# Optical Camera Communication as an Enabling Technology for Microalgae Cultivation +
## 5 Results
### Indoor Experiment  
Figure 8 shows the intensity radiation pattern emitted from the two selected regions at the container's surface. The columns represent the area's location, from left to right: the centre and the border. The rows represent each independent image's channel, from top to bottom: red, green and blue. Each graph shows the radiation pattern normalised to the maximum radiant intensity for azimuth angles between five and 175 degrees. The radiation patterns for the different concentrations are grouped following a colour gradient from lighter to darker as the concentration increases. Finally, for reference purposes, the Lambertian radiation patterns with n = 1 and n = 2 are shown. In the container's central region, the radiation pattern follows a Lambertian with n = 1 for red and green channels when no microorganisms are suspended in the water. In the blue channel, it deviates minimally for wider angles. It can be seen that as the biomass increases, the emission becomes more directive. Actually, in the case of the red and green channel, at the point of maximum concentration, it follows a perfect Lambertian pattern with n = 2. The reason behind this has a connection with _A. platensis_'s SPF and its absorption and scattering cross-section coefficients. _A. platensis_ is a planktonic filamentous cyanobacterium. Its cylindrical morphology gives the cell a highly directive SPF [33] (depending on the orientation). The absorption cross-section measured experimentally is moderately high compared to other microalgae organisms. Therefore, the absorption coefficient, measured experimentally in [25], contributes more to the light extinction than the scattering. This absorption causes the outgoing light at steep angles, which had travelled longer distances than the direct rays, to undergo a significant attenuation, which ultimately produces this directivity on the radiation pattern. On the other hand, in the blue channel, the radiation intensity decreases faster with the increase in biomass than the red and the green channel. This is to be expected given that the absorption cross-coefficient curve as a function of the wavelength is not flat, but intensifies by almost 1.7 times in the 350-450 nm region of the spectrum [25]. Regarding the emission from the edges, the skewness effect mentioned in Section 3.1 is observed. This emission also becomes more and more directive as the biomass increases. To better illustrate how the camera perceives the differences in the radiation between both regions (centre and border) at a certain angle and to understand the reason behind selecting these two regions for the analysis, Figure 9 shows some examples of the images captured during this experiment. In this figure, framed in a blue rectangle, the interface that abruptly separates the centre and the border region is highlighted (Figure 9a). Furthermore, this region's evolution with respect to the increase in concentration is shown in the boxes below. Figure 8: Red, green and blue radiation diagram of small areas located at the centre and the border of the container. Figure 9: Illustrative example of the results obtained in the indoor experiment. The location of this interface on the surface depends on the container's geometric construction and moves with the point of view. In thicker containers, this interface gets closer to the centre, and consequently, the border region increases significantly in size. On the other hand, as the container is viewed from a tighter angle, this interface moves towards the edges (Figure 9c). In this case, the central region extends smoothly towards the borders without reaching them. This interface's location should be taken into account in the design of the communications link because it delimits two areas with notable differences in light emission that will ultimately impact the SNR. In this setup, the power received from the edges is generally less than from the centre. Therefore, if the sampling occurs at the edges, the signal would be affected by a lower SNR.