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612 BROLLY December
LIGHT-INTENSITY REQUIREMENTS
Phenomenal sensitivities to light have been attributed to the imageorthicon tube. In fact recognizable pictures have been produced with a light intensity (incident) of less than 1 foot-candle. But under such low light conditions the picture is marred by a snowy or rainy appearance of the picture field, an effect which has come to be called noise in television parlance.
Three factors affect the light intensity required for television pickup. They are : the sensitivity of the image-orthicon tube, the amount of amplification used, and the lens aperture or speed. Considering these points in reverse order, the following facts are important.
The phenomenal sensitivities and operation with low light levels are usually achieved with large lens apertures of the order of //2.0 or better. Such large apertures produce very shallow fields of focus, so they are not practical for production of programs. Apertures of the order of //ll are more practical and such a reduction of aperture (//2 to//ll) requires more than 30-fold increase of light intensity to produce the same intensity of image on the screen inside the image-orthicon camera tube.
It might seem that since we amplify the television impulses created by the camera tube, we could increase this boosting almost indefinitely to compensate for weaker illumination. But there are practical limits to this also. At extremely low light levels the electrons released by photoelectricity in the camera tube become so few that the so-called shot effect produces noise or rain effects which seriously mar the picture. The same effect also may take place in the first amplifier tube of the camera chain. Under these circumstances further amplification increases the noise or rain along with the picture. So we have no alternative but to use more light.
Now with regard to the image-orthicon tubes themselves, we find quite a range of sensitivities. There are three different types of image-orthicon tubes. They all look alike and the principal difference among them is in the treatment of their photoelectric screens. Of the three, the one which usually exhibits the phenomenal light sensitivities has considerable sensitivity in the deep and infrared part of the spectrum. This is an unavoidable result in the present stage of development of the art of photosensitization. This red sensitivity is no great handicap to televising of sports events but it is a definite drawback t6 studio programming where true representation of colors,