Re: image brightness

[P3D Glen Murray <grmurray@xxxxxxxxxxxxxx>]

Greg wrote (about ghosting)

>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.

I don't see any difference between ghosts from polarizer extinction and ones
from de-polarization.  The end result is the same, some of the wrong image
is getting through.  The whole slide would suffer the same amount in either
case, and in both cases the ghosts would be more evident in areas where they
were superimposed on a darker area of the other image.

>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.  
-----snip-----
>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.

I didn't mean the contrast in each image, I was talking about the contrast
range between the "transmitted" and the "blocked" images.  For that to
change, the extinction of the polarizers would have to change, or the amount
the light being depolarized, or both.  Does the extinction % change with
brighter and brighter light, or does the % remain constant at all light levels?
I see, though, that if the ghost is increasing intensity at the rate of the
bright areas, and the darker areas are increasing intensity at a slower
rate, then it would get worse.  So, there should be some optimum light level
based upon the
1. extinction of the polarizers   
2. efficiency of the screen   
3. ambient light on the screen   
4. greatest difference in contrast in portions of images that overlap
5. retinal contrast retention range    :)  okay, okay   sorry
at which the worst ghost would be just below the threshold at which it could
be viewed.  Right.  My brain hurts.  

      Hey, good timing for all this stuff about ghosts, too!!!!!   

Glen
............grmurray@xxxxxxxxxxxxxxxxxxxxxxxxxx


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