This is part of a series that supplements our webinar on PolarCam and PolarView Software
One of the most active applications of Imaging Polarimeters and Polarization Cameras is measuring stress and birefringence in glass, and has multiple applications in machine vision setups. Using active illumination, typically used for in-line production measurements for birefringence, PolarCam precisely measures and maps mechanical stress in components in real time.
We measured the birefringence of a test optic with our camera using this setup:
The light source at the left illuminated a diffuser to spread the light out evenly at the polarization state generator —basically a linear polarizer and a retarder—so that we could generate uniform circular polarized light. Then we inserted the optic under test, in our test case it was a 1-inch optic in a ThorLabs mount with a locking set screw, between the polarized light source and the camera. If the locking set screw of the mount is too tight, stress is induced in the glass that alters its birefringence. This “stress-induced” birefringence is imaged and measured in real-time by the pixelated polarization camera.
Without a micropolarizing snapshot camera, you would have to use multiple cameras (one for each direction of polarization) – in other words: more complicated, bulky and difficult to calibrate. Or you could use a single camera and rotate a polarizer precisely between several snapshot images –but this won’t work for capturing dynamic changes to your test optic because acquiring the snapshot images takes too long. By measuring in real time, you are able to create videos of the changes in birefringence while you make fine set screw adjustments.
Real time videos of birefringence using Polarcam
The images above are single frames from a video captured by PolarCam from that test setup described above. As the set screw at the top is tightened, you can see the stress (in red and yellow) build up at the kinematic mounting points. In this case, it reached a maximum of about 70nm of birefringence that we measured.
Unlike a single-direction polarizing filter, when you’re using a PolarCam, it doesn’t matter what the angle of polarization is—in applications where glare reduction is the concern, Polarcam will reduce the glare. It’s the same thing for birefringence. With other polarimeters, you have to be really careful about the angle of the polarizers in your test setup. With the snapshot micropolarizer cameras, you don’t have to worry about that.
This is an aspheric lens with a chip out of it in the lower left quadrant of the circular optic.
That chip caused the stress field to change on the lens, as illustrated by the blip on the bottom left of the birefringence image. Even if the lens is rotated, the measurement doesn’t change—because the Polarcam captures four linear polarizations at once.
Birefringence in industrial applications
This image shows how we can measure birefringence in eyeglasses.
What you see on the left is the 4D polarization workstation with a pair of reading glasses, and the PolarView analysis software running on the laptop to the right of the workstation. Simply place the glasses on the stage and with a little autoscaling you get an instant map and tabular data of Birefringence like shown by the inset on the right. Most eyeglasses that people wear, whether reading or prescription, have lots of stress from their mounts. You can use PolarCam to measure this stress very easily.
To learn more about how the PolarCam can enhance imaging for your team, you can view the full webinar by using the link below.