What it shows:
The simplest method of controlling light to form an image is to use an opaque mask with a pinhole in it. Rectilinear propagation of light explains all (nearly). A video camera is substituted for the old prototypal shoe box so that an entire audience can see the pinhole image "live."
How it works:
An extremely light-sensitive video camera 1 sans lens substitutes for the pinhole camera box and film. The front of the Newvicon is the image plane and, as this sits right behind the faceplate of the camera body, it is necessary to use an extension ring to position the pinhole a reasonable distance from the image plane. 2 Over the front of the extension ring is stretched a piece of aluminum foil which is held in place with a rubber band. A sewing needle 3 is used to prick a tiny pinhole into the foil. Despite the incredible sensitivity of the video camera, the subject to be imaged (usually a person) must be brightly illuminated - two 500 watt spotlights at about arm's length from the subject work well. Just as our brain automatically inverts the upside down image on the retina, so does the electronics in the camera.
Setting it up:
The camera sits on a tripod at approximately eye level. Each of the spotlights is mounted on its own tripod, also at eye level, and flank the camera to each side. So as not to obscure the blackboard, it's best to set up the whole arrangement off to the side of the lecture hall.
Begin the demonstration with a regular lens in place so that the audience has a sense of what to expect to see on the video projector (or monitor) and this also allows you to aim the camera properly before you turn it into a pinhole camera. Obviously the spotlights must be turned off at this point and, depending on the amount of ambient light, it may be necessary to have the lens stopped down considerably (typically f/8). Next, turn the camera off (this is very important - otherwise the Newvicon may be irreparably damaged), remove the lens and replace it with the extension ring. Lest the audience thinks you're just replacing the lens with yet another, you can first poke your finger through the ring to demonstrate that, indeed, it is only a ring and not optics you're mounting on the front of the camera. With the extension ring screwed in place, stretch a small piece of aluminum foil over the front of the ring and hold it in place with a rubber band. Turn the camera back on (it's easy to forget this and later wonder if your pinhole is not large enough when you don't see an image on the monitor). Turn on the two spotlights and you're ready to make the pinhole with the needle. Have an extra needle and additional foil handy in case you drop the former or mess up the latter. You may have to adjust the camera aim slightly if you didn't get the pinhole in the center.
Try adding two or three pinholes, or vary the size to see the effect on resolution. Note though that the auto-gain of the camera will thwart your predictions about image brightness. Prof. Roy Glauber has added a fun twist to this demonstration by having the subject (himself) hidden from direct view of the audience by a screen. While the camera operator is busy removing the camera lens, Glauber dons a Groucho Marx mask which the audience doesn't see until the pinhole has been made.
1 The Ikegami model ITC-410 with option S4075 (Newvicon™) is our choice for low light level applications such as this. It's rated sensitivity is 0.106 lux scene illumination and 0.01 lux faceplate illumination.
2 Since the image size is 2/3" (16.9 mm) on the diagonal, the range of 20 to 40 mm extension gives an image comparable to a wide-angle to telephoto format.
3 In a pinch you can use a sewing pin, but a needle is sharper and will produce a smaller (better) pinhole.