Polymer materials are widely used in various fields such as automotive lighting, outdoor lighting, optical communications and more due to their ease of production and low cost.

Usually Plexiglas plates are used in transmission configuration and todays they are more and more used for edge-lit solutions. Therefore the refractive index of this material becomes an important parameter for optical designers.

Different methods based on ellipsometry or the dispersion by a prism are used to measure the refractive index of glass samples. However, these techniques could not be used for PMMA or Polycarbonate samples because their refractive index varies constantly to such a point that two optics made in PMMA in similar conditions will not have exactly the same refractive index.

Then, Light Tec has developed an optical system dedicated to the refractive index measurements of optical plastic plates (PMMA) with absolute accuracy of ±0.001 and with high repeatability (better than 1%).

Example of applications:

  • Head and rear lamps from automotive industry
  • Outdoor lighting

Automotive head lamp - Refractive index measurement on PMMAOptical engineering: lightings application

Refractometer: Technical specification
Type of measurementRefractive index
Wavelengths445 nm, 532 nm and 638 nm
Accuracy+/- 0.001
Weight100 Kg
Advantages• High precision
• High repeatability


Refractive index measurements example

First, the refractive index of a N-BK7 window is measured at a specific wavelength in order to calibrate the refractometer.

Example 1:
  • Standard: N-BK7 flat window Φ1’’
  • Thickness: 1mm
WavelengthTheoretical refractive indexMeasured refractive index
638 nm1.5149
1.515 ± 0.001
532 nm

1.51951.520 ± 0.001
445 nm1.52581.526 ± 0.001

The measurement precision is ±0.001 and the instrument repeatability is better than 1%.

After the calibration, the refractive index of the optical sample is measured at ±0.001.

Example 2:
  • Optical plate made in PMMA
  • Thicknesses: 1mm, 2mm, 3mm and 4mm
WavelengthMeasured refractive index
638 nm1.481 ± 0.001
532 nm1.484 ± 0.001
445 nm1.490 ± 0.001