The increased popularity of imaging technology over the last decade has spurred the demand for a wide variety of lenses that can provide optical designers with images suitable for all types of analysis. One such example is the telecentric lens, which is frequently used in the machine vision industry for measurement and alignment applications. In order to understand what makes telecentric lenses ideal for machine vision, it is important to look at what it means to be telecentric, compare telecentric lenses to conventional lenses, and examine the most common applications involving telecentricity.
WHAT IS TELECENTRICITY?
Interested in a short video on telecentricity?
View EO Imaging Lab Module 2.2: Telecentricity for more info.
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.
Classification 1: Object-Space Telecentricity
It occurs when the system stop is placed at the front focal plane of the lens, resulting in an entrance pupil location at infinity. A shift in the object plane does not affect image magnification.
Figure 1: 0.5X Object-Space Telecentric Lens (Note the Parallel Chief Rays in Object Space) [View Larger Image]
Classification 2: Image-Space Telecentricity
It occurs when the system stop is placed at the rear focal plane of the lens, resulting in an exit pupil location at infinity. A shift in the image plane does not affect image magnification.
Figure 2: 0.5X Image-Space Telecentric Lens (Note the Parallel Chief Rays in Image Space) [View Larger Image]
Classification 3: Double Telecentricity
Also known as bilateral telecentricity, it occurs when the system stop is placed at the common focal plane, resulting in both the entrance and exit pupils being located at infinity. Shifting either the image or object planes does not affect magnification given that double telecentric systems are afocal.
Figure 3: 0.9X Double Telecentric Lens (Note the Parallel Chief Rays in Both Image and Object Spaces) [View Larger Image]
TELECENTRIC LENSES VS. CONVENTIONAL LENSES
Figure 4: Reduced Perspective Error in Telecentric Lens vs. Conventional Lens [View Larger Image]
Perspective error, also called parallax, is part of everyday human experience. In fact, parallax is what allows the brain to interpret the 3-D world. In other words, we expect close objects to appear relatively larger than those placed farther away. Conventional lenses are those which exhibit this phenomenon, wherein the magnification of an object changes according to its distance from the lens (Figure 4). This occurs because the chief rays in this system are not all parallel to the optical axis (Figure 5). Telecentric lenses, by contrast, optically correct for parallax so that objects remain the same perceived size independent of their distance, over a range defined by the lens.
There is a common misconception that telecentric lenses have a larger depth of field than conventional lenses. Realistically, telecentricity does not imply large depth of field, which is only dependent on f-number and resolution. With telecentric lenses, objects still blur farther away from best focus, but they blur symmetrically. This symmetrical blurring holds the centroid position constant, allowing for accurate edge and feature location even when the image is not in focus.
Advantages of Telecentric Lenses
- Reduction or elimination of perspective error
- Reduction in distortion
- Increase in image resolution
- Uniform image plane illumination - An additional advantage of image space telecentricity is that it can lead to extremely uniform image plane illumination. The normal cos4θ falloff in image plane illumination from the optical axis to the edge of the field is removed, since all chief rays have an angle of θ with respect to the image plane.
- Constant magnification independent of shift in image and/or object planes
Disadvantages of Telecentric Lenses
- Use of large aperture optical elements in the region of telecentricity (image space or object space) in order to provide a non-vignetted field of view
- Use of more optical elements (due to the complex design) than conventional lens systems
- Increase in cost and weight because of large aperture and more optical elements
Figure 5: 75mm FL Conventional Lens (Note the Chief Rays in Both Image and Object Spaces are NOT Parallel) [View Larger Image]
Despite the disadvantages inherent to the increased complexity of telecentric lens design, the numerous benefits make telecentric lenses a popular choice in a variety of applications. In particular, telecentric lenses are commonly used in machine vision applications where software analysis is simplified and more accurate because of the reduction of parallax. In addition, general applications range from inspecting pipes to measuring object thickness.
Application 1: Alignment of Jumper Pins
As electrical components become smaller and smaller, the level of precision needed in aligning them becomes that much greater. When dealing with such minute detail, the perspective error created by a conventional lens becomes a more prevalent factor.
Figures 6 - 7 show a series of pins as imaged through a telecentric lens and a conventional lens. Notice how the conventional lens images the sides and bases of the pins that are off-axis (Figure 7). Comparatively, the telecentric lens only images the tops of the pins regardless of their location on the image plane (Figure 6).
Figure 6: Telecentric Lens Imaging Jumper Pins
[View Larger Image]
Figure 7: Conventional Lens Imaging Jumper Pins
[View Larger Image]
Application 2: CCD Based Measurement
CCD based measurement systems can be used to measure the spacing and/or size of a number of objects on an electrical or mechanical component. The precise measurement of objects (such as a pin) or features, or their separations, is accomplished through the use of measurement software. This type of software uses centroiding algorithms in the calculations of object separation and size. A telecentric lens is ideal for this application because extended objects will appear symmetrical (Figure 8), whereas the image from a conventional lens will be elliptical (Figure 9). Using a telecentric lens for this type of edge detection analysis results in an accurate circular fit to the pin, reducing error in the prediction of its center.
Figure 8: Telecentric Lens Imaging Extended Object
[View Larger Image]
Figure 9: Conventional Lens Imaging Extended Object
[View Larger Image]
Application 3: Metrology
Many metrology systems also depend upon telecentric optics. A profile projector is one example of such a system. Profile projectors are used to measure an object, or a feature within an object, by projecting an image of the area under test onto a screen. This projected image is then either compared to a "gold standard" reference at the proper magnification. This type of measurement requires equal magnification on two separate object planes for the comparison to be accurate - a task well suited to telecentric lenses.
Application 4: Microlithography
Microlithographic lens systems are used in the etching of integrated circuits onto wafers. The features inherent to these circuits are routinely sub-micron in size and getting smaller with every new generation of microlithographic equipment. The size of these features, along with their absolute locations, must be controlled to small fractions of a micron. This problem is intensified by the overlay necessary when numerous resist exposures and etches are required in the production process.
- Lenhardt, Karl, and Bad Kreuznach. "Optical Measurement Techniques with Telecentric Lenses." Technical and Scientific Contributions - Schneider Kreuznach. Web. http://www.schneiderkreuznach.com/pdf/div/optical_measurement_techniques_with_telecentric_lenses.pdf>.
- "Telecentric Lenses: Basic Information and Working Principles." Telecentric Lenses Tutorial - Opto Engineering. Web. 23 Apr. 2011. http://www.opto-engineering.com/telecentric-lenses-tutorial.html>.
- Greivenkamp, John E. "Telecentricity." Field Guide to Geometrical Optics. Vol. FG01. Bellingham, WA: SPIE, 2004. 33-35. Print. SPIE Field Guides.