Electrical read-out from optical alignment devices

It is often desirable or essential to obtain from an optical alignment device an electrical output which represents, in sense and magnitude, the error of alignment. This can be used to operate a recording meter, in order to give a permanent record over a period, or an indicating meter for convenience or if the measuring head is in an inaccessible position. Also, it can in some cases be used as the input signal to a servo-mechanism which corrects the error. Three methods of obtaining such an output are described with reference to two types of alignment device, and some experimental results are given.