## 6 Discussion and Conclusions  
Our concept suggests to use a cheap and inexpensive multi-channel spectrometer chip (AS7341) to measure photosynthetically active radiation in the form of PPFD. The additional RLQI information about the spectral distribution in the red, green, and blue light regime facilitates a better statement about the validity of a correlation between the measured PAR and the expected GPP. Our experiments prove that the PAR can be derived using the output from eight channels with a mean error of about \(14\,\mathrm{\SIUnitSymbolMicro m}^{-2}\,\mathrm{s}^{-1}\). We have used a multiple linear regression to determine a transfer function of the spectrometer output signals to the PPFD showing a high adj. \(R^{2}\) of 0.991. This proves that there is a linear correlation between the channel output and the desired PPFD value. Hence, a multi-channel spectrometer can be used for this purpose. The coefficients were obtained through a set of measurements at different lighting conditions. In this data set, low light conditions are over-represented which may negatively influence validity of the model at high PPFD. To increase the accuracy of the sensor, more measurements under real conditions have to be performed. In addition to the calculated PPFD, the RLQI provides information about the light distribution in the three spectral regions. The digital outputs of the channels provide a good representation for several light scenarios with known spectra. We see a maximum absolute deviation between the expected and measured RLQI of 4%. Measurements taken below some dense leaves show, as expected, an RLQI with a peak in the green region. Records at clear sky scenarios also agree well with those measured by [PERSON] et al. Despite this result, care has to be taken in interpreting the RLQI. An exact binning of the spectrum as done by the RLQI can not be accurately achieved with broad-band sensors by design. While the combination of the AS7341 channels fit well to this case, there is, for example, still a slight overlap of the channel sensitivity in the blue and red region, but their output is fully counted to the green region. This results in a slight overestimation of the green incidence. A full compensation of that error is to our knowledge not possible, although reduced when a thorough calibration is done for a known and limited set of light conditions. The IC ams AS7341 shows that a cheap, commercially available spectrometer can be used as PAR and light quality sensor. It is able to cover the desired PAR wavelength region between \(400\,\mathrm{nm}\) and \(700\,\mathrm{nm}\) and required intensities from \(0\,\mathrm{\SIUnitSymbolMicro m}^{-2}\,\mathrm{s}^{-1}\) to \(1600\,\mathrm{\SIUnitSymbolMicro m}^{-2}\,\mathrm{s}^{-1}\). The introduced RLQI accounts for the fact that theses sensors do only provide a limited spectral resolution, which, however, is sufficient for a lot of applications. In fact, its ability to supply additional spectral information beyond the PAR region, as done with the near infrared channel, can provide valuable information with regard to the plant growth. It is worth noting that the additional channel in the infrared region improves the accuracy of PAR determination via the MLR according to our experiment. As the channel's sensitivity lies outside the PAR region, there should not be any contribution. It has to be  
Figure 7: Measurement of direct sunlight on a clear summer day (**a**) and under a leaf (**b**). The measured PARs with both sensors are noted in the graph. The spectral channels are normalized to the highest count number of all channels. The RLQI values do not sum up to \(100\,\mathrm{\char 37}\) because of rounding errors. clarified whether this is due to imperfections of the used reference sensor LI-190 or the AS7341 itself, being still sensitive in the not desired NIR region. A further improvement in accuracy can be done by examining the different sensitivity and integration settings, the AS7341 provides.