How It's Done
For centuries people had envisioned "instantaneous vision from a distance," or "television," but it was not until the invention of the telephone that practical proposals for such a system were put forth. Some of these early ideas resembled modern CCD or CMOS image sensors, however rather than scanning an image, each element required its own wire connecting it to a corresponding light gate or dimmable bulb on a reproducing screen. Obviously something like this would have been completely impractical for distances beyond a few feet, if it could even be made to work at all with the technology of the time.
It was during this time that a young German inventor by the name of Paul G Nipkow (pronounced Nip-kov) came up with the idea of the disc that bears his name.
The Equipment
Nipkow Disc
The Nipkow disc is an opaque disc made of a thin, rigid material (usually aluminum), with a spiral of holes punched in its outer rim. The holes are evenly distributed around the circumference of the disc, with each hole exactly one hole-width closer to the center than the previous hole. If an even, diffused light is placed behind a portion of the disc and the disc spun slowly, each hole would appear as a line, traveling either vertically or horizontally depending on whether you are viewing the side or the top of the disc, respectively. If the disc is spun much faster, those lines would appear to stack up on top of (or next to) each other and form an evenly lit space called a raster, due to the persistence of vision.
Now, imagine that light behind the disc is flashing rapidly and in step with the disc's rotation. The pattern of light/dark it would produce can be modified to cause a recognizable image to appear on the surface of the disc.
However this system has several drawbacks. The image it produced was very small (about the size of a postage stamp in most cases), and the viewing angle was limited so than only one or two people could view it at a time. Also the reproduced image would be very dim, and would be best viewed in subdued lighting.
The vast majority of mechanical television sets produced use some variation of the Nipkow disc, primarily due to its ease of construction.
An illustration of a 30-line Nipkow disc taken from "Television for the Amateur Constructor" by H. J. Barton-Chapple. The image would appear within the dotted square.
Lens Disc
The lens disc is essentially the same as a Nipkow disc, but with lenses in place of the holes. Lenses capture much more light than a simple pinhole and can focus the light onto an external surface, such as a ground glass screen. However, rather than a soft diffused light, lens discs require a bright, point source of light to project onto a screen. This required the use of a different type of neon lamp, called a crater lamp.
It is much heavier than a Nipkow disc and more difficult to construct, not to mention there is always the danger of poorly secured lenses flying off at high speed! Several American home television receivers using lens discs were produced in the 30s, including the Western Television Empire State model and the Model 41.
Not only can it produce a brighter image, but also a larger image- more than several square inches. J.L. Baird used lens discs in his earliest television cameras (see image) to get around the relatively poor sensitivity of photo detectors and the low gain of the valve-based electronics of the time.
An 8-line double-spiral lens disc as used by Baird in his earliest experiments.
Mirror Drum
Mirror drums consisted of a thick disc or cast aluminum drum with a set of mirrors mounted on the edge facing outward. The mirrors were all perfectly aligned radially (pointing away from the center of the drum), but each one was canted slightly sideways to reflect a narrow beam of light onto a screen. Each mirror corresponded to one line of the image.
Mirror drums were used only on the most expensive receivers, partly due to their size and difficult construction, but also because they required a more complex light source. In order to get a light beam that was both bright and quick enough to display pictures, an incandescent light was paired with a Kerr cell. The Kerr cell modulated the light, which was then focused through lenses to create a small spot on the screen.
Variants of the mirror drum survive to this day in some bar code scanners and laser printers.
Mirror Screw
Mirror screws were one of the most interesting and artistic designs of the early mechanical scanners. They consisted of a stack of long rectangular metal plates, polished to a mirror finish on the edge of one of the long sides. A hole is drilled through the center of the plates, and they are bolted together with a shaft running down the middle. Each plate is rotated a few degrees beyond the previous one, until the last plate marks a full revolution relative to the first one.
Mirror screws require a long, thin light source placed at a distance parallel to the shaft. When the screw is slowly rotated, it causes one tiny segment of the light source to appear on the edge of a mirror plate, then travel across it to the other side. If the plates are properly aligned, once the spot reaches the end of the first mirror, it will appear on the opposite edge of the next mirror, and so on.
Mirror screws don't project the images, rather the viewer looks directly at the screw. The picture appears on a plane sitting just in front of the shaft.
A Mervyn 30 line mirror drum on display at the Early Television Museum in Hilliard, OH.
A Bell Labs 60 line mirror screw
All these technologies share one thing in common: they become more impractical as the line count increases. There were several more advanced designs of mechanical image scanners intended for higher definition systems which are not covered here. For more information on those systems, see Peter Yanczer's archived site.