Arrangement for and method of determining the amount of dampening agent on a printing-image carrier

An arrangement for determining the amount of dampening agent on a printing-image carrier, said arrangement comprising:

• (a) A radiator source for directing radiation at the carrier;
• (b) first means for generating first signals in response to radiation which returns from the carrier and has a predetermined wavelength at least approximating an absorption wavelength of the dampening agent;
• (c) second means for generating second signals representative of the amount of dampening agent on the carrier, said second means being arranged to receive said first signals and to relate a preselected first signal generated while the carrier is substantially dry to another first signal generated while the carrier is damp, and said second means being designed such that the second signals are derived from the relationship between said preselected and other first signals, said second means being further arranged to receive and process at least one glancing signal so as to compensate for errors due to radiation which is directed towards the carrier from said source and glances off the dampening agent applies to the carrier;
• and
• (d) additional means for generating reference signals in response to radiation of said predetermined wavelength which impinges upon said additional means directly from said source, said second means being arranged to receive said reference signals and to form ratios of said preselected and other signals with respective ones of said reference signals, and said additional means being designed to generate a reference glancing signal, said second means being arranged to receive said reference glancing signal and to form a ratio between said one and reference glancing signals.

A method of determining the amount of dampening agent on a printing-image carrier, said method comprising the steps of:

• (a) irradiating said carrier;
• (b) measuring a characteristic of radiation of a predetermined wavelength returned from said carrier while substantially dry to obtain at least one dry value of said characteristic, said predetermined wavelength at least approximating an absorption wavelength of said dampening agent;
• (c) applying said dampening agent to said carrier;
• (d) measuring said characteristic for radiation of said predetermined wavelength returned from said carrier while damp to obtain at least one damp value of said characteristic;
• (e) establishing a relationship between said dry and damp values;
• (f) generating a signal based on said relationship and representative of the amount of dampening agent on said carrier;
• and
• (g) correcting for radiation which glances off said dampening agent.