How to select the industrial lens in the machine vision system

The lens of an industrial camera is an important component in a machine vision system, and its quality will directly affect the overall performance of the machine vision system. The function of the lens is to image the target on the photosensitive surface of the image sensor. One, the lens parameters The main parameters of the lens include field of view, aperture, focal length, depth of field, working distance (object distance), working field of view, etc. When designing a machine vision system, it is necessary to choose a lens whose parameters match the user's needs. 1. Field of view The field of view is the angle formed by the two sides of the maximum range through which the measured object can pass through the lens with the center of the lens as the vertex. According to the size of the field of view, the lens can be divided into standard lens, wide-angle lens and telephoto lens. 2. Aperture (F) The aperture is a device used to control the amount of light entering the photosensitive surface of the camera through the lens. In terms of quantity, the ratio of the focal length of the lens to the clear aperture is used to express the size of the aperture. Apertures are f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, etc. 3. Focal length (f) Focal length, also called "focal length" or "image distance" by some people, is the distance between the clear image formed on the imaging plane such as the film, CCD or CMOS chip from the center of the lens, denoted by f. 4. Depth of Field After the subject is in focus, a clear image can be obtained in a certain range before and after the object. This range of the front and back distance is the depth of field. For a fixed focal length and shooting distance, the larger the aperture, the smaller the depth of field, and the smaller the aperture, the greater the depth of field. For a fixed aperture and shooting distance, the longer the focal length, the smaller the depth of field, and the shorter the focal length, the greater the depth of field. 2. Distortion Distortion is also called distortion, which is a phenomenon in which the straight line at the main axis in the plane of the measured object becomes a curve after being imaged by the optical system, thereby changing the geometric shape. Distortion does not affect the clarity of the image. The types of lens distortion include barrel distortion, pincushion distortion and linear distortion. Ordinary consumer-grade digital cameras have about 1% barrel distortion and 0.4% pincushion distortion. Three, lens classification 1. According to whether the focal length is adjustable, the lens is divided into zoom lens and fixed focus lens. 2. According to the size of the field of view, the lens is divided into standard lens, wide-angle lens, fisheye lens, telephoto lens, etc. Fourth, the interface standard of the lens The lens interface size is divided into three types: F type, C type, CS type. The F-type interface is a universal bayonet interface, generally suitable for lenses with a focal length greater than 25mm. The distance from the contact surface between the lens and the camera of the C-mount to the focal plane of the lens is 17.526mm. The distance between the contact surface of the CS-type interface lens and the camera to the focal plane of the lens is 12.5mm. The distance between them is a 5mm adapter ring. With the adapter ring, a C-mount camera can be used with a CS-type lens. Five, lens accuracy calculation When the chip size has been determined: Focal length/working distance=chip size/working field of view, Working field of view=chip size*working distance/focal length, or: Working field of view = chip size * magnification, Lens length or width direction accuracy = working field of view / camera chip length or width resolution. If the length and width of a 1.3 megapixel camera is 1280*1024, and the working field of view in the long direction is 300mm, the corresponding camera accuracy is: 300/1280≈0.2mm.