JPH0623693B

PURPOSE:To obtain an accurate measurement result constantly by performing relative evaluation of average power in reference to a detected maximum power from received power which is obtained by collecting time-series waves scattered to the rear. CONSTITUTION:A light-emitting element 6 is driven through a timing clock generation circuit 8 and a transmission circuit 7 and pulse-shaped infrared rays from the element 6 are emitted to a target W through a transparent protection plate 3. Then, scattering light which is scattered by the target W is received by a light-receiving element 9 through a transparent protection plate 2 and an optical lens 4. Reception voltage for each pulse hit from the element 9 is integrated at a reception circuit 10 and a reception pulse integration voltage V (V1 - Vn) which is integrated for each specified cycle is sampling-output. In a maximum value detection circuit 11, a maximum reception pulse integration voltage value Vm out of an integration voltage V is detected and an average value Va of the integration voltage V is detected at an average value detection circuit 12. Then, a ratio Q with the average value Va is obtained in a division circuit 13 in reference to a voltage value Vm and a measurement value is converted to a decibel value in a logarithmic conversion circuit 14.