The Photron Crysta is a high-speed polarization camera for the two-dimensional analysis of birefringence occurring internally within transparent and semi-transparent materials or fluids.
Why polarization? There are many ways of imaging what is occurring on the outside of an object; including high-speed photography, multispectral imaging, digital image correlation (DIC), infrared and conventional 25 or 30 frames per second (fps) imaging to name but a few. But what happens if you want to see what is taking place within the object, beneath the objects surface? By visualizing an objects birefringence, the Photron Crysta can measure the stress occurring within the materials being tested.
What is birefringence? Well, transparent, or crystalline materials can be classified as either isotropic, where refraction is symmetrical, so the same regardless of the direction the light passes through it, for example glass. However, some materials are anisotropic – meaning they have a physical property that has a different value when measured in a different axis – wood is a good example of anisotropic, it has two distinctly different axis and is stronger along the grain than across it. Calcite, is an excellent example of a anisotropic material, where light is refracted into two rays (ordinary and extraordinary) that depend on the polarization and propagation direction of light. One ray will typically travel slower than the other. This difference is defined as the retardation.
Fig. 1. High-speed sensor construction of Photron Crysta.
As mentioned, the Crysta is a high-speed 2D polarization camera where a 1MP CMOS sensor, comprising of 1,024 by 1,024 20µm pixels, has a pixelated polarizer array made from photonic crystal affixed directly to the sensor. This photonic crystal lattice features pixels, in groups of four square, each having a different polarization axis at 0˚, 45˚, 90˚ and 135˚. By applying phase shifting algorithms with circularly-polarized incident light, birefringence phase difference and azimuthal angle can be measured to enable us to quantify and measure physical stress in transparent and semi-transparent fluids and solids at full resolution operation to 7,000 frames per second (fps), and reduced resolution to 1.3 million fps.
Fig. 2. Relationship between retardation and stress
When comparing the Crysta to existing polarization systems already available on the market, the main advantage is the Crysta’s ability to see the whole picture, not just one line at a time, or one specialized view that takes minutes to compile. The Crysta lets you see your entire area of interest, thousands of times a second and with global shutter times as short as 369 nanoseconds. Plus, in addition to capturing the 2D area scan high-speed birefringence data showing what is happening inside the block of acrylic being drilled, we can also record polarizations axis data and ‘traditional’ high-speed images data alongside the retardation information.