Machine vision (MV) is defined as the technology used to provide imaging-based analysis for such applications as automatic inspection, process control, and robot guidance in industry. All the information collected by a machine vision system comes through a lens. The correct choice of lens can reduce image processing requirements and improve system performance. Software cannot correct the effects of a poorly chosen lens. The following are some of the variables that the lens designer needs to take into consideration.
The object and image points of a lens system are said to be conjugate points. The ratio of object distance to image distance along the principal axis of a lens is called the conjugate ratio. For an object exactly at the focal point of a lens, the conjugate ratio is infinite. For an object placed at twice the focal length of the lens, the image is formed at twice the focal length and the conjugate ratio is 1. Depending upon the specific machine vision application, this ratio may be fixed or variable.
The focal length of a lens is a measure of how strongly it converges light. In air, it is the distance over which initially parallel light rays are brought to a focus. A lens with a shorter focal length bends the rays more strongly, bringing them to a focus in a shorter distance.
The magnification of a lens is the ratio of the size of the image to the object conjugate. If the focal length of a lens increases for a specified magnification, both object and image conjugates increase by the same ratio.
Field Of View
The field of view (FOV) is the object area that is imaged by the lens onto the image sensor. It must cover all features to be measured, with additional tolerance for alignment errors. It is good practice to allow some margin (usually about 10%) for uncertainties in lens magnification. Features within the FOV must appear large enough to be measured. This minimum feature size depends on the application.
The working distance is the distance from the front of the lens to the object. In machine vision applications, this space is often needed for equipment or access. In general, the longer the working distance, the larger and more expensive the lens.
The f-number (f/#) of a lens describes the angle of the cone in which the light rays form an image. The larger the cone angle, the smaller the f/#. The f/# determines three important parameters – the brightness of the image, the depth of field (depth, such as within holes, at which the image remains clear) and the resolution of the lens (minimum size and separation of the objects it can clearly differentiate). A lens with a small f/# forms a high-resolution image but with a small depth of field. A lens with a large f/# has a larger depth of field but its resolution may be limited by diffraction effects, in which case the image is blurred even at best focus.
No matter how carefully the lens designer calculates the exact numerical values of each of these variables, the lens must be fabricated within precise specifications to meet the performance standards of the machine vision application. Esco Optics offers the expertise and state-of-the-art fabrication equipment to meet these exacting standards.