Polarization is a sample orientation effect, independent of the instrument.
HunterLab tests for polarization as part of our sphere design process and do everything we can to minimize it. However, there are always trade-offs between other optical characteristics and polarization.
In the design of the optical path in all sensors, there will be a requirement to implement a 90 degree turn at some point and typically mini-spheres, mirrors or fiber optics are used to do this. These optical elements have some polarizing effects of their own and always interact with a polarizing sample.
- The USXE has a small sphere after the lens to point the collected signal (minimizes polarization but reduces signal counts) towards an air path (efficient transmission of light signal) to the monochromator grating (some polarization). This requires a high power lamp to do this.
- The CQXE and UVIS use fiber optics (reduces efficiency but does not polarize significantly.
- The USPRO uses a mirror (some polarization) and an air path (most efficient at transmitting light) to feed signal to the monochromator grating. Signal level is maintained by multi-bursts of the lamp.
Overall the USVIS (newer design than CQXE) and USPRO are the best choices for minimizing polarization effects.
If the sample is highly polarizing, the best approach to minimize the effect of polarization contributing to variability in in color measurement is:
- Fix the orientation of the polarized sample and always make measurements of product standard and sample colors at that same orientation angle.
- Take readings of a highly polarizing sample in groups of 2 with a 90 degree rotation between each reading. This will average out polarization effects, minimizing the variability.