Density control for photographic enlargers

Claim 1

A CONTROL DEVICE FOR PHOTOGRAPHIC ENLARGERS IN ORDER TO PRODUCE A PRINT OF PREDETERMINED DENSITY COMPRISING: (A) A TIMER EQUIPPED WITH AN ADJUSTABLE ELEMENT ADAPTED TO BE SET BY AN OPERATOR TO PRESELECT EXPOSURE TIMES, (B) AN ANALOG COMPUTER OPERABLE IN RESPONSE TO PRESETTING BY THE OPERATOR TO GENERATE A VOLTAGE PROPORTIONAL TO THE LIGHT FLUX REQUIRED TO PASS THE LENS OF THE PHOTOGRAPHIC ENLARGER TO PRODUCE THE PRINT OF PREDETERMINED DENSITY, (C) SAID ANALOG COMPUTER COMPRISING INDICATING MEANS PROVIDED WITH TWO SCALES MOVABLE RELATIVE TO EACH OTHER AND EACH PROVIDED WITH A POINTER, THE FIRST SCALE BEING GRADUATED TO DENOTE MAGNIFICATION RATIOS, THE SECOND SCALE BEING GRADUATED TO DENOTE NEGATIVE FILM AREAS, AND A THIRD SCALE CALIBRATED IN EXPOSURE-TIME VALUES AND IN REGISTRY WITH THE ADJUSTABLE ELEMENT OF SAID TIMER, (D) AN ELECTRICAL NETWORK INCLUDING A RHEOSTAT AND A POTENTIOMETER, EACH ONE CONSISTING OF A RESISTOR AND A CONTACT MOVABLE RELATIVE TO EACH OTHER WITH THE CONTACT OF SAID POTENTIOMETER BEING CONNECTED TO THE POINTER OF SAID FIRST SCALE AND SIMULTANEOUSLY MOVABLE BY AN OPERATOR WHEREBY SAID POTENTIOMETER IS ADJUSTED TO PRODUCE A VOLTAGE IN ACCORDANCE WITH A PRESELECTED MAGNIFICATION RATIO AS INDICATED BY THE POINTER ON SAID FIRST SCALE, AND ONE END OF EACH RESISTOR BEING INTERCONNECTED; (E) MEANS FOR IMPRESSING A PREDETERMINED INPUT VOLTAGE BETWEEN THE CONTACT OF SAID RHEOSTAT AND ONE END OF THE RESISTOR OF SAID POTENTIOMETER, (F) MEANS TO DERIVE AN OUTPUT VOLTAGE FROM THE CONTACT OF SAID POTENTIOMETER AND THE INTERCONNECTED ENDS OF SAID RESISTORS, (G) AND THE POINTER OF SAID RHEOSTAT AND THE ADJUSTABLE ELEMENT OF SAID TIMER BEING CONNECTED TOGETHER TO CAUSE SAID RHEOSTAT TO BE AUTOMATICALLY ADJUSTED TO VARY THE OUTPUT VOLTAGE WHEN THE OPERATOR PRESELECTS A DESIRED EXPOSURE TIME ON SAID THIRD SCALE.