The term "immersion coefficient" is a scaling factor that is wavelength dependent, which compensates for the change in sensitivity that occurs when an instrument is calibrated in air, but used in a media of different refractive index, such as water. This is needed because the change in refractive index as photons enter an irradiance diffuser governs what portion of the photons enter— as opposed to being reflected—and the portion of photons once inside the collector are reflected back out. This coefficient is empirically determined and differs according to the design of the collector. It is measured in our laboratories for each irradiance collector design.
A reference describing how immersion coefficients are determined is found in Hooker and Zibordi, “Advanced Methods for Characterizing the Immersion Factor of Irradiance Sensors,” http://journals.ametsoc.org/doi/abs/10.1175/JTECH1736.1
When used with a radiance measuring device, this coefficient can be calculated based on ray tracing and is primarily a refractive effect where the field of view of the device changes due to refraction. The immersion coefficient for radiance radiometers is therefore calculated.