NBI Optical Filters in Minimally Invasive Medical Devices

The integration of the narrow band imaging (NBI) technique in highly miniaturized minimally invasive medical devices is presented. NBI provides a more reliable sensing performance for
in vivo
studies on tissues as compared to approaches based on white light illumination. NBI uses a selective filtered light with peak transmission at 415 (blue) and 540 nm (green). The blue light improves the visualization of the superficial mucosal layer, while the deeper penetration of the green light highlights the vascular patterns of the subepithelial vessels. The optical filters are based on a multilayer thin-film stack, using the Fabry–Perot configuration with titanium dioxide (TiO
₂
) and silicon dioxide (SiO
₂
). The blue light-emitting diode (LED) combined with the blue filter results in a maximum central wavelength at 414 nm, full-width half-maximum (FWHM) of 19 nm and maximum relative transmittance of 21%. The green LED combined with the green filter yields maximum peak intensity at 536 nm, FWHM of 30 nm, and maximum relative transmittance of 35%. RF-sputtering was used for the deposition of NBI optical filters. The refractive index and extinction coefficient of the TiO
₂
and SiO
₂
thin films were characterized and the green and blue filter designs were experimentally validated.