Originally Posted by GlenC
I guess I don't thoroughly understand the process. I haven't had time to do mine yet, when I get my new spectroradiometer, I plan to go there.
My take on this process is to reduce glare from the tube/lens, not cover the image. Striping the outer edge of the Red/Blue lens would essentially be hooding the lens to restrict light output to the image only. Am I wrong in thinking if the striping was to cover part of the image, you affect the projected image, like side blanking?
One thing I have noticed with my Mits is the Red and Blue tubes are not aimed at the center of the screen. Because of this, the image on the tube face is actually at the inner edge of the tube. This results in a large un used area of the tube from the image edge to the outer tube edge. Not good for stray light.
Yes, indeed that's where the confusion lies. Because you're striping the exterior of the lens, you are not in (or really anywhere near) a focus plane. It seems illogical because you would at first assume that by striping the lens, that the active image would be totally obstructed, as would say blanking the image. You are actually applying the tape across the active portion of the image, however because you are not in a focus plane, it only blocks light output generally, not specifically.
For instance, if you were to apply tape to block light on the interior of the lens (which is a focus plane I believe) or on the tubeface itself, clearly you would then essentially "crop" off that part of the image. But on the outside face of the lens you're really far from any focus plane, so that blockage of light is non-specific. You can see this very easily by yourself by just taking one finger and putting it in the light path right in front of a lens (not touching the lens of course, but as close as practicable). You can see that you don't see a giant shadow of your finger on the screen where it is blocking light, but rather just a slight decrease in light output from that tube across a general area of the screen, because you're not in a focus plane. Try moving your finger around, and you'll see the general effect move to different parts of the screen accordingly, but again in a very generalized way. You won't actually be cropping anything from the image, even though you are blocking part of the light that is part of the *active* image. Now, if you use TOO much material to block TOO much light, such as your whole hand across a significant portion of the lens, you will actually start to lose part of the image, but notice how the edge of the shadow boundary is very large at the screen, and totally out of focus (you can't make any shadow puppets!), that's because you're not in a focus plane. If you went right up to the screen, then you could make shadow puppets (or if you had miniature hands and could somehow stick them BEHIND the lens at the tubeface!) because you would be very near/at a focus plane.
It's not the stray light that is being affected, but you are actually impeding light from the ACTIVE image from reaching the screen. It does seem counterintuitive because you look at a large piece of tape across part of the lens, and you'd think you'd have that part of the image cropped off, but that's not the case.
The pictures linked on the Keohi site illustrate this, and indeed some will stripe farther in on the lens, but leave some space at the edge of the lens, such that it actually crosses the lens (hard to explain) rather than just to come in on one side, while still remaining heavily towards one side of the lens. Okay, that last sentence sounds indecipherable, but it's late and I'm tired and I've been reading Hegel so my brain doesn't work! Maybe it will make more sense in the morning. Or maybe not!
As for the unused phosphor area, that's a different question, and that won't affect the grayscale uniformity (at least not really at all significantly), but one thing that can be done for that (on AC units) is to mask the tubeface. Guy Kuo described this once, it might be buried somewhere in the CRT forum archives, I don't see it on Keohi unfortunately. But basically he just used some black material and cut it out in the shape of the active-image trapezoid to block off all the unused phosphor. I don't know how significant an improvement this would be, I've never bothered to do this myself, but this would affect ANSI performance, and not have any impact on grayscale uniformity.
Hope that helps out!