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FIGURE 2. In an on-axis interferometer (above) aberrations in the optics cancel completely. If tilt is introduced (below), the reference an test beams are no longer common path. Aberrations propagate through the system that cannot be accurately calibrated out. Retrace error and shear between the test and reference beams result, both significantly reducing measure- ment accuracy. The effects are particularly pronounced when measuring high NA optical elements. To obtain the increased spatial resolution necessary to measure larger local slopes, even greater tilt must be introduced, generating greater retrace error.

Figure 2 In an on-axis interferometer (above) aberrations in the optics cancel completely. If tilt is introduced (below), the reference an test beams are no longer common path. Aberrations propagate through the system that cannot be accurately calibrated out. Retrace error and shear between the test and reference beams result, both significantly reducing measure- ment accuracy. The effects are particularly pronounced when measuring high NA optical elements. To obtain the increased spatial resolution necessary to measure larger local slopes, even greater tilt must be introduced, generating greater retrace error.

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Creating a Fizeau interferometer that is capable of dynamic measurement, however, has proved a significant challenge. The difficulty lies in maintaining the on-axis design while coding the source beam into two orthogonally polarized beams from the reference and test surfaces. Dynamic interferometry is available on several widely used interferometer configurations. For measurement of flat and focal optics, a Fizeau interferometer configuration (Figure 1) offers several advantages:
Creating a Fizeau interferometer that is capable of dynamic measurement, however, has proved a significant challenge. The difficulty lies in maintaining the on-axis design while coding the source beam into two orthogonally polarized beams from the reference and test surfaces. Dynamic interferometry is available on several widely used interferometer configurations. For measurement of flat and focal optics, a Fizeau interferometer configuration (Figure 1) offers several advantages:
FIGURE 3. Using “Synchronous Frequency Shifting,” only in-phased beams (green) of opposite polarization (solid and dotted) remain at hig! contrast and generate fringes.
FIGURE 3. Using “Synchronous Frequency Shifting,” only in-phased beams (green) of opposite polarization (solid and dotted) remain at hig! contrast and generate fringes.
FIGURE 4. Retrace error vs. Resolution for an f1.5 sphere. As tilt is increased to provide better spatial resolution, retrace error also increases, limiting performance for tilted beam systems. The FizCam 3000 achieves high spatial resolution and minimal retrace error. To demonstrate, the convex surface of an f 1.5, A/10 reference sphere was measured with induced tilt varying from 0 waves (on-axis) to 300 waves. When approximately 250 waves of tilt were applied to the test beam, as is common with Spatial Carrier systems, peak-to-valley retrace error in excess of 1.5 waves was generated. In systems with variable tilt a smaller tilt angle can minimize retrace error but only at the sacrifice of spatial resolution.
FIGURE 4. Retrace error vs. Resolution for an f1.5 sphere. As tilt is increased to provide better spatial resolution, retrace error also increases, limiting performance for tilted beam systems. The FizCam 3000 achieves high spatial resolution and minimal retrace error. To demonstrate, the convex surface of an f 1.5, A/10 reference sphere was measured with induced tilt varying from 0 waves (on-axis) to 300 waves. When approximately 250 waves of tilt were applied to the test beam, as is common with Spatial Carrier systems, peak-to-valley retrace error in excess of 1.5 waves was generated. In systems with variable tilt a smaller tilt angle can minimize retrace error but only at the sacrifice of spatial resolution.