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Photo Electric Cells
By Richard Van Slyker
The surprising growth in recent years of photo-electric cells (phototubes), as applied in the industrial field may be easily understood when the versatility of the photo-tube is known, when seen in the countless uses to which it has been applied. A review of its multifold uses show that it ranges from automatic counting and sorting of various sized objects to the most delicate comparisons of colors and measurements of densities and of semi-transparent objects with an almost unbelievable degree of accuracy and fidelity.
As mentioned in a previous chapter, photo-tubes when used in an amplifying circuit require two types of amplifiers, the static and dynamic. Where we deal with slow gradual or abrupt changes of light intensity the static type of amplifier is desirable in one or more modified forms because of its easily controlled range of action and simplicity of construction. However, in some cases it is desirable to incorporate in the amplifier design an oscillating circuit so as smaller light intensities of a pulsating nature may be handled with a greater degree of sensitivity. This addition of an oscillating circuit permits a conventional design of dynamic amplifier to be used in gaining additional volume.
Again in the classification of phototubes the uses of the photo-tube is divided into several classifications irrespective of whether the tube is photo-electric, actino, or photo voltic.
A LIGHT SOURCE B LENS C SUBJECT D PHOTO CELL E LIGHT RAY
The divisions are:
(A) — Where the subject is interposed between the light rays and the photo-tube. (Fig. 1).
(B) — Where the reflected rays of the subject are directed to the phototube (Fig. 2) .
(C) — Where the measurement of light intensity is such as to permit direct exposure to the photo-tube (Fig. 3).
It is the intention of this article to try and present to the projectionists some representative uses of the photo-tube as shown in the above classifications, outside of the immediate use of the motion picture industry.
In the accompanying photograph (Fig. 4) a means of automatic control of the mixing, or, titration of
Nine
FIG. 1