| 摘要 |
PURPOSE:To enhance the accuracy of measurement of thickness and to enlarge an area serving as a subject for measurement. CONSTITUTION:Laser beam La for exciting photothermal vibration is intermittently emitted toward the upper surface 2a of a diaphragm portion 2 from a laser light source 3. This intermittent period is varied with frequencies generated by a frequency generator 11. Laser beam Lb for detecting displacement is similarly applied to the upper surface 2a of the diaphragm portion 2 from a laser light source 6 and the reflected light Lb is detected by a light detecting portion 7. Displacement caused by photothermal vibration is detected from the speckle displacement of the reflected light Lb. The frequency of the laser beam La when the displacement detected is at its peak is found as a resonance frequency and an arithmetic and control processing portion 10 accurately calculates the thickness of the diaphragm portion 2 according to the resonance frequency and the known properties of the diaphgram portion 2 such as the material of which it is made. |
