Trying to measure the diameter of a large hole in a thick steel plate using a CCD camera objective and a standard objective, one can notice different results focusing either at the upper or to the lower end of the hole.

The reason for this occurrence is that the hole is larger than the entrance pupil of the CCD objective, so that the objective has to “look” both to the left and the right, and up and down to see the rim of the hole due to the perspective projection on to the CCD-chip. The hole therefore seems to be larger at the front of the steel plate, than looking through the hole at the back.

This problem can be solved optically by having the rays pass the object plane at an angle of 90° over the entire object field.

The name given to a set of optics designed to produce such a ray construction is named “telecentric.”

Telecentric objectives work very successfully where either the distance is not exactly defined, or the length of the optical path varies during the measurement process.

What are some applications?
Telecentric objectives are mostly used in machine vision applications. Some examples are as follows:

• Computer integrated manufacturing - gauging and statistical process control
• Profile or contour gauging
• Large field coordinate measuring machines
• Inspection of IC leads and packages
• Gauging and process control of precision manufactured small parts
• Printed circuit board inspection and defect detection
• Inspection of electrical or mechanical connectors
• Wire bonding machinery