60 line Nipkow disc camera

History

In 1930, Rene BARTHELEMY experimented in his C.D.C. laboratory in Montrouge, two techniques of catch of sight, which it uses for exploration in 30 lines, each one having its advantages and its disadvantages.

One of it, also used by John L. BAIRD in London, is the 'Flying Spot'. With this technique, the subject to be televised is plunged in total darkness and a thin spot of intense light coming from an arc lamp , sweeps it face, exploring all contours.

See below a photo taken during such scanning experiment (studio of Marc CHAUVIERRE.1932).

The reflected light on the face was then collected by several photocells placed on both sides of the subject and those generated a weak variable electric current which was amplified and transmitted to the receivers. The great disadvantage of this technique, which did not survived anyway, was the peremptory necessity to plunge the scene to be televised in a total darkness. The technique of catch of sight in ambient light, is based on a different principle. This time, the scene to be televised is violently enlightened and the light of the projectors, reflected on the elements of the scene, is collected by a unique photocell placed behind the camera's Nipkow disc .

By those times, the main inconvenient of this technique, is the enormous amount of light necessary to sensitize the single photocell. This was to compensate the great loss of light through the Nipkow scanning disc. Effectively, the smaller the holes of the disc , the greater the loss of light. Thus the early 60 lines broadcast program claimed a very big quantity of light, equivalent to 15.000 luxes.

One of the deals that one have to face with, when he want to reproduce mechanical television images, is to be able to generate a video signal compatible with old mechanical receivers or actual prototypes. Effectively, nowadays no contemporary television broadcasting system is compatible with what existed during early times.

The television camera presented here, was the first machine I built for my Mechanical Television Workshop. It was a must, because it was the unique way to provide a video signal to my 60 lines receiver, before I got and uses the Aurora multistandad video converter.

The camera design and realization required more than 1500 work hours and required various knowledges in optics, photomultiplier characteristics, mechanics and electronics.

Characteristics of the prototype

- This camera uses the catch of sight in diffuse light technique. It is equipped with a 60 holes Nipkow disc driven by a synchronous motor at 1500 rpm.b

- The sensitivity of the machine is considerably higher (+1.000.000) than that of its ancestor, thanks to the use of a photomultiplier tube to replace the old photocell. Because of its great sensitivity, the camera circuitry include an electronic protection to prevent an accidental overload of ambient light that may damage the photomultiplier tube. In the same way, the backdoor actuates a switch with the opening, in order to protect the photomultiplier from an inopportune entry of light inside the machine.

- This mechanical camera is the only operational machine of this type in France. A cathodic monitor can be connected to it, for purposes of image control and demonstration.

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