DIY Video Projector Part II

[remp]

Thats a good point

Any thoughts on other types of glass or other transparent material.

[woneill]

Hi Guys,

Ok, you got me - yes you CAN have an image bigger than the lens and still project an image.

e.g. a "pin-hole camera" can focus the image of the sun onto a screen, and it is probably safe to say that the sun is generally bigger than most pin-holes...

However, if you want to catch MOST (if not ALL) of the light from the image produced by an LCD panel, and not just a tiny fraction of it, then you must either use a very big objective, or use some form of lens before/after the panel (at least as big as the panel) to focus the emerging light rays towards the smaller objective.

In effect, in almost all projectors, there is an optical stage (collection of lenses) that is as big as the image being projected. In the OHP, it is the fresnel under the glass panel, in other projectors it comes before/after the image.

The two issues many people seem to be objecting to are:

1) my statement inearlier postings that the objective should be bigger than the image. (This WAS too general - my bad - I should have talked about having an optical stage bigger than the image.)

2) the use of a fresnel after the panel rather than before it. (This seems to be a philosophical issue. I still maintain that the image degradation by such a lens placed after the panel is much less (when the fresnel is placed properly) than the degradation that occurs in trying to use an equivalent fresnel to focus light "through" an LCD panel from behind.)

Most of the TFT panels around today are not good optical filters - just hold them up to a lamp at the other end of the room and look at the lamp through the panel - in most cases you will see scattering and a diffuse image of the lamp due to MLA layers designed to collect as much light from a diffuse CCFL source as possible and funnel it through to the front with the widest viewing angle possible. This is not 100% compatible with the concept of forcing light through from behind at a very specific angle needed for it to reach the smaller objective in front of the panel.

Some LCD panels were specifically designed for OHP work, and they are much better in this respect. Others weren't...

As in all science, theory takes you half-way there, and practicalities take you the rest. I apologise, if in being too general I was giving bad advice.

Bill.

P.S. I still think the optimal arrangement is to have the light entering the LCD panel from behind, perpendicular to its surface, and then having the emerging light focussed towards a powerful, optically corrected objective by a low power, large diameter lens. (The lower the power, the less aberations etc.) And, as always fresnel lenses are poor compared to a nice optical glass lens.

[woneill]

Xblocker,

You mentioned how large format cameras generally have larger FL lenses than the smaller ones. The main reason for this is perspective.

In 35mm photography, a "normal" lens is usually quoted as 50mm. This lens gives roughly the same field of view (perspective angle) as the eye - producing a "normal" looking picture.

Wide-angle (35 mm FL and below) give a much wider field of view, and telephoto (70mm and up) give a much narrower perspective. (Never do a facial portrait with a wide angle lens - noses can become enormous...)

For the larger format cameras, the image is projected onto a larger area of film, and the dimensions of the cameras are such that to achieve a "normal" perspective, you need a longer focal length lens. The larger the format, the longer the lens...

Bill.

[Electro]

Using an LCD screen and a very, very bright light to project the image on a white object has some cons.

1) Needs a very bright light.
2) Bright light produces a lot of heat.
3) Fan needs to be used to cool off the light emitting device.
4) Needs a large lens
5) LCD screens cost to much.
6) Visible black lines on the projector when sitting 6 feet away from the source.


There are so many other cons when using a LCD and light emitting device. I think using lasers might be of some interest. Lasers act like electron tubes but lasers can be much finer in detail. Laser diodes, found in CD players and laser pointers, prices are coming down. There are red and green laser diodes but blue laser doides are coming soon. The next best way besides using laser diodes is LEDs. LEDs can handle a lot of voltage around 10 times their orignal forward voltage. Laser diodes can not handle this amount of voltage.

Lasers work on scheme of pulsing a high voltage to a light emitting device for a short period. LEDs can be great for projecting like a laser. For more information go to How Lasers Work

LEDs come all sorts of colors such as red, yellow, green, orange/amber, purple, and blue. The colors need to emit all the colors in the rainbow are red, green, and blue. Each color has its own brightness so adjustment is needed for accurate color.

I found a circuit in my books that uses a LED to be used as a laser. I will post it if anybody have some interest.

Below is a reference:
Iannini, Robert E. "Build Your Own Laser, Phaser, Ion Ray Gun & Other Working Space-Age Projects". TAB Books. 1983

[Undream]

Originally posted by Electro

1) Needs a very bright light.
2) Bright light produces a lot of heat.
3) Fan needs to be used to cool off the light emitting device.
4) Needs a large lens
5) LCD screens cost to much.
6) Visible black lines on the projector when sitting 6 feet away from the source.

1: This one is true. Whaddya know, you are 16.667% correct.
2: They don't produce a lot of heat. in fact, I even accidentally bumped into my MH bulb that had been burning for 45 minutes and didn't get burned.
3: Actually, they don't need to be directly cooled. Marklar's setup has a single fan mounted in the back, and isn't even pushing air upon the bulb. Most of these bulbs are designed to be in closed fixtures like street lights and things like that.
4: completely and utterly incorrect. I can use my ~3" diameter overhead projector lens to project my 10.4" diagonal LCD panel. You are just plain wrong. Want an even smaller lens? no problem. you can find one that will work. I just decided to use my 5 7/8" lens because it seems to work slightly better, and I want to use parts in my projector that can all be purchased again for cheap.
5: $100 is too much? How much would you spend on LEDs or Lasers? More importantly, how many man hours would you put in trying to get something like that to work? its all relative, I don't think this is an issue.
6: Put it slightly out of focus. Problem solved. Personally, I'll be sitting 12' from the screen, so, I'll be able to sharply focus without problems.

[jvisaria]

electro,

no offense, but we have been over this before in the old thread. You might want to check out the laser projector thread if you are interested in that route. My personal opinion is
DIY+moving parts=breaks a lot.

[xblocker]

woneill,
agreed, my point was someting different. I mentioned the relation between math formulas and physical reality. One the mathematical side, we could theoretically get shortest FL and biggest magnification, but not one the physical side, 'cause real lenses (the thicker they are) unfortunatly produce many undesirable side effects, which can be only eliminated with a bunch of correcting elements! That's why it IMHO would be better not to choice a FL below the diagonal of the panel.

xblocker

[Undream]

Latest from the frontlines...

you'd be surprised what you can do with a cordless drill, a 45 cent protractor, a hand saw, and a calculator. damn I need some decent tools :\



Lens mount/front assembly. turned out darn good.

from the front

here it is, not fully mounted, but, just slid on so you can see what its gonna look like. Not beautiful, but, it should work. Still have to mount 4 focusing bolts/wingnuts, cut some vents for the LCD panel, figure out how I'm going to get the LCD panel remote to work, put on one or two more braces, and then of course, mount/install the four feet that its going to be standing on. Sucker is heavy altogether. really heavy.

[Jason]

IMG tags are now turned on... they are off by default when creating new forums...

[Undream]

Thanks Jason.