Hue based Polariscope

d'Polariscope fitted with a loading machine; Hue Polariscope result

d’Optron is continually researching into new technologies as part of our research culture. One such is the use of hue as a means of determining phase difference, birefringence and stress from color photoelastic fringes. Since hue provides color visualization as seen by the human eye, it provides a better means of fringe identification as in Michel-Levy chart. With the advent of digital technologies, this approach can now be digitized. However, digital images represent color as RGB components and usually it is difficult to directly glean any useful information from these values due to different variations. Instead, we proposed using Hue as a means of quantifying colors and hence quantitatively measuring phase difference and birefringence (or stress). Furthermore, since hue ranges from 0 to 360, it appears to be closely linked to the phase which spans the same range. However, as with any color-based approach, hue does not have a quantitative measure which in traditional interferometry is provided by the wavelength.

However, since the hue is wrapped at 2π intervals akin to phase – this interval can be calibrated with the appropriate path difference. This calibration depends on the illumination and recording wavelengths. To enable accurate measurement, an RGB LED with known central wavelengths and a monochrome camera are used to record the images three R, G and B images. Hue was then deduced using standard RGB to Hue algorithm, which provide the Hue in the range of 0-360 or normalized to span the range of 0 to 1 to correspond to fringe order in normal polariscopes.


The system used was the same as our d’Polariscope fitted with a loading machine as shown in figure above. This allows us to not only measure phase using hue but also use the d’Polariscope for comparison. The initial results from the phase shift d’Polariscope and the new hue based polariscope is shown in the figure below for three different loads. It is clear that we get similar results and with suitable calibration can convert hue values to phase values relatively easily .

Note the phase results are wrapped while the Hue results can be easily unwrapped. More details are available with us if you would like to work with us on this further.

Phase shift results for three different loads

Hue polariscope results