Measuring Refractive Index
ACW reports the refractive index (RI) of many adhesives and coatings. In all cases, we report the RI of cured film which generally is 0.02 to 0.03 RI units greater than that of the uncured resin. Our measurements are made with an Abbe refractometer on 25 micron or thicker films or with a prism coupler on thin films coated on glass or bulk films. The RI is not dependent on film thickness, but it is dependent on temperature. Temperatures are controlled either by a circulating water bath (Abbe refractometer) or by a thermally regulated prism housing (prism coupler).
Both of our RI instruments operate at four wavelengths. The multi-wavelength data is solved by a Sellmeier function or a Cauchy function, both of which will provide excellent RI predictions for wavelengths within the range of measurements and good RI predictions when extrapolated. One of the lasers in our prism coupler operates in the Near IR (854 nm) which results in very good RI predictions for the Near IR communication wavelengths.
The plot below shows RI results for one of our high RI LuxNIL® resins. The measured RI values are plotted as the circles, and the fit of the Cauchy function is the line. With the measurement at 854 nm, we can have high confidence in the RI predicted for the 900-1000 nm range where LiDAR sensors operate. .