All machine vision systems create a high-quality picture to perform the essential measurements. The lens or lens combinations utilized in the vision system significantly impact the image quality generated. It influences a variety of other things, including the speed that can be reached, measurement precision, and the downstream analyses’ repeatability and dependability. Although fixed focus lenses are available, metal housings and focus mechanisms are used in most machine vision lenses. Therefore when choosing a lens for a machine lens, you need to ensure they are high quality, such as the MachVis Lens Configurator.
Here are some of the considerations to make when choosing a machine vision lens:
- Size of the sensor and pixel
The most appropriate lens and the sensor used in the camera have a direct link. Sensor and pixel sizes, for example, are critical parameters. Therefore, to eliminate shading and vignette, the lens must light the whole sensor region. It must also be capable of resolving pixel sizes. The higher a lens’ optical resolution, the more intricate structures it can replicate.
- Consider the quality
The MTF (modulation transfer function) about resolution and the optical distortion determine the optical quality of a lens. The detail of an item should be duplicated over around 4 pixels to resolve the details and enable precise edge detection. As a result, the needed magnification is determined by the desired object resolution and pixel size. The ideal lens would provide a picture that completely matched the item, with all of its intricacies and brightness changes. In actuality, because lenses operate as low pass filters, this is never totally achievable. MTF (Figure 1) classifies the degree of attenuation of each particular frequency or detail, and every lens has a point where the modulation is zero. This limit is sometimes referred to as the resolution limit, and it is measured in line pairs per millimeter (lp/mm).
Unsurprisingly, given the large range of sensor types and resolutions available in machine vision cameras and the numerous applications that may be used with them, there is also a broad array of lenses accessible, including several specialized lenses.
Listed below are some of the common lenses in the market today:
- Standard resolution lenses
This is the most often used lens type for sensors with a resolution of less than one megapixel. It is the most common lens type for sensors with less than one megapixel. These lenses are normally offered in conventional fixed focal lengths ranging from 4.5 to 100 mm. They are optimized for focusing to infinity, with an MTF of 70 – 90 lp/mm with minimal distortion and vignetting. Images with a shorter focal length are more likely to have ‘fisheye’ distortion.
- High-resolution lenses
Compared to ordinary lenses, these types of lenses provide better image performance. They typically have focal lengths up to 75 mm, an MTF of more than 120 lp/mm, and very low distortion (0.1 percent). They’re especially well-suited to cameras with tiny pixels or applications that need accurate measurements.
- Macro lenses
These types of lenses are accessible for tiny fields of vision that are about equivalent to the camera’s sensor size. They are optimized for ‘close-up’ focusing and are specified in terms of magnification relative to the camera sensor. They lack flexibility since the iris and working distance cannot be changed while having excellent MTF characteristics and little distortion.
- Large format lenses
On the other hand, large format lenses are necessary when a camera’s sensor is larger than what can be handled by C-mount lenses. Line scan applications are the most prevalent usage for them. Several distinct components, including focusing adapters, helical mounts, and spacers, are frequently used to produce large format lenses.
- Telecentric lenses
Specialized metrology applications are utilized to reduce dimensional and geometric discrepancies in pictures. Because they collimate the light entering the lens, they do not suffer from distortion. As a result, pictures with constant magnification and no perspective distortion are produced, regardless of object distance. The front aperture of the lens must be at least as large as the field of vision due to the collimation of the light. Large-field-of-view lenses will be more costly as a result of this