## 2 General Sensor Specifications  
This study focuses on the design of an inexpensive sensor for photosynthetically active radiation with regard to a later integration in a solar-powered wireless sensor node. The sensor system should provide accurate information about the environmental conditions that are relevant for plant growth. The use case is long-term studies with a node density of more than three nodes per 100 m\({}^{2}\). Consequently, costs and easy deployment, as well as power consumption and weather resistance, are important aspects of the design. Besides the light intensity in the form of PAR, information about the incident light spectrum should be provided. With regard to the photosynthetic activity of plants, temperature and humidity are commonly recorded by wireless sensor nodes and are also integrated in the proposed system. However, they are not further discussed in this publication. The size and weight of the sensor-node plays an important role in the deployment at different canopy levels, e.g., by attaching it to smaller branches, as required. We have found the size of a matchbox (\(50\,\mathrm{mm}\times 35\,\mathrm{mm}\times 15\,\mathrm{mm}\)) for the full system and a weight below \(50\,\mathrm{g}\) to be convenient. To the knowledge of the authors, there are no commercially available PAR sensors that could be integrated in such a small sensor device. Size-wise, the PAR sensor MQS-B from the company _Walz_ with its cylindrical shape, a \(14\,\mathrm{mm}\) diameter, a height of only \(16\,\mathrm{mm}\), and a weight of \(32\,\mathrm{g}\) would fit into such a system, but it lacks a weatherproof housing and the ability to be integrated in a sensor node. Furthermore, it does not offer information about the light spectrum, and with a price exceeding 100 euro, costs rapidly increase when numerous nodes should be deployed. PAR sensors that potentially fit into the desired sensor node have been built in a do-it-yourself approach by [PERSON] et al. (2015), [PERSON] et al. (2016), and [PERSON] et al. (2017) who provide detailed instructions. Their approach uses a photodiode, together with a diffuser and optical filters, to match the sensor response to the desired PAR spectrum. This solution lacks the potential to gather data about the light quality and requires a careful choice of the optical filters. Both quantities can be obtained with a spectrometer, as done by [PERSON] et al. (2014). However, this approach can not be miniaturized to fit into the desired dimensions and would also exceed a reasonable budget per sensor node. Our approach is to use a commercially available sensor that provides at least three channels distinct in their spectral sensitivity while, as a whole, covering the full \(400\,\mathrm{nm}\) to \(700\,\mathrm{nm}\) PAR spectrum. With additional information about the spectral distribution in the red, green, and blue region, the light quality can be further evaluated. The output of sensors with a higher spectral resolution than the three regions could be down-converted to the desired three intervals. Based on this concept, a number of standard RGB or CCD sensors and spectral color sensors can be considered as alternatives for sensing PAR and light quality. As these sensors are commonly used in mobile devices for imaging, spectral identification, and color matching applications, their price and size are reasonably low and, due to the integrated optical filters, costs and effort for external periphery are cut down significantly. In addition, the sensor candidates should fulfill the requirements for an integration into the planned sensor node: First, the sensor needs a high dynamic range, permitting a measurement of very low and high light intensities. It has been reported that the PAR, expressed in photosynthetic photon flux density (PPFD), can get as low as \(3\,\mathrm{\SIUnitSymbolMicro m}\,\mathrm{m}^{-2}\,\mathrm{s}^{-1}\) below the canopy (K  
mobile devices for spectral identification and color matching applications.