In a nutshell, the function of a television camera is to convert a two-dimensional image into a one-dimensional electrical signal. and the function of the receiver is to convert the signal back to an image. This conversion is accomplished by scanning: dividing the image into an array of small picture elements (pixels), stepping through them in sequence (left to right in each row then top to bottom row by row), converting the brightness of each pixel to a voltage, and transmitting the resulting sequence of voltages. The receiver reverses this process, placing each received pixel back in its proper place in the image. It accomplishes this by scanning a spot of varying brightness across the image plane in synchronization with the scanned spot in the camera.
The heart of the camera is the scanning disk. It consists of a spiral pattern of holes (24 in this picture, 30 in the actual disk) which scan the image onto the photodiode, and a single synchronizing hole which produces a pulse when the first scanning hole begins its scan.
As can be seen from the drawings and photos, the camera is built around a square housing which encloses the disk to keep out stray light and provide mounting for the various components. The capacity of our facilities limits the maximum size we can conveniently fabricate to about 4 in. This gives a disk diameter of 3.5 in with the radius of the outermost scanning hole being 1.5 in or 38 mm. For a scanned area 1 cm high this makes the radius of the innermost hole 28 mm and places the optical axis at a radius of 33 mm.
Obviously, for best picture quality we want to divide the scanned image into as many pixels as possible. However, as we increase the number of pixels, we must either reduce their size or increase the image area.
If we reduce the size of the pixels, we must reduce the size of the holes in the scanning disk. But smaller holes let through less light, and less light produces smaller photocurent, which in turn requires more amplification and is more susceptible to noise.
Larger image area means a larger apparatus, but it also means a larger photodetector. Larger photodetectors are more expensive and slower (because of increased junction capacitance). We have chosen a 1 cm square photodetector as the starting point for our design.
With the disk and photodetector size we have chosen, we can put 20 holes in the disk before successive holes overlap the detector. This is about the absolute minimum for usable resolution (the earliest "commercial" mechanical systems had 24). We can increase this by masking off part of the detector, but once we get past about 30 holes, the reduction in light becomes excessive. We have chosen a 30 hole disk for this lab.