Selecting the Correct Lens is Critical for a Machine Vision System to Perform its Intended Role

Measuring, evaluating, and detecting defects in targets are important functions of machine vision systems. These functions reduce or eliminate human errors and improve measurement accuracy and stability.

The core principle is to collect light signals into the camera through the machine vision lens, and then convert the light signals into electrical signals for processing by the image processing system. According to the captured image data to extract the characteristics of the target, the machine vision system performs various computations. The action of on-site equipment is controlled based on the results of the analysis, and the lens is an important part of the machine vision system. The quality of its imaging performance directly affects the effectiveness and feasibility of the algorithm.

Selecting the right lens is critical for a machine vision system to perform its intended role. The lens selection process involves gradually elucidating the parameters of the lens.

As an imaging component, the lens usually cooperates with the light source, and camera to form a complete image acquisition system. The choice of lens is affected by the requirements of the entire system. Generally, the following aspects can be considered in the analysis: The type and characteristics of the object being inspected; the depth of field or focal length; the load and detection distance; and the operating environment.

Wavelength and Zoom Capability

It is usually easiest to first determine the operating wavelength of the lens and whether it requires a zoom. If the magnification needs to be changed during the imaging process, a zoom lens can be used; otherwise, a fixed focal length lens is appropriate.

For the operating wavelength of the lens, the common range is within the visible light spectrum, though other wavelengths may also be used. Key considerations include whether additional filtering is required, whether monochromatic or polychromatic light is used, and whether stray or scattered light can be effectively reduced. Clarifying these questions allows for a balanced evaluation to determine the appropriate operating wavelength for the lens.

Prioritize Special Requirements

Depending on the characteristics of the actual application, special requirements may need to be prioritized and clarified first. For example, is there a need for measurement capabilities, should a telecentric lens be used, or is a large depth of field required? Depth of field is often overlooked, but it is a critical factor in any imaging system.

In many cases, such as pipeline inspection, a zoom lens can be used to achieve a greater depth of field. Varifocal lenses are similar to varifocal lenses and are used in applications where the focal length changes frequently. These lenses are usually driven by motors, allowing for smooth movement across the focal plane. By using this lens, the entire pipe and each section can be scanned and the focus can be adjusted to help identify each defect. However, unlike a varifocal lens, the working distance of a zoom lens can also be varied and can be repositioned as needed.

Working Distance and Focal Length

Working distance and focal length are often considered together. Generally, this approach can be used: first, determine the system's resolution to calculate the magnification based on the pixel size of the CCD. Estimate the approximate object distance to give the spatial constraints. From that, determine the focal length of the lens. Therefore, the focal length of the lens is related to the working distance, system resolution, and CCD pixel size.

Aperture and Interface

The lens aperture primarily affects the brightness of the image plane. However, in modern machine vision systems, the final image brightness is determined by a combination of several factors: aperture, camera gain, integration time, and illumination. Therefore, there are various options for adjusting the necessary image brightness.

The lens interface refers to the connection interface between the lens and the camera. The two must be compatible, and if they are not directly compatible, an adapter may be required.

Cost and Technological Maturity

Taking all the above factors into consideration, if multiple solutions meet the requirements, which can be evaluated the cost and technology maturity to make a balanced and optimal choice.

For example, when selecting a lens for a coin inspection imaging system, the constraints include a camera CCD 2/3”, pixel size of 4.65 μm; C-mount with a working distance greater than 200 mm, and a system resolution of 0.05 mm. A white LED light source is required.