| 摘要 |
PURPOSE:To measure thickness of a film with a high accuracy by receiving a movement of a scanning light beam by a position detecting light receiving element, and executing a feedback control so that said movement becomes a triangular wave signal of a driving source. CONSTITUTION:A light beam which has been scanned repeatedly by a scanning mechanism 20 is made to branch into two by an optical branching device 17, and one of them travels straight as it is and scans a gap part to be measured, and the other light beam is received by a position detecting light receiving element 18, and an electric signal corresponding to a position of a light beam spot is outputted, and amplified by an amplifier 19. This signal is sent to a driving circuit 22 of the scanning mechanism 20, compared with an output signal of an oscillator 21 and amplified, and the scanning mechanism 20 is driven so that its difference becomes zero. Accordingly, the light beam for scanning the gap to be measured is brought to feedback driving so that its movement always coincides with a triangle wave signal of a driving source, and a scanning speed becomes constant, therefore, an error caused by a variation of the scanning speed is not generated.
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