Phase Measuring Deflectometry

Phase Measuring Deflectometry, often abbreviated as PMD, is an optical method that allows to numerically reconstruct the shape of a specular object. It works by studying the reflection of a known pattern on the object.

Snellium developed its own implementation of the technique, allowing the measurement of both shape and curvature.

PMD Working Principle

A classical setup for phase measuring deflectometry is composed of a screen, that projects a pattern onto the object, and a camera that captures the reflection of this pattern. In a calibrated system, the reflection image is sufficient to reconstruct the shape of the object.

Snellium's implementation is based on the PMD variant called stereo-PMD, which uses two cameras visualizing the object at the same time to solve the reflection equations and determines the 3D location of the object surface.

Combined measurements

The shape and curvature properties of an object are strongly linked together. In a perfect case, each one of them could be computed from the other. However, in practical situations, this is not the case: error and noise in the measurement on one of them make the construction of the other incorrect.

To overcome this issue, most of the shape and curvature measurements are done separately. Snellium's solution does both of them at the same time, without sacrificing precision on any of them.

Even more interesting, the quality obtained on the measurements allows the execution of a post-processing step, that improves the quality of the shape based on the curvature data.