Optical parameters such as the attenuation coefficient are important environmental indicators of water bodies, including ocean, lakes, and rivers. The accurate measurement of the optical parameters of water bodies is crucial for ocean color remote sensing, ocean carbon cycle research, and ocean primary productivity assessment. Recently, the lidar technology has been gradually employed in the measurements of the marine chlorophyll concentration, scattering layer, optical attenuation coefficient, etc. However, the currently available lidar technique is mainly based on the time-of-flight principle, putting high demands on the laser energy, laser pulse width, and signal sampling rate to achieve large-depth and high-spatial-resolution measurements. In this work, we have designed and developed a Scheimpflug lidar (SLidar) system based on the Scheimpflug imaging principle (Fig. 1) for attenuation coefficient measurements in water bodies. The SLidar technique features a high range resolution, low cost, low maintenance, and wide wavelength selectivity, which can provide a novel approach for the quantitative detection of the water-body attenuation coefficient.
The water-body SLidar system (Fig. 2) mainly includes three parts: transmitting, receiving, and control units. The transmitting unit comprises a 450 nm multimode laser diode and a collimating lens. The 15-mm collimated laser beam is transmitted into a water tank. The backscattered light is collected using a receiving lens and then detected using a 45° tilted CMOS camera with an interference filter for ambient light rejection. The distance between the transmitting and receiving units is ~100 mm to satisfy the Scheimpflug principle. During measurements, the laser diode is on-off modulated and synchronized with the exposure of the CMOS camera, which alternately acquires the laser beam image and the background image. Thus, the dynamic subtraction of the background signal is achieved. In addition, the pixel-distance relation (Fig. 3) in water-body measurements is evaluated by considering the refraction effect at air-glass-water interfaces. The SLidar system is utilized to measure the attenuation coefficient of water bodies in a laboratory with different mass concentrations of fat emulsion (Intralipid). The slope method and Klett method are used to quantitatively retrieve the attenuation coefficient of the water body.
A water-body experiment is conducted under laboratory conditions. The transmitted laser beam in tap water is imaged using a CMOS camera. After pixel binning, background subtraction, and pixel-distance transformation, the range-resolved lidar profile is obtained (Fig. 4). Experiments with different mass concentrations of the fat emulsion are performed to examine the performance of the SLidar system. The fat emulsion solution with a mass concentration of 0.2 g/mL is diluted to 4 g/L, which is then added to the tap water to simulate different optical properties of the water body. During the measurement, the lidar signal is continuously recorded (Fig. 5), while the diluted fat emulsion (4 mL in total) is subsequently added to the water tank four times. The mass concentrations of the fat emulsion in the mixed water are then calculated, i.e., 0.17, 0.35, 0.52, and 0.69 mg/L. As shown in Fig. 6, the water body becomes inhomogeneous after adding the fat emulsion. Moreover, as the mass concentration of fat emulsion increases, the intensity of the lidar signal at a close distance first increases but then decreases rapidly. The attenuation coefficient profile is obtained using the Klett method (Fig. 7). The attenuation coefficient fluctuates significantly after the addition of the fat emulsion. As the fat emulsion and water body are fully mixed, the water body is almost homogeneous and the attenuation coefficients obtained using the Klett method at different distances are nearly the same. Furthermore, the attenuation coefficient of the water body generally increases with increasing mass concentration of fat emulsion (Fig. 8). The mean value of the attenuation coefficient under homogeneous conditions is evaluated. With an increase in the mass concentration of fat emulsion, the attenuation coefficients inverted using the Klett method and slope method increase correspondingly (Fig. 9). In addition, the correlation coefficient between the attenuation coefficient and mass concentration of fat emulsion reaches up to 0.997, successfully proving the reliability of the measurement result.
In this work, we design and develop a SLidar technique based on the Scheimpflug imaging principle for attenuation coefficient measurements in water bodies by utilizing a 450 nm laser diode as the light source and a CMOS image sensor as the detector. In addition, the pixel-distance relation in water-body measurements is calibrated by considering the refraction effect at air-glass-water interfaces. The SLidar system is used for water-body investigations in a laboratory with different mass concentrations of fat emulsion, and the slope method and Klett method are employed to retrieve the attenuation coefficient of the water body. Experimental results show that the SLidar technique can capture the dynamic changes in the water body as well as retrieve the water attenuation coefficients, which are consistent with the concentrations of the added fat emulsion. These promising results successfully demonstrate the great feasibility of using the SLidar technique for quantitative water body measurements, paving a way for open-water measurements.
