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J. Eur. Opt. Society-Rapid Publ. 21, 19( 2025)
Figure 3. Interferograms for a specimen( asphere) positioned in different positions around the Cat’ s Eye reference position.( a) Sweep along the optical axis( z).( b) Sweep perpendicular to the optical axis( lateral sweep in x) in the plane of the Cat’ s Eye position.( c) Sweep perpendicular to the optical axis in a plane 1 mm away from the Cat’ s Eye position.
moved perpendicular to the Cat’ s Eye position, the interference patch moves across the screen. This is shown in Figure 3b and 3c. The further away the apex of the specimen is from the optical axis, the further the interference patch is shifted to the side of the screen and the more it is deformed. For a specimen in the plane of the Cat’ s Eye, the interference patch turns round, when it comes close to the center, for the specimen’ s apex in a plane further away, the interference patch stays rectangular.
It has to be noted that the interferogram of the TWI has due to the special interferometer design a significant fringe density in the Cat’ s Eye position in contrast to classical interferometers( see Fig. 3a( z = 0 lm)). Due to the model-based evaluation approach, it does not disturb the process and can easily be corrected by subtracting a reference wavefront.
When looking into the relation between specimen’ s position and the interference patch, it can be seen that the position of the rectangular interference patch is related to the specimen’ s position perpendicular to the optical axis( compare Fig. 3c). Because of that, the position of the rectangular interferogram near the Cat’ s Eye position can be used for lateral alignment. In Figure 4a, the position of the patch’ s center-of-mass and along the x-axis of the screen is shown in relation to the lateral displacement of the specimen for three different axial specimen positions with rectangular shaped interferogram. For a lateral centered specimen, the pixel value of the center-of-mass on the screen can be identified and used for specimen adjustment. It has to be noted that the relation between patch center-of-mass and lateral specimen position is nearly identical for all three shown axial positions. This might be due to the patch distortion at the edges of the screen, where the shape gets limited by the optical system of the TWI( compare Fig. 3c) and which is an equal behaviour for all cases with rectangular patch.
The size and shape of the interference patch can also be used for pre-aligning the specimen to the Cat’ s Eye reference position. Further away from the Cat’ s Eye, the interference patch is a small rectangle that becomes larger, when the specimen is moved closer to the Cat’ sEye. Inclose proximity to the Cat’ s Eye, the interference patch becomes circular and the interference fringe density decreases( compare Fig. 3b). In Figure 4b, the number of pixel