Projecting Part of the LCD

dracul

Member
2003-07-30 3:55 am
Brazil
My 15 inch lcd arrived with burn marks on the third right side. Setting it at native 1024 x 768 I plan to use the remainder left side of the screen that is clean.

I will ofcourse centre this smaller image on the fresnel. How does this affect the placements of the optical elements. In order to project only this portion of the lcd do I need to bring the objective closer to the lcd?
 
move the objective?

No, the distance between the LCD and the projection lens is a product of the throw distance (lens to screen), and the lens focal length. It does not change for different LCD sizes. What will change is your screen image size.

I would suggest adding a mask over the part of the LCD you don't want to see on the screen.
 

dracul

Member
2003-07-30 3:55 am
Brazil
ok. thanks.

Now when people say a smaller lcd requires less light I assume they mean only if using condenser. I say this because it seems the light to fresnel distance also cannot chance regardless of lcd size. So a 2 inch lcd will have the same light passing through it as a 15 inch lcd per given square inch correct?
 
when people say...

When people say that a smaller LCD needs less light, they are just repeating a myth!

You need a certain number of lumens on your screen if you want to be able to see a decent image. If you send those lumens through a big LCD or a small LCD, that does not affect the results on the screen. What does make a huge difference is the size of the screen image. A 120" diagonal image spreads the light out over four times the area as a 60" diagonal image, because the image size expands in both dimensions.

The one aspect of LCD size that is important is that the LCD color filters and the first polarizing filter have to absorb more than 80% of the light. (That is the part of the light with polarization that doesn't match the direction of the polarizing filter, and the light hitting each color filter that doesn't match that filter's color.) That means the LCD surface has a constant input of energy that must be removed, or it will overheat. With a 250 Watt MH lamp and a properly designed condensor system, a 5" LCD will have to dissipate the same number of Watts as a 15" LCD. But the 5" LCD will get 9 times as many Watts per square inch, because it has 1/9 the area of a 15" LCD.

That is the challenge of using a small LCD: You have to concentrate all of the light through that small area by using the right reflector, lenses, and fresnels. Then you have to make sure you keep the LCD cool enough to operate. Commercial projectors avoid this problems by using small LCDs that do not have color filters or polarizing filters. Instead, they use three monochrome LCDs with seperate filters that can operate at high temperature. All the LCDs do is to twist the polarization of light passing through, which results in very little absorbed energy in the LCD.
 
Guy: since you seem to be knowledgeable on optics and small panels, can you suggest a place to get high temperature resistant precondensers? I have read that it should be the same size as the reflector(an Ikea napkin ring in my case(about 4" in dia? 110mm I think) and a 250W MH ED28 bulb)...
 
sources

>[heat] resistant precondensers?

Rolyn Optics has some in their catalog, but they are not cheap: The smallest (114 mm diameter) start at $66.

Actually, the precondenser lens does not have to be the same diameter as your reflector. It just has to cover the same angle from the center of the arc. If both reflector and precondenser lens are the same distance from the arc, then they should be identical diameters. But the reflector could be twice the distance and twice the diameter. Rays getting reflected should reach all the way out to the edge of the lens. Otherwise, the outer edges of the lens will receive less light than the center. Draw a picture showing where the rays go, and you will understand.
 
ok, so I'm using the IKEA napkin ring and an ED28 bulb and the reflector is right against the bulb so it looks like in this instance my precondensor would be the same size as the reflector? and if it was further away from the bulb the lens would need to be bigger? is it possible to use a fresnel as a precondensor? thanks for your most valuable info.... does it make a difference the size of the LCD you are illuminating?(using psone/verge), I wouldn't think so but.....
 
>[So] my precondensor would be the same size as the reflector?

Ideally, but that rather misses the point: You put a pre-condensor lens close to the lamp arc so it can capture a lot of the light and send it toward the condensor fresnel. If much of the light falls beyond the corners of the fresnel, then it is wasted. If it converges the light so much that the corners are dark, then you've gone too far. Also, the light has to "appear" to the fresnel as if it was coming from a point at the focal distance away, if you want it to come out the other side parallel. So you have to design the whole system to work together: Focal length & diameter of the pre-condensor lens, focal length of the condensor fresnel, size of the LCD, are all adjustable parameters. There is another thread on this forum all about this. I think it is "Understanding the Pre-condensor", or something similar.

The reflector is just a cheap way to get about 20% more light into the condensor system. You do have to insure that reflected rays get to all parts of the fresnel, or you will get much dimmer corners. If your reflector covers the same angle, or a wider angle than the lens, this will not happen. If it covers a narrower angle, then you might have a problem.

One very critical factor for a spherical reflector, is that the lamp arc must be placed at the center of curvature. I don't know how an ED28 fits an Ikea reflector. Is the center of the lamp arc equidistant from all points on the reflector surface?

>is it possible to use a fresnel as a precondensor?
Yes, but it better be a pyrex glass fresnel! An acrylic fresnel would melt because pre-condensor lenses go close to the lamp.

>does it make a difference the size of the LCD you are illuminating?
Of course! The biggest difference is the resolution. 5" LCDs are around 640 by 480. 7" and 8" LCDs tend to be around 800 by 600. 14" and 15" LCDs are all 1024 by 768. Blowing up a 5" LCD to a huge image size will look pretty pixellated. Another difference is that 15" LCDs with a 220 mm or shorter fl condensor fresnel will not benefit much from using a pre-condensor lens. (It just doesn't capture much more light.) Smaller LCDs will benefit. 5" LCDs really need a pre-condensor lens. Finally, with the same amount of light going through the LCD, a small one needs much more cooling because it absorbs more energy per square inch.
 
thanks for all the info guy .... will try to find that thread about precondensors.... and yes, the ikea fits the ed28 very well, needs to be place right aginst the lamp but it did make a noticeable difference in my brightness and also seemed to even out the light distribution.... just got done stripping my 15" so I think I'll let the 5" one sit for a while.... right offhand I'd say that a 5" is more difficult to do (well) than a 15" !