| 摘要 |
PURPOSE:To prevent the titled device from deviating from a detecting range of a focus against a sudden fall of a photodetecting output, by stopping a movement of an objective lens, and holding it, when the intensity of reflected light from the surface of an object has been below a prescribed value, in a microscope, etc. CONSTITUTION:When an object surface 6 to be observed exists in a focal position of a lens 5, photodetecting signals A, B from photodetectors 14, 15 are equal, therefore, a divided output becomes '0', and a lens driving part 23 is not driven. When an extremely large uneven part exists on the object surface 6, the photodetecting signals A, B become both small, but when an added output (A+B) becomes smaller than a reference value D, an analogue switch 31 is turned off, therefore, an abnormal output value from a divider 20 is not inputted to an integrator 21, and an integral output is maintained for a prescribed time (a discharge time of the integrator 21). When an irradiated position to the object surface 6 returns to the normal state within this time, the photodetecting signals A, B become large, and the automatic focal follow-up is continued. |
