DIY Video Projector Part II

[Guy Grotke]

redd38:

That stuff you listed will work fine. If you get a 200 to 220 mm fl condensor (first) fresnel (such as lumenlabs or 3dlens), then put it that same distance from the lamp arc. With this kind of geometry, a pre-condensor lens would not be worth the trouble.

If you get a 330 mm fl condensor fresnel, then the right pre-condensor lens would help gather more light. But, I would just use a 220 and not bother with a pre-condensor lens.

You can pay a lot for an optical-quality spherical reflector from places like Edmonds, but again I would not bother. Try a polished stainless steel mixing bowl from one of the DIY projector sites. They are under $10 precut to fit your Ushio lamp.

Don't forget UV & IR filtering, and one or more fans for forced-air cooling.

[jgr]

hey guy, with a spherical reflector, would you have to not use a condensor fresnel and reflect the front part of the light arc back to the reflector? (kind of like car headlights work)

[Guy Grotke]

Car headlights are parabolic, with the lamp filament a bit deeper into the "dish" than the focal point. That makes the resulting beam spread slightly.

A spherical reflector is used with the lamp arc placed at the exact center of curvature. Every ray that hits the reflector is reflected right back through the lamp arc and out the other side. The condensor fresnel "sees" all of this as a single bright point source. If the distance from the lamp arc to the condensor fresnel is exactly the focal length of the fresnel, then parallel rays will come out the other side of the fresnel. A field fresnel can then be used to focus all of those parallel rays to a point inside the projection lens.

There are other possible ways to do it, but any design that can't focus most of those rays into the projection lens will not give you a bright screen image.

[jason_ehl]

Hello All,

I have been reading through these posts for two days now trying to gain some insight into building a custom video projector.

I am currently working on a project where a very small projector is needed to fit within an internal cavity...a robotics app.

There are a few products coming out at the end of the year...the pocket projector type video projectors. A friend saw one at a trade show this week and said that the mitsubishi one...although small, is still very dim in a dark room.

I would like to try my hand at a home made version before paying 1000+ for a product that doesnt quite seem there yet.

I have a 2.5" lcd screen from accelvision...their products are quite good.

I have been looking at the luxeon v portable l.e.d. for a light source.

Aside from that I would need a lense assembly that can fit within a 1.5" diamter cavity.

I cant really go with a lamp due to the regual heat/fan issues.

Based on what I have read I still need to spec a fresnel lense (x?) and the lense...as well as filters?.

What I ultimately need to do is produce a 2-3' image on a screen if possible with the projector being a few feet away from the screeen surface.

I am not expecting crisp bright home theater pictures...but I would like to know if it is possible to rig something up that is at least on par with the pocket projector forecasts.

Any help would be greatly appreciated.

Thank you in advance

Jason

[jason_ehl]

P.S.

I have a cnc mill...so getting some of the tolerances for component placement shouldnt be too terribly difficult...

Just to reiterate the space constraints are quite tight.

Just to give some background

[Guy Grotke]

Scaling down from the size of my screen, a 4000 - 5000 lumen light source should give you a nice image in a dark room, not so nice in a well-lit room. If you can find that in an LED, then you might be able to make it work. (But I doubt that you can: LEDs are LESS efficient than Metal Halide lamps, so they make more heat per lumen.) You could do it with a 40 - 50 Watt MH lamp & ballast. If that is too bulky, then you might consider an exotic car headlight. Or maybe just a small 100 Watt halogen lamp with a good heat mirror to cut the IR.

You could use a fresnel condensor system, but your LCD is small enough that you could use a regular convex lens instead. surplusshed.com has several in that diameter or larger for about $6 US.

I think your main difficulty will be the tight space: You won't have much room for long focal lengths. A parabolic reflector eliminates the need for a condensor fresnel. You can put it very close to the LCD. The projection lens would have to be around 75 mm fl, to get a 27.5" image from 3 feet. The LCD to lens distance would be around 81 mm. Not bad. With this design, a 90 mm fl field lens placed about 10 mm before the LCD would direct all the light into your projection lens.

[jason_ehl]

Guy,

Thanks for the tips.

As you can tell I am new to this end of design.

I did a few more hours of research after my original posts and read your comments about MH light sources.

Any idea if there are dc versions? The system is needs to be portable and I have a limited 24v dc supply onboard.

With approx. 7.2 aH of juice I fear a MH bulb and a dc to ac convertor isnt going to result in long viewing times.

Any alternative you can think of?

Other question I have is...due to the small space...what is the likelyhood of a triplet magnifier vs. a projection lense working?

[Guy Grotke]

There are DC MH lamps, but they all take more like 90 volts. I think you are stuck with halogen tungsten. I am sure you could find a wide assortment of very bright marine halogem lamps that run on 24 VDC. Color LCD projectors are very inefficient: As I said earlier, I think you will need 50-60 Watts of lamp to get a viewable image. You can do the math...

Regarding the projection lens: Does the image need to be perfect? Maybe a little barrel distortion would look okay for this application. In that case, you could just use a simple planoconvex or positive meniscus lens. Hard to beat $6 from surplusshed.com.

If it does need to be perfect, then you need a projection lens. A magnifier will not be optimised for a wide enough field of view. The problem is that you need something with around 75 mm fl to get the image size at the throw distance you mentioned. It will have to cover the 63 mm wide LCD from a distance of 81 mm, so it will need a pretty wide angle FOV. Maybe a large format camera lens?

[jason_ehl]

Guy,

Thanks again for the help.

This part of the project is turning into its own little nightmare.

I was wondering if you had any links for acceptable yet small diameter projection lenses. I need the overall diameter of the lense assembly to be no larger than 1-1.25"...case included in that spec.

I know they exist...I saw a infocus lp120 today with a pretty small lense on the front end.

Have you come across a supplier for any such beast?

[Guy Grotke]

look at these at surplusshed.com:

http://www.surplusshed.com/detail.cfm?ID=L1196
http://www.surplusshed.com/detail.cfm?ID=L3277
http://www.surplusshed.com/detail.cfm?ID=L3278

The first is a simple lens that would fit within a 1" hole, and give you the size image & throw distance you mentioned, with a 2.5" LCD. But it will not have an image that focusses at the center and the corners at the same time.

The latter two are good multi-element lenses, but their mounting may be too large for you. Their light paths would not be all that large, so maybe you could mount one sticking out from the surface of your robot. Actually, at these prices (ie. $7 each) you could get two to mount on the "head": Use one for projection and the other for machine vision.

What about the 2.5" LCD you mentioned? That is not going to fit in a 1.25" wide chamber!