The satellite measures the sunlight in the visible spectral region reflected by the earth. Hence the measurement signal depends on the irradiation hitting the reflecting layers. After subtraction of the radiometer offset Co the remaining Count behaves proportionally to the irradiance I. Under utilization of these proportionalities the relative reflectivity p can be defined as standardized backscattering value:
p := C-Co /1. (1)
In the following only relative differences not absolute values of the reflecting properties are important therefore it is sufficient to divide by the cosine of the sun zenith.
The standardized backscattering values of clouds usually exceed those of the Earth's surface, excluding the case of snow. Thus it is possible to identify the occurrence of clouds. If the reflectivity for a completely cloudy pixel pc and the reflectivity of the unclouded ground (and ocean respectively) pg is known, the cloud index n as a degree of cloudiness can be defined as
n = (P - Pg)/(Pc - Pg). (2)
If the maximum and minimum of the standardized backscattering values of a pixel is selected as reference values, the Cloud index takes values in the range of 0<=n<=1.