Definition of radiometric quantities Sample Clauses

Definition of radiometric quantities. In the present document the terminology of “remote-sensing reflectance”, 𝑅𝑟𝑠, is used where Xx(𝜆, 𝜃, 𝜙) 𝑅𝑟𝑠(𝜆, 𝜃, 𝜙) = 𝐸0+(𝜆) 𝑑 (1) 𝑑 where 𝐸0+(𝜆) is the spectral downward plane irradiance, also called “above-water downwelling irradiance”, and Xx(𝜆, 𝜃, 𝜑) is the water-leaving radiance, defined, e.g. see [9], as the component of above-water directional upwelling radiance that has been transmitted across the water-air interface in the upward direction measured by the sensor and defined by above-water viewing nadir angle 𝜃 and azimuth angle 𝜑 – see Figure 1 and Figure 2 of [6] for a schematic of the geometry and the measurands respectively. Although the independent variables (𝜆, 𝜃, 𝜙) may be dropped from this notation for brevity it is noted that 𝑅𝑟𝑠 has both spectral and angular variability (as well as time and horizontal space variability). 𝑅𝑟𝑠 can be very simply related [10] to other popular radiometric quantities such as normalized water-leaving radiance, 𝑛𝐿𝑤(𝜆, 𝜃, 𝜙) , and water- leaving radiance reflectance, 𝜌𝑤(𝜆, 𝜃, 𝜙). Since there are diverse and ambiguous definitions and usages for these terms, we propose to follow here the terminology of the Ocean Optics Web Book [ xxxxx://xxxxxxxxxxxxxxx.xxxx/view/atmospheric-correction/normalized-reflectances], which is closely aligned with [9]. 𝑅𝑟𝑠 can also be “corrected” for BRDF effects [11] to yield radiometric quantities such as “exact1” normalised water- leaving radiance, with less sensitivity to sun and viewing geometry. Alternatively, 𝑅𝑟𝑠, can be converted/extrapolated to a nadir-viewing geometry, e.g. equation (5) of [12] using a “view correction” or “Q-factor” but without the correction for scattering angle effects associated with the sun zenith angle, “f-factor”. The nadir- viewing geometry facilitates comparison with measurements from in situ instruments and from satellite missions, provided that the latter are also corrected to a nadir-viewing angle. Except for Case 1 waters, there is no consensus at present on how to perform such viewing angle correction and it is expected that the state of the art will evolve in time as knowledge and tools improve. We therefore recommend that the in situ reflectance measurements be recorded and archived in the acquisition geometry and that software tools be made available to extrapolate to different viewing geometries both for in situ and for satellite measurements. In a validation context the viewing angle difference...