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If machine vision is considered the eyes of artificial intelligence, then industrial camera lenses are the eyes of machine vision. In a machine vision system, the primary function of the lens is to align the imaging target with the photosensitive surface of the image sensor. All image information processed by the machine vision system is obtained through the lens. The quality of the industrial camera lens directly affects the overall performance of the vision system. Reasonable selection and installation of the lens is an important part of machine vision system design. So, how should we select an industrial camera lens for the machine vision system?
The selection of an industrial camera lens is a relatively complicated process. It requires an understanding of the specific purpose of the lens, the usage environment, and several experimental operations to determine the final parameters. When the actual specific usage scenario is uncertain, do not rashly purchase a lens. This may result in the inability to utilize the imaging function of the lens, yielding unclear images and thereby affecting product inspection quality. In severe cases, it could lead to project development failure and increased costs. When selecting an industrial camera lens, we should pay attention to the following aspects:
When selecting an industrial camera lens, working distance and focal length are often considered together. Generally, you can follow this idea: first, determine the system’s resolution. Combining the CCD pixel size will give you the magnification, and combining it with spatial structure constraints will roughly estimate the object-image distance, and further estimate the focal length of the industrial camera lens. Therefore, the focal length of the industrial camera lens is related to the working distance of the industrial camera lens and the system resolution (and CCD pixel size). When choosing the focal length, distinction between fixed focal length and zoom focal length should be noted:
Fixed focal length: The focal length is fixed and does not change. It can be divided into two types: with aperture and without aperture. With aperture: The aperture size of the industrial camera lens can be adjusted. The aperture size should be adjusted according to changes in ambient light. Without aperture: It refers to a fixed aperture, where the light transmission is fixed and unchangeable. It is mainly used in situations where the light source is constant or the industrial camera has an electronic shutter.
Zoom focal length: The focal length can be adjusted as needed, allowing the image of the subject to be enlarged or reduced. Commonly used zoom lenses include six times and ten times zoom. There are three adjustable lenses: adjustable focal length, adjustable focus, and adjustable aperture. There are two adjustable lenses: adjustable focal length, adjustable focus, and automatic aperture. Under the premise of fixed lens specifications and focal length, the field of view angle of CS mount lenses will be greater than that of C mount lenses.
The image size of the selected industrial camera lens should be compatible with the camera’s photosensitive surface size, adhering to the principle of “large compatibility with small”—the camera’s photosensitive surface cannot exceed the image size indicated by the industrial camera lens—otherwise, the image quality at the field edge cannot be guaranteed. The requirements for image quality mainly focus on MTF and distortion. In measurement applications, special attention should be paid to distortion.
Industrial lenses are divided into manual aperture lenses and automatic aperture lenses based on the aperture. The selection is mainly based on whether the ambient light is stable. For stable environments, a manual aperture can be chosen, which is fine after one adjustment; otherwise, an automatic aperture lens should be selected (it must be paired with an industrial camera with an automatic aperture lens socket) to achieve automatic adjustment of the image brightness. Control signals for automatic aperture lenses are divided into DC and VIDEO control modes, namely, DC voltage control and video signal control. When selecting types of automatic aperture lenses, attention should be paid to the connection method of the industrial camera automatic aperture lens socket and the driving method switch of the automatic aperture lens, ensuring good coordination among the three.
As a type of machine vision lens, industrial lens specifications generally include 1/3", 1/2", and 2/3", among others. Under certain circumstances, the relationship between the lens focal length and the field of view angle is as follows: the longer the focal length of the lens, the smaller its field of view angle; under a fixed focal length, the relationship between the lens specifications and the field of view angle is: the larger the lens specifications, the larger its field of view angle. When the lens object distance is fixed, as the focal length of the lens increases, the image range becomes smaller, but image details become clearer; conversely, as the lens specifications increase, the image range enlarges, but image details become more blurred.
Besides the four more important factors mentioned above, there are many factors and parameters to consider when choosing an industrial camera lens, such as lens interface, imaging depth of field, wavelength, etc. All these need to be comprehensively considered based on the actual situation to find a suitable industrial camera lens.
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