# Machine Learning Powered Microalgae Classification by Use of Polarized Light Scattering Data
## 2 Materials and Methods
### Experimental Setup  
The experimental setup is shown in Figure 1, and it was designed to measure the polarized light scattering of individual microalgae. The light source (S) emitted linearly polarized light with a wavelength of 532 nm. Then, the linearly polarized light could be modulated into different states of polarization (SOP) with a polarized state generator (PSG), which consisted of the half-wave plate (HW1) and the quarter-wave plate (QW1). With lens (L1), the modulated light was focused into the sample pool (P) full of filtered seawater. In the sample pool, the microalgae were suspended with a stirrer rotating at the speed of 200 rounds per minute.  
Scattering happens once microalgae pass the scattering volume which is the intersection volume of the optical systems of the illuminating path and receiving path [18]. Then, the scattered light is received at the backward 120 deg, and this angle has been proven to be sensitive to the microstructure of microalgae [8]. Subsequently, the scattered light passes through the lenses (L2 and L3), and the pinhole (PH) of 100 um, so as to collimate the light and confine the size of the scattering volume to 100 um. Then, the measurement of individual microalgae can be realized if the concentration is lower than 10\({}^{5}\) cells per mL. The collimated light after L3 can be analyzed by the polarization state analyzer (PSA), to finally get the Stokes vector \(S\)[23], as Equation (1),  
\[S=\left[\begin{array}{c}I\\ Q\\ U\\ V\end{array}\right] \tag{1}\]  
\begin{table}
\begin{tabular}{c c c c c} \hline
**No.** & **Name** & **Phylum** & **No.** & **Name** & **Phylum** \\ \hline
1 & _Skeletonema costatum_ & Bacillariophyta & 19 & _Cyclotella meneghtiniana_ & Bacillariophyta \\
2 & _Platformomas subordiformis_ & Chlorophyta & 20 & _Tetransellnis_ & Chlorophyta \\
3 & _Thalassioira pseudonana_ & Bacillariophyta & 21 & _Isochrysis zlanyiangensis_ & Chrysophyta \\
4 & _Chattonella marina_ & Chrysophyta & 22 & _Chatecoros muncleri_ & Bacillariophyta \\
5 & _Ampara_ & Bacillariophyta & 23 & _Phaecocystis globosa [PERSON]_ & Chrysophyta \\
6 & _Thalassioira rotula Meunier_ & Bacillariophyta & 24 & _Isochrysis CCMV3180_ & Chrysophyta \\
7 & _Isochrysis galluna_ & Chrysophyta & 25 & _Karenia mikmotoi_ & Pyrrophyta \\
8 & _Thalassioira weissflogii_ & Bacillariophyta & 26 & _Porphyridacae_ & Rhodophyta \\
9 & _Cyclotella meneghtiniana_ & Bacillariophyta & 27 & _Chatecoros gracilis_ & Bacillariophyta \\
10 & _Isochrysis_ & Chrysophyta & 28 & _Leptocylindrus_ & Bacillariophyta \\
11 & _Nizzola clostarium_ & Bacillariophyta & 29 & _Zoxamthella_ & Pyrrophyta \\
12 & _Pavlova viridis_ & Chrysophyta & 30 & _Heterosigma akashiuro (Hada)_ & Xanthophyta \\
13 & _Amphidinium carterae [PERSON]_ & Pyrrophyta & 31 & _Scrippsiella trochoidena_ & Pyrrophyta \\
14 & _Nannochloropsis_ & Chlorophyta & 32 & _Cyclotellaryptica_ & Bacillariophyta \\
15 & _Chaetoceros curvisetus_ & Bacillariophyta & 33 & _Cryptomonas_ & Cryptophyta \\
16 & _Dunaliella hardaul_ & Chlorophyta & 34 & _Talytomas helgandica_ & Chlorophyta \\
17 & _Trichdesmtium_ & Cyanophyta & 35 & _Cococolithorphids_ & Chrysophyta \\
18 & _Chaetoceros deblils Cleve_ & Bacillariophyta & & \\ \hline \end{tabular}
\end{table}
Table 1: Detailed information of samples used in this work.  
Figure 1: The schematic diagram of the experiment setup.  
where \(I\) is the light intensity, and \(Q,\ U,\ V\) are the residual intensity of \(0^{\circ}\) linear polarization, \(45^{\circ}\) linear polarization, and right-handed circular polarization, respectively. The three normalized polarization parameters, \(q,\ u,\ v,\) are defined as Equation (2),  
\[q=\frac{Q}{I},\ u=\frac{U}{I},\ v=\frac{V}{I} \tag{2}\]