| 摘要 |
A method and apparatus for determining a range of sight which are capable of monitoring and correcting the operability of the instrument. A light emitter directs an emitted light beam to a measurement zone and a light receiver receives a received light beam from the measurement zone. The emitted light beam and the received light beam cross each other in the measurement zone. A deflector deflects at least a portion of the emitted light beam downstream of the measurement zone along a light transmitting path and directs the portion of the emitted light beam into the received light beam at a side of the measurement zone opposite from the light receiver. An optical shutter intermittently closes the light transmitting path in a temporally controlled manner.
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