PURPOSE: To increase the efficiency of converting an energy beam signal into an optical signal by directly applying a phosphor or similar material to an optical fiber such as an image guide without installing a glass member between the phosphor and a light-detecting system.

CONSTITUTION: An optical signal conversion system consists of an image guide 3 the front surface of which is coated with a reflection-preventing film 1 and a phosphor 2. It efficiently transmits light produced in the phosphor 2 when an energy beam becomes incident upon the phosphor 2 to a light-detecting system 4. The thickness of the reflection-preventing film 1 is properly adjusted so that an energy beam 5 has sufficient intensity when it reaches the phosphor 2. Besides, components of light produced in the phosphor 2 traveling in given direction are reflected by the reflection-preventing film 1 in order to increase the signal transmitted to the image guide 3. Consequently, it is possible to increase the efficiency of converting an energy beam signal into an optical signal.