## 6 Methods of Microalgae Cultivation Monitoring
### Light Monitoring  
The type of illumination used, the light intensity reaching the microalgae culture, the wavelength, and the photoperiod are decisive factors that largely determine the technological efficiency of the cultivation process for photosynthetic species [154]. The optimal light intensity for most microalgae is between 50 and 500 \(\upmu\)mol photons/m\({}^{2}\)-s, which corresponds to photosynthetically active radiation (PAR) in the range of 400 to 700 nm. Direct sunlight can provide up to 2000-2500 \(\upmu\)mol photons/m\({}^{2}\)-s on a clear day, which corresponds to 100-200 klux in equatorial regions [154]. On cloudy days, this value can drop to 20-50 klux, which is still optimal for efficient production of microalgae biomass. It should be noted that the conversion of illuminance expressed in lux (lx) to \(\upmu\)mol photons/m\({}^{2}\)-s within the PAR range requires consideration of the type of light source and a conversion factor that depends, among other things, on the source. Typical conversion factors (lx/\(\upmu\)mol photons/m\({}^{2}\)-s) are around 54-57 for sunlight, 74-80 for white fluorescent lamps, 60-75 for white LEDs, and 120-150 for sodium lamps. PAR accounts for about 40% (in W/m\({}^{2}\)) of the visible solar spectrum [154]. Among the sensors used to monitor microalgae culture under controlled conditions, PAR photometers play a key role [155]. The data obtained with this light detection method provides precise information about the wavelength range specifically used by microalgae and cyanobacteria for photosynthesis, which in turn affects biomass growth and the accumulation of target cellular metabolites [155]. Another way to describe the lighting system is to use the correlated colour temperature, expressed in Kelvin (K) [156]. This approach is considered simple, facilitates standardisation of the culture, saves energy, and can be used to control the chemical composition of the biomass. It is an effective tool for both research and industrial microalgae cultivation [156]. Common methods for measuring light intensity in microalgae biomass production systems are typical light sensors (photometers) [157]. Depending on the cultivation system, they can measure the light reaching the surface of an open pond or the transparent wall of a closed photobioreactor (PBR). In such cases, the photon flux density that penetrates the active volume of a PBR and reaches a selected point in the culture medium is modelled theoretically [158]. The models are often based on Lambert-Beer's law with experimentally determined extinction coefficients. The photon flux density is usually measured in the PAR region using planar or spherical quantum sensors [159]. It is important to consider  
Figure 3: PBR system with multiparameter monitoring. the relationship between the light intensity reaching the cell, the exposure time, and the complex interactions with other process parameters, including mixing, cell concentration, and incident light intensity [160]. However, a crucial variable affecting microalgae growth is the light intensity provided by the operator inside the culture, rather than the intensity falling on the surface of the culture substrate or the transparent wall of the PBR. The illumination intensity is attenuated before it reaches the microalgae cells due to absorption and self-shading effects caused by dense cultures inside the bioreactor [161]. Modern measurement systems allow the continuous monitoring of the light intensity reaching the cells of the growing microalgae population and the tracking of temporal changes in this parameter. Underwater light metres and photometers are used for this purpose. A typical sensor for underwater measurements is the Apogee SQ-520 (Apogee Instruments, Logan, UT) [155]. This digital PAR sensor is mainly used in PBR monitoring, where precise control of light intensity is crucial for optimising the photosynthesis of microalgae.