The appropriate selection of a lens has a crucial impact on the overall performance of a machine vision system. Only when the optical lens and image sensor are well-matched can the imaging system's resolution and detection accuracy be fully utilized. Therefore, when selecting a lens, it is necessary to comprehensively consider several key technical parameters, especially factors such as chip size, resolution, focal length, and aperture parameters.
Before formal selection, it is necessary to understand common lens types. Industrial lenses generally include ordinary perspective lenses, telecentric lenses, wide-angle lenses, fisheye lenses, and special spectral lenses for near-infrared applications.
In most machine vision applications, ordinary perspective lenses are the most widely used. These lenses can create perspective imaging effects similar to the characteristics of human vision, i.e., nearby objects are imaged larger, and distant objects are imaged smaller. From an optical principle perspective, the human eye itself is also a typical perspective imaging system; therefore, ordinary perspective lenses have good versatility in applications such as appearance inspection and positioning recognition.
In the process of selecting a suitable lens, the following technical factors play a decisive role:
Chip size and image circle diameter: It is necessary to ensure that the lens image circle can completely cover the sensor target surface to avoid vignetting and image quality degradation.
Resolution and Pixel Size: The lens resolution should match the camera's pixel size to fully utilize the camera's imaging capabilities.
Focal Length Selection: Directly affects the field of view and working distance, it is a key parameter for determining the size of the inspection area.
Aperture Opening and Light Source Conditions: Aperture size not only affects the amount of light entering the camera but also relates to depth of field and image sharpness. It should be comprehensively evaluated in conjunction with the ambient lighting conditions.
Only through a systematic analysis and reasonable combination of the above factors can a stable, reliable, and high-precision imaging foundation be built for the machine vision system.
Lens focal length and chip size
In an optical system, the distance between the optical center of a lens and its focal point is called the focal length, usually expressed in millimeters (mm). Ideally, parallel incident light rays passing through the lens will converge at the focal point to form a sharp image. The focal length (F) of a lens is primarily determined by its optical structure and the refractive index of its materials.
Does lens focal length affect image quality?
Focal length is an important parameter that affects the field of view and magnification of an image. Generally, the longer the focal length, the more pronounced the telephoto characteristics of the lens, the smaller the field of view, and the larger the relative size of the object in the image. The large telephoto lenses commonly seen in sports photography or news reporting utilize longer focal lengths to achieve clear, magnified images of distant targets.
Conversely, wide-angle lenses and fisheye lenses typically have shorter focal lengths, allowing them to cover a larger field of view, but also introducing more pronounced perspective distortion. In machine vision applications, the choice of focal length directly impacts the size of the detection area and the imaging accuracy.
Lens parameters and applications
In practical applications, lens focal length is not an isolated parameter, but is determined by factors such as the imaging chip size, the size of the object being measured, and the working distance. To facilitate selection, most lens manufacturers provide focal length calculation tools on their official websites, helping users quickly obtain recommended focal length ranges based on specific application conditions.
Imaging Differences at Different Focal Lengths
By comparing the imaging effects of different focal lengths, it can be found that as the focal length increases, the effective field of view of the lens gradually decreases, while the magnification effect is significantly enhanced. This imaging characteristic is particularly important in applications such as precision measurement and detail inspection.
Therefore, in the design of machine vision systems, the choice of focal length must be matched with the chip size and the specific application scenario. Only with reasonable configuration can an optimal balance be achieved between field of view, resolution, and detection accuracy.
Aperture and Light Source Conditions
In machine vision systems, aperture settings directly affect image brightness and imaging quality, making them a crucial factor that cannot be ignored during lens selection and system debugging. Aperture is typically expressed as an F-number, defined as the ratio of the lens's focal length to the effective aperture diameter, used to describe the size of the aperture opening.
The effect of lens F-number on image quality
The larger the F-number, the smaller the aperture, and the less light enters the image sensor. In applications with limited lighting conditions, appropriately increasing the aperture helps to improve image brightness and ensure the visibility of image details.
On the other hand, a smaller aperture effectively increases the system's depth of field and, to some extent, suppresses optical aberrations such as vignetting. However, an excessively small aperture will introduce significant diffraction effects, thereby reducing image sharpness.
Therefore, in practical applications, each lens usually has an optimal F-number range for imaging performance. This range represents a balance between depth of field requirements and diffraction effects.
In summary, the choice of aperture should match the lighting conditions of the specific application scenario. While ensuring sufficient image brightness, avoid setting the aperture too large or too small to obtain stable, clear, and consistent imaging results, providing a reliable foundation for subsequent image processing and measurement.
Lens Selection Checklist
Before finalizing the lens model, it is recommended to confirm the following key elements based on system compatibility and application requirements:
Lens Mount: Ensure the lens mount type (e.g., C-mount) is fully compatible with the industrial camera interface to guarantee a stable and reliable mechanical and optical connection.
Resolution Matching: Verify that the lens's optical resolution can meet the pixel requirements of the imaging chip, preventing the lens's insufficient resolution from limiting the overall imaging performance of the system.
Focal Length Selection: Ensure the lens focal length matches the chip size, working distance, and actual field of view to guarantee complete and clear imaging of the target area.
Image Circle Coverage: Confirm that the lens's image circle diameter completely covers the sensor size to avoid vignetting or edge brightness attenuation.
Aperture and Lighting Conditions: Ensure the lens's aperture range is suitable for the on-site lighting environment, balancing brightness with image quality and depth of field requirements.
Selecting an industrial lens is not a simple matter of matching a single parameter; rather, it requires a systematic engineering approach that comprehensively considers the imaging chip, inspection accuracy, working environment, and overall system performance. From ordinary industrial lenses to high-precision telecentric lenses, appropriate optical configurations not only significantly improve image quality but are also a key foundation for achieving stable and highly consistent machine vision inspection. As industrial inspection evolves towards higher precision and reliability, scientific and standardized lens selection will continue to provide crucial support for unleashing the performance potential of machine vision systems.
