Method and device for measuring the quantity of water dampening the face of an offset printing plate

A device for measuring the quantity of water dampening a face of an offset printing plate comprising:

• a light source for projecting a light beam at a suitable projection angle to the face of the offset printing plate;
• a first light sensor angularly disposed with respect to said face of said offset printing plate at an angle equal to said projection angle of said beam projected from said light source so as to receive the light beam directly reflected from the face of said offset printing plate;
• a second light sensor disposed substantially normal relative to the face of said offset printing plate so as to receive diffused portions of the light beam reflected from the face of said offset printing plate;
• and
• a display and operation means for receiving outputs from said first and second light sensors so as to compute a quantity of water dampening the face of said offset printing plate and displaying an indication of the quantity of water, said display and operation means receiving said outputs by detecting an output A.sub.SAT from said first light sensor when the face of said offset printing plate is essentially saturated with dampening water, detecting an output A.sub.DRY from said first light sensor when said face of said offset printing plate is essentially dry, detecting an output from said second light sensor and determining the quantity of dampening water according to the following equation:
• ##EQU7## wherein A is the output from the first light sensor representing the quantity of dampening water present, A'=.alpha.A, .alpha. is the variation rate of the output A from the first sensor, B'=.beta.B, .beta. is the variation rate of output B from the second sensor expressed as b.sqroot..alpha. and b is a constant having a value between 1.0 and 2.0.