Stopping Time with TDI

What if you could stop time? In addition to being able to eat as much as you want without consequences, there is also a compelling case for it in machine vision.

Let’s say you are running a web process of some kind. Unlike discrete components like bottles or cans, a web is a continuous flow of material like paper, steel, aluminum, or fabric – just to name a few. And let’s say you have a vision system to inspect this web (probably with a line scan camera to handle the continuous flow of material), but you simply can’t get enough light into the camera for a good quality image. Lack of sufficient light is a common problem in web inspection (stay with me – I promise to return to why you might want to stop time).

Continuous sheet of metallic material

Continuous sheet of metallic material

A line scan camera might be running at around 50,000 lines per second. That means that each line has only 20 microseconds (or 1/50,000th of a second) to gather the light of the photons that pass through the lens to the image sensor during that time. Also, most line scan cameras have thousands of pixels per line, each one looking at a small area of the web (perhaps 10 microns on average). With a good lens, this means that the 20 microseconds of the photons coming from just a 10 x 10 micron area are going to be gathered in a single pixel. This doesn’t leave very many photons per pixel to be converted into voltage (photon to voltage conversion is the primary purpose of the image sensor in the camera). One way to get more light is to increase the brightness of your light source, but there is both a practical limit and a technology limit to how much light you can generate. The other way to get more light is to spend more time gathering photons from the light source you already have. But to slow your web to gather these photons would slow the manufacturing process which would cost your company money. What if you could stop time (I told you I would get back to this)? We can’t stop time, but what if you could “effectively” stop the web, without “actually” stopping the web, to allow for more time for the sensor to integrate the available light? This is what a TDI camera does.

TDI stands for Time Delay Integration. TDI technology has been around for many years, and Teledyne DALSA has been a pioneer of this technology. If you understand how a line scan camera works, think of a TDI camera as a line scan camera with 100 or so successive lines. You can see an example of this on our website here. Once the camera starts grabbing images, the first line of the camera will grab a line on the web that is positioned right under that first line of the TDI camera. Then, as the web moves, the line that was previously under the first line of the camera, is now positioned under the second line of the TDI camera. What the TDI camera does is move the image from the first line into the second line.  It does this in lock-step with the motion of the web.  Because the web has moved, the second line of the TDI camera is now looking at the same area on the web that the first line of the TDI camera was previously looking at. The second line of the TDI camera now captures an image of that area and “adds” that image to the image from the first line. So, for a standard line scan camera running at 50,000 lines per second, the camera has 20 microseconds to gather the light. A TDI camera has 20 microseconds times the number of lines (or TDI “stages”), to gather light. So with just a two-line, or two “stage” TDI camera, we have doubled our integration time – meaning we have converted more photons to voltage, and made the image twice as bright. But a typical TDI camera has many more than two lines of integration. The number of lines can vary but, on average, you will increase the brightness of the image by a factor of 100 without an additional investment in lighting or slowing your process!

In my next post, I will discuss some of the details of the TDI operation and synchronization to the web motion.



About Glen

Glen is Sales & Applications Support Manager for the Eastern US. Glen has been in the vision industry throughout his whole career.
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