Industrial camera, image acquisition card, lighting source, analysis software and industrial lens are important components of machine vision inspection system, among which the key component is industrial lens. If the visual inspection system wants to play a full role, the following five factors must be considered before selecting industrial lens:
The working environment of machine vision inspection system is different, and the industrial lens selected will be different. Because the reflection coefficient of the object, light, temperature, vibration and pollutants will affect the normal operation of the lens. If it is in a high temperature environment, it may be due to the thermal expansion of the optical elements in the lens. Not all lenses can adapt to temperature changes. When detecting hot objects, use lenses with long working distance, that is to say, we should select the appropriate lens according to the working environment of the visual inspection system.
Detection object category and characteristics
The ability of lens to analyze object features depends on whether the contrast of features is strong. In order to determine the resolution of the system, or the analytic characteristics of an object, we can use the image force method such as lunge's method. The resolution of the lens is determined by its ability to identify the line coupling or point distance of a specific width under the specified light conditions. Resolution is usually displayed in the form of images by the module conversion function (MTF).
Depth of field or focal length
Depth of field effect (DOF) refers to blurring caused by moving objects. DOF is the depth of the object completely within the focal length range, and the allowable movement of the object under the ideal focusing state, moving forward and backward from the focal length. When the object is placed closer or farther than the working distance, it is out of focus, which will affect the resolution and contrast. For this reason, DOF matches the specified resolution and contrast. When the depth of field is fixed, the DOF can be increased by reducing the aperture of the lens (i.e. increasing the F / #) value, and light enhancement is also required.
Many camera lenses need to be loaded smoothly, but when the distance between the object and the lens is limited, changing the distance between the lens and the image can change the working distance. The loading limits, including protective enclosures for harsh environments, must be flexible enough to be adjustable according to the working distance.
The distance between the object and the front of the camera lens when the image is in focus. It limits the space required by the vision system and the devices that work with it. Within the limits, the working distance can be changed by refocusing the lens.
Therefore, appropriate lens selection plays a very important role in machine vision detection system.