be present at input terminal 4 logic stage 9 will now divert the voltage previously present at output terminal 6 to a connection 11 to logic stage 10 logic stage 10 is connected to the photosensor 2 in transmitted light barrier 24 by input terminal 5 if the film 12 illustrated in fig3 has already been developed photosensor 2 will receive the light emitted by source 22 because developed films are practically transparent in the infrared range in this case a signal will accordingly be present at input terminal 5 if logic stage 10 is accordingly provided with a signal by way of connection 11 and input terminal 5 there will also be a signal at output terminal 7 but no signal at output terminals 6 and 8 if the film 12 is undeveloped and will accordingly have a blocking effect in the infrared range the photosensor 2 will receive no light and there will be no signal at input terminal 5 in this event a signal at connection 11 but no signal at input 5 there will be a signal at output terminal 8 in this state there will be no signal at output terminals 6 and 7 controls 3 can be considered a logical component with two input and three output terminals to ensure unambiguous results here transmitted light barrier 24 will be ideally positioned where it can act on the edge of the film.