If you look at some olf B&W TV footage, you frequently see bright images 9such as light reflecting from metal surfaces0 as turining to black. is this an artifact of the TV system in use back then? What is this called?
It’s called flare, and it’s a common characteristic of the old image orthicon tubes in the cameras that were in use then.
It was not only shiny metal surfaces that caused this. Objects such as a white shirt-front or white paper could also cause this. So newsreaders and front of camera presenters would have their notes on yellow paper and not wear white shirts.
Why, exaclty, did it turn black? I’ve seen photographs where the sun has burned black its portion of the film, is it a similar phenomenon?
A technical explanation from 1939:
The present methods of Television depend upon the iconoscope, invented by Dr. V. K. Zworykin of Westinghouse and RCA, for its operation. This conventional type of iconoscope consists of an evacuated glass tube within which there is a plate covered with a mosaic composed of many tiny photosensitive globules. . . . When the scanning beam strikes the globules of the mosaic, however, secondary electrons are emitted by the globules, which in a large measure depend on the positive charge induced by the action of the optical image. The current resulting from these secondary electrons is collected by the collector electrode, which is back of the mosaic plate and constitutes the signal current. Because of the high electron velocity in this beam, however, there are more secondary electrons emitted than are produced by the scanning beam; and since, with an insulated plate at a given potential, the number of electrons leaving the plate must be equal to the number liberated, some electrons fall back in a shower onto the mosaic; and as their distribution is not uniform over the mosaic they produce ‘dark spots’ in the image.