Photoelectric document authenticating apparatus with age and color compensation

Claim 1

IN A CIRCUIT FOR AUTHENTICATION OF PAPER SECURITIES THE COMBINATION OF MEANS TO PROJECT LIGHT ONTO A PAPER SECURITY TO BE TESTED, A REFERENCE PHOTOCELL TO VIEW A PARTICULAR AREA OF THE PAPER SECURITY TO BE TESTED, CIRCUIT MEANS CONNECTED TO THE REFERENCE PHOTOCELL TO PRODUCE AN OUTPUT VOLTAGE DEPENDENT UPON THE AMOUNT OF LIGHT DETECTED BY THE REFERENCE PHOTOCELL, A PLURALITY OF TEST PHOTOCELLS ELECTRICALLY CONNECTED WITH THE OUTPUT VOLTAGE FROM THE CIRCUIT MEANS TO VIEW OTHER AREAS OF THE PAPER SECURITY TO BE TESTED, A SEPARATE SECOND CIRCUIT MEANS IMPRESSED WITH A CONSTANT INPUT VOLTAGE ELECTRICALLY CONNECTED WITH EACH TEST PHOTOCELL COMPRISING A PAIR OF ZENER DIODE CIRCUITS EACH HAVING ONE END IN COMMON TO THE VOLTAGE POTENTIAL DETERMINED BY THE TEST PHOTOCELL AND ONE CIRCUIT HAVING ITS OTHER END CONNECTED TO THE CONSTANT INPUT VOLTAGE TO THE SECOND CIRCUIT MEANS AND THE OTHER CIRCUIT HAVING ITS OTHER END CONNECTED TO GROUND, SAID ZENER DIODE CIRCUITS BEING OPPOSITELY DIRECTED TO PASS CURRENT ONLY WHEN THE VOLTAGE POTENTIAL THEREACROSS EXCEEDS A PREDETERMINED VALUE SO THAT AN ACCEPT TEST IS PRESENT WHEN NEITHER ZENER DIODE CIRCUIT CONDUCTS CURRENT, AND MEANS TO SENSE ALL THE ZENER DIODE CIRCUITS ELECTRICALLY CONNECTED WITH ALL THE TEST PHOTOCELLS TO DETERMINE WHETHER THE PAPER SECURITY TESTED IS AUTHENTIC.