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The internal losses are due to absorption and the small but finite losses suffered in the numerous internal reflections due to deviations from the prescribed, cylindrical fiber cross-section and minute imperfections of the core-jacket interface.

These losses depend on fiber diameter and length, absorption coefficient, the mean value of the loss per internal reflection and last, but not least, on the angular distribution of the incident light.

Explicit expressions ( integral averages ) are given in the literature.

Lacking reliable data for some of the variables, we are relying on experimental data of about 20 percent internal losses for 1/4-inch long, small ( 5 - 10 M ) diameter fibers.

This relatively high value is probably due to the small fiber diameters increasing the number of internal reflections.

Since we are considering here relatively small diameter ( 1 - 1.5 inches ) fiber plates, their average thickness can be kept below 1/4 inch and their internal losses may be assumed as 15 percent ( per plate ).