Photographic exposure meter

Claim 1

AN EXPOSURE METER FOR USE IN DOCUMENT PHOTOGRAPHING APPARATUS HAVING A PHOTOGRAPHING STATION FOR RECEIVING DOCUMENTS, MEANS FOR ILLUMINATING THE DOCUMENT WHEN IN SAID PHOTOGRAPHING STATION, MEANS FOR VARYING THE INTENSITY OF AND FOR EXTINGUISHING SUCH ILLUMINATING MEANS, AND AN EXPOSURE STATION FOR RECEIVING A DOCUMENT IMAGE RECORDING MEDIA TO RECORD AN IMAGE OF EACH ILLUMINATED DOCUMENT, SAID EXPOSURE METER COMPRISING AN ELECTRICAL BRIDGE CIRCUIT FOR COMPARING RESISTANCES, TWO PHOTO-CONDUCTIVE CELLS OPERATIVELY INTERCONNECTED IN SAID BRIDGE CIRCUIT AND FORMING OPPOSED RESISTANCE ELEMENTS THEREIN, MEANS FOR CONFINING THE FIELD MEASURED BY ONE OF SAID CELLS TO A GIVEN AREA OF THE PHOTOGRAPHING STATION, MEANS FOR IRRADIATING SAID CELLS WITH A SUBSTANTIALLY CONSTANT FLUX OF RADIANT ENERGY TO MAINTAIN SAID CELLS IN AN ACCOMMODATED STATE, MEANS RENDERING SAID IRRADIATING MEANS EFFECTIVE WITH RESPECT TO SAID ONE CELL ONLY WHEN SAID DOCUMENT ILLUMINATING MEANS IS EXTINGUISHED, MEANS FOR BALANCING SAID BRIDGE CIRCUIT WHEN THE INTENSITY OF ILLUMINATION OF A DOCUMENT IN SAID PHOTOGRAPHING STATION AS MEASURED BY SAID ONE CELL IS SUFFICIENT TO PROVIDE CORRECT EXPOSURE FOR PHOTOGRAPHING SUCH DOCUMENT ON SAID RECORDING MEDIA, AND MEANS FOR INDICATING WHEN SAID BRIDGE CIRCUIT IS BALANCED.