Automatic exposure control circuit for photomicrographic apparatus

Apparatus for specimen photomicrography including a lamp source, focusing optics and a film chamber, said focusing optics being adapted to image a specimen in the film plane of the chamber;

• shutter means for selectively exposing the film in said chamber for an adjustable time interval at least equal to a predetermined value to enable use of a lamp source having an intensity which is insufficient to unduly heat and/or damage a specimen;
• means for sequentially determining the feasibility of obtaining a properly exposed picture of a specimen and for thereafter operating the shutter means to expose the film for a time interval commensurate with a proper exposure comprising:
• a normally deenergized solid state switching circuit having an input and adapted to generate an output when a signal of a predetermined threshold level is applied to its input;
• threshold circuit means;
• photoelectric transducer means positioned in the optical path between said source and said shutter so as to be activated by light from the specimen;
• first and second lamp means;
• settable time delay means for developing a predetermined adjustable signal level after a delay interval associated with the film speed of the film in said chamber;
• means for charging said time delay means with an adjustable charging current having a magnitude which is related to the film speed of the film in said chamber;
• motor means for opening said shutter means;
• normally deenergized means for closing said shutter;
• operable actuable shutter release means for energizing said motor to open said shutter;
• relay means and first switching means responsive to the state of said shutter means to energize said relay means when said shutter means is closed and to deenergize said relay means when said shutter means is open, said relay means having first movable switch means for coupling said transducer means between said charging means and the input of said threshold circuit means and for coupling the input of said solid state switching means to the output of said threshold means and said first lamp means to the output of said solid state switching means whereby said threshold circuit means develops an output at least equal to said threshold level when said transducer means is illuminated by light of a sufficient level to cause said solid state switching means to light said first lamp means to indicate the feasibility of a proper exposure;
• said relay means including second movable switch means responsive to deenergization of said relay means for coupling said transducer means between said adjustable time delay means and the input of said solid state switching means and for coupling the output of said solid state switching means to said shutter closing means whereby said shutter closing means is energized to close said shutter means after said predetermined time delay;
• said relay means including additional movable switching means responsive to deenergization of said relay means for illuminating said second lamp means and for decoupling said first lamp means from said solid state switching circuit to assure that said first lamp means is energized only during a feasibility testing interval and said second lamp means may be energized only during a film exposing interval.