In industrial machine vision systems, industrial camera lens is equivalent to the human eye, and its main function is to focus the optical image of the target on the light-sensitive surface array of the image sensor (camera). All the image information processed by the visual system is obtained through the lens, and the quality of the lens directly affects the overall performance of the visual system. Here are 23 important technical terms related to machine vision industrial lenses.
1, distortion
Can be divided into pillow distortion and barrel distortion, as shown below:
2. TV distortion:
The value calculated as a percentage of the actual side length of the distorted shape and the ideal shape
3. Optical magnification
4, Monitor zoom
5. Resolution MTF
It shows the interval between the 2 points that can be seen 0.61x the used wavelength (λ)/ NA=resolution (μ)
The above calculation method can theoretically calculate the resolution, but does not include distortion.
※The use wavelength is 550nm
6. Resolution
The number of black and white lines can be seen in the middle of 1mm. Unit (lp)/mm.
7. MTF (Modulation Transfer Function)
The spatial frequency and contrast used to reproduce the shade changes on the surface of the object during imaging.
8. Working Distance
The distance from the lens barrel to the object
9. O/I (Object to Imager)
The distance between the object and the image is the length between the object and the image.
10. Image circle
Image size φ, you need to enter the camera sensor size.
11. Camera Mount type
C-mount Lens: 1" diameter x 32 TPI: FB: 17.526mm
CS-mount Lens: 1" diameter x 32 TPI: FB: 12.526mm
F-mount: FB: 46.5mm
M72-Mount: FB manufacturers are different
12. Field of view (FOV)
Field of view refers to the range of the side of the object seen after using the camera
The longitudinal length of the effective area of the camera (V) / optical magnification (M) = field of view (V)
The lateral length of the effective area of the camera (H) / optical magnification (M) = field of view (H)
＊The field of view on the technical data refers to the value calculated from the general value of the light source and effective area.
The vertical length of the effective area of the camera (V) or (H) = the size of one pixel of the camera × the number of effective pixels (V) or (H).
13, depth of view
Depth of field refers to the distance of an object after imaging. Similarly, the range on the camera side is called focus depth. The value of the specific depth of field is slightly different.
14, focal length (f)
The distance from the rear main point (H2) of the f (Focal Length) optical system to the focal surface.
15、F/NO
When the lens is from an infinite distance, the brightness represents a value that is smaller and brighter. FNO - focal length / incoming aperture or 効 caliber - f/D
16, effective F
The brightness of the lens at a limited distance.
Effect F s (1 s optical multiply) x F s
Effect F - Optical multiply / 2NA
17、NA（Numerical Aperture）
The NA on the side of the object is sin u x n
Na' on the imaging side s sin u'x n'
The image below shows the entry angle u, the refractive index n on the side of the object, the refractive index 'n' on the imaging side
NA - NA' x Magnification
18, edge brightness
Relative lightness refers to the percentage of central and peripheral light.
19, far-heart lens
A lens in which the main light is parallel to the lens light source. There is the far heart on the side of the object, the far heart on the imaging side, the far heart on both sides and so on.
20. Telecentric
Telecentricity refers to the magnification error of the object. The smaller the magnification error, the higher the Telecentricity.
Telecentricity has a variety of different uses. It is important to grasp Telecentricity before using the lens. The chief ray of the telecentric lens is parallel to the optical axis of the lens,
If the telecentricity is not good, the effect of using the telecentric lens is not good; Telecentricity can be simply confirmed with the following figure.
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 magnification 2 = allowable error value / (NA x optical magnification)
(Using 0.04mm Permissible COC)
22. Ventilation plate and resolution
Airy Disk refers to the fact that a concentric circle is actually formed when the light is concentrated through a lens without distortion. This concentric circle is called Airy Disk. The radius r of Airy Disk can be calculated by the following calculation formula. This value is called resolution. r= 0.61λ/NA The radius of the Airy Disk changes with the wavelength. The longer the wavelength, the more difficult it is for light to concentrate on one point. Example: NA0.07 lens wavelength 550nm r=0.61*0.55/0.07=4.8μ
23, MTF and resolution
MTF (Modulation Transfer Function) refers to the change in density on the surface of an object, and the imaging side is also reproduced. Indicates the imaging performance of the lens, the degree of contrast of the imaging and reproducing object. To test the comparison performance, a black-and-white interval test with a specific spatial frequency is used. The spatial frequency refers to the degree of change in density at a distance of 1 mm.
As shown in Figure 1, the black and white matrix wave, the black and white contrast is 100%. After this object is photographed by the lens, the change in the contrast of the image is quantified. Basically, no matter what lens, there will be a decrease in contrast. The final contrast is reduced to 0%. , Can't distinguish between colors
Figures 2 and 3 show the changes in the number of spatial peri-wave waves on the side of the object and on the imaging side. The horizontal axis represents the number of waves in the space per week, and the vertical axis represents the brightness. The contrast between the object side and the imaging side is calculated by A and B. MTF is calculated by the ratios of A and B.
The relationship between resolution and MTF: Resolution refers to how 2 points are separated from each other. Generally from the value of resolution can be judged the lens is good or bad, but in fact MTF and resolution has a great relationship. Figure 4 shows the MTF curve of two different lenses. Lens a has low resolution but high contrast. Lens b has low contrast but high resolution.
