Moderate Absorption and Low Scattering: Near-infrared Illumination for Clear Imaging through Highly Turbid Water

Abstract

Optical imaging through highly turbid water ($>$50 NTU) presents significant challenges due to strong photon scattering effect that impedes effective detection of light signal. Using Monte Carlo simulation, it is found that moderate light absorption can improve image quality, while photon scattering is unequivocally detrimental. To achieve clear imaging in highly turbid water, the wavelength of illumination light has been red-shifted from the traditionally utilized blue-green to near-infrared, which has moderate absorption and low scattering. Experimental results demonstrate that near-infrared illumination is helpful to the detection of target while visible illumination causes the loss of target information. This provides a simple optical imaging method for extracting signals from targets in highly turbid water, without the requirement of complex equipment. Furthermore, the negative effect of backscattering background under forward-illumination mode on imaging contrast is confirmed, and the back-illumination mode is proposed to eliminate its interference. Finally, the number and movement of fish are successfully observed in real time through the simulated aquaculture pond, using the near-infrared lateral-illumination, and the research has the potential to be applied in various turbid underwater imaging scenarios.