High-Speed 2D Polarization Camera For Measurement Of Stress In Transparent And Semi-Transparent Materials And Fluids

San Diego, CA – High-speed cameras have become an established tool in the modern workplace. Their ability to record at hundreds or thousands of frames per second for subsequent slow motion replay and analysis has made them an important tool for applications such as ballistics, materials testing, automotive safety testing, and fluidics. Their capabilities are significantly increased when they are coupled with innovative non-contact measurement techniques such as digital image correlation (DIC) to provide invaluable information as to what is occurring on the surface of a solid, or through particle image velocimetry (PIV) in a seeded fluid or gaseous flows. 

But what happens when it is not possible to use tiny seed particles in a fluid or gas, or you wish to see within an objects exterior to understand what is occurring inside a material? Providing the subject is a transparent or semi-transparent material or fluid, polarization can be utilized to see the stresses occurring within these materials. Anyone who has worn a pair of polarized sunglasses has likely seen the normally invisible weird and colorful patterns in the widows of their car. These patterns are known as birefringence, and are evidence of residual stress generated during glass production process such as the laminate.

Imagine the ability to not only see, but to measure this stress, in real time and across the entire glass surface. Then imagine how useful it would be to record this phenomena in high-speed as the specimen is subjected to different stress, be it induced through compression, heat, pressure or any other means.

To a certain degree, this can already be achieved, but with definite caveats, with existing technology having to rely on slower record rates utilizing rotating polarization filters, or time consuming point measurement techniques that can scan only a small ‘sub-sample’ of the materials full surface, requiring generalizations be made to produce a ‘guesstimate’ as to what is occurring in the test sample as a whole.

To address these issues, Photron, and partners Photonic Lattice have developed two different digital 2D birefringence measurement systems, one, the CRYSTA, a high-speed dynamic measurement camera to scan high-speed  events at speeds as fast as 1.3 million frames per second (fps). The second system, named the KAMAKIRI, is a high accuracy static scan system that uses a five megapixel CMOS cameras with three different bandpass filters to very accurately scan a sample as large as a piece of A4 paper.

Both systems utilize a custom engineered two-dimensional camera system that utilizes a photonic crystal polarizer array directly attached onto Photron’s proprietary high-speed CMOS sensor technology. By coupling this polarizer array directly to the sensor itself, we make it almost impervious to vibration, with each polarizer corresponds directly to each of the CRYSTA’s individual 20µm (5µm in the case of the KAMAKIRI) square pixels. These polarized pixels are clustered in groups of four, each having a different axis orientations of 0˚, 45˚, 90˚ and 135˚ in a clockwise direction, and one polarization datum is calculated through the detected light intensities of these four pixels.

This sensor technology, when coupled with the CRYSTA’s high-speed parallel read-out circuitry, enables us to continually measure two-dimensional polarization data at speeds ranging from 30 to as fast as 1.3 frames per second (fps), with exposure times, independent of the framing rate, down in to the nanosecond range.

In addition to being able to record, display and measure the retardation taking place in the material or fluid, the Crysta can also display polarization direction/axis data, as well as the original monochrome high-speed data, making the Crysta a dual purpose tool for capturing high-speed video as well as measuring retardation in any birefringent material or fluid that transmits a minimum of thirty percent light.

Links to video:

BB striking acrylic, recorded at 15K fps: https://youtu.be/oKVcennAFJg
Flow Visualization of rheological fluid 250fps @ 50X mag: https://youtu.be/yDOWz9jgHvk