Design of a wide-angle objective lens system in capsule endoscopic use

Abstract

Endoscopic screening inside of the small intestine requires a compact wireless imaging system providing wide field of view, bright images with high resolution to gather much diagnostic information. The first element with large diameter is commonly showed in the wide-angle objective lens system. However, in the capsule endoscope (CE), the optical system should be minimized to save the space for residual CE components. Wide-angle objective lens for CE system is reported in this study. The first element of the proposed system has a compact size in diameter while providing the wide field of view (FOV) of 160 deg with f-number 2.8 and total length of 5 mm. Relative illumination in the entire fields is maintained over 0.6. In terms of the image quality, the modulation transfer function (MTF) exceeds 0.3 at 180 lp/mm calculated by the 1920 X 1080 1/6” CMOS sensor’s pixel size of 1.4 um. Final design has 5 plastic lenses for mass production and aspheric surface fabrication. All surfaces are set as even asphere to correct aberrations, and the order of aspheric coefficients are restricted to prevent complex surface. The optical performance of the system is discussed in detail. The lens design for narrow-band imaging(NBI) is also suggested to enhance the contrast of the intestinal surface structure. By modify the system for white light imaging, the lens for NBI is optimized based on the wavelengths of 410 nm and 540 nm.