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Technology of Optocrafts wave-front sensors

Optocraft delivers powerful metrology components, modules and instruments. Highest emphasis and dedication is put on quality and functionality. On this page we introduce the fundamental technologies of wave-front sensors and some exemplary applications.

The Shack-Hartmann principle

The original idea was published by Johannes Hartmann in 1904. For the measurement of the imaging quality of optics he suggested to position a hole mask in an optical beam and to observe the characteristic dot pattern on a screen or photographic film. From the position of the dots he was able to conclude  the quality of the objective under test.

Roland Shack further developed this technique by using microlenses instead of holes. By doing so he improved the light efficacy and opened the way to automated wave-front evaluation by using digital cameras.

Hartmann - objective tests

Wave front  - objective test

Wave front inclination

Objective quality  - spots

The fundamental principle, however, stays the same: the local inclination of the wave-front across the microlens array causes a distortion of the spot array. Each spot is deviated from the local optical axis of its microlens depending on the ray direction. Thus, the spot position is a measure for the wave-front inclination and the ensemble of all measurement points can be used to reconstruct the wave-front by using numerical methods.

Why Shack-Hartmann?

Thanks to the geometrical optical principle, Shack-Hartmann sensors work without a simultaneously needed reference wave-front. This is a big advantage compared to interferometry. Also, the requirements concerning environmental conditions and the light sources are much more relaxed:
• "Single shot" technique, i.e. wave-front evaluation of single camera frame is possible
• High intrinsic stability
• Usable within broad spectral range
• Usable with low coherent light

Additional to the general properties, Optocraft's sensors offer:
• Extreme dynamic range
• High basic accuracy

Why Optocraft's SHSLab?

Based on the metrology software SHSWorks and the sensor heads SHSCam, Optocraft's wave-front measurement system SHSLab boosts the potential of wave-front sensors with some extraordinary properties, extreme dynamic range, exceptional basic accuracy, stability and reliability.

Extreme dynamic range
By means of the hardware and software components, extreme wave-fronts can be measured:
• Linear range (absolute tilt): wave-front tilts with angles of +/-15 degree and more can be measured without ambiguity.
• Dynamic range: wave-fronts with extreme asphericity can be measured. The local radius of curvature can be as low as 10 mm, i.e. the local refractive power can be 100 Dpt.
Due to the single shot evaluation this does not compromise the high evaluation speed of SHSLab.

Excellent basic ccuracy
SHSLab offers an excellent basic accuracy which indicates what measurement error SHSLab shows when illuminated with a perfectly flat wave-front but without (!) subtracting a reference wavefront. The basic accuracy of a SHSLab HR, i.e., is specified to be less than lambda/90 (rms) at 540nm. This property is particularly helpful in applications of laser beam or laser system testing where a referencing of the wave-front sensor with an ideal wave-front is often difficult or impossible.

High reliability and measurement speed
SHSLab gives reproducible and accurate results even under difficult environmental conditions. The evaluation rate ranges from a few Hertz up to around 1000 Hertz, the latter for specific high speed versions of SHSLab.


Applications of SHSLab

Beside you will find interesting notes on typical applications of SHSLab. Please ask us in case you have further questions or in case you have a specific metrology task in mind that you would like to discuss with Optocraft.