New-Tech Europe | April 2018

changes. This variation slightly defocuses the image, or allows the lens elements to tilt in the barrel. The selection of a suitable material for the lens mount, or lens elements that compensate for this expansion are possible solutions to overcome this problem. Board mounted lenses come off the shelf with a fixed aperture, and have the advantage of no moving parts. A focus adjustment is achieved by screwing the lens in and out of the mount. The adhesion of a glass and metal lens to a metal mount provides a simple and stable lens setup. M12 mount lenses are small and light (even if made of fully formed glass) as compared to C-mount lenses, and the effect of shock and vibration is significantly reduced. Production Techniques Although the mechanical design of the lens has major impact on usability in stereo applications, the quality of the lens assembly plays an important role, too. In the beginning, all the lens parts need to precisely meet the gauge of the lens barrel. For example, if the lens barrel is too wide, and the lens element is too small, the lens will move inside the barrel and possibly tilt during use. The next step is the precise assembly of the lens elements in the barrel. A fixed lens that is tilted in the barrel leads to a degradation of the image quality. An unstable lens will lead to a degradation of image quality, or cause defocusing with respect to the orientation of the lens during vibration, or movement of the camera system. A precise production process is necessary to avoid loose or tilted lenses. The part-to-part accuracy of the fundamental optical parameters like focal length and aperture

Figure 3: A spring guides the lens elements in the correct position

focus in place so that these lenses can withstand vibrations up to 10G, and 70G of shock. The iris is produced as a fixed iris plate instead of moving blades. Usually, different F-stops are available, but then there is less variety with a fixed iris, than with a variable aperture. Most of the C-mount machine vision lenses are made of glass elements and metal bodies. Therefore, changes in temperature are a minor problem as glass and metal have relatively small coefficients of expansion (ϒ). Consequently, imaging performance is constant over a wide temperature range. In contrast, M12, M8, or boardmount lenses are more likely to be made, in part, from plastic. Plastic lens elements can change their shape over variations in temperature and this property changes the optical performance. A plastic barrel can expand or shrink over temperature

Lens manufacturers have different solutions to overcome the issue of lens shift. One solution is the insertion of a spring between the lens and the locking screw; this method applies force on the lens elements to keep them in their correct position. This elegant solution allows the focus to be adjusted and the spring absorbs the shock; whereas a more rigid system tends to break. Nominally, these lenses deliver a shock-resistant performance, with the optical axis displacement of no more than 10µm* at an orthogonal acceleration of 10G. In M12 mount designs, for example, a rubber ring can be used instead of a spring, achieving similar results. Another solution is “ruggedized” lenses; here, the single lens elements are glued to the lens body, and the entire lens set is moved for focus adjustment with a lockable jam nut. The nut tightly locks the

Table 1: Lens Types and Operating Temperature Range

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