I brought up this topic before, but nobody's talked about it lately. Basically, using a fresnel is the one thing preventing me from getting a super crisp image. When projected, fresnels display lots and lots of "rings". This amplifies the screen door effect a lot. People may not even notice them because they mish w/ the screen door, giving it an effect that looks like a bunch of tiny:
/\
\/
shaped pixels. In effect, it lowers your resolution, thus making it pretty much pointless to use anything higher than XGA.
I know nothing about optics, and I know people have discussed this kind of thing a while back, but has anyone gotten a large panel to work w/o a fresnel?
Could a series of large condenser lenses work? kind of like this:
@ < ) - - - ( < | ) - - - () <
Does that make senese? The @ is the bulb, the < is light expanding, the ---- is a focused beam, and the ( and ) are convex and concave lenses, and the last () is the projection lens. The | is the LCD.
/\
\/
shaped pixels. In effect, it lowers your resolution, thus making it pretty much pointless to use anything higher than XGA.
I know nothing about optics, and I know people have discussed this kind of thing a while back, but has anyone gotten a large panel to work w/o a fresnel?
Could a series of large condenser lenses work? kind of like this:
@ < ) - - - ( < | ) - - - () <
Does that make senese? The @ is the bulb, the < is light expanding, the ---- is a focused beam, and the ( and ) are convex and concave lenses, and the last () is the projection lens. The | is the LCD.
See http://physics.bu.edu/~duffy/java/Opticsa1.html
It's a small java applet that lets you mess with optics.
There is also a free program called OSLO Edu that can be used to build simple optic devices, don't remember the url right now but do a google search on OSLO edu.
It's a small java applet that lets you mess with optics.
There is also a free program called OSLO Edu that can be used to build simple optic devices, don't remember the url right now but do a google search on OSLO edu.
Great applet - Thanks!
Here's the link for the Oslo program: Sinclair Optics
I haven't tried it yet, but it looks like fun.
Garry
Here's the link for the Oslo program: Sinclair Optics
I haven't tried it yet, but it looks like fun.
Garry
Lifter:
Is your LCD resting directly on the ohp stage? try elevating it and refocussing. Then the projection lens should be focusing on the LCD, and not the fresnel. Just an idea.
Is your LCD resting directly on the ohp stage? try elevating it and refocussing. Then the projection lens should be focusing on the LCD, and not the fresnel. Just an idea.
Does that actually work? I've been meaning to try that (actually, I'm going to have to lower the fresnel because the LCD is built into where the stage glass used to be. Someone said that it makes the screen door effect worse however.
Well, I have an OHP lcd panel, and if I take it off, and refocus on the fresnel, I get rings. The LCD panel frame lifts the actual LCD an inch or so off the actual stage.
I would recommend against lowering the fresnel, as I think it is probably positioned such that the bulb is at the focal point of the lens. Try moving the LCD, if at all possible. And the worse the screen door effect is, the better your focus is. If I find the effect bothersome (*I* have a 640x480 panel
), I just defocus very slightly and it's much better. Good luck!
I would recommend against lowering the fresnel, as I think it is probably positioned such that the bulb is at the focal point of the lens. Try moving the LCD, if at all possible. And the worse the screen door effect is, the better your focus is. If I find the effect bothersome (*I* have a 640x480 panel
), I just defocus very slightly and it's much better. Good luck!Thanks. Moving the LCD is going to be a problem. I'll see what happens when I lower the fresnel, but if it doesn't work I'll have to figure something out.
Ok, I guess I'll have to figure something out. Arrrghhh. Your absolutely right. Lowering the fresnel didn't work because there needs to be a set distance between the fresnel and the condenser lens that's over the bulb. I spent a LOT of time retrofitting the top of my OHP to house my LCD. Maybe I can take a crash course in MDF, carbon fiber, or whatever. I don't know any material (or tools) that I can use to turn a bare LCD into a "projection panel" if you know what I mean.
well, if you want to keep the LCD where it is, see if mabye you can raise the entire OHP stage, and lower the fresnel equal amounts(in order to maintain the same fresnel->bulb distance). Then you'll still keep your clean LCD install, and hopefully get rid of the rings! How you would raise the top of the overhead is beyond me though.. 
- Status
- Not open for further replies.