###### Abstract  
We propose a novel approach to measure photosynthetically active radiation (PAR) in the form of photosynthetic photon flux density with an inexpensive, small multi-channel spectrometer sensor, with integrated optical filters and analog-to-digital converter. Our experiments prove that the combination of eight spectral channels with different optical sensitivities, such as the sensorchip in use (AS7341, ams), derive the PAR with an accuracy of 14 umol m\({}^{-2}\)s\({}^{-1}\). Enabled by the sensor architecture, additional information about the light quality can be retrieved which is expressed in the reduced light quality index. A calibration method is proposed, and exemplary measurements are performed. Moreover, the integration in a solar-powered wireless sensor node is outlined, which enables long-term field experiments with high sensor densities and may be used to obtain important indexes, such as the gross primary production. photosynthetically active radiation; PAR; light quality; light sensor; ams AS7341; spectrometer; multi-channel spectrometer; photosynthetic photon flux density +
Footnote †: journal: Sensors  
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does not cover large areas. From our point of view, the drawbacks of the system with handheld devices could be circumvented with sensors that are small, cheap, self-sustaining and easy to deploy. Sensors with such properties can be installed at numerous places (in the range of hundreds) within the observed area. Such systems, when equipped with wireless communication, are often referred to as \"smart dust\" and are running on batteries or solar cells over long periods of time. It may cover areas significantly larger compared to the manual approach, although not as large as a satellite based systems. On top, it would enable the user to study light intensity at different canopy levels to derive the GPP when deployed in horizontal and vertical space. Beside the light intensity being an important factor in plant growth, the light quality plays an important role, as well, but is not reflected in PAR. The light quality considers the spectral composition of the incident light. It is usually described in a ratio of the integrated intensities of blue (400 nm to 500 nm), green (500 nm to 600 nm), and red (600 nm to 700 nm) intervals [5; 6]. Evidence of the influence of the spectral composition is commonly collected in laboratory experiments. For example, plants were exposed to various light quality scenarios but at the same intensity, showing a distinguishable growth behavior [7; 8; 9]. Plants may heavily differ in the thickness of their abaxial-face epidermis and leaf thickness when exposed to different light spectra [7]. [PERSON] et al. have studied the effect of light in the red or blue regimes on biologically inactive and active photoreceptors, such as phytochrome phototropin [10]. Other studies examine the plant growth under varying illumination conditions for different species [11; 12]. These studies were conducted in either greenhouse or laboratory facilities, but similar effects have already been observed in studies outside a controlled laboratory environment, such as the study by [PERSON] and [PERSON], in which apple orchards were covered by differently colored hailnets, and a change in plant and fruit growth was observed [13]. The light quality is usually measured with expensive spectrometers that can also be used to determine the PAR [6; 14]. However, the nanometer precise data set gathered by these devices is not always needed to obtain information about the light quality and is, therefore, often reduced to the three light regimes of blue, green, and red. In such cases, we see the opportunity for this to also be achieved with a inexpensive sensor chip in contrast to the usual approach of obtaining the light quality with a high resolution spectrometer. To measure PAR and determine the light quality, we have designed, built and tested an inexpensive sensor, using a commercially available spectrometer sensor.