136 J. Eur. Opt. Society-Rapid Publ. 21, 12( 2025)
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Figure
19. Measurement of birefringence of FS freeform lenses: OPD for( A) ground state,( B) after PJP, and( C) after the final annealing step.
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Figure
20. Investigations of structural changes of FS bulk material:( A) XRD measurement( B) FT-IR measurement.
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Figure
21. Surface roughness measurement of FS freeform lenses:( A) visualization of the Alvarez geometry and indication of the positions for roughness measurements and comparison of( B) waviness and( C) roughness for ground and PJP state [ n = 2 ].
mode of hydroxyl( OH �)[ 34, 35 ]. After annealing, the overall spectrum is at the same level with the reference measurement, which highlights the importance of the additional thermal post-treatment.
4.2.2 Waviness and roughness
It is essential to reduce the waviness and roughness significantly during PJP in order to obtain a functional optical surface. Therefore, the waviness and roughness of the freeform samples were measured before and after PJP at two defined positions( y = ± 10 mm, labelled P( peak) and V( valley)), where the nominal surface prescription exhibits its global maximum and minimum, as shown in Figure 21( A).
Figure 21( B) illustrates the mean Sq values, accompanied by error bars that highlight the slight discrepancy in Sq values between the two measuring positions. This could be attributed to the local differences in sample thickness and curvature between this positions. The effective thickness as per the Alvarez design is 4 mm( at V) versus 6 mm( at P).