PURPOSE: To measure a real time a high reliability by using an array-like fiber and a lattice-like electrode, and preventing a pseudo signal of a real time measurement by an adoption of a high detecting sensitivity photodetector and an ionized ion by vacuum ultraviolet rays, respectively.

CONSTITUTION: Light to be measured passes through lattice-like electrodes 8, 9, and irradiates a photoelectron converting surface 4. On this surface, the light is converted to an electron, amplified by a specified channel in a microchannel plate 1, and thereafter, a fluorescent plate 5 is made to emit a light. In this case, an irradiating position of the light on the photoelectron converting surface 4 and a light emitting point on the fluorescent plate 5 correspond to each other by one by one. As for the light of the fluorescent plate 5, the greater part propagates in an array-like optical fiber 10 being near the light emitting point, and it is detected by a photodetector 11, converted to an electric signal being proportional to its intensity, and outputted. A positin on the photoelectron converting surface 4 irradiated by the light, and the photodetector 11 correspond to each other by one by one. Also, an intensity of the light on the photoelectron converting surface 4 appears as magnitude of an output of the photodetector 11, and a spectrum of an incident light is obtained by displaying successively an output of the photodetector.