What is a Telecentric Lens ?
An important goal in machine vision applications is taking measurements in a consistent, accurate, and precise manner. This can include measuring the dimensions of manufactured parts to guarantee they are within the design tolerance, or measuring the width of electric tracks in a PCB. For these applications, telecentric lenses can deliver a high level of precision.
Images from a Telecentric Lens
A conventional image created by a lens will present a magnification of the object that will depend on the distance between the object and the lens. So, objects that are closer to the lens will create a larger image than objects farther away. This magnification (added to other aberrations as distortion) will affect how a machine vision will measure a particular object. So if you are measuring the dimensions of tracks’ width (as in our previous example), and the PCBs are not aligned exactly the same or they are displaced a little bit from your optical system, they can create measurement errors due to these changes in magnification.
With a telecentric lens, these magnification errors are eliminated. This is because the image created by a telecentric lens has a magnification that is constant regardless of the distance between the object and the lens. In basic terms, a telecentric lens creates an orthographic image of our object.
An orthographic view is a two-dimensional representation of an object, kind of like drawing just the side-view of an object. So, the orthographic view of a cylinder would look like a square from the side-view and as a circle from the top-view. The way we create an orthographic view is by drawing parallel lines that are perpendicular to the view that we are observing. As shown in fig. 2.
So how does a telecentric lens achieve this? A telecentric lens projects all its chief rays parallel to the optical axis. In other words, it can only receive light that is perpendicular to the lens. You can imagine a telecentric lens as a lens with a very narrow field of view. So two objects, one in front of the other, would appear to be side by side on a telecentric lens. As a matter of fact, it would be almost impossible to tell which object is in front of the other. This makes taking measurements more consistent and less prone to errors. As shown in figure 3
Telecentric Lens Design
A very common telecentric lens design is a double-telecentric system. In this configuration, we have a front optical element whose entrance pupil is located at infinity and its stop is placed at the lens’ back focal length. We can also use a back optical element whose exit pupil is located at infinity and its front stop is placed at its front focal point (we basically place the second lens stop at the same location as the front element’s stop). In figure 4 we show a double telecentric configuration designed for one of our clients. Lenses of this type are used in chip manufacturing lithography systems.
A second telecentric design has the telecentrical ray path in image space as the layout below shows. This design type is used in laser scanning systems.
In Summation: How do I know if I need a telecentric lens?
When the distance between the lens and the object is not known or changes constantly (i.e. vibrating conveyor belt)
When you need to measure the profile of an object
When you need to measure features of an object
When object at different distances needs to be constantly evaluated