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Durango Calibration

Phase Shift Calibration

Durango phase shift calibration has changed fundamentally from what it was when the calibration tutorial was produced. Previously, calibration consisted of assigning a control signal (usually voltage) to each phase step. Now, calibration consists of determining a polynomial representation of phase angle versus voltage. With this change, it is no longer necessary to recalibrate when changing phase algorighms. Also, the calibration tutorial is no longer of much use.

At this point, we have not yet added a graphic interface to the calibration. This makes it difficult for users to know what is going on, especially if the calibration fails.

Most phase shifters, including those offered by Diffraction International, use voltage controlled PZT elements to translate a mirror and produce thereby the desired phase shift. PZT translators are non-linear. They also exhibit hysteresis, meaning that the phase angle at a given voltage will depend on the recent history of the voltage signal.

The figure below shows the behavior of a typical PZT element upon repeatedly cycling the voltage. The curve depends on the maximum (and minimum) voltage values. Just as the curve changes depending on the maximum voltage value, it would also change if we were to alter the minimum voltage level.

PZT Calibration Diagram

Durango acquires images only along the lower edge of the curve where the control voltage is increasing. We label the voltage range limits as "Floor" and "Ceiling". Because the curve is bent most severely at its lower end, we impose a "Low" limit (higher than "Floor") for purposes of calibration and image acquisition. Durango will still cycle the control voltage all the way to the "Floor" or else we would only have moved the problem to another voltage range. We could similarly restrict the voltage range to "High" at the upper end, but this is less useful unless we were to be acquiring images along the upper edge of the curve.

During the acquisition of a phase measured interferogram, Durango optionally cycles the voltage once to establish the desired immediate history. You may notice the fringes shifting very quickly as this happens.

Pixel Aspect Ratio Calibration

Durango expects that digitized image pixels will be square, but includes a pixel aspect ratio (PAR) parameter to correct the computed results in cases where this is knownt to not be the case. When testing spherical optics, it is common to allow a bit of despacing, resulting in a few circular fringes that are subsequently subtracted (as power) in the data analysis. If an incorrect pixel aspect ratio is used, then these circular fringes will be somewhat elliptical and, after subtracting power, you will be left with residual astigmatism that is not a property of your test optic.

Pixel aspect ratio calibration is an non-issue for digital cameras. The horizontal and vertical pixel pitch (center-to-center spacing) is a specified physical characteristic of the sensor chip.

If your interferometer uses an analog video camera, then the image data from each horizontal line of the sensor is converted to a time varying analog signal which is subsequently digitized by the frame grabber. The frame grabber should, in most cases, yield nominally square pixels. However, this will depend on the accuracy of the pixel clocks in the camera and in the frame grabber. Some high end frame grabbers, such as the National Instruments PCI-1410, allow the user to adjust the pixel aspect ratio by fine tuning the frame grabber clock rate.

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Page updated: 11 November 2014

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