Laser removal of material from workpieces

Claim 1

Apparatus for engraving a printing surface by means of a laser beam which scans said surface, said apparatus including a beam intensity control circuit responsive to a varying image-representing signal to provide a varying beam intensity at said surface and, when the intensity is above an excavation threshold for the material of said printing surface, to cause different amounts of material to be removed from different parts of said surface, said beam intensity control circuit additionally comprising: at least one conductive probe positioned in a region of ionisation close to a part of said printing surface from which material is being excavated by means of said laser beam; a probe circuit in which said probe is connected, said probe circuit passing current when said probe detects ionisation of said material; means for applying to said beam intensity control circuit a test signal which changes in value during calibration; calibrating means provided with a reference signal the value of which represents the image signal level which is desired to correspond to the excavation threshold for the material, said calibrating means operating in a first mode to control the beam intensity in accordance with a progressively changing test signal and operating in a second mode to control the beam intensity jointly in accordance with the image-representing signal and a calibrating signal; and means responsive to the value of the test signal at the excavation threshold during operation of said calibrating means in the first mode to determine the value of the calibrating signal as a function of the difference between the value of the test signal and the reference signal level.

Claim 8

A method of ascertaining, for apparatus in which the beam of a laser engraves a workpiece, the value of a beam-intensity calibrating signal which corresponds to a beam intensity at the engraving threshold for the material of the workpiece, comprising placing at least one conductive probe close to a part of the workpiece on which the laser beam is incident, progressively altering the value of the calibrating signal and detecting the passage of current through a circuit containing the probe to ascertain the value of the calibrating signal for which there is a change from non-ionisation of the gaseous medium adjacent the said workpiece part to ionisation thereof, or vice versa.

Claim 9

A method of ascertaining, for apparatus in which the beam of a laser engraves a workpiece, the value of a beam-intensity calibrating signal which corresponds to a beam intensity at the engraving threshold for the material of the workpiece, comprising placing at least one conductive probe close to a part of the workpiece on which the laser beam is incident, progressively altering the value of the calibrating signal and detecting the passage of current through a circuit containing the probe to ascertain the value of the calibrating signal for which there is a change from non-ionisation of the gaseous medium adjacent the said workpiece part to ionisation thereof, or vice versa.