High-precision machine vision applications depend on the production of high-quality, reproducible images, whether for inspection or measurement purposes. This means that images must have sufficient resolution and correct definition of the areas of interest for inspection or measurement to occur. Each element of a machine vision system has an important role to play in the overall result, but the optical device is the heart of it since it is responsible for forming the image on the camera sensor.
The construction characteristics of the optical system within the lens (such as the size of the lens radius, the distance between the lenses, or the type of glass used) as well as the distance between the lens and the sensor have a direct impact on the image quality.
Factors affecting lens performance
Once the field of view of the application is determined, resolution and image quality are the most important selection criteria. Most machine vision lenses are endocentric, fixed-focus lenses that provide the same perspective as the human eye. With perfect optics, all light rays incident on the sensor are focused from a single point at the object to a single point at the image. However, all lenses suffer from imperfections that influence the resolution and quality of the image produced by the lens. We find for example:
Physical defects. Surface defects will cause light rays to focus at a different point, resulting in blurring.
Chromatic aberrations. The angle of refraction of light through a lens depends on the wavelength, resulting in colored bangs on all images produced with white light with blurring at the edges of the image.
Spherical aberrations. Most lenses are spherical. Thus, the focal point of each ray passing through the lens depends on the distance from the center of the lens. The resulting images will become increasingly blurred as they approach the edges.
Spatial distortion. All lenses suffer from some distortion in which the image is stretched or compressed in a non-linear fashion, making accurate measurements difficult. In general, shorter focal length lenses suffer more distortion than longer focal length lenses, because the light strikes the sensor from a wider angle.
Non-uniform illumination and shading across the image. With any lens, the brightness of the image is reduced toward the edges and this is called vignetting. Cos4 vignetting occurs because the light has to travel further to the edge of the image and reach the sensor at a lower angle. Mechanical vignetting occurs when the light beam is blocked mechanically, usually by the lens mount.
Factors such as diffraction can also affect image quality. Diffraction results from the slight bending of light as it passes near the edges of the lens opening. The smaller the aperture, the greater the diffraction effect. Diffraction causes a point of light to spread out in what is called a blurred circle, which reduces the sharpness of the image. The overall optical quality of a lens is defined by the MTF (modulation transfer function), which mathematically describes the comparison between the contrast of detail at the edges of an object and the contrast of detail in its visual representation.
Making corrections
There are several ways to deal with the problems described above. Surface defects can be minimized by paying special attention to lens construction, grinding and polishing. If no color information is required, chromatic aberration can be avoided by using monochromatic lighting.
A monochromatic LED source is used in conjunction with a suitable filter to filter out any extraneous colors. The most common color used here is red, as it is the most economical solution. Alternatively, white light can be used with a suitable bandpass filter.
If image color information is required, color-corrected lenses are available in which several optical elements are combined to compensate for wavelength-dependent variations in light refraction.

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