The invention relates to phase shifting devices, and more particularly to a phase shifting device for projected fringe contouring without the need for expensive, highly accurate mechanical components required in prior phase shifting devices.
Surface profile measurement by non-contact optical methods has been extensively studied because of its importance in fields such as automated manufacturing, component quality control, medicine, robotics, solid modeling applications, and other fields. In most of the known methods, a known periodic pattern, such as a sinusoidal grating, is projected on the surface to be measured. The image of the grating is deformed by features of the surface, detected, and analyzed by various well known techniques to determine the profile of the surface. For example, see U.S. Pat. No. 4,641,972, issued 2/10/87 to Halioua, et al.
The projected grating usually includes either a pattern of straight, parallel grid lines or a pattern of straight, parallel, sinusoidally varying intensity grid lines. If the test surface departs from being flat, then the image viewed by the detector or camera will have curved and/or non-parallel non-equally-spaced intensity patterns. The degree of departure of the curved and/or non-parallel, non-equally-spaced intensity patterns observed by the camera or detector from the projected grating pattern is mathematically related to the height of the test object relative to a flat reference surface. If the angle between the projector and the camera is known and other geometrical factors such as the distance from the camera to the test object and the distance from the projector to the test object are measured, then the height distribution of the test object can be calculated.
Phase shifting projectors used for projected fringe contouring generally are simple slide projectors, with the slide having therein a grating pattern with sinusoidally varying intensity. This type of slide projector is much less expensive and produces more illumination than laser-based interferometric projectors, which also can be used. The phase modulation required for the slide projector can be accomplished using a high precision mechanical device to translate the slide laterally relative to the incident beam of the projector. The period of the grating is typically less than 500 microns, and movements of the grating typically have to be less than 125 microns per acquired video frame, with a positional accuracy of about 2-3 microns for each step. Such prior slide projectors for projected fringe contouring apparatus require complex, high precision, expensive mechanical devices to achieve shifting of the grating slide with this level of accuracy.
Thus, there is a need for an improved, simplified phase shifter of lower complexity and lower cost than the phase modulating projectors used in prior projected fringe contouring devices.