A 3D optical profilometer using a compact disc reading head

Abstract

The design, construction and performance of a novel optical profilometer based on the laser reading head of a compact disc player is described. The instrument is capable of self-guided scanning along arbitrary contours by utilizing the focus and radial tracking signals from the sensor head. The sensor is mounted on an x, y, motorized stage. The radial tracking signal provides the feedback for the angular position of the sensor, ensuring that the optical axis of the sensor is always perpendicular to the profile preventing the signal loss that occurs in conventional profilometers due to deflection of the light beam. The focus signal has a sensitivity in the nanometre range which makes the precision of the stages used to perform the scanning movements the dominant limiting factor for the measuring accuracy. The sensor head intermittently follows the contour, collecting datapoints at a predetermined lateral resolution. The profile is digitized in the form of x, y, and coordinates for each scanpoint. The accuracy of the instrument was verified by scanning standard measures with flat and circular cross sections. The mechanical structure of the prototype instrument was purpose built for the measurement of rigid corneal contact lenses. Various lenses have been measured and profile graphs are illustrated.