DIY Video Projector

[xblocker]

Myren.
you're right, my classical OHP drawing isn't quite correct. I know OHPs have spherical reflectors, but they are bouncing the lightsource back not right INTO the filament, but directly BESIDE it.
BTW, to overcome the problem with a parabolic reflector+fresnel, how about a blind directly in front of the lightsource, so that only reflected light hits the fresnel?

xblocker

[prjctr_builder]

if i you do use a metal halide lamp in your projector, how do you condense the light???????
i know that if i get a headlight projector lights, the condensors are already factory installed.

hehe

[fender4]

Myren,
The number under the MH in the graph is "115" lumens per watt. Sorry if it didn't show up properly. Using a 400 watt MH, following this data, would produce 46000 lumens. That is pretty close to the 40000 lumens you are getting with your 400watt MH. Just wanted to straighten that out.

Also, make sure to look into the orthogonal parabolic or elliptical reflectors (remp's post a couple of days ago). These make much more sense for long filament bulbs, like MH's, since the true focal point of the reflector actually only covers a small part of the filament. Without an orthogonal reflector, light produced outside of the true focal point will travel in a slightly aberrant path....leading to inefficiency (and that is the enemy! )

For those who have experience with OHPs:
Don't some of the OHP's use a bulb, a small reflector (parabolic perhaps), and a short focal length PCX lens to disperse the light? If so, what are the advantages/disadvantages of this as opposed to a strict reflector style light setup? Just curious.

-f4

[remp]

Fender4

I have looked harder at that orthogonal reflector and found a better description at US patent # 5037191. The inventor claims it is the best thing since sliced bread and as well as collecting light to a point he says the point is infinitely small. They dont get much smaller than that. I have also checked out radiation from a hot body such as a light source and all the light leaves at 90 degrees only excepting if the source has a non smooth surface in which case thee is radiation from the lumps in non 90 degrees but the majority is at 90 degrees. Could be a useful reflector.

[xblocker]

fender4,
as i said 2 posts before, OHPs have spherical reflectors. Some have an additional PCX-lens directly ober the bulb, some not. PCX lens is a simple condensor, which shares his job with the fresnel. More of lightrays can be refracted and a smaller overall size of projector case can be reached.

Richard,
could you point out a little more this 90 degree thing ?

xblocker

[MuzzMan]

I found some details about the Delta 67 and Delta 77 lenses I have been trying to use with my 5" LCD for rear projection:
The focal lengths are 67mm and 77mm respectively (could have guessed!)
They are designed to be used with a 5" raster diagonal CRT, which is absolutely perfect, but......
They are intended for use with a CURVED crt faceplate (radius 350mm), so they are no good for our purposes at all!

So I now have to go back to trying to design a multi element lens setup using PCX or DCX simple lenses, or see if another complex lens like the delta is available surplus.
I know some people have bought other lenses surplus intended for rear projection (I remember Delta 20, fujinon or something?).
Has anyone managed to get a completely in focus image out of one of these that would indicate it is designed for a flat CRT? Most of these lenses would have been intended for 5" to 7" image size on the CRT.

[piGuy]

I had a few questions regarding some comments on optics. Myren, you said that a spherical mirror will reflect light at the focus back to the focus? How is this? IIRC, a light at the focus will result in parallel light reflecting outwards. And prjctr_builder, I'm not so sure a semi-permeable mirror will work. A two way mirror works by contrast. You have a light on one side and darkness on the other side. If you're on the light side, you see the light reflect back. If you're on the dark side, you see the light come in. If you put in a two way mirror, the light will reflect back towards the light source. If I'm not making sense, think of it this way: A two way mirror could be reversed and it will still work the same way. It all has to do with the lighting on each side. That's why an interrigation room is well lit, and the observation room is much dimmer. For an example, take a pop-tart wrapper. It looks reflective normally, but if you put it up to the sun, you can see through it, because there's greater contrast. I didn't take a close look at your setup, but I'm not sure if there would be a way to incorporate a two way mirror that would keep the light from the bulb from reflecting back. I don't really have much experience with two way mirrors (other than learning about them briefly in physics), but I'm personally not aware of a mirror that actually only lets light in one way. Sorry if someone else has already addressed this. This thread is so long its impossible to remember everything that's been said. :-)

BTW, I know I had talked about my distaste for an LCD based projector, but just how good of results are you guys getting? Has anyone tried a side by side comparison between a homemade and commercial LCD projector? (I think Myren mentioned this once) I haven't tried building one, but I suppose that the length of this thread would indicate that the results are better than ****-poor.

[remp]

Xblocker,

The 90 degree thing is to do with this orthogonal reflector I found at the US patent database #5037191. I mentioned this reflector several pages ago and Myren thought there might be large inefficiency. I thought he could be right in which case the reflector would be useless but looked at radiation leaving a hot body. If you look at an ordinary light bulb it seems to emit the same amount of light all the way round. Except the fillament end of course. So I looked at theoretical radiation from hot bodies and it is at 90 degrees to the surface. If the surface is not dead smooth which is the case in practical devices there is always some radiation at odd angles but the majority is at 90 degrees to the surface. This orthogonal reflector allows you to use a long light source takes all the 90 degree radiation and focus that to a very small point. Dimensionless point the inventor says. I have never seen one of these reflectors but looks fairly easy to make.

[Marklar]

Well I gave up finding the bridge rectifier so i just took the light and ballast out of my OHP. My OHP was rated at 7000 lumens but I looked at the sylvania site and looked up the bulb it is 49,000 lumens. Today i tried a 250W MH i wraped it in foil and cut a hole about 1.5 inches across placed a 5" magnifing glass in front of it and it made a very bright spot about 6" across when i stuck my hand in front of it i put it in my OHP and got it to project the lcd on the wall but it wasnt bright enough. This bulb has about 30,000 lumens less than the origional so I found a compact 400W MH that GE makes that puts out 50,000 lumens and its half the size of the 250W so its going to be easier to cram it in there.

[remp]

Marklar

You can make a bridge rectifier from 4 diodes for about a dollar. If you cannot locate the diodes I can post you 4 diodes made into a bridge. Be prepared though that even if you bridge has failed it could have been taken out by parts after the bridge. Your system uses DC on the light I would guess hence the bridge to change your household AC voltage into DC voltage. Invariably after the bridge is a capacitor to store/smooth the Dc. They fail as well. But failure in bridge rectifiers is fairly common so you could be lucky. Let me know if you want a drawing or some parts