## 5 Conclusions  
A design was presented in this paper for a low-cost narrow-waveband sensor for the collection of PRI data. The design and low cost of the sensor presented in this paper facilitate the unattended deployment into larger networks to cover broad scales in a manner that fits contemporary data needs and budget constraints. Such networks hold potential value for the calibration and validation of remote sensing data. For example, fine-spatial, _in situ_ measurements of PRI may be an important source of information for the assessment of algorithms used to subdue atmospheric and surface BRDF effects present in airborne and tower-based data or to gain a better understanding of fine structure-function relationships.  
Calibration of the instrument using a calibrated light source resulted in a high correlation coefficient and the instrument response was very linear. Under moderately low-light conditions, the instrument performed well and showed reductions in turf reflectance in both wavebands, when the turf was illuminated with a combined LED/Halogen light of approximately 350 \(\upmu\)mol photons m\({}^{-2}\cdot\)s\({}^{-1}\), representing conditions such as may be encountered outdoors on a cloudy day; However, no significant change in PRI was noticed. Using PRI observations of the upper strata of a Douglas-fir canopy for times-of-day between 8 a.m. and 4 p.m., about 23% of variation in EC-derived daily stand-level LUE estimates were explained. The finding is significant given the difference in scale between the EC and PRI instruments and limited temporal coverage over the season. However, using the data available, diurnal trends in the PRI signal could not be well explained, and some are likely due to factors not related to xanthophyll-induced down-regulation of photosynthesis, including BRDF effects and changes in incident light quality. Further research is needed to investigate the sensitivity of the bandwidth and center-wavelength of the interference filters to varying incidence angle. The latter may require use of low-cost optics to improve the sensor cosine response and collimate incoming radiation before passing through the interference filters. Variations in bandpass center wavelength across sensors of the same make and model have also been noted by [PERSON] _et al._[15]; however, investigations as to how this issue may be resolved for low-cost acquisition of PRI data have not been found so far.  
Figure 11: Change of the photochemical reflectance index (PRI) over time. Two largest discrepancies between observations and model fit represent days with lengthy periods of low-light conditions in mornings that caused visible noise in PRI data. \(R^{2}=0.8\) would have been achieved by selecting a shorter time span, e.g., 10 a.m. to 2 p.m.  
A word of caution is appropriate for interpretation of the PRI index. Unlike other normalized difference vegetation indices, such as the Normalized Difference Vegetation Index (NDVI) that is used to assess vegetation structure or vigor from canopy reflectance data, both channels of the PRI index are sensitive to changes in photosynthetic down-regulation. In fact, since xanthophyll-induced down-regulation of photosynthesis relates to changes in PAR, observations in a single waveband can also explain significant portions of the total amount of variation in photosynthetic down-regulation. Moreover, an incorrect calibration can correlate with EC-derived LUE, for reasons other than actual reflectance changes caused by (de-)epoxidation of xanthophyll and associated conformational changes. Ideally, calibrations should be conducted frequently using calibration standards and results should be quality checked to ensure PRI remains constant with changing illumination intensity at a constant color temperature. Other concerns on the use of PRI are the potential influence of phenology [13; 28] and physiological mechanisms, besides the xanthophyll cycle, that result in a photosynthetic down-regulation, such as chloroplast avoidance movement [29] and chlorophyll content [30].