- Basics of machine vision optics
The object area imaged by the lens is called the field of view (FOV). The FOV should cover all features that are to be inspected with tolerance for alignment errors. Features within the FOV must be large enough to be measured. In alignment and gauging applications, the lens is also responsible for presenting the image in a fixed geometry that is calibrated to the object’s position in space. The working distance (WD) is the distance from the front of the lens to the object being imaged.
The depth of field (DOF) is the maximum object depth that can be maintained entirely in focus. The DOF also determines the amount of variation in the working distance that can be allowed while still achieving an acceptable focus.
The sensor size is the size of a camera sensor’s active area, typically specified in the horizontal dimension. The primary magnification is the ratio between the sensor size and the field of view. With primary magnification held constant, reducing the sensor size reduces the field of view and increasing the sensor size increases the field of view. If the sensor is large enough, it will exceed the size of the image circle that is created by the lens, creating blank spots in the corners of the lens that are known as vignetting.
Resolution is a measurement of the vision system’s ability to reproduce object detail. Figure 2 (a) shows an image with two small objects with some finite distance between them. As they are imaged through the lens onto the sensor, they are so close together that they are imaged onto adjacent pixels. If we were to zoom in, we would see one object that is two pixels in size because the sensor cannot resolve the distance between the objects. In Figure 2 (b), on the other hand, the separation between the objects has been increased to the point that there is a pixel of separation between them in the image. This pattern—a pixel on, a pixel off, and a pixel on—is referred to as a line pair and is used to define the pixel limited resolution of the system.