To develop high-performance machine vision systems, an understanding of how to select the appropriate machine vision lens is needed. Lenses are a critical component in an imaging system. The right lens is needed to maximize the camera and board-level imaging device’s capabilities. Without the correct lens, image quality may be compromised.
Two types of distortion should be considered when selecting a machine vision lens: radial distortion and perspective distortion. Radial distortion is an optical error or aberration that results in a difference in magnification at different points in the image. Sometimes distortion can be corrected with software that identifies where each pixel is supposed to be and then moves it to that position.
Perspective distortion, or parallax, is the illusion that the further an object is from the camera, the smaller it appears through the lens. It’s caused by the lens having an angular field of view. One way to minimize perspective distortion is to keep the camera perpendicular to the field of view and to use a telecentric lens. Telecentric lenses maintain magnification over depth of field and, as a result, eliminate perspective distortion.
Because pixel size continues to be reduced on camera sensors, the resolution of a machine vision system is often limited by the lens and not the camera sensor. Lower resolutions result in distant objects being merged together into adjacent pixels and the separation between them is no longer apparent.
Resolution measures the machine vision system’s ability to reproduce object detail. The resolution of a lens is expressed as a frequency in image space related to the size of the pixels. A camera sensor with a 3.45 μm pixel size represents a pixel-limited resolution of 145 lp/mm (line pairs per millimeter). The machine vision lens used will need to resolve at least this frequency for the system to obtain maximum resolution.
Contrast is the separation in intensity between black and white. When considering two lenses of the same resolution, image quality is determined by how each lens preserves contrast. Higher lens resolutions make it harder for the lens to produce a high-contrast, sharp image.
Measuring a machine vision lens’ performance is not as simple as measuring contrast at a given resolution. Contrast is also dependent on the field position (or image height). To determine a lens’ performance, more variables must be considered. The lens’ working distance, f/#, wavelengths of light, and the size of the image sensor used with the lens must be reviewed.
Lens manufacturers provide Modulation Transfer Function (MTF) curves to compare lenses. These curves should take into account the intended use of the lens and manufacturers should be willing to provide MTF curves at detailed specifications upon request.
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