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目前在航测作业中用得最多的印象机是五灯印象机和紫光印象机。前者根据底片黑度调节五个灯的亮度（有的机子中间灯泡可以上下移动），后者靠关掉一部分紫光灯的办法来满足底片黑度不均匀性的要求。这两种调光办法
At present, the most used impression machine in aerial survey operations is a five-light impression machine and a violet impression machine. The former adjusts the brightness of the five lamps according to the blackness of the negative film (some of the middle light bulbs can move up and down), and the latter can meet the requirement of non-uniformity of the blackness of the negative film by turning off a part of violet light. These two dimming methods