Imaging Performance of Telecentric Lenses

When designing a machine vision system, changes in magnification, with respect to working distance can disrupt calibration of that system. When it's important to maintain a magnification throughout a small change in working distance, using a telecentric lens is a better choice over a fixed focal length lens. Telecentric lenses eliminate all parallax error and any change in magnification that comes with working distance change. For any applications such as gauging in metrology, where accuracy is incredibly important, using a telecentric lens is the better choice. This scene is designed to show how a telecentric lens would view an object in comparison to a fixed focal length lens. There are four different main features of the scene: a train, people sitting on park benches, trees, and telephone lines. When viewed through the telecentric lens, you cannot tell the direction that the train is moving, or how far away it is from the lens. It never changes size, regardless of where it moves, and you can only see the front of the train. Viewed through the fixed focal length lens, you can see the side of the train, and you can clearly tell which direction it is moving; whether forward or backward. Looking at the people on the park benches, through the telecentric lens you can only see one set of people. With the fixed focal length lens, you can see that there are three sets, but the apparent distance between each park bench is much different. While the benches are equidistant from one another, the apparent distance between the first two benches appears much greater than the apparent distance between the last two. Looking at the trees, there are a few different things to notice. From the telecentric lens you can tell that there are four different tree trunks in the image and you can easily judge the height difference between the trees. There's clearly one tree that stands out as being taller than the rest. Through the fixed focal length lens, you can only count three tree trunks, because one of them is obscured, due to the parallax error in the lens, and also cannot judge the relative heights because of the magnification change with distance. Looking at the telephone lines, through the telecentric lens, we can only see one of them, and you can clearly tell how well they are stacked. Using the fixed focal length lens, you can see all of the telephone lines, but you can't see their tops due to the parallax error with respect to the trees, and you also cannot tell how tall they are, or how well aligned they are. Telecentric lenses are fantastic where any sort of measurement engaging needs to take place. Much more can be learned from other videos that we have done on telecentric lenses, and the white papers available on our website.