| 摘要 |
A laser inspection system is provided having an absolute closed loop dc system in which the power gain is automatically stabilized. In order to stabilize the power gain of the system and provide a closed loop absolute reference dc level, a sample of the radiation from the laser beam utilized to scan the material being inspected is applied to a photomultiplier tube for generating a reference pedestal signal which is dependent on the intensity of the laser beam and is free of the influence of the characteristics of the material being inspected. The reference pedestal signal is applied to a comparator which develops a control signal which is applied in a single closed feedback loop including in effect, the laser source and the light collection and detection system within the loop. This loop includes the photomultiplier tube and its high voltage power supply to which the control signal is applied for stabilizing the gain of the system. Accordingly, if the sensitivity of the receiver varies, the photomultiplier drifts, drift occurs in the power supply, and/or the laser beam intensity drifts, the feedback loop will detect such changes and compensate for them, to produce an absolute, stable dc system.
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