1. A calibration artifact for calibrating a machine vision measurement system, the calibration artifact comprising:

a substrate; and
a plurality of concentric rings on one surface of the substrate, each ring of a different pre-defined size.

2. The calibration artifact of claim 1 wherein the change in the size of any two adjacent rings is different than the change in size of any other two adjacent rings.

3. The calibration artifact of claim 1 in which each ring has an inner edge and an outer edge.

4. A method of calibrating a machine vision measurement system, the method comprising:

placing a calibration artifact including a series of concentric rings under a camera of the machine vision measurement system;
choosing a magnification level;
measuring the size of a first largest ring in pixels;
measuring the size of a second largest ring in pixels;
comparing the sizes;

and determining, from the comparison, the actual diameter of said ring.

5. The method of claim 4 in which each ring is of a predetermined different size and wherein the change in the size of any two adjacent rings is different than the change in size between any other two adjacent rings.

6. The method of claim 4 further including determining a first average of the measured size of the first largest ring in pixels and the measured size of the second largest ring in pixels.

7. The method of claim 6 further including measuring the size of a third largest ring in pixels and determining a second average of the measured size of the third largest ring in pixels and the measured size of the second largest ring in pixels

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