Test & measurement
DECENTRED POWER PROFILE If the refractive power profile of a lens is decentred with respect to its optical zone, a double peak will occur at the centre of the power map. SHSWorks detects the rim of the lens and so determines its centre position. For the analysis of lenses with a decentred optical zone, it is possible to laterally shift the entire power profile in a given direction. In the example below, a shift of 0.27mm suppresses the central double-peak in the power map.
Figure 4
CONCLUSION In most cases, the spot-based sagittal power map is a good choice and therefore has been chosen as default in SHSWorks.
It is not recommended to use any power map for detection of flaws or aberrations in the shape of the lens. For this purpose, the wavefront is best suited, as power maps are only derivatives of the more immediate wavefront data. In addition, the wavefront offers the possibility of a Zernike decomposition.
When considering diffractive lenses, each zone of the lens generates a separate wavefront. Depending on the lens design, these wavefronts may overlay on the wavefront sensor, so that measurement of a diffractive lens can be difficult or simply not possible.
ACCURACY OF THE POWER MAP CALCULATION The graph below (Figure 5) shows the radial sagittal power profile of a meniscus lens, averaged in the azimuthal direction. The orange line results from a direct measurement in transmitted light configuration, as is the case in Optocraft ophthalmic instruments. The blue line results from a very precise measurement (accuracy 2µm) of the curvature radius of the front and back of the same lens. Together with
the centre thickness of the lens, these radii were used to model the lens in Zemax. The blue line represents the result of a raytracing simulation. The graph below shows an excellent agreement of the direct measurement in transmission and the simulation based on the measured lens dimensions.
In conclusion, this demonstrates that a direct measurement in transmission correlates very well with the highly precise, indirect measurement based on the curvature radii.
Optocraft SHSInspect instruments are widely used in R&D and production when performance, reliability and efficiency counts most. Based on the state-of-the-art wavefront sensor SHSLab, Optocraft offers measurement modules, turnkey instruments and automated tools for a great
variety of applications in the field of optics and optical systems, such as: objective lens testing; testing of optical elements and windows; surface shape measurement; and inline measurement and automation.
Optocraft provide wavefront sensors and inspection systems that are distinguished by their high speed, single-shot measurements, excellent unreferenced accuracy, extreme dynamics and broad spectral ranges. They are also able to measure wavefronts with very strong higher order aberrations. They can measure large tilt angles and strongly defocused beams. They offer high intrinsic stability and reliability, powerful, customisable evaluation software and are versatile and flexible in usage. Optocraft’s systems are in operation in many demanding customer applications.
Micro-Epsilon
www.micro-epsilon.co.uk
Figure 5 22 March 2024 Instrumentation Monthly
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