Video high-density disk surface testing with a laser diode and a position sensor

A surface tester was developed for the VHD (video high-density disk) using an optical technique. A 10-microm diam beam from a laser diode irradiates the disk surface, and a photosensor detects the angle and the intensity of a reflected beam. Defects on the surface are found by observing changes in the photosensor signal. A microscopic examination of the disk surface revealed that the tester can detect defects larger than 10 microm. The tester classifies defects by their category and size and displays a defect map on a monitor screen. The data obtained are utilized for improving the VHD manufacturing process.