| 摘要 |
PURPOSE:To measure the moving extents of a lens, used to read information on a video disk, in the optical axis direction and its orthogonal direction independently, simultaneously and accurately, by using a detection prism having a critical angle to a reference light beam in incident luminous flux. CONSTITUTION:An objective lens 1 whose moving ectents are to be measured and which is arranged movably in the optical axis direction X and the direction Y at right angle to paper face is irradiated with light from a light source device 2. Light passed through the lens 1 is made incident to a detection prism 26 through a condenser lens 25. The reflecting surface 27 of the prism 26 is set nearly at a critical angle to a reference light beam in the incident luminous flux, and reflected light from the reflecting surface 27 is made incident to a photodetector 28. The detector 28 is provided with photodetection areas 28A-28D devided in a direction alpha parallel to the incidence surface of the reflecting surface 27 and a direction beta at right angles to it. The moving extent of the lens 1 in the optical axis direction is measured from the sum of and difference between photodetection outputs of both the areas across the direction beta of the detector 28, and the moving extent in the direction at right angles to the optical axis is measured from the sum and difference between the detection outputs of both the areas across the direction alpha independently and simultaneously. |
