Industrial camera and lens selection skills

1. Why does the field of view become smaller when adding an extension ring?

1. Comprehension of vision formula

Focal length = object distance X camera size / camera field of view
Although the image distance is always greater than the focal length, the physical definitions of the two are similar. When the object distance is at infinity, the focal length and the image distance are equal, and the focal length can be equivalent to the image To understand this formula, that is, the ratio of the image distance to the size of the photosensitive chip and the ratio of the object distance to the field of view are equal.
That is: focal length / photosensitive chip size = object distance / field of view, so it is easy to understand

2. Analysis of reduced field of view after adding ring

After the camera is confirmed, the size of the camera remains unchanged. Compared with the fixed-focus lens, if the field of view is to be smaller, the relative object distance will be smaller. Adding an adapter ring will make the object distance smaller and the image distance larger.
Note: It is equivalent to seeing things close to people, and of course the field of view becomes smaller

2. In the following test bench, an extension ring is added, and the image must be clear, so how should the height of the camera be adjusted?

1. Test bench scene

(The picture above is from Baidu Pictures)

2. The relationship formula between image distance, object distance and focal length is as follows

(1/object distance) + (1/image distance) = (1/focal length)

3. Analysis

We added an extension ring, and the focal length remains unchanged, which means that the image distance becomes larger, and our object distance is correspondingly reduced. Therefore, the camera should move down, get closer to the object, and reduce the object distance.

3. After the camera model is determined, how to choose the lens according to the product size

1. Camera model parameters

Assuming that glue defect detection is required, considering cost and practicality, 2 million color cameras have useful imaging for the visual detection of common glue defects in CCD and MEMS products, and the cost is appropriate.
Baser's 2 million color camera, sensor chip size: 7.2X5.4mm

2. Determination of field of view

Generally, the product should be located in the center of the entire image, and the image is slightly larger than the product.
Suppose a customer's product is 40mmX40mm, then the field of view can be determined to be 50mmX50mm, which is relatively large and may require a high working distance.

3. Lens selection

According to experience, the larger the working distance, the larger the corresponding focal length. At this time, you should choose a smaller focal length, such as below 25mm. How much focal length to choose depends on how much object distance space our mechanical structure has. The calculation is as follows:
object distance = field of view X focal length / photosensitive chip size
(1) The short side of the lens with 50mm focal length
reaches 50 field of view: the object distance needs to be 462.96mm
long 50 field of view on the side: the object distance needs to be 347.22mm
(2) The lens with a focal
length of 35mm reaches 50 on the short side: the object distance needs to be 324.07mm and the
field of view on the long side reaches 50: the object distance needs to be 243.05mm
(3) the lens with a focal length of 25mm
reaches 50 on the short side Field of view: The object distance needs to be 243.05mm
The long side reaches 50 Field of view: The object distance needs to be 231.48mm (4) The short side
of the lens with 12mm focal length reaches 50 Field of view: The object distance needs to be 111.11mm The long side reaches 50 Field of view: The object distance needs to be 83.33mm

4 Conclusion

In the visual structure of the machine, the mechanical distance should not exceed 200mm. If the camera is determined, a lens with a focal length of 12mm should be selected

4. Basic information and principle of telecentric lens

1. Application scenarios of telecentric lens

Precise measurement of mechanical components

2. Features of telecentric lens

(1) Magnification change due to object displacement
(2) Image distortion
(3) Perspective error
(4) Poor image resolution
(5) Uncertainty in object edge position due to light source geometry

3. Requirements for magnification stability

【1】In 2D measurement and most industrial robots, only the orthogonal view of the object is needed (only the top view plane, no side view) 【2】When working, the
vibration of the machine will cause the object to vibrate
【3】Many objects The depth of field is very large, more than a few centimeters
[4] The same photo, measured at different heights at the same time, under the condition of ensuring clarity, the ratio of the image size to the actual size should remain unchanged

4. Definition of telecentricity

When the object remains within a certain range, the size obtained by the telecentric lens will not change with the displacement of the object. This range is called "depth of field" or "telecentric range".

5. After the camera is confirmed, how should the lens be selected?

1. Determine the focal length according to the field of view and object distance

2. Optical size

The reason why the camera can image is because the lens hits the light reflected by the object on the CCD chip, and the mirror size of the lens is larger than the size of the CCD

3. Dimensions of chip type

4. Lens interface

(1) C mount: one of the standard mounts of the lens
(2) CS mount: the only difference from the C mount is the distance from the lens positioning surface to the image sensor, the C mount is 17.5mm, and the CS is 12.5mm. C-mount lens and CS-mount camera need to add a 5mm adapter ring.
(3) F interface: When the CCD size of general industrial cameras exceeds 1, use the F interface.