Distortion and the Telecentricity Specification
This is Section 5.2 of the Imaging Resource Guide
As discussed in prior sections, the elimination of both parallax and distortion play large roles in determining the quality of a particular telecentric lens. While distortion’s specifications are discussed in much more detail in Sensor Relative Illumination, Roll Off and Vignetting, the distortion of telecentric lenses can be specified in two different ways: TV distortion or geometric distortion. Both are generally classified as a percentage value, but TV distortion values will almost always be lower than geometric distortion values, which can be misleading. When a telecentric lens is specified with geometric distortion values, the value that is given is at the maximum field height of the maximum sensor size that the lens is capable of. In the case where the lens has monotonic distortion, the value that is specified will be the highest. However, in the case of wave distortion, it is important to look at the plot (as in Figure 1) to determine how the distortion is actually characterized.
Figure 1: Non-monotonic, or wave distortion, typical of telecentric lenses.
The other relevant specification is telecentricity, which is generally specified in degrees, and can be thought of as the residual angular field of view of the lens. Unfortunately, just as no lens has zero distortion, no lens is perfectly telecentric. Figure 2 shows a plot of the telecentricity for a 1X telecentric lens.
Figure 2: A telecentricity plot for a typical telecentric lens.
The plot in Figure 2 shows three different lines, each representing the telecentricity at different wavelengths (red, green, and blue). The most important nuance to note about the plot is that the telecentricity varies with wavelength, meaning that the accuracy of a measured part can change depending on the wavelength (color) of light that is used to inspect the part. While this variance is small in an absolute sense, it is important to consider when designing systems that require the highest possible accuracy. For these systems, it is best to use monochromatic illumination, preferably the wavelength where the telecentricity was optimized in the design process. Learn more about using proper illumination on Choose the Correct Illumination.