Typical Interferometer Setups
This page explains some of the interferometric measurement techniques commonly used in the optics industry. For more detailed information on these setups, and many more, you may download a copy of ZYGO's Guide to Typical Interferometer Setups.
This setup is used for measuring surface flatness of plano optical elements such as mirrors, prisms, and windows. The test object must be held so that the surface under test can be aligned in two axes of tilt, such as with ZYGO's two-axis accessory mount and a self-centering element holder.
Transmitted Wavefront Quality
This setup is used to measure distortion of a plane wave transmitted through the optic under test. It is typically used for windows, filters, and prisms. In addition, glass and other transparent raw material may be examined for homogeneity. Since the measurement beam passes through the component being tested, it need only be positioned approximately perpendicular to the optical axis of the measurement beam. No special mount or alignment is required.
Spherical Surfaces - Convex
Convex spherical surfaces are examined for surface figure and irregularity using the setup shown. To select the optimum transmission sphere, two criteria must be met. First, the radius of curvature of the convex surface under test must be less than the back focal length of the transmission sphere; and second, the radius of curvature of the test surface , divided by the clear aperture (the R/number) should be approximately equal to the f/number of the transmission sphere. Six-inch diameter transmission spheres are available for this application. Adjustment of the test surface is typically achieved with a 3-axis mount.
Spherical Surfaces - Concave
A transmission sphere transforms the interferometer's output beam into a precise shperical wavefront for the evalutation of spherical surfaces and lenses. A concave spherical surface is examined for surface figure and irregularity (deviation from the best-fitting sphere) by placing its center of curvature coincident with the focus of the transmission sphere. This usually-difficult alignment is simple and takes only seconds using the interferometer's Quick Fringe Acquisition System. Adjustment of the test surface is typically achieved with a 3-axis mount.
Lens or system quality can be measured by examining the distortion of the transmitted wavefront. For a lens system focused for infinity, the lens can be placed in the interferometer's collimated measurement beam and a high-quality concave or convex sphere can be used to retroreflect the transmitted wavefront. By rotating the optical axis of the lens under test with respect to the axis of the measurement beam, performance at various field angles can be determined.
Lens/System Performance - Alternate Method
For lenses or systems with finite conjugates, or for systems with apertures larger than the sample beam of the interferometer, an alternate setup, as shown, may be necessary. The alignment of the elements, however, may be significantly more difficult, and errors due to misalignment may be introduced.
ZYGO's Guide to Typical Interferometer Setups
This 16-page booklet illustrates and discusses a number of the most common measurement setups for use with a ZYGO interferometer system. You can download the Guide as a PDF document right now!
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