Dampening system on an offset printing press with a device for

Abstract

Means for measuring and correctively changing the thickness of a film of water on the plate of an offset printing press which includes a light source forming a spot of light on the plate with light pick-up means including a photocell producing an output signal in response to remitted light. A disc carrying two filters in symmetrically spaced sectors is interposed in the path of the light reaching the photocell. The first filter is a band pass filter which passes a wavelength slightly separated from a maximum light absorption band of water. The second filter passes a wavelength which is well separated from the maximum light absorption band of water and which lies in a region of where there is minimum change of absorption as a function of film thickness. As a result the photocell is subjected to an alternating series of 'measuring' light pulses and 'reference' light pulses through the respective filters. Means are provided responsive to the pulsating output signal of the photocell to provide a direct indication of film thickness. In a preferred form of the invention a water fountain roller is driven at a variable speed, and a servo system is interposed between the photocell and the motor which drives the fountain roller for constant corrective variation in speed to maintain the film thickness automatically at a desired level.