## 3 Experiment Results and Discussions
### Chlorophyll a Concentration Sensing  
We also performed a chlorophyll \(a\) concentration sensing experiment with the same optical measurement setup shown in Figure 3. Before the measurement with an environmental water sample, we calibrated to clarify the correlation between chlorophyll \(a\) concentrations and output photovoltages. As calibration solutions of chlorophyll \(a\), we prepared eight different concentration calibration solutions. The prepared chlorophyll \(a\) concentrations were 0, 5, 10, 20, 50, 100, 200, and 500 \(\upmu\)g/L, which are shown in Figure 12. With the naked eye, the concentration differences cannot be distinguished. In the  
Figure 11: Turbidity measurement results based on St-DVB copolymer microbeads solution calibration for samples at the 8 places indicated in Figure 7.  
Figure 10: The measurement results for calibration solutions of St-DVB copolymer microbeads shown in Figure 9. PD1 is placed perpendicular to LED1, and PD3 is placed at the bottom of the sample. measurement, the measurement setup and procedure are the same as what we used for the turbidity measurement, except for the LED. For chlorophyll \(a\) measurements, we utilized a blue LED instead of a red LED for the excitation of fluorescence. The blue LED was set to the LED2 position in Figure 1b. The measurement results are shown in Figure 13. In the measurement, the photovoltages were detected by PD3 only because detection by PD3 yielded better results in turbidity measurement. Moreover, we set a red filter sheet on PD3 to consider only the fluorescence around 685 nm in the measurement. For convenience in the red filter setup, we chose PD3 for the detection. As a result, we were also able to obtain good linearity between chlorophyll \(a\) concentrations and photovoltages. In other words, with the same simple optical measurement setup, we can easily detect both the turbidity and the chlorophyll \(a\) concentration. Commonly used commercial chlorophyll \(a\) sensors measure water soluble uranine (fluorescein disodium salt, C\({}_{20}\)H\({}_{10}\)Na\({}_{2}\)O\({}_{5}\)) fluorescence intensity for calibration, and measurement results are then converted to chlorophyll \(a\) concentration. However, by using our measurement setup, we can directly calibrate with the chlorophyll \(a\) that is dissolved in ethanol and diluted with water. In chlorophyll \(a\) concentration sensing with an environmental water sample, zero calibration is necessary to consider fluorescence intensity only. We performed chlorophyll \(a\) concentration measurements with the same environmental water samples, which were used in previous turbidity measurements. The measured chlorophyll \(a\) concentrations are summarized in Figure 14. The chlorophyll \(a\) concentration was measured successfully with the same optical measurement setup applied to turbidity sensing. The chlorophyll \(a\) concentration is also one of the more important quantities in defining water quality. For instance, the turbidity of Sample No. 4 showed the highest value, but the chlorophyll \(a\) concentration of it was not high. This means that the environmental water sample at the Sample No. 4 position was only muddy and not polluted by phytoplankton. Figure 12: The 8 different concentration calibration solutions of chlorophyll _a_: From left to right, the concentrations are 500, 200, 100, 50, 20, 10, 5, and 0 ug/L. Figure 13: The measurement results for chlorophyll \(a\) solutions shown in Figure 12.0 PD3 placed at the bottom of the socket was used for detection. In the chlorophyll \(a\) concentration measurement, we used a red filter in front of PD3 to effectively detect fluorescence. PD3 is also used for turbidity detection, and we performed the turbidity measurement again with a red filter on PD3 to investigate influence of the red filter installation on turbidity measurements. In the measurements, we measured photovoltages and NTU turbidities for kaolin calibration solutions shown in Figure 4 by using our measurement setup and a commercial sensor, respectively.