size slide images and took light meter
readings at
> the screen.

Very good procedure.  Most people speculate, judging brightness by what they
recall from viewing, and this all too often results in erroneous
impressions.  There are many variables, such as viewing angle, particular
reflectance characteristics of a specific screen, human recollection, etc.
A light meter is the best way.

You sound like a very meticulous person, and probably have been very careful
to insure the accuracy of your measurements.  As such, the following
comments are directed more towards others who may want to measure projector
brightness, and do not want to inadvertantly introduce errors into their
measurements.

When comparing two or more projectors, you want to make sure that all
conditions are equal.  With the very directional characteristics of most
nondepolarizing screens, an incident light reading is preferable to a
reflected reading off of the screen, which adds screen characteristics to
projector performance.  Only a slight angle difference can make a
significant difference in the reading you get off of a screen.

The meter should be secured in a fixed position, such as taping it to a
tabletop.  You are quite correct that you want to match image size, not
projector distance when you compare.  Be aware, however that the same
projector fitted with different focal length lenses may yield different
brightness levels for the same image size, due to differences in the
efficiencies and apertures of the lenses.

Lamps can vary, some types more than others.  Halogen type lamps usually
emit more than 90% of the light just before burnout that they did when new.
Non-halogen conventional tungsten filament lamps, however, usually emit less
than 50% of the light just before burnout than what they did when new.  The
best procedure if testing and comparing for brightness is to relamp with new
lamps just before the test.

Polarizers vary in efficiency, and also change with age.  For complete
objectivity, you might want to measure each projector's output both with and
without its polarizers installed, to determine how much of a role a specific
projector's polarizers play in its brightness.

> 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.

Wattage is only one of several factors that determine how much brightness
you can get out of a bulb when used for projection applications.  I recall a
small "pocket" filmstrip projector that had only a 25 watt bulb that was
brighter than a similar projector fitted with a 100 watt bulb.  The
difference in this case, was that the 25 watt version had its own reflector
built into the bulb, which was designed for maximum efficiency, while the
100 watt bulb had to rely on a separate "one size fits all" external
reflector, which was made with a "compromise" radius for use with several
different lamp/filament configuration combinations.

Another factor is the physical size of the "apertures" or "restrictive
holes" through which the light must go.  Higher wattage lamps usually have
filaments that are larger in area than lower wattage ones.  There is a
maximum size for every projector design through which the light can travel.
This is determined by such things as reflector diameter and radius, the film
gate aperture (size of the opening for the filmstrip or slide), lens
diameter, etc.  If a projector lamp filament (or the reflected image of it)
is larger than the smallest limiting aperture in the system, you will get no
more light through than this limit, no matter how high the wattage of the
lamp.

JR




 

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