Using MiniFIZ Laser Fizeau Interferometer

Interferometry of flatness, sphericity and wavefront transmission

Each interferometer setup example shows a MiniFIZ laser interferometer mainframe, a part being tested, and the optical accessories required to make the interferometry setup work. The accompanying text explains how the setup is used.

For many interferometry situations, the interferometer mainframe and the optics accessories may all sit on one vibration isolation table, with the measurement beam oriented horizontally. However, in many other cases the setup illustration will show the interferometer in a vertical orientation, either upward or downward looking. This popular setup is conducive to ergonomic, fast changes of test pieces, and uses less space than a horizontal interferometer configuration. We illustrate the vertical orientation of the surface measurement system where our customers most use this facet of the versatile MiniFIZ interferometer.

While the MiniFIZ family of laser interferometers is ideal for the testing of Surface Flatness, Transmitted Wavefront, Parallelism (or Wedge), and Sphericity, many times the microstructure, roughness or waviness of surfaces must be tested. For such surface measurements, we recommend the MicroXAM, an interference microscope that functions as an optical, 3D surface profilometer. MicroXAM has the dense lateral resolution necessary to assess microstructure accurately.

Surface Flatness

Flatness measurement setup.

This interferometer setup is used for the measurement of surface flatness of plano 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.

Suggested Accessories for Surface Flatness testing: Phase Shifting Interferometer, Transmission flat, 4%; Mount, 2-axis (tip-tilt); Self-centering element holder.

Plano Transmitted Wavefront

Wavefront transmission setup

This setup is used to measure distortion of a plane wave transmitted through the article under test. It is typically used for windows, filters, and prisms. In addition, glass and other transparent raw material may be examined for homogeneity evaluation. Since the measurement beam passes through the article under test this article need only be placed nominally perpendicular to the optical axis of the test beam. No special mount or alignment is required.

Suggested Accessories: Phase Shifting Interferometer, Transmission flat, 4%; Mount, 2-axis (tip-tilt); Reference flat, 4%.

Parallelism (Wedge)

MiniFIZ Wedge setup

Wedge can be measured using the MiniFIZ interferometer. One technique requires no auxiliary optics because interference is obtained between wavefronts reflected from the two surfaces of a transparent element. The element must only be placed nominally perpendicular to the mainframe output beam. The optical and mechanical wedge can be calculated from the resulting interference pattern. There are a variety of other techniques, some with a fraction of an arc second reolution, which can be used to measure wedge using a phase measuring MiniFIZ.

Suggested Accessories: Phase Shifting Interferometer; Mount, 2-axis (tip-tilt); Self-centering element holder.

Prism Testing

The measurement of prisms with high equivalent reflectivity, i.e., >40%, is carried out simply by inserting an attenuation filter of suitable aperture between a 4% transmission flat and the prism under test. Contact ADE Phase Shift for techniques for measuring corner cubes.

Suggested Accessories: Phase Shifting Interferometer; Transmission flat, 4%; Attenuation filter, Mount, 2-axis (tip-tilt); Self-centering element holder.

Transmission Sphere Selection

Transmission spheres

Transmission spheres of various f/numbers are available as standard items, and other may be obtained on special order. If the speed (R/number) of the surface under test does not match the f/number of the transmission sphere being used, one of two situations will occur. If the R/number is smaller than the f/number, the interferogram will not fully cover the entire aperture of the surface under test. If the R/number is larger, the interferogram will be smaller than full size. However, the Mainframe’s f/number zoom can be used to bring the image on the viewscreen to full size over a 6X range in the latter situation. In order to select the optimum transmission sphere f/number to fill the aperture of a concave or convex surface, the R/number of the surface to be measured is calculated using the following formula:

R/Number= Radius of curvature of surface under test/ Clear aperture of surface under test

Suggested Accessories: Phase Shifting Interferometer; Transmission flat, 4%; Mount, 3-axis; Self-centering element holder.

MiniFIZ configured for concave surface metrology

Overfill of a concave spherical lens

Concave Surface Figure

A transmission sphere transforms the Mainframe output beam into a precise spherical wavefront for the evaluation of spherical surfaces and lenses. A concave spherical surface is examined for surface figure and irregularity, i.e., the deviation from the best-fitting sphere, by placing its center of curvature coincident with the focus of the transmission sphere. Adjustment of the surface under test is typically provided by a 3-axis mount.

Suggested Accessories: Phase Shifting Interferometer; Transmission sphere; Mount, 3-axis (tip-tilt); Self-centering element holder.

Convex Surface Figure

Convex surface measurement set-up

Convex spherical surface are examined for surface figure and irregularity using the setup shown. In order to select the optimum transmission sphere, two criteria must be met. Firstly, the radius of curvature of the convex surface under test must be less than the back focal length of the transmission sphere; and secondly, the radius of curvature of the surface under test divided by the clear aperture, i.e., the R/number, should be approximately equal to the f/number of the transmission sphere. Six –inch diameter transmission spheres are abailable for thei application. Adjustment of the surface under test is typically provided bay a 3-axis mount.

Suggested Accessories: Phase Shifting Interferometer ;Transmission sphere, diverger; Mount, 3-axis; Self-centering element holder.

Non-Contacting Spherometer Radius and Figure

In addition to surface figure and irregularity, the radius of curvature of either a concave or convex spherical surface can be measured. The center of curvature of the test surface is interferometrically made to coincide witht the focus of the spherical wave emanating from the transmission sphere. In this arrangement, shown in the first illustration, the fringe patteren provides information not only about the figure igure andirregularity of the surface under test, but also about the precise locationof the center of curvature of the test surface with respect to the focus of tht transmission sphere. The surface under test is then translated along the optical axis of the interferometer until the urface under test coincides with the cfocus of the spherical wave from the transmission sphere, as shown in the second illustration. This is known as the “cat’s eye” position. The fringe pattern again provides a very sensitive indicator of the point where the surface coincides with the focus. The distance that the surface under test is translated, Rx, is equal to the radius of curvature of that surface. The digital radius slide provides a convenient readoutof this distance. This technique is accurat4e to 10microns or 0.1% whichever is larger. For cases where extreme accuracy is required (1micronor 0.001%) an Interferometric Radius Slide is available that inputsdistance measurements directly to the interferometer’s processor and is corrected for focus errors.

The third illustation shows the equivalent surface figure and irregularity and radius of curvature of a high quality spherical or cylindrical surface can be determined is a function of:

a. The R/number of the surface under test.
b. The accuracy of judging fringe straightness.
c. The resolution and accuracy of the radius slide readout.

Suggested Accessories: Phase Shifting Interferometer; Transmission sphere; Mount, 3-axis; Self-centering element holder; Digital radius slide.

Various Aperture Diameters

A number of situations may require the use of a different interferometer aperture diameter. For example, a single interferogram does not provide complete coverage when a plano article, tested at normal incidence, is larger than the interferometer’s aperture diameter. In this case, an interferometer aperture dieameter at least equal to the test article isneeded for full coverage with one interferogram.

Conversely, very small plano elements can produce interferograms too small to be useful. The Mainframes 4X zoom obviates this problem except for small elements, even though a 4-inch aperture interferometer is used.

Unlike plano surfaces, concave surface of any size can be examined in their entirety without depending oupon the interferometer aperture diameter. This is accomplished by generating a diverging measurement wavefront of an f/number equal to, or somewhat faster than, the R.number of the test surface.

Convex surfaces, which must be placed in a converging measurement beam, may require a larger interferometer aperture. For convex surfaces of large size and/or long radius, it may be necessary to begin with a larger aperture converging beam produced by the appropriate transmission sphere.

Aperture Converters

ADE Phase Shift offers a 4inch to 6inch (100mm to 150mm) aperture converter. The converter accepts standard 150mm elements. The converter permits full resolution measurement of 150mm flats, and greater flexibility in measuring convex surfaces.

Flatness measurements

Concave measurements