# FluoC Spec 2--An Automated Field Spectroscopy System to Monitor Canopy Solar-Induced Fluorescence
## 3 Results  
In this section, we first show the results from radiometric calibrations done in the lab and field, tests on the light leak of FluoCSpec 2, and the relationships between dark signals and temperature and integration time. We then present an example of the SIF measurements in the field and a network of ground SIF observations (FluoCNet) that has been established. The results from the estimation of calibration factors under various integration times suggest that we can normalize calibration factors with the integration time. The calibration factors collected at different integration times (300 ms, 400 ms, and 500 ms) share the same shape (Figure 2A), and it is clear that the calibration factor is a function of integration time (Figure 2B). We show that the ratio between two calibration factors scales with the inverse of the integration time:  
\[\frac{Cal_{1}}{Cal_{2}}=\frac{IntTime_{2}}{IntTime_{1}}, \tag{7}\]  
For example, the ratio between calibration factors collected at 500 ms and 400 ms is \(\sim\)0.8 (400/500). This means that we can normalize the calibration factor using the integration time, and use the calibration factor collected at one integration time to estimate the calibration factors at other integration times. Since the integration time is optimized and may change in every measurement cycle (10 min), the result here demonstrated that a single calibration factor normalized by the integration time can be used in Equation (4) to calculate Radiance = Cal \(\times\) (Signal \(-\) Dark). It is important to test the light leak of a spectroscopy system as leaks represent noise from other sources. We found that FluoCSpec 2 is \"light-tight\" when the spectrometer is kept in the enclosure (Figure 2D). It is also important to use fiber optics with a metal or black jacket, which help to block ambient light. Potential components along the light path between the tip of the bare fiber and the detector at the spectrometer include the shutter and the spectrometer itself. When we closed the shutter and pointed the fiber to the port of an integrating sphere (Figure 2C), an incoming radiation two times the peak solar intensity caused a change in the raw DN of 3-4 across the spectrum (1 s integration time). That means if any leak had occurred, it was about 0.003% of the actual signal (in comparison, SIF was about 1-5% of the signal). The in-field calibration uncertainty was less than 5%, on average (Figure 3). For the sake of simplicity, we only show the performance (difference in % radiance) from the 5 nm moving window. The assumption of a smooth calibration factor is valid as we can see from the true calibration factor in Figure 3A. The artefact in the calibration factor was obvious when the spectral resolution was high, especially in the O\({}_{2}\)A band. We hypothesize that this is related to (1) a small spectral shift between two spectrometers; (2) a lower SNR in the O\({}_{2}\)A band. When the moving window was 5 nm, the fine structure in the O\({}_{2}\)A band was gone, and the calibration factor was close to the true values. Comparing the true radiance with the estimated radiance from the in-field calibration shows that the error was on average 2.94% of the total radiance value and varied from 1.25% to 4.25%. We demonstrated that the dark current is a linear function of the integration time under a fixed temperature (Figure 4). In Figure 4A, we measured the dark signal under various integration times (100 ms, 400 ms, 700 ms, 1000 ms, and 2500 ms) at \(-\)10 \({}^{\circ}\)C. First, the variation of DN in each dark spectrum was small (\(\sim\)0.06%), suggesting a near uniform dark response. Second, it was possible to characterize the dark DN by the integration time. We also show the importance of keeping the detector temperature low and stable in Figure 4C,D. The difference between 20 \({}^{\circ}\)C and \(-\)10 \({}^{\circ}\)C in the dark current at an integration time of 100 ms was \(\sim\)120. The changes in DN as temperature rose were nonlinear.