Unlike IOPs, Apparent Optical Properties (AOPs) depend both on the medium and the ambient light around it. For example, sunlight shining on the ocean can change quickly as a cloud passes overhead and casts a shadow on the ocean surface.
Each AOP can be broken down into a ratio of radiometric variables. There are three main categories of AOPs used in radiative transfer: 1) diffuse attenuation coefficients, 2) reflectances, and 3) average cosines. These represent changes of light across depth, the ratio of light coming into versus off of a surface, and the ratio of spectral plane to scalar irradiances, respectively.
Diffuse Attenuation Coefficients provide a simple measure of how light decreases with depth into the ocean (or another medium, such as the atmosphere). The rate at which light changes with depth depends on the IOPs of that medium.
Imagine standing in shallow seawater. How far down can you see? Can you see your toes? Can you barely see your hand just below the surface of the water? A commonly used oceanographic sampling tool, the Secchi Disk, employs a similar technique. Invented in 1865 in Italy, the Secchi Disk applies the principles of the Diffuse Attenuation Coefficients, by lowering a black and white disk into the water and then noting the depth at which the disk disappears from view.
Note that there are several different types of Diffuse Attenuation Coefficients. However, the one most often used for oceanographic purposes is associated with Downwelling Spectral Plane Irradiance, providing a metric of how light changes with increasing depth.
For PACE science, the reflectance value we are most interested in is Remote Sensing Reflectance (Rrs). Rrs is the ratio of the light going into the ocean (the spectral plane downwelling irradiance at the surface) to the light coming out of the ocean (water-leaving irradiance). For example, Rrs does not include light reflected off the sea surface.
As noted earlier, water-leaving radiance is only a small fraction (~10%) of the light received by an earth-orbiting satellite. The majority of light is scattered from the atmosphere or reflected off the ocean surface.
Now here’s the brain teaser…Rrs is used to understand optical properties of the ocean, but both of the measurements used in its calculation are taken above the surface of the ocean. The PACE satellite does not take “underwater” measurements. How do the PACE scientists do it? To find out, read on to the next section – What Does this Mean for PACE? .
Want to get even more technical? Check out the OceanColor Web’s Data Product page for Remote Sensing Reflectance related algorithms, references, and data access points.
Our last type of AOPs are Average Cosines. There are three main cosines relevant for ocean science used in radiative transfer calculation: (1) average cosine of radiance distribution; (2) average cosine of downwelling radiance; and (3) average cosine of upwelling radiance. Each of these are calculated by dividing Spectral Plane Irradiance by Scalar Irradiance. Remember, the difference between these two measurements is that while Scalar Irradiance represents the sum of light from all directions, Spectral Plane Irradiance takes the angle of the light into account. In this way, Average Cosines tell us about the angular distribution of light.
For more details and equations check out the AOP section of the Ocean Optics Web Book .