What it shows:
Like visible light, invisible infra-red radiation can be refracted by lenses to produce an image on a screen. Indeed, a slide projector designed for visible light is used as the imaging device; a heat- sensitive screen makes the invisible IR image visible.
How it works:
Our IR source is a 1000 watt "lantern slide" projector 1 from which we have removed the special heat-absorbing glass in the condenser assembly. The slide to be imaged is some kind of lettering, like the name of the course (Science A-29, for example). 2 An opaque (to visible light) black glass IR filter 3 is positioned in front of the projection lens so that no visible light falls on the screen. The screen is a black liquid crystal sheet 4 which turns green to blue, depending on the amount it is heated. A small 15W fluorescent light illuminates the screen so that the audience can see the color changes - within a few seconds, the name of the course appears in blue on the screen.
Setting it up:
The entire setup fits on one of the large (4.5' or 5' long) lecture carts. The projector-screen distance (about 70 cm) is a compromise to have enough heat to produce colors on the liquid crystal screen and yet have the image large enough for the entire audience to see. Depending on the ambient room temperature, we use either the 20-25°C or the 25- 30°C sheet. If it's exceptionally warm in the lecture hall, the lower-range sheet will already be green at room temperature and, although not much additional heat is required to turn it blue, the image contrast will not be as great.
Remember to remove the heat reducing glass filter in the projector before attempting this demo.
We suggest starting without the IR filter and an additional regular white screen placed in front of the liquid crystal screen. Not only does this allow you to focus, 5 but the audience can also see what the slide image is like in visible light and thus will have a better idea of what to look for in the invisible. Having done that, place the IR filter in front of the projector lens so that the audience can see that there is no (visible) light coming out of the projector. Now remove the regular screen and within a few seconds the liquid crystal screen should show the same image. The fluorescent light can be on during all the preliminaries. In any case, don't use an incandescent light bulb to illuminate the liquid crystal screen!
1 Slide King II, Charles Beseler Co., East Orange, NJ
2 The "slide" is made from 1/16" thick aluminum sheet cut to the standard 3.25" × 4" size. A large opening in the middle is covered with brass stenciled letters. This (all metal) scheme is quite durable to the heat it is exposed to, and one needn't worry about melting the slide!
3 The filter is 6" in diameter and comes from a surplus Army Sniper scope (purchased for $5.95 in 1984 from Herbach and Rademan (H&R Co.), 401 E. Erie Ave., Philadelphia PA 19134-1187).
4 The sheet measures 6" × 12" and is part of a set available from Edmund Scientific (E.S. #61160). The set consists of five sheets in 5°C ranges: 20-25°C, 25-30, 30-35, 35-40, and 40-45. These are also available in 12" × 12" sheets as well as other temperature ranges. The sheet is taped onto a multi-purpose "screen" made of 14" × 20" 1/8"-thick Masonite® fitted into a saw-cut slot of a 2×4 piece of lumber. These "screens" are painted white on one side and black on the other.
5 Focusing the visible light on a screen a few centimeters in front of the IR screen happens to work out well since the focal length for the IR will be a bit longer than for the visible (remember, red refracts rotten).