(You would almost certainly not use the Radar or Sonar spins under this heading)

• A pulse of carrier wave is dispatched at intervals determined by the clock. 

• The energy will spread out (inverse square law) and be partially absorbed by the medium. These diminutions in signal strength must be taken into account.

• A portion of the energy will be reflected by any discontinuities that exist on a line from the transmitter perpendicular to its aperture. The return time and strength will be noted by the system.

• The next pulse will be dispatched in a different direction (perhaps by using a rotating aerial, or perhaps by using an array of emitters and introducing a progressive phase shift into the signals supplied to them so as to do a kind of diffraction grating in reverse) so as to return information about discontinuities in a different part of space.

• A CRT has an electron beam making a sweep under the control of the timebase, which is synchronised to start just as the pulse starts its journey.

• The mixed signal is fed to the grid of the CRT so as to affect the brightness of the trace. Reflected signals cause an increase of brightness.

• The timebase is organised to sweep the beam across the screen in a direction related to the direction in which the signal is transmitted. In this way a map is built up.

Radar

The transducer is a radio aerial, often letterbox-shaped so as to give a wide diffraction pattern vertically and a narrow pattern horizontally (remember that pattern width varies inversely with slit width). Since the resulting image is a map, this gives good resolution in the plane of the map while ensuring that everything in a vertical direction is picked up.

Ultrasound 

The transducer is a piezo-electric crystal, which vibrates when a pd is applied across it (and vice-versa). 

Sonar

The transducer is a piezo-electric crystal, which vibrates when a pd is applied across it (and vice-versa). I don't think you sweep through different angles with sonar, but I'm not going to take the time off to look just at the moment.

Suitable for CCTV cameras, camcorders, infra-red cameras, etc.

• A CCD chip consists of an array of independent photodiodes, each one constituting a pixel.

• Incident radiation (visible or I.R.) releases a number of electrons on a given pixel proportional to the strength of the radiation and how long it is on for.

• Periodically the charges are harvested onto a capacitor that develops a voltage proportional to the charge and therefore to the radiation intensity. This potential is converted to binary by an ADC and the value stored in a memory chip.

• Control circuitry sees to it that a particular memory location always stores the information about the radiation intensity on a particular pixel. The circuitry polls the pixels in order across the chip and then moves down to the next row but one, and so on, scanning every other row. Then it goes back and does the intermediate rows. It mustn't stop, because the charge accumulated is proportional to the elapsed time as well as the intensity.

Note that you can do smoothing in reverse to extract detail from an image that isn't at first sight in the original. Suppose you have a 3 x 3 grid, but the resolution of your telescope or whatever is such that you can only see four squares at once, and the number you get back is the sum of the four squares.

To make life easier, we'll allow the grid to be surrounded by a notional ring of squares with 0 in them - off the picture, as it were:

So the telescope / radar / scanner looks first at abfg, then at bcgh and so on. There are 16 such combinations, and the unprocessed image information might be

i.e. a+b+f+g returns a value of 1, b+c+g+h returns a value of 4 and so on. Try to reconstruct the original 3 x 3 grid surrounded by the notional ring of 0s. Remember that a, b, c, d, e, f, j, k, o, p, t, u, v, w, x and y are all 0. Here is the answer.

Much use is made of this technique in processing images from Hubble.

There is a system in which you have a thick lead plate with holes in it, so angled that only those X-Rays travelling in a particular direction get through, and yet another system in which the X-Rays setting off have to get through two holes in lead plates, thereby ensuring that they are going in a particular direction.  These rays are said to be collimated. I will try to find out more about how you form the very sharp images obtained in a CT scan. I suspect that what happens is that you have a collimated beam shining on to a photomultiplier tube on the opposite side of the apparatus, and that you move the beam (and subject) so that it passes through all  the bits of the subject one after the other. This produces a varying output from the photomultiplier tube, which you sample each time the beam is shining through a different bit of tissue. Software later sorts all this numerical data out into a series of images of 'slices' of the body.

Here are the issues I would like to find out about:

• How many beam/detector pairs are there in the 'doughnut'
• Do the beam/detector pairs go round and round as the subject goes through the machine or do they oscillate?
• Does the subject go through in a series of steps, giving circular sections, or does (s)he go through continuously, giving a spiral pattern of exposures?
• How do they prevent the central axis of the subject getting a higher dose of X-Rays than the outer parts?

I've found nothing very useful on the internet in a short surf. If anyone comes up with something good, perhaps he could alert us all?