This is Section 5.1 of the Imaging Resource Guide
Perspective errors, also called parallax, are part of everyday human experience. In fact, parallax is what allows the brain to interpret the 3-D world. We expect closer objects to appear larger relative to those placed farther away. This phenomenon is also present in conventional imaging systems in which the magnification of an object changes with its distance from the lens. Telecentric lenses optically correct for this occurrence so that objects remain the same perceived size independent of their distance from the lens over a specific field of view defined by the lens (Figure 1).
For many applications, telecentricity is desirable, as it provides constant magnification (object size does not change) over a range of working distances, virtually eliminating perspective angle error. This means that object movement does not affect image magnification, allowing for highly accurate measurements in gauging applications. This level of accuracy and repeatability cannot be obtained with standard lenses.
Figure 1: The difference between a telecentric lens and a fixed focal length lens.
Telecentricity is a unique property of certain multi-element lens designs in which the chief rays are collimated and parallel to the optical axis in image and/or object space. A key characteristic of telecentricity, then, is constant magnification regardless of image and/or object location. There are three classifications of telecentricity depending upon the optical space(s) in which the chief rays exhibit this behavior.
Figure 2: A conventional lens vs. a telecentric lens, looking down on cylindrical objects. The telecentric lens can only see the top of the object, while the non-telecentric lens sees the entire cylinder, and it appears tilted due to parallax.
Type 1: Object-Space Telecentricity
In a system with object space telecentricity, movement of the object toward or away from the lens will not result in the image getting bigger or smaller, and an object which has depth or extent along the optical axis will not appear as if it is tilted. For example, a cylindrical object whose cylindrical axis is parallel to the optical axis will appear to be circular in the image plane of a telecentric lens. In a non-telecentric lens this same object will appear tilted; the top of the object will appear to be elliptical, not circular, and the sidewalls will be visible (see Figure 2). An object space telecentric lens design is shown in Figure 3.
Figure 3: Object space telecentricity (note the parallel chief rays, in bolded lines, in object spaces).
Type 2: Image-Space Telecentricity
In systems with image space telecentricity, moving the image plane to focus or intentionally defocus the system will not change the image size; this property is fundamental to today’s micro lithography industry where tolerances on feature size are routinely below a tenth of a micron. An additional advantage of image space telecentricity is that it can lead to extremely uniform image plane illumination. The normal cos4q falloff (discussed in Sensor Relative Illumination, Roll Off and Vignetting) in image plane illumination from the optical axis to the edge of the field is removed, since all chief rays have an angle of 0° with respect to the image plane. In sensors outfitted with microlenses, image space telecentricity can improve the fill factor of a pixel and lead to a higher quality image.
Figure 4: Image-Space Telecentricity
Type 3: Double Telecentricity
Also known as bilateral telecentricity, double telecentricity occurs when the system stop is placed at the common focal plane, resulting in both the entrance and exit pupils being located at infinity (Figure 4). Shifting either the image or object planes does not affect magnification given that double telecentric systems are afocal.
Figure 5: Double telecentricity (note the parallel chief rays, in bolded lines, in both image and object spaces).