Optical Interference Filters Combined with Thin Film Residual Stress Compensation for Image Contrast Enhancement

We propose two single-wavelength notch filters and one dual-wavelength (480 and 620 nm) notch filter to enhance image contrast. The stack structure of the notch filters was designed as (Ta2O5/SiO2)4Ta2O5 in Essential Macleod thin film simulation software. Dual-electron-beam evaporation with ion beam-assisted deposition was used to prepare optical interference filters with different center wavelengths. A multilayer notch filter with a center wavelength of 620 nm was deposited on the front surface of the glass, and then a notch filter with a center wavelength of 480 nm was coated on the rear surface of the same glass. The proposed dual-wavelength (480 and 620 nm) notch filter is a combination of two single-wavelength notch filters coated on a double-sided glass substrate to compensate for residual stress. The transmittance, residual stress, and surface roughness of the proposed notch filter were evaluated using different measuring instruments. The experimental results show that the residual stress of the dual-wavelength notch filter could be reduced to 10.8 MPa by using a double-sided coating technique. The root-mean-square (RMS) surface roughness of the notch filters was measured by using a Linnik microscopic interferometer. The RMS surface roughness was 1.80 for the 620 nm notch filter and 2.09 for the 480 nm notch filter. The image contrast obtained with the three different notch filters was measured using an optical microscope and a CMOS camera. The contrast value could be increased from 0.328 (without a filter) to 0.696 (dual-wavelength notch filter).