Solar radiation enters the Earth's atmosphere with a portion being scattered by clouds and aerosols.

Processing, archiving and distributing Earth science data
at the NASA Langley Research Center

How do I convert the numbers stored in the files to physical quantities?

At Level 1A, the 14 most significant bits (MSB) directly represent the raw digital count from the camera's Charge-Coupled Device (CCD). The 2 least significant bits (LSB) of the 16-bit data values are data quality indicators (DQI). A DQI of 0 means the data meet all specifications; 1 means some degradation in quality has occurred but the data are still valid; 2 means the data should not be used for scientific analysis; and 3 means the data are not usable. At Level 1B1, the image data and the data quality indicators are stored as separate fields. The image data values represent "scaled radiances", which means that they must be multiplied by a scale factor to obtain radiance in units of Watts/square meters/steradian/micrometer. These band-by-band scale factors are stored in the swath metadata of L1B1 files, and are found within the attributes of the swath structures. The data quality indicators follow the same convention described above.

For Level 1B2 data, the image data and the data quality indicators are once again packed into 16-bit words. The 14 MSB's represent "scaled radiances", and the 2 LSB's are DQI values that follow the same convention described above. The conversion from scaled radiance to radiance (Watts/square meters/steradian/micrometer) requires multiplication by a scale factor. These band-by-band scale factors are stored in the grid metadata of L1B2 files, and are found within the attributes of the grid structures. Note that certain high values of the scaled radiances at L1B1 and L1B2 are reserved for fill, as described in the Data Product Specification Document.