Re: preliminary summary on ghosting

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

John Bercovitz corrects:

>Now that's funny; I thought the purpose of the ribs was to _spread_
>the light so it didn't primarily go back to the projector.

Indeed, I had it backward.  I stand corrected.  From Da-Lite:

"Gain from a front projection screen is not governed by the density of
 the diffusion material but by the degree to which its reflectivity is
 allowed to be directional. The more specular or mirrorlike a front
 screen becomes, the more its gain will increase and its viewing angle
 will shrink."

So the fact that the screen is aluminized is responsible for the higher
on-axis brightness vs. a matte white screen (specular vs. diffuse
reflection).

"The second way that profiled screens are different is that their designs
 can permit them to disperse light asymmetrically.  To see how this works,
 let's first consider a screen which Da-Lite makes called Super
 Wonder-Lite. This is a soft, aluminized front projection screen which has
 a series of straight, parallel ribs embossed onto its surface. 

 The first thing the ribs do is render portions of the surface not flat.
 And if these raised ridges are oriented vertically, they will present to
 the projector a series of beveled slopes alternating with an intervening
 series of flat planes. Light rays which fall upon the plane portions of
 the surface will be reflected according to the law of specular reflection
 which states that the angle of incidence equals the angle of reflection.
 Thus light arriving at a 15o angle from the left will bounce off the
 screen at "an equal and opposite" angle of 15o to the right. 

 Rays falling onto the ridges will also obey the specular reflection law
 but, because their incidence angle to the screen is altered when the area
 they strike is sloped, their reflectance angle will be commensurately
 shifted. If the face of a rib rises from the surface of the screen at
 an angle of 20o , then the same 15o light ray we considered above will
 have an incidence angle of 35o (15+20) and thus will bounce off the
 screen at a 35o angle to its surface. Since the Super Wonder-Lite
 screen has a pitch of 42 ribs/inch, about half of its active surface
 area is sloped and the other half flat. In a sense, therefore, it is
 two screens in one: each has a high gain (due to its metallic coating
 which is much more mirror-like than the standard matte white diffuser)
 and a resultantly narrow viewing angle. But because one narrow viewing
 angle is aimed at the center of the audience field (this is all the flat
 parts of the screen) and the other narrow viewing angle is aimed at the
 edge of the audience field (all the sloped portions), the combination of
 the two produces a front projection screen with a gain of 2.5 and a
 horizontal half-angle of approximately 35o"

So, a flat (non-lenticular) aluminized screen  will have an even narrower
working angle, but be brighter on-axis than the lenticular screen.

	-Greg W.


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