How to choose the right telecentric lens

Having fixed working distance and aperture, telecentric lenses are classified by their magnification and image circle. Choosing the right telecentric lens is easy: we must find the magnification under which the image fit the sensor.

Example. We need to measure the geometrical feature of a mechanical part (nut) using a telecentric lens and a 2048 x 2048, 5.5 µm sensor. The nut is inscribed in a 10 mm diameter circle with 2 mm uncertainty on the sample position. What is the best choice?

Given the camera resolution and pixel size (2048 x 2048 pix, 5.5 µm), the sensor dimensions are calculated to be 11.26 x 11.26 mm.

The FOV must contain a 12 mm diameter circle, hence the minimum magnification required is 0.938X.

The Opto Engineering® TC23009 telecentric lens (M=1.000X, image circle 11 mm) would give a FOV of 11.26 mm x 11.26 mm, but because of mechanical vignetting the actual FOV is only an 11 mm diameter circle. In this case, if a more accurate part placement cannot be guaranteed, a lens with a lower mag or a larger image circle must be chosen. Using the Opto Engineering® TC2MHR016-x lens (M=0.767X, image circle 16.0 mm) we find a FOV of 14.68 x 14.68 mm which is a very close match.