I'm thinking of using the fresnel lenses from my Dukane Starfire which
is a high quality OHP designed for using ohp lcd panels, so do you think it's ok for me to use that for my DIY enclosed project?
i'm using the ushio 400 w. retro bulb and want to just put the lcd after the fresnel as it is used in the ohp. (the fresnels are stuck together as one piece)
is there a huge difference using the "sandwhich" method? where the lcd is between the two fresnels?
is a high quality OHP designed for using ohp lcd panels, so do you think it's ok for me to use that for my DIY enclosed project?
i'm using the ushio 400 w. retro bulb and want to just put the lcd after the fresnel as it is used in the ohp. (the fresnels are stuck together as one piece)
is there a huge difference using the "sandwhich" method? where the lcd is between the two fresnels?
oh, so it's not about focus, only effect the distribution of light.
well, i'll try it with one, and it there are hot spots, i will fork over some more money.
but i've already spent alot factoring all the parts. we'll see.
thanks for the reply.
well, i'll try it with one, and it there are hot spots, i will fork over some more money.
but i've already spent alot factoring all the parts. we'll see.
thanks for the reply.
split vrs unsplit
It is always about getting as much light as possible into the projection lens. But there are a couple of other factors:
1) You can do some optical keystone correction with a split design by tilting the field fresnel (the one after the LCD). This helps in case your projector can't be mounted with the projection lens right on the line perpendicular to the center of the screen. (Try to avoid this if you can. You will have enough trouble without going to look for more!)
2) Your focus might be a bit sharper using a non-split design (both fresnels before the LCD). Fresnels are not very good optical devices, so sending the image (as opposed to just the light) through a fresnel is not optimal. On the other hand, I have used the 3dlens.com 550 mm fl fresnel in a split design and the image was very sharp.
3) With a non-split design, the rays pass through the LCD in a converging cone. That means light through the center has a "viewing angle" of 0 degrees, but out towards the corners the angle is much more pronounced. With some LCDs you may see visual artifacts at such angles, like color shifts or dark areas. This is made worse with a short focal length projection lens (ie. 300 mm fl) versus a long throw lens (ie. 450 mm fl), because the angles are farther from 0 degrees. It is also worse for large LCDs (ie 17") versus small LCDs (ie. 7").
4) With a split design AND a field fresnel that matches your LCD-to-lens distance, you can put the lamp arc at the exact focal length of the condensor fresnel. That gives you mostly parallel rays going through the LCD, so there are no viewing angle artifacts.
It is worth trying it both ways to see which works better with your LCD and optics.
It is always about getting as much light as possible into the projection lens. But there are a couple of other factors:
1) You can do some optical keystone correction with a split design by tilting the field fresnel (the one after the LCD). This helps in case your projector can't be mounted with the projection lens right on the line perpendicular to the center of the screen. (Try to avoid this if you can. You will have enough trouble without going to look for more!)
2) Your focus might be a bit sharper using a non-split design (both fresnels before the LCD). Fresnels are not very good optical devices, so sending the image (as opposed to just the light) through a fresnel is not optimal. On the other hand, I have used the 3dlens.com 550 mm fl fresnel in a split design and the image was very sharp.
3) With a non-split design, the rays pass through the LCD in a converging cone. That means light through the center has a "viewing angle" of 0 degrees, but out towards the corners the angle is much more pronounced. With some LCDs you may see visual artifacts at such angles, like color shifts or dark areas. This is made worse with a short focal length projection lens (ie. 300 mm fl) versus a long throw lens (ie. 450 mm fl), because the angles are farther from 0 degrees. It is also worse for large LCDs (ie 17") versus small LCDs (ie. 7").
4) With a split design AND a field fresnel that matches your LCD-to-lens distance, you can put the lamp arc at the exact focal length of the condensor fresnel. That gives you mostly parallel rays going through the LCD, so there are no viewing angle artifacts.
It is worth trying it both ways to see which works better with your LCD and optics.
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