Detailed explanation of industrial lens terminology

In a machine vision system, an industrial lens is equivalent to the human eye. Its main function is to focus the optical image of the target on the photosensitive area array of the image sensor (camera). All image information processed by the vision system is obtained through industrial lenses, and the quality of industrial lenses directly affects the overall performance of the vision system. The relevant technical terms of the machine vision industry lens are explained below.

I. Telecentric Optical System:

Refers to an optical system whose principal rays are parallel to the optical axis of the industrial lens. The light emitted from the object towards the lens is kept parallel to the optical axis, even outside the axis, which is called the object-side telecentric optical system.

Second, telecentric lens:

Telecentric lens refers to an industrial lens whose main light is parallel to the light source of the lens. There is object telecentricity, like party telecentricity, bilateral telecentricity. General industrial lens

The main ray is at an angle to the lens optical axis, so when the workpiece moves up and down, the size of the image changes.

        The main object side of the telecentric head on both sides, the image side is that the main ray and the optical axis are parallel to the variable aperture, which can obtain a high depth of field, which is more stable than the object telecentric head. It is most suitable for measuring image processing optical systems. However, the cost of large-scale object-distance telecentricity is very high. Only the object-side principal ray parallel to the lens axis changes the workpiece up and down, and the size of the image will not change. Necessary conditions when using coaxial epi-illumination. The head is just like the main light of the square parallel to the optical axis of the lens

The camera side can absorb changes in photographic magnification even if there are individual differences in installation

Used for color shift compensation, cameras should have used this lens

Third, the characteristics of telecentric optical system:

Pros: Smaller size. Reducing the number of lenses can reduce costs.

Disadvantages: When moving the surface of an object up or down, it will change the size or position of the object.

Advantage: When moving the surface of the object up and down, it will not change the size or position of the object. When using coaxial lighting. Smaller size available

Disadvantages: larger than standard lens size when coaxial lighting is not used

4. Telecentricity:

Telecentricity refers to the magnification error of an object. The smaller the magnification error, the higher the telecentricity. There are various uses for telecentricity, and it is important to grasp telecentricity before using the lens. The main light of a telecentric lens is parallel to the optical axis of the lens. If the telecentricity is not good, the use effect of the telecentric lens is not good. The telecentricity can be easily confirmed with the following figure.

V. Resolution (μm):

The scale of the optical system capability indicates the maximum logarithm of the black and white stripes that can be observed and observed in 1mm when the black and white grid pattern is observed through the lens. The resolution is the closest distance measurement value between two points before it cannot be identified. For example, a resolution of 1 μm means that the closest distance between two points before it cannot be identified is 1 μm. The following is a formula for calculating the theoretical resolution based on the lens's non-phase-diffraction light diffraction.

6. Resolution (Lines / mm):

The resolution refers to the number of recognizable black and white stripes in a 1mm area in the image in the black and white network map lens. The unit of resolution is line / mm. For example, 100 line / mm represents the recognizable black and white pitch of 1 / 100mm (10 μm). The width of the black and white lines is 1/200 mm (5 μm).

7. Horizontal TV resolution (TV lines):

The total number of black and white horizontal lines in the width corresponds to the height value of the vertical height of the TV screen. The vertical to horizontal length ratio of the screen is usually 3: 4, so the total number of bars in the horizontal width is 3/4. The horizontal resolution of the TV is 240TV lines, and the total number of horizontal widths of the TV screen is 320 lines. When measuring the resolution of a lens, a set of black and white lines should be considered as one line, but in terms of TV resolution lines, a set is considered as 2TV lines.

Distortion (%):

Distortion is a straight object off the optical axis, and lens aberrations when presenting a curve. Lens distortion is also called lens distortion, which is a general term for perspective distortion inherent to optical lenses. It can be divided into pincushion distortion and barrel distortion. The straight line toward the center is called pincushion distortion. Barrel Distortion.

Nine, TV distortion (%):

The image on the TV screen is distorted. The closer the value is to zero, the higher the animal energy.

Ten, TV distortion:

The calculated value is the percentage of the distorted shape of the actual side length and the ideal shape.

Eleventh, the marginal efficiency of the aperture efficiency (%):

Aperture efficiency is the brightness difference between the optical axis of the imaging disk and the surrounding area when shooting a uniformly bright object with the lens.

12. Shading (%):

Obscuration is the brightness difference between the center and edge of the TV screen when using the lens and CCD-TV lens to shoot a uniformly bright object, the unit is percentage (%). This percentage is usually calculated using the power ratio of the light receiving component to the CCD component. Masking refers to the overall performance of the lens and TV lens. Telecentric optical systems can be used to reduce masking.

Thirteen, color difference:

In lens optics, the location of the image and the magnification of the image vary with the wavelength of the light. Different wavelengths of light have different colors, which is called color distortion. The distortion on the optical axis is called color distortion. The difference in magnification is called magnification color distortion.

14. Working distance (WD) (mm):

Working distance refers to the distance from the first working surface of the lens to the measured object.

15. O / I (Object to Imager)

OI refers to the distance from the object to the knot image plane.

Sixteen, focal length f (mm) rear focal length / front focal length

Focal length is the distance from the main point of the optical system to the focal point. The distance from the vertex of the last lens to the back focus is the back focus. The distance from the vertex of the first lens to the front focus is the front focus.

17. Depth of field:

Depth is when the object moves back and forth from the best focus. The distance between the closest point and the furthest point where the sharpest focus appears. The depth range on the object side is called the depth of field. Similarly, the range on the camera side is called the depth of focus. The value of the specific depth of field is slightly different. 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)

The theory of the image through the lens will form spots. An acceptable blurring condition on a clear image is called an acceptable circle of diffusion.

18. Depth of focus:

Depth is the distance between the closest point where the sharpest focus appears and the farthest point when the CCD moves back and forth from the best focus. The depth range on the image side is called focal depth.

Nineteen, back intercept (mm):

The distance from the front of the lens mount plate to the image.

Twenty, C mount specifications:

Name Standard outer diameter Number of screw threads (for 25.4mm) Back intercept

U1 25.4000mm 32Threads 17.526mm

Twenty-one, numerical aperture NA, NA ':

When the half angle generated by the object on the incident light hole is u and the refractive index is n, n x sinu is the numerical aperture (NA) on the object side.

When the half angle generated by the object on the exit aperture is u 'and the refractive index is n', n 'x sinu' is the numerical aperture {NA ') on the image side.

NA = n x sinu NA '= n' x sin u '

The higher the NA, the better the resolution and brightness of the lens. As shown in the figure below, the incident angle u, the refractive index n on the object side, and the refractive index 'n' on the imaging side: NA = NA 'x magnification

For Macro lenses, NA = M / 2 xF NA '= 1/2 xF NA = NA' x optical magnification NA '= NA x optical magnification

Twenty-two, F value F No:

This value refers to the brightness of the lens. This value can be obtained by dividing the focal length of the lens by the effective diameter of the object side (the diameter of the incident light hole Dmm), or it can be calculated using NA and the optical magnification (β) of the lens. The smaller the value, the brighter the lens. F No = focal length / incident aperture or effective aperture = f / D

23. Effective F No:

This value is the brightness of the lens within a specific distance, and refers to the brightness in actual operation. The higher the optical magnification (β), the darker the lens.

Effective F = (1 + optical magnification) x F #, effective F = optical magnification / 2NA

24. Optical magnification β:

The ratio of object size to image size.

β = y '/ y

  = B ／ a

  = NA ／ NA '

  = CCD lens element size / field of view actual size

25. Optical magnification:

Magnification means that the size of the original imaging area of the subject can be changed by adjusting the lens. Optical magnification is the magnification that can be changed by an optical lens. Relationship between main points and imaging: Magnification refers to the ratio of imaging size to the object.

26. Electronic magnification:

The electronic magnification is the magnification of the image when it is displayed on the monitor screen compared to the CCD.

27. Display magnification:

Display magnification The magnification at which an object is presented on the display through a lens.

Display magnification = (optical magnification β) x (electronic magnification)

 (Calculation example) Optical magnification = 02x, CCD size 1/2 "(diagonal 8mm), display 1/4":

 Electronic magnification = 14 x 25.4 / 8 = 44.45

 Display magnification = 0.2x44.45 = 8.89 (times) (1 inch = 25.44mm)

※ Some of the above simple calculations may change depending on the scanning status of the TV monitor.

28. Field of View (FOV):

The field of view refers to the range of the object side when the camera is used.

Camera length (V) / optical magnification (M) = field of view (V)

Horizontal length (H) / optical magnification (M) of camera effective area = field of view (H)

The 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).

(Calculation example): Optical magnification = 0.2x, CCD size 1/2 "(length 4.8mm, width 6.4mm}:

 Field of view length = 4.8 ／ 0.2 = 24 (mm)

 Width = 6.4 ／ 0.2 = 32 {mm)

29. Resolution:

Shows that you can see the interval of 2 points 0.61x using wavelength (λ) / NA = resolution (μ). The above calculation method can theoretically calculate the resolution, but does not include distortion. ※ Using wavelength is 550nm

30. Resolution:

The number of black and white lines can be seen in the middle of 1mm. Unit (lp) / mm

31. MTF (Modulation Transfer Function):

The spatial frequency and contrast used to reproduce the changes in the shade of the surface of an object during imaging.

Thirty-two, imaging circle:

For imaging size φ, enter the camera sensor size.

33. Camera Mount:

C-mount: 1 "diameter x 32 TPI: FB: 17.526mm, CS-mount: 1" diameter x 32 TPI: FB: 12.526mm, F-mount: FB: 46.5mm, M72-Mount: FB varies by manufacturer .

34. Edge brightness:

The contrast degree refers to the percentage of the illuminance in the center and the illuminance in the periphery.

35. Ventilation plate and resolution:

Airy Disk means that when the light is focused a little through a lens without distortion, it actually forms a concentric circle. This concentric circle is called Airy Disk. The radius r of the 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 to concentrate the light. Example: NA0.07 lens Wavelength 550nm r = 0.61 * 0.55 / 0.07 = 4.8μ

36. MTF and resolution:

MTF (Modulation Transfer Function) refers to the change in the density of the object surface, and the imaging side is also reproduced. It indicates the imaging performance of the lens and the degree of contrast of the imaging reproduction object. To test the contrast performance, a black-and-white interval test with a specific spatial frequency was used. The frequency of the space frequency refers to the degree of light and shade change at a distance of 1 mm.

Black and white matrix waves, the contrast of black and white is 100%. After this object is photographed by the lens, the change in the contrast of the imaging is quantified. Basically, no matter what lens, the contrast will decrease. The final contrast is reduced to 0%. , Can't make color difference.

The change of the spatial frequency of the object side and the imaging side is shown. The horizontal axis represents the frequency of the space frequency, and the vertical axis represents the 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 and B.

The relationship between resolution and MTF: Resolution refers to the interval between two points that are separated and recognized. Generally, the quality of the lens can be judged from the value of the resolution, but the actual MTF has a great relationship with the resolution. Figure 4 shows the MTF curves for two different lenses. Lens a has low resolution but high contrast. The lens b has low contrast but high resolution.

37. Macro lens:

Enlarged photography without close-up rings or close-up lenses. The lens designed for close-up photography has a limited distance (= light emitted from the objective lens, focused at a certain distance)

38. CCTV lens:

Suitable for a wide range of observations, not suitable when strict precision is required, infinity (= light emitted from the objective lens, unfocused, parallel progression)

39. Zoom lens:

Variable focal length lenses, magnifications, and imaging ranges can be easily changed. It is suitable for situations where you need to find the most suitable shooting conditions (shooting distance, focal length of the lens) for easy operation. A zoom lens does not cause a focus position shift, and a zoom lens causes a focal position shift.

Forty, imaging circle:

The size of the imaging circle in the optical system, the size of the imaging circle = the diagonal size of the CCD, is the same as the CCD size.

Forty-one, rear zoom lens:

It is installed in front of the CCD, without changing the working distance, and expanding the field of view. The F value decreases, the resolution and contrast decrease, and the focus will be slightly inaccurate.

Forty-two, front zoom lens:

Installed in front of the lens, the working distance will change, the brightness will not change, and the field of view will be enlarged.