Projector - Near Infrared Light Source

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Hi, I am working on science project and I have to expose the surface of a special material to an "image" made of near infrared light (700 nm to 800 nm). The image must have a good resolution (at least 640x480) and its size must be relatively small (between 3" and 10").

I thought building an homemade projector would be a cheap solution to get good results. However, since the optics and LCD of projectors are designed to be used with visible light (~400 nm to 700 nm), I am not sure that it would work as well as I want it to. I was wondering if anyone of you would have the absorption spectrum of a standard transmissive LCD panel to see if it absorbs a lot of light in the range I am looking for (if it does, perhaps a DMD chip from Texas Instrument would do a better job). I am also concerned that some IR filters might be included on some of the optical parts. I think I shouldn't have much problem with the lenses since the wavelengths that I would use are pretty close to visible light. Most projector lamps emit a lot of IR light, so I guess I could only use a lowpass filter to cut all visible light. Please correct me if I am wrong.

I would also like to know if standard projection lenses can be used to throw a well-focused image at only a few inches from the projectors (since I want a 3" to 10" max image size). Would it require additional optics?

Thanks a lot for your help, this forum is awesome!

color filters & LCDs

I know (from experience) that all of the photographic filters (ie. Wratten from Kodak, etc.) pass a lot of IR. They block some part of the visible spectrum, but they pass as much or more IR as they do of their specific visible light color. (So you can make a very nice IR pass filter from a red filter and a blue filter together.)

I don't know about the IR behavior of the color filters used in LCDs. You could try making one of those IR pass filters I mentioned, and then measure what gets through.

The easiest way to do what you want would be to get a used commercial three-LCD projector and replace the three external color filters with IR pass filters. (These projectors use small LCDs without built-in color filters.) You could actually just block the light to two of the three LCDs, and send IR through the third LCD. Make sure you don't remove the polarizer, or you won't get an image projected. If the projector has a dichroic IR filter or reflector, then you might need to replace that with a longer wavelength filter or a non-dichroic reflector so you get more IR to the LCDs. But you can try it first: Even with those IR-removers, a 10" image will be very bright.

Projecting a 10" diagonal image may be a problem too. You can't form an image if the "screen" is closer than the focal length of the projection lens. You might have to change to a shorter focal length (higher "power") simple lens.

If you want to build a custom projector, you might look at:
They have some monochrome LCD kits for as low as $199. Then you would just build a standard DIY projector but add an IR pass filter.
Google it.

Silkscreen is the way they put the white lettering all over printed circuit boards. Its also used as a stylised art method of printing a photograph.

I assume you want a black/white equivalent from the infared. Silkscreening can create a white image on overhead film for example. You shine the light through the film and light passes the non-white areas. Very cheap and no "projector" required.

A similar process is used to etch printed circuit boards. You paint photo resist on the copper side of the board and use a silkscreen trace with a UV light. When you etch the copper the etchent cannot get through the areas that wern't broken down by the uv (under the white pattern). Microprocessors are created in a similar fashon.

Should work with a heat lamp.
silkscreen is a pain!

And it's pretty much obsolete: If you need to project a single still image, just print on a piece of transparency film with a laser printer. Then you can cut it down to size, put it in an opaque frame, and use it instead of an LCD in a DIY projector. I think black laser printer toner will block near IR just fine.

In fact, if that is all you need, then you should just shoot a 35 mm photo of a negative image you want using high-contrast film. Then mount that shot in a 2" by 2" slide frame and stick it in a standard slide projector. (Slide projector lamps put out more IR than visible light.) Add an IR pass filter, and you're done. In fact, you will get at least 24 shots on your roll of film, so you could make a whole bunch of different slides in case you need to try multiple images.
I am already able to make still images with a photographic slide and an old Kodak projector that is filtered with a longpass filter (>700 nm). Now I am looking for video (IR video).

A projector that uses LCDs that do not have any built-in filters is a good idea, I will try to see what is available. It is still hard to find LCD and polarizer absorption information though.

Any idea where I could get a shorter focal length lens that would otherwise be much like any projection lens?

Thanks again for your help.
projection lens

If you start with a small LCD and want a projected image around the same size, then the lens is easy: You can use a simple DCX lens. If you make the LCD to lens distance equal to the lens to screen distance, then you get a 1:1 image.

You will still need fresnels to condense the light into the lens, so I would base the design on available fresnels. You can get 200 mm fl fresnels from (#A260). If you put a pair of these 20 mm before the LCD, then a 100 Watt Halogen lamp (high IR) would go 200 mm from the fresnels. The field fresnel will focus the light to a point about 180 mm past the LCD, so that is where you should put the lens. For a 1:1 image size, the lens to screen distance will also be 180 mm. So you can calculate the focal length you need:

1/fl = 1/180 + 1/180 = 2/180 = 1/90

So you need a 90 mm fl DCX lens, at least 2" in diameter. has a 91 mm fl 54.8 mm diameter DCX (#L6453) for $4.

A small mono LCD is not really expensive or difficult to find. A controller that can run it is. That's why the Earth LCD website is very useful. They have complete kits of LCD & controller in many odd sizes and types.
Another little-known IR tool

It's not well known, but almost all CCD cameras are even more sensitive to near IR than to visible light. They add IR cut filters to good cameras. Otherwise, the IR gets through the red, green, and blue pixel filters and blurs the image.

Most video cameras see IR even with the cut filter, if you add an IR pass filter over the lens. You can make them even more sensitive if you can open them up and remove the IR cut filter (usually looks like a flat disk of green-tinged glass).

So you can use a video camera to view you IR projection in realtime. You can test your video camera by looking through it at an IR remote control. It should look like a very bright point source when you push a button on the remote.
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