Method and apparatus for detecting a fluorescent area on a sheet of paper

Claim 1

Apparatus for detecting on a moving sheet of paper the presence, in predetermined areas, of marks which become fluorescent when exposed to ultraviolet radiation, comprising a source of ultraviolet light, a source of current modulated at a frequency of about 10 KHz for supplying current to said ultraviolet source, first filtering means for filtering ultraviolet light produced by said source and directing said ultraviolet light on said sheet to produce fluorescent radiation from said marks, second filtering means for filtering said fluorescent radiation from said marks, a first photo-electric element sensitive to fluorescent radiation and disposed on the same side of said sheet as said source of ultraviolet radiation to receive filtered fluorescent radiation from said marks to produce a first-signal, first circuit means for detecting, demodulating and amplifying said first signal received from said first photo-electric element, a second photo-electric element disposed between said first filtering means and said sheet to receive directly ultraviolet light from said source filtered by said first filtering means to produce a second signal, second circuit means for detecting, demodulating and amplifying said second signal, means coordinated with movement of said sheet to produce first and second reading windows, first trigger means controlled by said first signal and said first reading window producing means to produce a first output signal measured on a first predetermined area of the sheet, second trigger means controlled by said first signal and said second reading window producing means to produce a second output signal measured on a second predetermined area of the sheet, and means controlled by said second signal for regulating the operating level of both of said trigger means.