31 August 2023 tbs.pm/80075

RALPH W. HALLOWS, our technical expert, puts you “in the picture” with this description of how the TV camera works — and why it can see better than the human eye

From TV Mirror for 21 November 1953

THOUGH it is a much younger member of the moving picture family, the TV camera has now learnt pretty well all the tricks known to its elder brother of the cinema. It can swing through a “pan,” “zoom” from distant view to close-up, change quickly from one lens to another by means of its turret, move smoothly about the studio on its “dolly” or go where it is wanted on its outside-broadcast mounting.

In one important respect it has the ciné camera beaten: it can make good pictures in much poorer light, for its sensitiveness is almost uncanny. You have probably been surprised sometimes when watching a football match on the TV screen to hear the commentator say “The ball’s gone over to the far side of the field and the light is so bad that I can’t tell you what’s happening.” Though he cannot see, you can — as plain as a pikestaff on your screen, thanks to the keenness of the camera’s

A motorised camera “dolly” in action at Lime Grove. The dolly (as the wheeled base is called) is powered by an electric motor and steered by a tiller at the back. The cameraman can “pan” (swing his camera from side to side) and raise or lower himself by means of pedals

Where the light falls

The TV camera makes its pictures and passes them on to you in a most ingenious and interesting way. It’s not easy to explain, but I think I can give you an idea of it without asking you to wade through a sea of technicalities. We’d better begin with the ordinary camera, such as you use for holiday snapshots.

If you examine an exposed film before it is developed (in the dark-room, of course), you find that it looks exactly like one that hasn’t been exposed. Yet it contains a recorded picture; you could put it away for months and the picture would still appear when the developer had got to work. Until it is developed, the picture is “latent,” or hidden.

When the lens of an ordinary camera focuses an image on to a film, light causes chemical changes to take place in each microscopic granule of the sensitised emulsion with which it is coated. The changes vary according to the brightness of the light; they are different in the white, grey and black parts of the picture. Their effects do not become visible until the developer has brought about further chemical changes.

The TV camera’s lens focuses images on to a different kind of sensitised coating. It is called a “mosaic” because, like a mosaic floor, it is composed of vast numbers of small, separate pieces. Each of the millions of tiny pieces in the TV mosaic is affected by light. But the changes which it produces in them are electrical and not-chemical.

The tiny pieces are called cells. Any cell on which light falls collects a little store of electricity —an electric charge. The brighter the light, the bigger the charge. No light, no charge.

The picture, then, is recorded in the form of millions of different electrical charges on the surface of the mosaic. Cells in the white parts have the biggest charges; in the very pale greys charges are smaller. And so it goes on through the whole range of shades from white to black, till in the dark grey parts charges are very small and in the blacks there are none at all.

As in the under-developed film, we have a latent image on the mosaic. Some means of “developing” it must be found before it can be used by the TV transmitter.

Fast-moving beam

The charges are positive. What makes it possible for electricity to light our homes, operate our TV receivers, run trains and do all the other useful things that it does today is the fact that anything with a positive charge is in a kind of uncomfortable, strained condition and thirsts for a negative charge of the same value to come and put it out of its pain. When that happens to a cell of the mosaic, the negative charge flows in, positive and negative cancel one another out, and the cell is discharged.

It is on those lines that the development of the latent picture on the mosaic is done. A narrow beam of electrons is made to pass over the surface of the mosaic in very much the same way as your eye moves over this page. The eye moves steadily along a line from left to right. Then it flashes rapidly back from right to left, travelling a little lower as it does so, to the beginning of the next line. Line by line it “scans” the column till it comes to the bottom, then it flashes up to the beginning of the next column.

One big difference is that the electron beam reads rather more quickly: it covers over 10,000 lines in every second!

Varying shades

Every one of the myriad electrons in the beam is a tiny chunk of negative electricity. Each positively charged cell in the mosaic takes in, as the beam touches it, just enough of them to quench its thirst and discharge it. The bigger its positive charge, the more electrons it takes.

And so the latent image is developed. The discharge of each cell in the mosaic causes a “whiff” of current to flow, the size of the whiff depending on whether a particular cell has recorded a white or a different shade of grey. A black leaves no record and so there is no whiff of current.

In ways too complicated to describe here, each whiff of current is collected as it occurs and turned into a corresponding impulse sent out by the transmitter.

That’s a very much simplified account of how the TV camera does its job. I think you’ll agree that it is a very wonderful instrument.