Re: image brightness

[P3D Gregory J. Wageman <gjw@xxxxxxxxxx>]

Glen Murry writes:

>   All this stuff about brighter projector bulbs increasing ghosting doesn't
>make any sense to me.  The ghosting is a product of the inability of the
>polarizers to work 100%, or the reflecting surface's inability to leave that
>polarization alone, right?

>From my (limited) experience, it's mostly a function of the polarizers.
The ability of the polarizer to block light of undesired polarization is
called 'extinction'.  Different polarizers have different ratings.   Since
ghosting is only a problem with the brightest areas of the scene when
immediately adjacent to dark areas, it follows that it's an extinction
problem (or lack of it).  If de-polarization were the problem, the whole
slide would suffer, not just the brighest areas.

>So, for any given screen/polarizer pair-up,
>you've got a certain contrast range between the images you're supposed to
>see, and the ghost images.  That contrast range remains constant with an
>increase or decrease in brightness, and when your eyes adjust to the bright
>images, the ghosts should look the same.

I question this assumption.  You're assuming that the darkest areas of
a slide increase in brightness proportionally with the brightest areas
when you switch to a brighter lamp.  From my personal observations, this
isn't true.  If we assume that the blackest (densest) parts of a slide
are opaque, then no matter how bright you make the light, black will
remain black (of course, there are other effects like radiosity that
I'm ignoring, and I doubt that a black slide is truly 100% opaque).
Conversely, as you dim out the lamp the darkest areas don't increase
in darkness, but the brighter areas do get dimmer, so clearly contrast
isn't a constant.

>Brighter bulbs just keep the
>projections from being too dark, which, after 5 1/2 stops (2 1/2 per
>polarizer, 1/2 for the screen) of light loss, would tend to be that way.

The best polarizers I've found transmit only 40%, and since you need
two to tango, you're starting off with only 16% of the original light
(40% of 40%).  The Da-Lite silver lenticular screen material has some
*gain*, not loss, however.  You can visit their Web site and read all
about this material.

        -Greg


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