The cameras, lenses and illumination used in a machine vision system all make significant contributions to the overall quality of the images that are produced. The rapid developments in CMOS image sensor technology over the last few years have created significant challenges for lens manufacturers. The move towards higher and higher sensor resolutions means that there are now many sensors with much smaller pixels, requiring higher resolution lenses. On the other hand, higher resolution sensors that maintain larger pixel sizes for higher sensitivity are often larger format and so will require larger format high resolution lenses. In addition, many applications that require lenses with very long focal lengths such as surveillance, sports, aerial photography, and photography on theme park rides are increasingly coming under the umbrella of machine vision and need to be addressed. A number of factors are important in lens design and these include lens resolution, spatial distortion and uniformity of illumination through the lens.
The ideal lens would produce an image which perfectly matches the object, including all of its details and brightness variations. In practice this is never completely possible as lenses act as low pass filters. The image quality of a lens, taking into consideration all aberrations, can be described quantitatively by its modulation transfer function. MTF is defined by the ability of a lens to reproduce lines (grids) with different spacings (spatial frequency in line pairs/mm). The more line pairs/mm that can be distinguished, the better the resolution of the lens. The loss of contrast due to the lens is shown in the MTF-graph for each spatial frequency (Figure 1). Large structures, such as coarsely spaced lines, are generally transferred with relatively good contrast. Smaller structures, such as finely spaced lines, are transferred with low contrast. The amount of attenuation of any given frequency or detail is classified in terms of MTF and this gives an indication of the transfer efficiency of the lens. For any lens there is a point at which the modulation is zero. This limit is often called the resolution limit and is usually quoted in line pairs per millimeter (lp/mm), or with some macro lenses in terms of the minimum line size in µm which also equates to the minimum pixel size for which the lens is suitable. The MTF deteriorates moving from the center axis of the lens towards the edges, which is an important consideration if the nominal resolution is required across the entire image. MTF can also vary depending on the direction of the lines at a point on the lens due to astigmatism and is also a function of the aperture setting at which it is measured, so care must be taken when comparing lens performance. Since a lens must be chosen so that the resolving power fits with the pixel size of the image sensor, the smaller the pixels, the higher the resolution required from the lens.