How it works… Camera Pick-up Tubes

“Television is the art of instantaneously producing at a distance a transient visible image of an actual or recorded scene by means of an electrical system of telecommunication.”

The wording of the British Standards Institution’s definition of our work, gives a somewhat unimaginative description of an art form that has, perhaps, a most complex method of construction to produce small black- and-white shadows, yet also has a high degree of human attraction.

The first process in any television system the creation of an optical image of the scene to be transmitted. This optical image is formed in the case of cameras by the camera lens, and is focused by it on to the face of the pick-up tube inside the camera. It is then the function of the pick-up tube to convert the light image into an electrical current which varies in magnitude, in proportion to the amount of light reaching the tube from each part of the scene at any given time.

The exchange of light for an electrical current is the principle upon which a photographic exposure meter works. The tiny light-sensitive cell in an exposure-meter gives a small electrical current in direct proportion to the average light value of the subject which it is seeing and registers this visually on a scale.

This device, invaluable for photographers, is useless for television because it cannot convey information about the detailed structure of the scene. In the past many methods were tried, using the light-sensitive or ‘photo-cell’ as it is called, to break up the picture into small parts and to allow the photo-cell to look at one part at a time in strict sequence and to register a separate value of current for each. This is similar to the way in which the eye ‘scans’ a printed page, gleaning different information word by word and line by line.

Baird scanned his scene by means of an opaque rotating disc containing a series of holes in the form of a spiral placed in front of his photo-cell. The spiral was arranged so that, as the disc revolved, each hole in turn swept across the picture, allowing the photo-cell to see a small, continuously varying part of the scene. When the last hole had completed its scan across the bottom of the picture, the first appeared and started the operation at the top again. The current from the photo-cell was proportional at any instant to the amount of light reflected through the hole from the scene.

Another method reflected light from the scene by means of a revolving drum containing mirrors set on the outside in a staggered formation. When the drum revolved at high speed, each mirror reflected into the photo-cell light from a slightly different part of the scene each time.

These mechanical methods, however, suffered from extreme clumsiness of operation and poor definition because of their inability to scan the scene in small enough elements. A ‘major break-through’, as the popular press would now call it, was the innovation of a vacuum tube with electrical scanning.

This tube, rather like a large valve, was developed in this country in the early 1930’s and brought the first high- definition television system in the world into operation in 1936. The tube was called an Emitron or Iconoscope and was cumbersome, insensitive and, by today’s standards, somewhat crude in operation, but was in fact the parent of most present-day camera tubes.

In appearance it consisted of a cylindrical glass tube about 14 inches in diameter, one end of which blossomed into a bulb about 7 inches in diameter. The bulb had a flat window on one side and inside, parallel to the window, was mounted a ‘target’ upon which the optical image of the scene formed by the lens was focused. The tube joined the bulb at a slight angle and contained a device like a gun, which in effect fired a tiny stream of electrically-charged particles at the target.

This fine beam of charged particles called ‘electrons’, was made to sweep over the target in straight lines one below. the other, starting from top left of the picture through to bottom right and back to top left again. This sweeping or scanning could be achieved electrically at enormous speed and in fine detail because electrons are so small that they are almost weightless. At first sight this may appear to be unconnected with our original problem, which was the inability of the photo-cell to distinguish between the brightness details of a picture. In order to see in what way this can be useful, we must consider more closely the function of the target.

The target (so called because the optical image lands there as well as the electron beam) is made up of a sheet of mica covered on the front with a mosaic of many thousands of tiny photo-cells, each one insulated from each other and able to work independently. The mica sheet is then backed by a metal plate.

The situation is now that the optical image projected on to the target by the lens is not falling on one photo-cell but on many thousands so that each one is receiving an amount of light in proportion to the brightness of that particular element of the original scene. Part of our problem has, therefore, been solved, as the scene can now be divided into many small elements and each photo-cell can give a separate value of current.

As always, of course, there is a snag. What was an advantage at first sight now becomes a problem, because as soon as light falls on to the mosaic all the photo-cells merrily start to produce a current which they store rather like a battery. This current is not all required at once, and this is where the beam of electrons comes into its own.

As it scans the mosaic, it acts like a switch and touches each photo-cell in turn, causing them to discharge their stored current to the metal plate at the back of the target.

This current can be collected and forms the signal output of the tube representing at any given instant the discharged current of one photo-cell and, therefore, the electrical equivalent of one picture element. As this is changing extremely quickly there is a continuous output from the tube which is called the ‘video signal’. This signal is amplified and processed in many ways to become the transmitted picture, which when re-created by the receiver builds ‘a visual image of an actual or recorded scene…’ as in the definition.

This then was the first attempt at high definition tele- vision and many improvements followed fairly rapidly. Perhaps the most significant was the separation, in the pick-up tube, of the two functions performed by the target of electrical image creation and scanning. This was achieved by placing in front of the target a semi-transparent sheet having photo-electric properties which gave an electrical magnification and greater sensitivity. This type of tube was called an image iconoscope. The construction of the target was also modified to have a greater electrical storage, i.e. a larger battery, which was then scanned in much the same way as before.

These tubes were in use for a number of years and were in fact still used after the war.

Still further increases in sensitivity have been made and more efficient and complex tubes are now used by most television broadcasting organisations. The ‘image orthicon’ which is used for television broadcasting cameras, is perhaps the most versatile and widely used tube today. It has such sensitivity that in certain cases it will give a reasonable picture by moonlight. Its method of signal production does, however, differ from the image iconoscope but the same basic principles apply to both.

Another new tube, the ‘Vidicon’, is used extensively in telecine machines and industrial cameras and indeed some of the most significant developments of recent years have been in the field of vidicon tubes which, because of their small physical size and relative cheapness (£25 as against £450 approximately for an image orthicon), have been much favoured and will doubtless become increasingly useful in a wider field when certain fundamental snags are overcome.

Television cameras have many uses in fields other than broadcasting and one of the most significant has been their introduction into the field of medicine. Here, their use is obvious as a means of allowing many hundreds of students and nurses to view on closed circuit a major surgical operation, often in colour, which only a few at a time could normally watch in an operating theatre.

Additionally, because of their sensitivity under certain modified conditions of working, television cameras can be used to intensify an X-ray image in order to keep the ‘dosage’ of X-rays to a minimum for the patient while still providing a satisfactory picture under deep penetration conditions.

The use of television cameras in space-satellites is only just beginning and recent developments have been rapid indeed, as has been similar progress for the armed forces. So, from the humble photo-electric cell and Baird’s scanning disc of the 1920’s, has grown a world-wide industry employing many hundreds of thousands of people of varying skills which, I think it is fair to say, can give pleasure, entertainment, instruction and a means of research on a scale quite undreamed of even 30 years ago.

Of the future we can only make an intelligent estimate based on the rate of growth to date and the present state of the art and science.
That international exchanges of vision and sound will take place on a day-to-day basis and that trans-Continental networks will be available for world events is obvious from present indications.

That many future transmissions will be in colour and possibly stereoscopic as well would be a natural extension to the reality of today’s programming.

That more and more use of closed circuit viewing will be likely perhaps for specific programmes on a wired ‘pay as you view’ service, and eventually as an added facility to the telephone service.

Whether television could be made cheap enough for domestic use ‘See if junior is asleep in the nursery’ type of thing is debatable at present. Certainly a flat picture-frame type of viewing tube is possible and indeed in development at present.

That we shall explore, remotely at first, the outer edges of space and the depths of the oceans is again highly likely. In all these projects, however, the television camera tube has played and will continue to play a vital part in what is, perhaps, the most exciting medium of communications of our age.