# An Optical Differential Method for Underwater Wireless Communication in Turbid Environments
## 3 Experiment  
In order to verify the theoretical predictions, experiments on underwater optical transmissions and optical communications are conducted in an optical dark room. Figure 2a shows the schematic diagram of the experimental setup. In order to compare the light intensity and color at the same level, a collimated light source is built. The light first passes through a tunable aperture and then enters a water tank. The tank is made of glass, which absorbs almost no light in the visible range. The light in the water is collected by a lens (lens 2) and transferred to an integrating sphere, which is connected to an optical fiber spectrometer (Type: SW2000, Company: EVERUPING Optics, Hangzhou, China). The lens and the integrating sphere are installed into an acrylic box that is waterproof and transparent. The acrylic box is mounted on a translational stage and can be moved along the direction of light propagation.  
The inset of Figure 2a shows the detail of the home-built collimated light source. Two chips of white-light LEDs with different colors (6000 K-CCT LED and 3000 K-CCT LED, XPG3535, Company: Cree, Durham, NC, USA) are soldered onto an Al substrate-printed circuit board. The emission spectra are shown in Figure 1a,b. The Al substrate is fixed on a heat radiator, which is made of Al fins so that the heat produced by the LEDs can be efficiently diffused into ambient light. In front of the LEDs, there is an acrylic diffuser to mix the light uniformly. The mixed light becomes a point-like light source after passing through a pinhole and then becomes a collimated light beam after passing through a movable lens (lens 1). Figure 2b shows images of beam spots in the air at different positions of the optical path. If \(l\) is defined as the distance between the input boundaries of the water tank and acrylic box, the red and green enclosed images show the beam spots when \(l\) equals 10 cm and 60 cm, respectively. As can be seen, the beam diameter expands only 18.18% after traveling a distance of 50 cm, indicating a negligible diverging angle (\(\sim\)0.229\({}^{\circ}\)).  
The wires of the LEDs are threaded past two holes of cooling fins and connected with an external driving printed circuit board with constant-current modulations. Two colors of LEDs are driven separately by pulsed modulation signals. A so-called digital color-coding method is used, which allows for precise manipulations of light luminosity and chromaticity within the color spaces defined by the Commission Internationale de l'eclairage(CIE) [24, 25]. To make a collimated light source, we use optical components, such as diaphragms, light mixing plates, etc., which will lose part of the light intensity (the collimated light source is only made to compare the communication level between light intensity and light chromaticity, and there is no need to use the collimated light source in actual communication). When both duty cycles are set to 100%, the luminous flux is collected by the integrating sphere when the positions of \(l=0\) are 0.003205 lm and 0.004288 lm for the 6000 K-CCT and 3000 K-CCT LEDs, respectively. The 3000 K and 6000 K LEDs were chosen because of their brighter emission and wider color gamut, and the digital color-coding method is suitable for LEDs of other colors (e.g., red-green-blue LEDs). Figure 2c demonstrates the precise manipulations of chromaticity of the compound light source using the biprimary color mixing technique of the digital color-coding method [24, 25]. The luminous flux of compound light source is kept constant at 0.0032 lm. The comparison between the experimental results (red asterisks) and the theoretical predictions (black circles) confirms the precision of the digital color-coding method.