Investigation on image signal receiving performance of photodiodes and solar panel detectors in an underground facility visible light communication system

For the safety, underground facilities are required to be inspected regularly, especially with image analysis. Traditional wireless and wired transmission techniques have a weakness of limited transmission range in narrow underground environments. In this study, a new image transmission method based on visible light communication (VLC) has been thus proposed. Two types of detectors as an image signal receiver have been tested and discussed in the following experiments. The photodiodes (PDs) are widely used as a common image signal detector in VLC technology, but image signal detection using solar panels (SPs) has not been studied. PDs have a higher sensitivity and faster response time but a limited detection area and high cost. Besides, PDs require the lens to focus light. On the other hand, SPs have much larger optical signal receiving areas and stronger optical signal capture capabilities. They can realize lens-free detection and are inexpensive. These features of PD were firstly verified in experiments with several receiving areas and angles of detectors. The experimental result revealed that PD had better image detection and recovery capabilities than those of SP. Then, we found that a larger receiving area obtained by using double PDs/SPs improved the brightness of the restored image. In a supplementary experiment, the influence of different RGB optical components on VLC, especially the VLC-based image transmission, has been investigated by using two-dimensional Fourier transform frequency analysis. We found that the red optical component significantly increased the intensity and energy of the restored image as the image low-frequency signals were larger than the restored image using ordinary mixed white light, and moreover, the blue optical component decreased the low-frequency part of the image.