Spark-discharge recording head with position sensor and control for imaging lithographic printing plates

Apparatus for imaging a lithographic printing plate, the apparatus comprising:

• a. means for supporting a lithographic printing plate;
• b. a discharge source including an electrode;
• c. means for positioning the discharge source close to the plate surface, comprising:
• i. a pneumatic position sensor in fixed alignment with the discharge source, which sensor comprises:
• a) air-discharge means for emitting a regulated flow of air toward the plate;
• b) sensor means proximate to the air-discharge means, for responding to pressure reflected from the plate surface;
• c) signal-generation means for generating a signal representative of the reflected pressure and whose magnitude indicates the distance between the discharge source and the plate surface;
• and
• ii. feedback control means in fixed alignment with the discharge source and position sensor, the control means being adapted for moving a subassembly comprising the position sensor and discharge source with respect to but without making contact with the plate surface and also with respect to the control means, thereby altering the position of the position sensor and recording head with respect to the plate surface in response to the magnitude of the signal so as to maintain a relatively constant distance between the discharge source and the plate surface;
• d. means for moving the discharge source and the plate relative to one another so that the discharge source scans over the surface of the plate;
• and
• e. means for causing the discharge source to produce, at selected points during scanning, spatial discharges between the discharge source and the plate that change the affinity of the plate surface for water and/or ink at such points, thereby producing image spots on the plate.