N00B needs help

[lone_wolf_84]

Hello, Where can I go to find information about what triplets and doublets are, what condensors are, what "focal length" i need, etc etc. I have been looking and cant find a definitive source for this information.
Thanks

[Guy Grotke]

There are lots of posts here about projectors people have built, and there are other forums on DIY LCD projectors. Read them! Some even have online stores or group buys with pretty cheap (and tested) components. Some have lots of public domain design examples, some sell plans for specific designs.

www.diybuildergroup.com has a "light engine tool" free excel download you can use to see what lenses, panels, and projection distances give you a particular size image.

Once you figure out what focal length lens to use, you can look for that as a singlet (single lens), a doublet (assembly of two lenses, selected to correct chromatic aberration), a triplet (assembly of three lenses, selected to correct more aberrations), or a multi-element lens. Try to find one at least 65 mm in diameter, and don't buy a curved-CRT projector lens to use with a flat LCD. Some people claim to get adequate results with a $6 singlet from surplus shed. Others spend hundreds. I suggest you start simple and then improve it.

A condensor is usually a Double Convex (DCX) lens about an inch from a light source, used to capture a wide arc of that light and bend it all into the projection path. Since the lamp is closer than the condensor's focal length, it works in magnify mode (as opposed to projection mode).

Here is a pretty good optics information website you can peruse:
http://hyperphysics.phy-astr.gsu.edu...enscon.html#c1

[lone_wolf_84]

ok, thanks. So a condensor lense, is that what a fresnel is? And what i ment by asking what a triplet was i ment what kind of lenses it used, same thing for the doublet.
Thanks

[ace3000_1]

Heya buddy, thats actually incorrect, they are normally PCX, they come in pairs or singles, (singles for what we need).

Here is a correct PCX condenser setup run in pairs.

Lone_wolf, have you seen the search page? use the forums search function to find the information you want, this forum covers prety much everything you need .

Trev

[Guy Grotke]

These DIY designs actually have two different, but cooperative optical systems:

1) The light engine, that sends light through the LCD panel in such a way that most of that light strikes the objective lens.

2) The LCD panel, objective lens, and screen.

You can project an image with just #2, but it will be very dim. This is the same as the 100" TV internet scam.

In #1 most designs use a big, short fl converging lens (ie. a 100 mm diameter 200 mm fl DCX or PCX) very close to the lamp to capture 33% of the output (or closer to 66% with a spherical reflector behind the lamp) and converge it to a lower fresnel the size of the LCD, a lower fresnel (ie. about 220 mm fl) to turn those diverging rays into a parallel beam that sends light through each pixel of the LCD, and an upper fresnel (ie. about 330 mm fl) to converge the resulting "modulated" rays toward the objective lens. Other common designs put both fresnels together below the LCD, so the rays are already converging before they go through the pixels. In one sense, all three light engine lenses are acting as a condensor. But I was referring just to the first lens in the light engine. Cheap fresnels are made from PVC, good ones are made from acrylic, and "way more expensive than we need" ones are made from glass.

In #2, an objective lens is used to focus the image of the LCD on the screen. You can do this with a simple DCX, PCX, or positive meniscus lens that has the right focal length. There are also more complex multi-element lenses that try to correct various aberrations, but those aberrations may not even be noticable in some projectors. (That's why I suggest starting cheap and simple, then adding improvements as needed.) Two aberrations to look for are chromatic aberration (red and blue fringes near the edges of the image), and spherical aberration (bulging edges). You can correct chromatic ab by upgrading to an achromat: Generally a crown glass DCX + flint glass PCV with a focal length ratio of 38:59 (optimal because of the different refractive indexes of the materials), and of course they have to have a combined focal length that is usable in your situation. For example, I am using a 4" diameter 200 mm fl DCX and a -308 mm fl PCV to get 570 mm combined fl, which is as close as I could get with cheap lenses from surplus shed. ($7.50 total cost) These correct the "prism effect" because the crown glass DCX bends the colors differently in one direction, and then the flint glass bends them differently in the other, so they recombine on the screen. I did this because I needed a combined fl I could not buy on the cheap. But you might be able to use an OHP objective lens, or one of the triplets you can buy from various DIY projector websites. I have seen 320 mm fl triplets for $29.

[lone_wolf_84]

hi, thanks for the replys, yes i have searched the forums, alot of forums, but either I dont know what to search for specificly or something, cause ive learned more in the three post's on this thread then I have the last 3 days of reading threads with mixed jargon in it. While i got your nice peoples attention (Which i really appreciate btw) i have an old bell and howell 3870a overhead projector, how many parts could i salvage fromt this? And i would be extatic if some could tell me or show me a link telling me what types of lens's are in a triplet.

Thanks

[Guy Grotke]

You can use it intact, just as it is. Assuming it still works, of course. If you get an OHP projection panel (ie. ebay $75-100) and lay it on the top, you are in business! Most of these have both VGA computer input and composite video input for watching TV & DVDs. (Look for at least 640 by 480 native resolution with 16 million colors. 1 million colors is ok but not as nice.) Many people are perfectly happy with this setup. (Typicall, people who like watching their big screen TV better than fiddling with a DIY projector. . )

This also assumes your OHP has the light source below the transparency surface. Some of the real cheap newer OHPs have the light source up by the lens assembly, and use a reflector below the transparency surface. Those don't work with LCDs. The polarization twists done by the LCD result in no light reflected back through the pixels.

Another option is to dissassemble it, to build more of a box DIY projector. You can use the fan, the fresnels, the lens assembly, maybe the focussing mechanism, the power cord & switch . You could start your development with the OHP lamp, but they don't last very long. I would replace that with a Metal Halide bulb, ballast, spherical reflector,and condensor lens. MH bulbs give you lots more light, less heat, and run 10000-20000 hours. Of course, they cost $20-40.

The next improvement is to go with a better LCD. 15" LCD monitors are down to the $200 range now, and have great parameters. You should get a model that somebody else has already tried, so you know it is easy to strip away the backlight and then get all the PC boards out of the way, etc. The OHP fresnels might not be quite big enough to completely light a 15" LCD (ie. 9" by 12"). You might need to buy some larger fresnels. If you used a 14" LCD monitor, then they should be big enough. (But most 14" panels have 50 msec response time, so they show some ghosts and trails when stuff moves fast.)

In general, start with what you have and then upgrade as needed. If you want more light, go to MH. If you want less image distortion, go a triplet. If you want higher resolution, higher constrast ratio, and better response time, go to a stripped LCD monitor panel.

[ace3000_1]

But I was referring just to the first lens in the light engine.

Guy, are you trying to tell me somthing? lol

I was only talking about the primary condenser aswell and you will never find a company who will sell you a DCX lens as a primary condenser because none are made. Sure maybe you will find one as a secondary for instance in a slide projector (very rare), but never as a primary. Primary condensers are either PCX or Asheric, the index of refraction on a DCX is too steep to be used as a primary condenser and it will therfore give you distorted light and a hotspot.

The diameter of a condenser also matters, the correct size for a 250w mh bulb with an arc of 25mm is 65-80mm, flooding a condenser with light will distort the image, not having enough light in a condenser will give you a dull image so this calculation needs to be taken very seriously in a design and calculated correctly for optimum results. The power factor of a condenser also plays a role, ideally in our setups, a short focal condenser with a low power has far less chromatic distortion then a short focal with high power.

Once these parameters have all been calculated out correctly only then a condenser will make the world of difference.

I gather you got your information from the DIY Builder Group, the only reason they use the DCX over there is because thats the only lens that JCB could find to work in his system, and its scientificly incorrect.

Here is a Aspheric condenser, infact these are the best, they work perfect in a singular formation with a spherical reflector, on a large panel. These are actually called condensers where as the PCX is an alternative.

http://www.edmundoptics.com/onlineca...productid=2454

And here is your PCX.

http://www.edmundoptics.com/onlineca...productid=1747

And your DCX

http://www.edmundoptics.com/onlineca...productid=1748

Notice how the DCX is designed for imaging while the other 2 are for light collecting?. The DCX is best used in a multi element lens system, ie: a triplet, not for collecting/condensing light.

Trev

[Guy Grotke]

Here is a simple discussion of various lens properties:

http://www.olympusmicro.com/primer/l...nsesintro.html

Here is a nice paper on Cooke Triplets:

http://www.willbell.com/tm/The%20Coo...r%20Lenses.pdf

Triplets are generally made with a negative lens (ie. a DCV) between two positive lenses (ie. DCXes). They are designed to correct chromatic aberration at more than just two wavelengths, and also to do some spherical aberration correction. Designing them is so complex, that everybody uses computer simulations now. There are too many variables to wing it: focal lengths, spacing beyween lenses, lens materials, etc. Just buy one! If you want to make your own, get a degree in optics.

You can assemble your own achromatic doublet: This corrects the chromatic aberration for red and blue, and makes it easier to get a longer focal length for a long-throw projector. But even a simple DCX can be used for low magnification applications. Most DIY projector applications should qualify as low magnification.

[Guy Grotke]

Thanks for the info, but I'm not sure I understand it all:

>the index of refraction on a DCX is too steep to be used as a primary condenser

What does the shape of a lens (DCX versus PCX, etc.) have to do with the index of refraction of the material it is made from? (ie. crown glass=1.52, flint glass=1.65) Did you mean the reflectance at the edges of the DCX lens gets too high?

>a short focal condenser with a low power has far less chromatic distortion then a short focal with high power

What do you mean by power? The lumens put out by the lamp?

Why would chromatic aberration matter in a condensor lens? We are not trying to focus the lamp's individual bits accurately. Or do they end up contributing to the total chromatic ab in the final image because you get blue rays travelling in a different direction than red rays? Wouldn't that very small difference in angle be much smaller than the effect of the non-point-source lamp arc?

I also noticed the edmunds 75 mm diameter condensor lens has a fl of 50 mm. So this is a very much stronger lens than the 200 mm fl lens I mentioned. Wouldn't this add more distortion?

I suspect that either could be made to work just fine in a DIY projector application. After all, they are being used in magnification mode, rather than projection mode. It should just be a matter of adjusting the distance between the lens and the lamp arc, until the light fills the lower fresnel. It would be interesting to see if anybody could tell the difference, or measure it with a light meter. On the other hand, that edmunds lens costs $47.90, and you can get a 100 mm diameter 200 mm fl DCX from surplus shed for $3.50. I think the difference between "good enough" and perfect might not be worth the price. Has anybody actually tried both in the same system?