## 4 Radiometric Processing Workflow
### Scene Reflectance Generation
#### 4.3.1 Reflectance Generation Based on Incident Irradiance  
The spectrometer radiances can be transformed into reflectance with the aid of incident irradiance measurements. The incident irradiance can be either estimated using atmospheric radiative transfer models (ARTMs) or measured using an irradiance spectrometer. ARTMs allow simulating incoming irradiance from the sun to top of the canopy as well as the influence of the atmosphere on the signal during its way from the canopy to the sensor. Input parameters include the time, date, location, temperature, humidity, and aerosol optical depth measured by, e.g., a sun photometer. ARTMs can be used to generate the irradiance necessary to calculate reflectance together with the radiance received by the sensor. One example is shown in [PERSON] et al. (2015), which used the SMARTS model ([PERSON] et al., 2015), parameterized with the aerosol optical depth measured at 550 nm with a Micro-Tops II sun photometer (Solar LIGHT Co., Philadelphia, PA, USA) collected in the study areas at the time of the flight, for hyperspectral pushbroom imagery at 575 m above ground level. The drawback of ARTMs for reflectance calculations is the need for sufficient parameterization of the atmosphere. This is particularly challenging for flights over larger areas, where the atmosphere might be heterogeneous, and under varying illumination conditions due to clouds. Due to the challenges with the ARTMs, the technologies measuring the incident irradiance using a secondary spectrometer are of great interest in the UAV spectrometry. The possible methods include using stationary irradiance or radiance recordings (e.g., of a reference panel or with a cosine receptor on the ground) or a mobile irradiance sensor equipped with cosine receptor optics mounted on the UAV. [PERSON] et al. (2015) used two Ocean Optics STS-VIS cross-calibrated point spectrometers. One of the spectrometers was measuring radiance reflected from the object on-board a multi-rotor UAV, and the second spectrometer measured the Spectralon panel on ground. This method is also referred to as a \"continuous panel method\", and is similar to setups of classical dual ground spectrometer measurements, which provide reflectance factors by taking consecutive measurements of the target and Lambertian reference panel (e.g., ([PERSON] et al., 2015)). [PERSON] et al. (2015) used a dual-spectrometer approach with two TriOS RAMSES point spectrometers. They calculated the relative sensitivity of the radiance and irradiance sensors and fitted a third-order polynomial to this ratio. They transformed the radiance measurements of the downwards-facing device to reflectance utilizing the simultaneous irradiance measurements of one upward-facing spectrometer equipped with a cosine receptor on-board the UAV. Lately, also consumer-grade multispectral sensors such as the Parrot Sequoia and Maia are shipped with an irradiance sensor ([PERSON] et al., 2015; [PERSON] et al., 2015). An upward-looking sensor equipped with a cosine receptor foreoptic is required to measure the hemispherical irradiance. In reality, the angular response of the cosine receptor deviates from a cosine shape (c.f. Sections 3.3.7 and 4 in ([PERSON] et al., 2015) and ([PERSON] et al., 2015)). The cosine error correction depends on the atmospheric state when the measurements were made, and the deviations typically become larger as the incidence angle increases, implying that measured irradiance is underestimated compared with an instrument with a perfect angular response. This is particularly important during times of low sun elevation (e.g., in high latitudes, and in the morning and evening). The underestimation may be corrected for, providing that the sky conditions during the measurements are known, and that the angular response of the instrument is known. [PERSON] et al. (2015) reported a deviation from a perfect cosine response of less than 2%.