Those characteristics in machine vision lens design

There are many important characteristics in lens design, including lens resolution, lens distortion, and illumination uniformity, which directly affect the performance of the machine vision system. The cameras, lenses, and lighting used in machine vision systems all make important contributions to the overall quality of the image. The rapid development of CMOS image sensor technology in the past few years has brought major challenges to lens manufacturers. Increasing sensor resolution means that many sensors now have smaller pixels and require higher resolution lenses. On the other hand, a high-resolution sensor that maintains a larger pixel size for higher sensitivity usually uses a larger format, and therefore requires a larger format high-resolution lens. In addition, many applications that require very long focal length lenses, such as surveillance, sports, aerial photography, and photography on theme park amusement facilities, are increasingly being included in the category of machine vision and need to be addressed. In the lens design, the lens resolution, spatial distortion and uniformity of illumination through the lens have an important influence on the performance of the lens. Modulation transfer function (MTF) An ideal lens can produce an image that perfectly matches the object, including all its details and brightness changes. In practice, this is by no means entirely possible, because the lens acts as a low-pass filter. Taking into account all aberrations, the image quality of a lens can be quantitatively described by its modulation transfer function. MTF is defined by the ability of the lens to reproduce lines (grids) with different pitches (line pairs/spatial frequency/mm). The more line pairs/mm that can be distinguished, the better the resolution of the lens. The MTF diagram for each spatial frequency shows the contrast loss caused by the lens. Large structures such as coarse spacing lines are usually transferred with relatively good contrast. Smaller structures, such as finely spaced lines, are transmitted with low contrast. The amount of attenuation at any given frequency or detail is classified by MTF, which represents the transmission efficiency of the lens. For any lens, there is a point where the modulation is zero. This limit is usually referred to as the resolution limit, and is usually quoted in line pairs per millimeter (lp/mm), or in the smallest line size (in μm) to quote some macro lenses, which is also equivalent to the minimum pixels required by the lens size. appropriate. MTF moves from the center of the lens to the edge and deteriorates. This is an important consideration if the entire image requires a nominal resolution. Due to astigmatism, MTF can also vary according to the direction of the line at a certain point on the lens, and is also a function of the aperture setting during measurement, so care must be taken when comparing lens performance. Since the lens must be to match the resolution to the pixel size of the image sensor, the smaller the pixel, the higher the resolution required by the lens. Lens distortion In addition to changes in resolution, all lenses will also be subject to a certain amount of spatial distortion. How to stretch or compress the image in a non-linear manner makes it very difficult to accurately measure the entire sensor. Although there are some software methods that can solve this problem, they cannot consider the physical depth of the object, so it is better to choose a good quality low-distortion lens instead of trying to correct these errors in the software. As a general rule, a lens with a shorter focal length will have more distortion than a lens with a longer focal length because the light hits the sensor from a larger angle. Using more complex lens designs can maintain low distortion, and many lens manufacturers have been working hard on their optical designs to enable them to reduce spatial distortion to the order of 0.1%. Illumination uniformity All images from the lens have vignetting, that is, the light intensity from the center to the edge of the image is reduced, which may affect the applicability of the lens. Mechanical shading is the shading of the edge of the image caused by the beam being mechanically blocked (usually blocked by the lens holder). This mainly occurs when the image circle (or format) of the lens is too small for the size of the sensor. All lenses are affected by "Cos4 vignetting", which is because the light must travel a greater distance to reach the edge of the image and reach the sensor at a shallow angle. This is also exaggerated when the angle focuses light on the non-sensitive part of the sensor, on a lens with a microlens on each pixel. This can be minimized if the lens is stopped by two fs. By improving the uniformity of illumination across the sensor, lens manufacturers can eliminate the need for light intensity compensation, which may introduce noise into the image. Lens interface Fixing the lens to the camera is achieved by using different standard lens interfaces. The most commonly used in machine vision applications is C-mount, which can benefit from a variety of lenses and accessories, including the ability to provide computer-controlled aperture and focus. The CS mount is not commonly used and is basically the same as the C-mount mount, but the flange focal length is shortened by 5mm. Smaller lens mount systems (such as S-mount) are usually used for board-level cameras and miniature cameras. These lenses only allow minimal adjustments. For large format sensors and line scan applications, larger F-mount systems can be used, although more powerful M42 mounts (sometimes called T-mounts) are increasingly used. But the large format lens does not support the ability to automatically control the aperture and focus. Telephoto lenses are also used in machine vision, with the longest focal length up to 600 mm. These large format lenses are mainly developed for professional photographers. They also include motorized apertures and zooms, and require special EF lens mounts. At present, more and more machine vision cameras are manufactured with EF mounting function and EF lens, and its novel optical functions are provided to the wider machine vision market through the recent direct distribution agreement.