RGB‐Color Image Analysis for Determination of Birefringence of Micro‐Films and Columnar Coatings

• Published in: Laser & photonics reviews 2023-03, Vol.17 (3), p.n/a
• Main Authors: Ryu, Meguya, Han, Molong, Grineviciute, Lina, Hu, Jingwen, Moein, Tania, Honda, Reo, Katkus, Tomas, Tobin, Mark J., Vongsvivut, Jitraporn, Tolenis, Tomas, Nishijima, Yoshiaki, Ng, Soon Hock, Juodkazis, Saulius, Morikawa, Junko
• Format: Article
• Language: English
• Subjects: Birefringence; Chromaticity; Color imagery; Compensators; Image analysis; Numerical analysis; Polarizers; red–green–blue; sculptured thin films; Silicon dioxide; Silicon substrates
• ISSN: 1863-8880; 1863-8899
• DOI: 10.1002/lpor.202200535

The birefringence analysis of thin ≈1μ$\approx 1\hspace*{3.33333pt}\umu $m SiO2 films deposited via evaporation at a glancing angle of 70∘$\hspace*{3.33333pt}{70}^{\circ}$ to the normal on resist pillar arrays on Si and nanopatterned SiO2 substrates is carried out by spectral and color (red–green–blue [RGB]) imaging modes. Retardance and birefringence of the films deposited over flat and structured regions can be distinguished with only ≈1% difference between neighboring regions for the visible spectral range using RGB numerical analysis of images. The Michel‐Lévy color map is used for color rendering of birefringence to make quantitative measurements by numerical RGB color filtering. It is shown that by using λ/2$\lambda /2$ at 530 nm waveplate inserted at π/4$\pi /4$ angle to the cross polarizer‐analyzer setup, the range of changes in chromaticity xy‐coordinates expands approximately twice upon angular rotation of a birefringent sample. This facilitates a better signal‐to‐noise determination of birefringence. The proposed method with λ/2$\lambda /2$‐plate color shifting can be directly used to determine birefringence from step‐like spectral features in reflection and transmission polariscopy. Direct measurement of birefringence (Δn=0.023${\Delta}n=0.023$) and retardance of SiO2 chevron film is carried out using Berek compensator as a benchmark. The birefringence analysis is proposed using the basic red–green–blue (RGB) color analysis under white light illumination. Retardance and birefringence is distinguished when only ≈1% variation was between neighboring micro‐regions. The method does not require separate measurements at each of RGB colors.