Measurement Principle - Rhopoint Instruments Optimap PSD 3 Manual

Measurement Principle

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Phase Stepped Deflectometry (PSD), a white light optical technique, uses a periodic
pattern with a sinusoidal waveform to measure the profile or curvature (K) of a surface.
The waveform is presented to the surface using a high-resolution display and the
reflected pattern captured by a high-resolution camera. This waveform acts as a ruler
over the surface allowing the ordinates of the light source to be determined as they
are proportional to the spatial phase of the sinusoidal pattern profile.
By "shifting" or stepping the waveform phase across the surface it allows an accurate
measurement of each point from the corresponding point per pixel on the camera.
Using the known geometric relationship between the display, the object surface and
camera, light rays reflected from the surface are spatially modelled to calculate the
direction of the normal at each point across the surface thereby allowing the profile at
that point to be obtained.
Displaying the sinusoidal waveform in horizontal and vertical directions across the
surface allows slope information to be determined in both orthogonal directions.
By differentiating the slope information curvature data can be produced allowing
accurate characterisation of the surface profile.
As curvature is the first derivative of slope and second derivative of amplitude it
combines both parameters to produce a more adapted quantity capable of being
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expressed in traceable SI units as m ¹.
Integration of this information allows the original 3D surface topology to be
reconstructed.
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