Adjusting the vision system to new requirements

Adjusting the vision system to new requirements
mady70 / shutterstock

Let’s imagine a situation where you want to speed up the production line which has plenty of cameras for visual inspection which work “on the fly” – without stopping the product for inspection. Could the existing camera systems continue working without any modifications? Possibly, but let’s go through the specifics.

To achieve system feasibility, we need to ensure that the camera and lens parameters are balanced. Two main things that will be affected are exposure time and depth of field. If the production line speed increases and exposure time stays the same, there is a possibility of seeing a motion blur on our images. Exposure time, in this case, is the time component in the path covered by the object in the image.

Adjusting the vision system to new requirements
Mihai_Andritoiu / shutterstock

The next logical step would be to decrease the exposure time to avoid this problem. We could either decrease the time on the camera or increase the physical opening of the lens – aperture. Opening the aperture will in fact cause more light to fall on the chip, but it will reduce the depth of field. Depth of field is the distance between the nearest and the farthest objects that are in acceptably sharp focus in an image. 

Why does the opening of the lens decrease the depth of field? An explanation is quite simple, having a smaller pinhole means that only direct rays of light fall on the sensor. As we open it further, reflected rays are coming more and more, which translates as a blurred image. In the cover image, we can see that only a few rows of the cookies are sharp, which means they are inside the depth of field. If you wish to explore this further, I recommend this article.

Let’s explore the exact relationship between the (production line) speed, aperture, and depth of field with a 12 mm lens.

Relationship between production line speed, aperture and depth of field

To keep the same exposure time, which effectively means the same distance traveled during the exposure time, we have to move somewhere along the gray curve.

If none of these options satisfy our requirements, we need to consider a lens with another focal length. Comparison between 8 mm and 12 mm lenses can be seen below.

Comparison between 8 mm and 12 mm lens

Changing the lens has its consequences, especially on the field of view – choosing a smaller focal length will give us a bigger field of view, which automatically means a smaller number of pixels per millimeter:

Different focal length lenses, depth of field

One more option would be to increase the light intensity if that is even a possibility. Of course, there is a chance that the initial depth of field was large enough that the speed increase will not have a significant impact on the vision system performance.

There is more than one solution to this problem and the purpose of this article was to give you a general overview of balancing between the different camera and lens parameters. To make an exact calculation use online calculators like Vision Doctor or contact us directly.