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Light Guide to Photomultiplier Coupler Kent Gerber U.S. Naval Research Laboratory, Plasma Physics Divi- sion, Washington, D.C. 20390. Received 12 November 1973. A simple design is given for enhancing the coupling be- tween plastic or glass light guides; and, in this case, an RCA 7265 phototube with an S-20 spherical section photocath- ode. Since the typical S-20 photocathode will reflect 30%, transmit 50%, and absorb only 20% of the 7000-Å light incident normally upon it, 1 the use of total internal reflection at both the photocathode-vacuum and window- air piano interfaces has become standard procedure. 2 With a curved (convex to the incoming light) photocath- ode, however, the incident light must be injected radially from the center of the photocathode window at or be- yond the critical angle for total internal reflection of 42° (n = 1.51) with the window normal. Figure 1 illustrates our design for the mounting of the fiber bundle in a coni- cal array at the critical angle. The mount is made in two parts, a cone with apex angle of 84° and a conical recepta- cle through which the fibers run and are epoxied in place. The array is then ground flat, polished, and at- tached directly to the phototube window with index 1006 APPLIED OPTICS / Vol. 13, No. 5 / May 1974
