## 2 Experiment Method
### Algae Spectral Information Extraction Method  
A total of 100 spectra (Figure 6b) were extracted from the captured hyperspectral images in the ROI (region of interest), sized 10 \(\times\) 10 (Figure 6a), for each sample. These 100 spectra extracted for each sample were averaged using post-processing and normalized into one spectral spectrum.  
The hyperspectral images taken for each sample were saved as radiance, or the intensity of light having a unit of W/m\({}^{2}\)/sr that has gone through the non-uniformity correction (NUC) process adopted by the hyperspectral sensor (microHSI) itself [16]. In this study, since hyperspectral images were acquired outdoors; radiation correction was performed through a white plate with 99% reflectance to obtain algae spectral information by utilizing reflectivity, which has radiance characteristics that are not affected by the intensity of the light source; and the reflectance was obtained through reflectance Equation (1), derived from the radiance of spectral information.  
\[\text{Reflectance}(\lambda)=\frac{\text{Radiance}(\lambda)}{\text{Reflectance}( \lambda)}\times 100=\frac{\text{Radiance}(\lambda)}{\text{Radiance}_{white~{}plate }(\lambda)} \tag{1}\]  
To extract the unique spectrum of algae that have undergone radiation correction, [PERSON]'s (2020) method for separating hazardous chemical properties was applied to algae. To exclude the effect of radiance due to conditions other than the characteristics of algae, the algae's unique spectrum was separated by removing the reflectance value of BG11, which is the base of the algae culture medium. This is shown in Equation (2).  
\[\text{Reflectance}_{algae}\left(\lambda\right)=\text{Reflectance}_{object} \left(\lambda\right)-\text{Reflectance}_{base}\left(\lambda\right) \tag{2}\]  
where Reflectance\({}_{algae}\left(\lambda\right)\) represents the unique spectrum of algae, Reflectance\({}_{object}\left(\lambda\right)\) represents the spectrum of algae culture medium, and Reflectance\({}_{base}\left(\lambda\right)\) represents the spectrum of BG11, and each result is shown in Figure 7.  
Figure 4: Micro HSI 410 Shark model (Corning, NY, USA).  
Figure 5: Hyperspectral image acquisition equipment ((**a**) HSI and gimbal setting—method for correcting the shake of the HSI; (**b**) hyperspectral system—device configuration for imaging algal samples; (**c**) white plate (99.9%)—white reference plate used for radiation correction; (**d**) sample setting—algal samples for collecting hyperspectral data.)  
Figure 6: Hyperspectral image extraction method ((**a**) region of interest—pixel range setting; (**b**) hyperspectral system—100 spectra for each pixel).  
The extracted spectra of green algae and blue-green algae were normalized through a normalization process, and the green and blue-green algae species were classified and grouped in order to extract the wavelength band with the maximum absorption wavelength peak value, and the absorption wavelength band of green and blue-green algae was analyzed. However, since water tends to absorb light, it was difficult to perform analysis using only the minimum wavelength band, so the trend of the slope change was analyzed by applying the differential to calculate the slope for each wavelength band. The equation for calculating the slope is as follows (Equation (3)):  
\[\mathrm{dy/dx}\ =\ \mathrm{d}_{normalized\ reflectance}/\mathrm{d}_{ wavelength} \tag{3}\]