Project using a 17" LCD panel

[janeketchen]

Project using a 17" LCD panel

Hi. I'm new to this forum but have spent several days reading what people have said in other threads. I thought I would do something similar to Inkog and start a thread documenting the progress of my projector. Hopefully, with help from some of the experienced members (Guy Grotke and Ace3000) I'll be able to get this thing up and running in no time. Also, this thread might benefit others aiming to do something similar to me.

I started with a busted Envision 1700e that I bought 3 years ago. The controller for the backlighting didn't work 100% of the time (I would have to turn it on for several hours, then unplug and replug the vga connector until the backlighting would turn on). I had seen the OHP projector made by the guys at Tomshardware and decided to pursue that. I bought a 3m 9100 knowing that a huge portion of my LCD would be cut off by the 10.5 by 10.5 inch stage aperture.

I then dissected the LCD monitor and managed to extract the LCD panel. I put the panel on top of the powered on the OHP only to find it wasn't bright enough.

The new plan is to do this the right way, with a metal halide lamp and a homemade enclosure.

Here are my current questions:
1. The 3M 9100 has a doublet objective lens. How can I figure out whether or not this lens would be usable in the new project?

2. Does the fresnel lens have to be as big (or bigger) as the 17" panel? More fundamentally, what exactly is the purpose of the first fresnel lens? I understand the light needs to be spread out before the LCD. Is this lens bending the light (which is coming in at many angles) into collineated light? If this is the case, I'm assuming the fresnels must be as large as my LCD.

3. Finally, I haven't seen a lot of projects with done with panels this large. Would pursuing this be just plain stupid? I have no limitations on space, so I don't mind making a large box.


If you could help with any of these questions, please jump in.

Thanks in advance.

 

[Guy Grotke]

expert advice?

I'm not an expert: I just read everything on a few forums and a few books on optics. So far the number of projectors I have built: 1!

But I'll give it a try for what that's worth...

1. The 3M 9100 has a doublet objective lens. How can I figure out whether or not this lens would be usable in the new project?

You could figure out the LCD to lens distance for your projector to calculate the needed field angle. Then you could try putting the beam from a laser pointer through your lens at that angle. I would try a mockup, instead: You can make a mockup of a well-lit LCD panel using transparency film of the same size with some fine lines & text printed on it. Mount it in a hole in the middle of a big piece of cardboard, and then put that up against a window about 12-14 feet from a white wall, screen, or hanging sheet. Then mount your lens in another piece of cardboard. During the day, that "LCD" should be bright enough in the darkened room, to project it on the screen using your lens 12-20 inches from the transparency. See how much of it ends up on the screen.

2. Does the fresnel lens have to be as big (or bigger) as the 17" panel? More fundamentally, what exactly is the purpose of the first fresnel lens? I understand the light needs to be spread out before the LCD. Is this lens bending the light (which is coming in at many angles) into collineated light? If this is the case, I'm assuming the fresnels must be as large as my LCD.

There are usually two fresnels. The first (the condensor fresnel) takes light from a point source (MH lamp arc) and refracts it to be mostly perpendicular to the LCD, and hopefully even over the entire surface. The second (the field fresnel) converges the light to focus it all into the projection lens. If the lamp arc is at the focal distance from the condensor fresnel, and the projection lens is at the focal distance of the field fresnel, then this is optimal. But you may have to fudge those distance a bit: Fresnels can go together at least 20 mm before the LCD, or split with the condensor fresnel at least 20 mm before and the field fresnel around 20-30 mm after the LCD. But the LCD to projection lens distance is fixed by the projection lens focal length and the throw distance, so you have to find fresnels that can work with that distance. In a split fresnel design, make them at least 1/2" larger than the LCD on all 4 sides. If you have to put them way before the LCD, then they have to be large enough so the converging cone of light fills the whole LCD where they intersect.

3. Finally, I haven't seen a lot of projects done with panels this large. Would pursuing this be just plain stupid? I have no limitations on space, so I don't mind making a large box.

You may have a difficult time finding a projection lens with a wide enough field angle for a 17" LCD. I use an expensive process lens that works for 11" by 14" film large-format cameras, for my 15" LCD. That would work for a 17" LCD. You already have the 17" LCD, so you should try the lens experiment above. If you can get it all on the screen, see if you could get a 15" LCD on the screen. Good 15" LCD monitors are now under $200. Or you could hunt on eBay to find a 10-12" overhead projection panel for less. And remember: A big image = a dim image, no matter what you use to project it.

 

[janeketchen]

thanks for your reply, guy (that rhymes)

More questions if Guy, or anyone else has the time to answer:

If a bigger image is necessarily a dimmer image, why are many people using 15" LCD panels? (besides the fact that there are no 13" LCD monitors). Would the difference between a 17" and 15" LCD be that significant?

A condenser and a condenser fresnel are too different things, correct? The condenser goes directly in front of the arc while the condenser fresnel should be near its focal length away from the condenser?

Lastly, and sorry if this is a dumb question. Are LED's still not bright/efficient enough? I read a lot of posts saying 'no,' but they were written in 2004. I thought maybe the technology had improved by now.

 

[janeketchen]

and yet another question

Sorry for yet another possibly dumb question.

Is a condensor fresnel just a field fresnel turned around? Is there any real difference between the two besides their focal lengths?

Looking around at fresnels, I see lumenlabs offers a lens kit that includes a fresnel and a triplet. Is this fresnel a field, condenser, or is it the two combined?