Patent GB-1403911-A (GB)
Title: Method and apparatus for testing optical components
IPC: G01N21/84
Filed: 1973-03-08
Abstract: 1403911 Testing optical components photoelectrically SIRA INSTITUTE Ltd 25 July 1973 [26 July 1972 16 Dec 1972 8 March 1973] 34851/72 58217/72 and 11386/73 Heading G1A An optical component is tested by scanning a focused beam of light across it so that the beam is influenced by the component, retroreflecting the beam back to the component so that it is influenced a second time and for an acceptable area produces a return beam coaxial with the incident beam, and separating and photo-electrically detecting the return beam. As shown, Fig. 1, a beam of light from laser 1 passes through a stop 2, a microscope objective 3 and a polarizing beam-splitter 9 to a collimator 23, an adjustable stop 4 and a focusing lens 5. The resultant beam is scanned in orthogonal directions by two rotatably mounted mirrors 53, 56 and is focused on the optical component 7 under test. Light passing through component 7 is received by a sheet 8 of retroreflective material and is returned, with a small amount of dispersion, back through the same area of component 7. A quarter wave plate 50 is arranged in front of the component 7 so that the returning beam is polarized perpendicularly to the outgoing beam thus enabling the returned beam to be separated by splitter cube 9 and applied to a silicon photo-voltaic cell 11. A flaw in the component 7 may cause the incident beam to be deflected away from retroreflector 8 so that no return beam is generated. Alternatively due to the dispersion of the retroreflected beam most of the light will not be incident on an area containing precisely the same form of flaw as that which influenced the original beam and thus a large proportion of the light will be influenced for the second time by the component 7 in such a way that it will not pass back along precisely the same path and will therefore be blocked by stop 4. Both these mechanisms produce a reduction in the detector signal when a flaw is scanned. The method may be applied to part finished components, e.g. lenses with only one face polished. The component 7 may be raster scanned or preferably spiral scanned this being achieved by rotating the mirrors 53, 56 by torque motors 55, 58 fed with quadrature sinusoidal voltages the amplitudes of which are linearly increased with time. The incident light beam is maintained in focus as it is scanned over the surface of the component by adjusting the position of focusing lens 5 by means of a stepping motor in response to control signals correlated with the amplitude of the scanning drive voltages. Alternatively focus may be maintained by a synchronized cam follower system or by a system directly determining the position of the portion of the surface of the component 7 being scanned e.g. by means of an air probe or optical system. The electrical output may be applied to a CRT for display of the flaws or may be analyzed by an arrangement using pulse height discriminators and counters to determine the severity and number of flaws. Reference is made to a known system for producing a smoothed waveform from the signal for use as a reference level. The component under test may also be a reflector, Fig. 3 (not shown) and spiral scanning may be achieved by rotating the reflector about its centre and scanning the incident beam radially.