In a machine vision system, the lens is analogous to the human eye, its primary function being to focus the optical image of the target onto the photosensitive array of the image sensor (camera). All image information processed by the vision system is obtained through the lens, and the quality of the lens directly affects the overall performance of the vision system. The following is a detailed explanation of the relevant technical terminology for industrial lenses in machine vision .
I. Distortion
It can be divided into pincushion distortion and barrel distortion, as shown in the figure below:
II. Television Distortion
The value is calculated as a percentage of the actual side length's distorted shape compared to the ideal shape.
III. Optical Magnification
IV. Monitoring and Magnification
Calculation method:
Example: VS-MS1 + 10x lens 1/2” CCD camera, image on a 14” monitor
A 0.1mm object appears as a 44.45mm image on the monitor.
*The above simplified calculations may vary depending on the scanning status of the TV monitor.
V. Resolution
This indicates that the interval between the two visible points is 0.61x, calculated using wavelength (λ)/NA = resolution (μ).
The above calculation method can theoretically calculate the resolution, but it does not include distortion.
※ Wavelength used is 550nm
VI. Resolution
The number of black and white lines can be seen within 1 mm. Unit: (lp)/mm.
7. MTF (ModulaTIonTransferFuncTIon)
Spatial frequency and contrast are used to reproduce the tonal variations on the surface of an object during imaging.
8. Working Distance
Distance from the lens barrel to the object
9. O/I (Object to Imager)
The distance between an object and its image is the length from the object to the image.
10. Imaging Circle
For the image size φ, you need to input the camera sensor size.
11. Camera Mount
C-mount:1"diameterx32TPI:FB:17.526mm
CS-mount:1"diameterx32TPI:FB:12.526mm
F-mount: FB: 46.5mm
M72-Mot:FB manufacturers vary.
12. Field of View (FOV)
Field of view refers to the range of an object seen from the side after using a camera.
Longitudinal length of the camera's effective area (V) / optical magnification (M) = field of view (V)
The horizontal length of the camera's effective area (H) / optical magnification (M) = field of view (H)
* The field of view in technical data refers to the value calculated from the general values of the light source and the effective area.
The longitudinal length (V) or (H) of the effective area of the camera = the size of one pixel of the camera / the number of effective pixels (V) or (H)
To calculate.
13. Depth of Field
Depth of field refers to the distance of an object after it has been imaged. Similarly, the range on the side of the camera is called the depth of focus. The specific values of depth of field vary slightly.
XIV. Focal Length (f)
f(FocalLength) is the distance from the rear principal point (H2) of the optical system to the focal plane.
15. FNO
The brightness value when the lens is at infinity; the smaller the value, the brighter the lens. FNO = focal length / incident aperture or effective aperture = f / D
XVI. Practical F
The brightness of the lens at a limited distance.
Effective F = (1 + optical magnification) x F#
Effective F = Optical magnification / 2NA
17. NA (Numerical Aperture)
NA = sinuxn on the object side
NA'=sinu'xn' on the imaging side
As shown in the figure below, the angle of entry u, the refractive index n on the object side, and the refractive index n' on the imaging side are...
NA = NA' x Magnification
18. Edge brightness
Relative illuminance refers to the percentage of illuminance in the center compared to the illuminance in the surrounding area.
19. Telecentric Lens
A lens in which the principal ray is parallel to the lens light source. Telecentric lenses can be positioned with the principal ray on the object side, the image side, or both sides.
20. Telecentric
Telecentricity refers to the magnification error of an object. The smaller the magnification error, the higher the telecentricity. Telecentricity has various uses, and understanding it is important before using a lens. In a telecentric lens, the principal ray is parallel to the lens's optical axis. Poor telecentricity will result in poor performance of the telecentric lens. Telecentricity can be easily confirmed using the diagram below.
21. Depth of Field (DOF)
Depth of field can be calculated using the following formula:
Depth of field = 2 x Permissible COC x Effective F / Optical magnification² = Permissible error value / (NA x Optical magnification)
(Using a 0.04mm PermissibleCOC)
22. Ventilation disc and resolution
AiryDisk refers to the concentric circles formed when light is focused onto a single point using a distortion-free lens. The radius *r* of the AiryDisk can be calculated using the following formula. This value is called resolution: *r* = 0.61λ/NA. The radius of the AiryDisk changes with wavelength; the longer the wavelength, the more difficult it is to focus light onto a single point. For example, for a lens with NA 0.07 and a wavelength of 550nm, *r* = 0.61 * 0.55 / 0.07 = 4.8μm.
23. MTF and Resolution
MTF (Modularity Transfer Function) refers to the tonal variations on an object's surface, which are also reproduced in the image. It represents the lens's imaging performance and the degree to which it reproduces the contrast of an object. Contrast performance is tested using a black-and-white interval test with a specific spatial frequency. The spatial frequency refers to the degree of tonal variation over a distance of 1 mm.
Figure 1 shows a black-and-white matrix wave with a black-and-white contrast of 100%. The change in contrast in the image after the object is photographed is quantified. Basically, regardless of the lens, a decrease in contrast will occur. Ultimately, the contrast decreases to 0%, making color differentiation impossible.
Figures 2 and 3 show the changes in spatial frequency between the object side and the imaging side. The horizontal axis represents spatial frequency, and the vertical axis represents brightness. The contrast between the object side and the imaging side is calculated from A and B. MTF is calculated from the ratio of A to B.
The relationship between resolution and MTF: Resolution refers to how well two points can be separated and perceived. Generally, the quality of a lens can be judged from its resolution value, but in reality, MTF is closely related to resolution. Figure 4 shows the MTF curves of two different lenses. Lens a has low resolution but high contrast. Lens b has low contrast but high resolution.
