FAQ: “Do you sell a depolarizer for my HunterLab sphere instrument? On occasion we test polarized samples and there is definitely a dependence on the orientation of the lens. I was wondering if adding a depolarizer would eliminate this phenomenon.”
A depolarizer will probably not help. We do test for polarization in our instrument sphere design and do everything we can to minimize it.
Most of our sphere sensors have pre-mixing spheres around the lamp to homogenize the emitted light. This well-mixed light is then pumped out into the 150 mm (6 in) diameter main sphere which further homogenizes the lamp illumination, minimizing polarization effects.
However, there are still elements in the optical path that can contribute some polarization that can produce an orientation bias in highly polarizing samples like you lenses. All spectrophotometers typically have gratings in the optical path that will have some slight tendency to polarize. Some optical paths also have mirrors which also have slight polarization tendencies.
To test for polarization in your sample, take a reading, then rotate the sample 180 degrees and take a second reading. If there is a significant difference, there may be an orientation effect, one of which is polarization.
The best approach to minimize the effect of polarization in color measurements of polarizing samples is:
- If the effect is small, say under 1 unit, take readings in groups of 2 with a 90 degree rotation between each reading. This will average out mild polarization effects. Some coated plastic films exhibit this behavior.
- If the effect is several units, you will need to identify the orientation of the polarized sample and always make measurements of product standard and sample colors at that orientation. Polarized lenses or the polarizing film used in polarized lenses are typical samples that require a fixed orientation for consistent measurement.