Telecentric Lenses

Telecentric lenses are used to provide images independent of an object’s distance, or to prevent wide ranges of angles of incidence. Applications for these telecentric optics include digital image sensors, machine vision systems, measurement systems that use charged-couple devices (CCDs), metrology equipment, and microlithographic camera systems. There are three basic types of telecentric lenses: object-space, image-space, and double telecentric. With object-space telecentric lenses, the entrance pupil is at infinity. With image-space telecentric lenses, the exit pupil is at infinity. If both the entrance pupil and the exit pupil are at infinity, then the lens is double telecentric. Specialty telecentric lenses are also available.

Object-space telecentric lenses are used in machine vision systems because the size and shape of the image remains independent of the object’s distance or position in the field of view. In other words, the movement of the object toward or away from an object-space telecentric lens will not cause the image to appear larger or smaller. Some blurring will occur, however. By design, the entrance pupil is infinitely far behind the object-space lens so that looking in the front causes the aperture to appear very far away. Like other types of telecentric lenses, object-space lenses are larger, heavier, and more expensive than other optics with the same focal length and f-number. Object-space telecentricity requires multiple components, and the lens must not be smaller than the largest object to be imaged.

Image-space telecentric lenses prevent image-plane movements that are designed to focus or defocus a system from changing the image size. Consequently, image-space telecentric optics are often used in microlithography and other applications in which feature-size tolerances are less than one-tenth of a micron. Image-space telecentricity promotes a very uniform illumination of the image plane, as well. The rays reach the zero angle of incidence at the image sensor or film. Because image-space telecentric lenses minimize the sensor’s angle-of-incidence dependence, they are usually located in the beam-splitter prism assembly. Both image-space telecentric lenses and double telecentric lenses may be optimized for digital cameras. In this way, these telecentric optics can avoid the color crosstalk that occurs with digital image sensors that have a color filter array.

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• Understanding Video and Imaging Equipment
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