DIY Video Projector

[rjacobs]

For video projection a 320 resolution would be enough. (a VCR does not output more than about 250 pixels) but for computer graphics it isn't enough.

I don't think Micro displays are an option because, altough optics in small sizes are easier and cheaper it will be more difficult to focus it correctly and to get enough light thrue the display without overheating it.

IMMO it's easier to use a (relative) large LCD and the optics used in an overhead projector. (Because of the large area at the base it's easy to cool and as everyone can see the optics of a overhead projector is not very high tech while still archiving good quality and brightness)

An overhead projector only uses 2 lenses and a mirror, for our application the mirror is not needed. (we can mirror the projection by turning the LCD) so we only need a bright light source, one fresnel lens (at least a big as your LCD) and one normal lens ( 150 ... 400 mm focal length depending on the size of your room and desired projection size)

I'm currently tring to locate (cheap) frensel lenses here in Spain to experiment. I'll keep you guys posted.

Robin

[vdi_nenna]

I was reading the specs of a projector last night and several models used LCDS 8" and 10". The native resolution was 800x600 on the 10" screen. I'd like to use a larger LCD screen for better resolution, but I don't know how to fit the image of a 10" screen through a 3" diameter projection lense. There are 7" screens w/ 16:9 aspect ratio. That's excellent for home theater. I'm sure it can be done, but I don't know how.

I have a source here in the USA for new frensel lenses. They have various sizes. http://www.edmundoptics.com
I have seen frensel lenses on surplus sites too.

[tim0]

Robin,

what you propose is exactly what i want to do, too.
Please let me know if you can find a supplier in Spain, or the general EU. I am in Germany, ordering from the US is no option right now given the strong dollar.
I am still very interested in this project and determined to do it but i have several audio projects with higher priority.

For the optics parts i was thinking of the spare parts supply of OHP manufacturers. I used to work at a hospital where we routinely got replacement parts from the manufacturers of the projectors. Should be no problem at least if you are some sort of company...

For the display i still play with the idea of using a regular 15" LCD screen like they are sold for computers. Remove the back wall and original backlight, put LED backlight in it and DONE.

Greetings
Timo

[Petter]

I used to do a lot of photography. What we always said was: You can't beat a bigger format. The reason for this was that lens quality was always a problem, and the larger film reduced lens effects. It also reduced film effects of course.

To corroborate: One of the sharpest production lenses ever made was Kodak Disc. It needed to be so sharp because the film was so small. Kodak realized their units produced terrible quality images and spent lots of money developing much improved films. The Japanese used the better film with a larger area and we all know the rest.

Why am I going on about this? I feel that a large LED (or other source) array, ideally direct view, but also projection must be better than passing a lot of light through (or reflecting off) a small area must be better.

I am very surprised that nobody has manufactured large direct-view diode screens.

Petter

[tim0]

Petter,

there are direct-view RGB LED screens. The problem seems to be that the LED, each representing a pixel, are so big, hence you need a large distance from the screen.
Besides, you need 480000 (fourhundredeightythousand) LEDs for a 800x600 screen. I don't want to assemble that, or buy so many RGB LEDs, for that matter. Then you need analog control for the color of each led. Not my project.

Here's the Philips LED wall:

http://www.projektor-beamer.de/rueck...n/alfresco.htm
http://www.vidiwall.philips.com/press22.htm

this seems to have 5 normal LEDs instead of the RGB led.
Advantage: digital control of the LEDs ?

Thanks for your post, reinforces my decision to use a big screen and lense.

Regards
Timo

[vdi_nenna]

You have a good point Petter. Bigger format is better. My brother shots color and black and white photography w/ a 120 format. The sharpness and color depth is where it's at.

There are very small RGB LEDs that this can be done with. But your distance would have to be at least 10' away from the screen. Also, one of the great challenges is contrast. How black can this get in other words. LEDs can be completely shut down. The best device is DLP. 800:1. Some of the best LCD screens only do 300:1 or 400:1 ratio.

But again, 300,000+ LEDs can draw alot of current @ 20mA each.

[Shaun]

Significant power savings can be achieved by pulsing the LEDs on and off. This will extend LED life. One can even drive them to peak current; possibly this may yield a perceived brighter picture.

Regards
Shaun

[rjacobs]

Leds are not an option for a DIY screen for several reasons.

a) It's a hell of a job to just connect all these leds.
b) Too expensive: You need 64000 RGB leds for a 320X200 screen, 256000 for a 640X400 and 480000 RGB leds for a 800X600 screen. If you use serparate Red, Green & Blue leds instead of those expensive RGB leds you need 3 times these numbers.
c) You will have to make all electronics yourself for driving the leds (in matrix configuration) at a speed of almost 4 Mhz/LED.

I've played with the idea a long time but even a monocrome red (cheapest color) it's just not do-able.

To pull something of like this you need massive resources (like Philips, Sony).

The ONLY way to go is projection.

Robin

[rjacobs]

Timo,

The easiest thing would be to find a (old) overhead projector; They have all the optics we need. (Frensel lens ,projection lens) and they have the system to adjust the focus already made.

Many years ago at my school, I allready saw a (monocrome) LCD display on a normal overhead projector. It worked perfectly.

Robin

[vdi_nenna]

You have to take into account that there is going to be some light bleeding into the room. Any extra light diminishes the image. This washes the image out. You should, somehow, block off any extra light around the LCD.

I want to add this for some visual help. It's a bite elementry, but optics is a tough subject.


http://www.opticalres.com/kidoptx.html#ControlLight

http://www.lighting-optics.com/techn...ers.htm#lenses

[Edited by vdi_nenna on 08-21-2001 at 09:46 AM]