Objective  Single-pixel imaging (SPI) technology has excellent application prospects in biomedical imaging because of its weak light detection ability and wide working band. In recent years, some research works on full-color SPI imaging methods have been reported one after another. The current technical route of full-color SPI is roughly divided into two directions: one is to use RGB trichromatic light sources for illumination, to capture the light intensity after modulation of different color light through a single-pixel detector respectively, and finally to synthesize full-color images through reconstructed RGB three-channel images. The other is to use a white light source for illumination and capture the color and spatial information of the target scene through a beam-splitting prism and multiple single-pixel detectors. With the rapid development of single-pixel imaging technology, some researchers have applied full-color single-pixel imaging technology to biomedical imaging systems. However, most of them only apply full-color single-pixel imaging to wide-field imaging systems without taking full advantage of the low light detection ability and a broad spectrum of single-pixel imaging technology. Moreover, most of the optical imaging systems built in the laboratory are only validation prototypes, and further modularization and miniaturization of them have yet to be developed. Therefore, it is necessary to combine full-color single-pixel imaging technology and endoscopic imaging technology to design and build a modular and miniaturized full-color single-pixel endoscopic imaging system. To this end, two full-color SPI endoscopic imaging systems were designed and built by combining the two main technical routes of full-color SPI with endoscopic imaging technology. The advantages and disadvantages of the two full-color SPI endoscopic imaging schemes were systematically analyzed. Moreover, the modular and miniaturized design and modification according to the proposed imaging scheme make it possible to match various laparoscopes for endoscopic imaging.

  Methods  Two modular full-color single-pixel imaging systems were designed and built based on Hadamard single-pixel imaging technology. The system's optical paths and modular schematics are shown in Fig.2 and Fig.4. The experimental setup used the group's self-developed four-band LED light source box as the light source, a digital micromirror device (DMD) as the spatial light modulator, and silicon-based photodetectors as the single-pixel detectors. The two full-color single-pixel imaging systems were used to perform endoscopic imaging experiments on the color bars. The reconstructed images of the color bars of the two imaging schemes were compared and analyzed using PSNR and SSIM as evaluation criteria (Tab.1). The imaging times of the color bar endoscopic imaging experiments of the two imaging schemes were also analyzed for comparison (Tab.2). Further endoscopic imaging experiments were performed on human intestinal models using a modular full-color single-pixel system.

  Results and Discussions   The PSNR of the RGB trichromatic light scheme was 18.5717 dB, SSIM was 0.6431, and total imaging time was 6.5506 s. The PSNR of the white light scheme was 18.4988 dB, SSIM was 0.6860, and total imaging time was 3.8726 s. The difference between the PSNR of the white light scheme and the RGB trichromatic scheme is only 0.0729 dB. However, the SSIM of the white light scheme is 0.0429 higher than that of the RGB trichromatic scheme. The measurement time of the white light scheme is shorter than that of the RGB trichromatic scheme by one-third, and the reconstruction time is shorter by 0.012 s. Therefore, the total imaging time of the white light scheme is 2.678 s faster than that of the RGB trichromatic scheme. From the experimental results, the white light scheme is more suitable for the endoscopic imaging system. The human intestinal model was imaged using the white light scheme, and the results are shown in Fig.7. Complex targets can also be imaged, which can meet the needs of endoscopic imaging.

  Conclusions  Two full-color single-pixel endoscopic imaging systems were designed and built. A quantitative comparison of the two proposed imaging schemes was made to analyze their advantages and disadvantages. By calculating the imaging quality and time, the white imaging scheme is more suitable for the full-color endoscopic imaging system than the RGB tri-color scheme. Meanwhile, the two imaging systems are miniaturized and modularized to be adapted to various types of laparoscopes for endoscopic imaging.