This has probably been discussed before, and I know it's probably going to involve a fresnel. so enough with the search smileys allready i'm on a 33.6k connection i've done the best I can !
anyway i've bought a 3400ANSI lumen OHP - projector works great a nice image at about 70" ( needs MH installing at some point though its very yellow )
the trouble is - as with most OHP's its bright in the center and dimmer towards the edges.
Whats the best way to sort this without sending light too far towards edges and wasting any ?
anyway i've bought a 3400ANSI lumen OHP - projector works great a nice image at about 70" ( needs MH installing at some point though its very yellow )
the trouble is - as with most OHP's its bright in the center and dimmer towards the edges.
Whats the best way to sort this without sending light too far towards edges and wasting any ?
getting uniform light from OHP
A little trouble-shooting::
Do you get a nice uniform white image when you don't have an LCD panel or a transparency on the OHP?
YES: Then it might be a problem with the angle of the light going through your LCD. OHPs put both fresnels below the LCD, so the light is converging toward the projection lens as it goes through the LCD. The angle towards the edges is the farthest from perpendicular. If it gets too far from 0 degrees, it may exceed the viewing angle of your LCD. The only way to fix that is to split the two fresnels and put the upper one about an inch above the LCD. That way you get all perpendicular 0 degree light through the LCD.
NO: Put a piece of white paper on the OHP tansparency surface. Does it get uniform light? If not, then your lamp is not at the focal point of the lower fresnel. Does your fresnel assembly come out for cleaning? Maybe it got flipped over at some point. Some OHPs have a 220 mm fl fresnel on the bottom and a 330 mm fl fresnel on the top. (They may be glued together.) If such an assembly is upside down it will not work right. Measure the distance from the lower fresnel to the bulb filament. Is it exactly 220 mm? (Some OHP use 200 mm on the bottom.) Maybe you have a substitute replacement bulb with a different height than the original. Can the bulb height be adjusted?
A little trouble-shooting::
Do you get a nice uniform white image when you don't have an LCD panel or a transparency on the OHP?
YES: Then it might be a problem with the angle of the light going through your LCD. OHPs put both fresnels below the LCD, so the light is converging toward the projection lens as it goes through the LCD. The angle towards the edges is the farthest from perpendicular. If it gets too far from 0 degrees, it may exceed the viewing angle of your LCD. The only way to fix that is to split the two fresnels and put the upper one about an inch above the LCD. That way you get all perpendicular 0 degree light through the LCD.
NO: Put a piece of white paper on the OHP tansparency surface. Does it get uniform light? If not, then your lamp is not at the focal point of the lower fresnel. Does your fresnel assembly come out for cleaning? Maybe it got flipped over at some point. Some OHPs have a 220 mm fl fresnel on the bottom and a 330 mm fl fresnel on the top. (They may be glued together.) If such an assembly is upside down it will not work right. Measure the distance from the lower fresnel to the bulb filament. Is it exactly 220 mm? (Some OHP use 200 mm on the bottom.) Maybe you have a substitute replacement bulb with a different height than the original. Can the bulb height be adjusted?
some good advice there, I took a look inside and yes the bulb height could be adjusted - one way makes the edges bluer and one yellower, but theres a point in the middle where the light becomes perfectly uniform - i now have a pretty workable 7foot image 🙂
Thanks
James
Thanks
James
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