Abstract: A touch-trigger probe is provided with a sensor (which may be a mirror (106), an optical fibre (206), or a bi-refringent element (307)) which changes state by vibrating or undergoing strain when a stylus connected to the probe contacts a workpiece. The sensor is typically provided on (in the case of the mirror (106)), or in (in the case of the optical fibre (206)) the stylus. The change of state of the sensor causes a change in the path length, polarization state, or intensity of light waves conveyed by the sensor. An interferometer may be provided to detect such changes. Other forms of electromagnetic waves may be used to the same effect.

Abstract: The invention concerns a focus detection circuit for an optical trigger probe. The probe has a laser light source, a beam splitter and a focusing lens system. The probe produces a light beam focused to a point at a distance d from the probe. Light reflected from the surface is reflected from the beam splitter onto two pairs of photosensitive detectors. The signals from the two outer detectors are added as are the signals from the two inner detectors, and the two sums are subtracted to provide a focus error signal which is passed to a zero crossing detector. When the light beam is focused on the surface, the difference between the sums of the detector signals will be zero and the zero crossing detector circuit emits an output pulse. To eliminate false trigger signals when the probe is some distance from the workpiece and the reflected light is minimal, a validating signal is generated by measuring the intensity of the relected light at a detector.