## 3 Obtaining PAR from a Color-Matching Spectrometer  
There are several possibilities to quantify PAR. The most common one is the photosynthetic photon flux (PPF), also described as the PPF density, called PPFD. It considers all incident photons per time and area, independent of their wavelength, in the range from 400 nm to 700 nm [21]. An alternative definition is the yield photon flux (YPF) which takes the plant growth behavior more closely into account. Here, each photon in the range of 360 nm to 760 nm is weighted according to its wavelength and the relative quantum efficiency curve of plants. Various measurement principles exist to obtain one or both of PPFD and YPF. A first option is the usage of a spectrometer, such as the popular LI-1800. These usually measure the spectral power distribution (SPD) over the full PAR range with a spectral resolution often better than 1 nm. The PPFD and YPF are then calculated by integrating the acquired SPD throughout the PAR spectrum and are, in case of the YPF, multiplied with a defined weighting. This principle allows an accurate PAR result even for narrow-band light sources. The second option is a quantum sensor, using a photo-diode and an optical band-pass filter for the PAR wavelength region. The photocurrent is given by the integral over the product of diode responsivity and incoming optical power at a given wavelength. Due to the non-ideal photodiode responsivity with respect to the desired sensitive area in the PAR region, these sensors are less accurate for narrow-band light sources compared to the approach using a spectrometer [21]. As the photocurrent  
Figure 3: Sensor node prototype with embedded AS7341 and circuitry, in comparison to a standard matchbox as a dimension reference. On top are six solar cells, the glass dome over sensor and the antenna.  
does not allow to deduce the sensed spectrum, only the PPFD can be monitored. With the lack of any spectral information and the non-ideal diode response curve, careful calibration is needed for the desired operational scenario, i.e., the correlation of the PAR to the GPP.  
For our proposed sensor, we are using an integrated circuit (IC) with a \(4\times 4\) photodiode array and two separated ones with dedicated functions builtin. The photodiodes in the array are paired, and their output is internally combined, integrated, and digitized. To determine the PPFD, it is enough to sum all integrated photocurrents generated by the channels that are sensitive in the PAR range. The overall spectral response can be optimized by weighting each diode output. Along with the technical effort, this method provides a reasonably accurate spectral resolution, of about 30 nm to 50 nm, which enables the user to detect scenarios with unfavorable light conditions that negatively influence the correlation between PPFD and GPP. It is conceivable that even a coarse YPF could be determined evaluating the channel signals individually. At least, the detection of an unevenly distributed light spectrum provides auxiliary information.  
The rough spectral information that is provided by the sensor can be used as a simplified measure for light quality. Since the sensor is not able to provide a nanometer precise spectral resolution, we consider the obtainable data set as reduced light quality index (RLQI). The RLQI can be a measure of light condition while obtaining the PPFD in parallel. It divides the PAR spectrum into three equally sized intervals for red (700 nm to 600 nm), green (600 nm to 500 nm), and blue (500 nm to 400 nm). The RLQI is expressed as the fraction of all diode outputs in the corresponding interval over the total sum. It is convenient to express the spectral distribution over the three regions as a percentage of the overall intensity, such as 20%-35%-45%, in the order of red-green-blue (r-g-b).