Publications Details

Effects of lighting on performance of CMM video probes

Tran, Hy D.; Claudet, Andre C.

CMMs equipped with non-contact probes, such as video probes, are becoming popular for a variety of 2-D or 2.5-D objects. The advantages of a video (or vision) probe include the ability to measure features which are either too small or too delicate for a touch probe. Unfortunately, vision-based probing systems do not have the same measurement accuracy as touch probe equipped machines. For example, a Moore M48 coordinate measurement machine has an expected measurement uncertainty of 0.2 μm (plus a scale dependent term) when using a touch probe (the actual repeatability is on the order of 0.03 μm). When the probe is changed to a Leitz LS1 vision system, the expected measurement uncertainty is 1.2 μm plus a scale dependent term. The decreased accuracy is due entirely to the change in probing method. Components of the error budget include environmental effects, choice of lighting, lens distortions, and stage 2-D accuracy. Lighting is a major contributor to the measurement error budget, especially when a bidirectional measurement needs to be made (for example, the width of a line, rather than the center location of a line). We report on the effect of the sensitivity of vision probing on an OGP Avant Apex 200 to different lighting conditions, both for unidirectional and bidirectional measurements.