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Apparatus set up for testing efficiency of the new crystal cell.
A New Light Modulator for Television.
With all mechanical television systems there has always been the same persistent demand for more light. Among the hither-to known light sources the neon lamp is the most common. Whilst it has the undoubted advantage that it is easy to modulate, the light efficiency is inadequate for modern television receivers, where only a small fraction of the light can be used. Mercury and sodium lamps are brighter and give a more pleasant colour, but fail to follow frequencies over 50 kHz. The Kerr cell, on the other hand, gives quite good light, although the actual efficiency is poor. It is, moreover, difficult to modulate with ordinary output voltages, and its characteristic is not linear so that it does not produce true black and white gradation.
The ideal of the television engineer would be a light source of high efficiency, easy to modulate, giving pure white light, and having a true linear characteristic. It must, in addition, have absolutely no lag, even at frequencies running into six figures. The prospects for further research in gas discharge lamps seem rather poor, as practically all combinations of gases, using both the negative plate and the positive column, have been unproductively tried. The more hopeful path lies in the direction of a light valve, such as the Kerr cell, but without the latter's short comings.
Working systematically on the lines indicated above, von Okolicsanyi, a well-known Hungarian television investigator, who is best known for his Mirror Screw scanner, searched for a light valve that would, in addition to the properties outlined above, fulfil the further demands that it should be cheap to produce, easy to handle and require no attention. This brought him to the conclusion that the substance needed should be a solid with a high Kerr constant and of great durability in order to avoid the inconvenience of liquids such as nitro-benzol. The solid must be sufficiently hard, but yet be easy to cut and polish. Lastly, the substance must he as clear as water and perfectly transparent.
Nature of the Crystal
Fig. 1.- Zinc sulphide crystal in its natural form.Click for modern image.
Okolicsanyi, there-fore, continued his search among inorganic crystals and tested many of these for electrical double refraction. This is known to occur to some degree in quartz, tourmalin and sodium chloride crystals. Electrical double refraction is a secondary effect in which the light is influenced indirectly by the electric field, i.e., through a dielectric. The static field directs the molecules of the crystal, according to the field strength, increasing or decreasing the angle of rotation in the polarised plane proportionately. It was found that such an effect was confined to cubic crystals, which could in turn be divided into two groups: the regular rock salt type, and the acentric group. in which the eventual solution of the problem was found, namely, the zinc sulphide crystal. (Fig. 1).
For experimental purposes a small plate of crystal such as shown in Fig. 2 was cut and ground. The dimensions were about 6 by 10 mm and 1 mm thick. This crystal plate was placed between two brass springs and the electrical potential applied to the two terminals, as shown.
Fig. 2. - Cut and ground crystal placed between metal conductors.
The crystal was then placed between two crossed Nicol prisms, and set up in a television receiver. To the great joy of the inventor a perfectly good bright image appeared, which seemed to him to fulfil the demands of an ideal light valve.
For commercial use the whole light valve is set up in a metal tube about 4 inches long, which contains the crystal, the Nicol prisms and two concentrating lenses. To prove the absolute superiority of the crystal cell, a receiver was constructed using a 90 hole Nipkow disc, which is optically the least efficient of all scanners. The disc is of 10 inches diameter, the holes being only one-tenth of a millimetre square. Using only a small 30 Watt lamp as the light source an image was projected on to a ground-glass screen, as shown in the photograph, the size being about one foot square.
Advantages claimed for the crystal cell over the ordinary nitro-benzol Kerr cell are:-
- Solid, therefore no evaporation or smell as in the case of nitro-benzol.
- Smaller, thus allowing the whole electro-optical system to be built into a single unit.
- Better light efficiency clue to closer spacing of the Nicols and crystal. Efficiency of about 35% compared with 10%
- Infinite durability.
- Linear characteristic.
- Smaller capacitance.
As a result of the lower capacitance, only one-tenth of the power needed compared with the nitro-benzol cell. As the cell can be placed directly in the anode circuit of the output valve a resistance of only 3o kΩis required (Fig. 3)
Fig. 3. - The circuit employed with the crystal cell.
The potential is about 400 Volts, so that the power needed can be calculated at about 5 Watts. This figure can be obtained with any powerful mains receiver.
The circuit arrangement employed is shown in Fig. 3. Okolicsanyi has therefore succeeded in finding a light valve that is suitable for any mechanical television receiver where a concentrated source of modulated light is required. This light source is more efficient in all respects than any other one previously known. It is, moreover, absolutely without lag. It is stated that measurements have been made, when it was found that the frequency curve shows no sign of falling off even at 350 kHz.
Even though the new cell is ideal for home television receivers from a technical point of view, the price is a further important factor in its favour. The price of the complete electro-optical unit, as shown in Fig. 4, is stated by the inventor to be under £5.
Fig. 4. - The crystal mounted between Nicol prisms. The two terminals are for the applied potential across the crystal.
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