Next-generation Fundus Camera with Full Color Image Acquisition in 0-lx Visible Light by 1.12-micron Square Pixel, 4K, 30-fps BSI CMOS Image Sensor with Advanced NIR Multi-spectral Imaging System

This paper presents a near-infrared (NIR) multi-spectral imaging system, which can be applied to a CMOS image sensor with fine pixels. Using the multi-spectral technology, NIR1: near 800 nm, NIR2: 870 nm, and NIR3: 940 nm in the NIR wavelength were acquired for a target image. Using this image sensor and imaging system and with the application of interpolation and color correction processing, a color image is reproduced by only multi-NIR signal without visible light (0 lx). We also developed a next-generation fundus camera, which employed this multi-spectral imaging system with a multi-NIR LED illuminator. This multi-NIR LED illumination system, which was also developed, is designed to emit light with high efficiency despite its size of 2.3 mm square in size. We applied this NIR multi-spectral camera module with the multi-NIR LED illuminator to the next-generation fundus camera; the retinal pigment appears progressively more transparent, revealing the underlying choroid.

Introduction

Recently, the CMOS image sensor (CIS) market crossed $11 billion per year. Its Compound Annual Growth Rate has been showing a double-digit growth each year because the digital camera has been widely used globally given that it is attached to almost all smartphones [1]. It is important to consider how best to utilize the small camera installed in the smartphone, which is not only used for taking pictures but also serves more important purposes. Si can perform photoelectric conversion from the ultraviolet to the near-infrared region. Digital still cameras usually have NIR cut filters, which allow the images to be viewed in visible light without acquiring an NIR wavelength [2]. However, a conventional digital camera with a common CIS can acquire a color image with only visible light (wavelength: 400 nm–700 nm). The near-infrared wavelength region is hardly utilized for photoelectric conversion. In these digital cameras, the near-infrared wavelength region is ignored [3]. Recently, some surveillance cameras could capture a black and white image with near-infrared (NIR) light, which uses only mono NIR wavelength [4]. Further, mono NIR wavelength is used in a 3D mapping sensor or in face recognition by KINECT [5]. Lately, in an industrial and a medical camera, multi-band spectral cameras are started to use. Such cameras use all band areas of the wavelength from ultraviolet to near-infrared [6]. However, the pixel size of the CIS should be designed to exceed approximately 5–10 μm square pixel size because of an inherent issue in the realization of spectral imaging technology [7].