1. Field of the Invention
The present invention relates to a novel optical arrangement for performing testing of an aspheric surface, viz., a surface that departs substantially from a spherical surface.
2. Description of the Prior Art
With technological progress in processing optical elements, more and more aspheric surfaces have been developed in optical systems for the reduction of size and weight and for the enhancement of the quality of the imaging optical systems. Consequently, the need to precisely test topographic and other properties of an aspheric surface has become very important.
Methods for testing aspheric surfaces can be classified into two categories: surface profilometric methods and optical interferometric methods. Traditional profilometric measurements use probes that contact the surfaces of the elements under test (xe2x80x9cEUTxe2x80x9d). These methods are time consuming and their accuracy is limited to lines that are traced on the surfaces of the EUT. Since some of the test element surfaces are delicate and can be scratched by the probes, interferometry is a better choice in testing most of these precision optical components.
Well-known interferometric methods include shearing interferometry, high density array interferometry, sub-Nyquist analysis method, long wavelength interferometry, two wavelength holography, computer generated holography, and multiple annular interferogram technique, etc. For a comprehensive understanding of these methods, interested readers can refer to a technical paper by James C. Wyant, xe2x80x9cInterferometric testing of aspheric surfaces, xe2x80x9dProc. SPIE 816, 616-636 (1987).
A drawback of conventional interferometry techniques when applied to testing aspheric surfaces is that the fringes in an interference pattern are in general too dense to be analyzed. Although null optics testing techniques, such as reflective and computer generated hologram null optics, and various non-null testing techniques, such as lateral shear interferometry, long wavelength interferometry and two wavelength holography, can be used to overcome this drawback, it would still be attractive to develop a simpler, cheaper and easier method to handle the testing.
It is therefore a primary object of the present invention to provide a new testing method to measure the profile of an aspheric surface. The test procedure involves immersing the aspheric EUT in a container of liquid crystal to produce interference patterns with a fringe density suitable for automated analysis.
It is another object of the present invention to provide a cheap, simple and nondestructive technique to test aspheric surfaces.
In accordance with the objects of the present invention, there is provided a liquid compensatory interferometric technique to achieve the testing and measurement. The technique is provided with a container that is filled with an index matching liquid crystal and sandwiched between a polarizer and an analyzer. An externally applied direct current (DC) voltage source generates an electric field which would orientate the liquid crystal molecular directions during the test. When a collimated laser beam passes through the container, it will be divided into ordinary and extraordinary beams (i.e. the orthogonally polarized o-rays and e-rays in optics terminology). As the aspheric EUT is not a uniaxial crystal, the interferogram formed by the analyzer behind the container will bear the information on the aspheric surface profile. After analyzing the interferogram, the user can obtain the profile of the aspheric EUT.