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 can I tell where the valid data begins and ends in L1B2 products?

The L1B2 product contains a hierarchy of metadata fields to assist you in determining which data are relevant. All L1B2 files begin with block number 1, even though block processing rarely begins that early. Refer to the file metadata attributes "Start_block" and "End block" in order to locate valid data. Note: The missing underscore in End block is real. The L1B2 per-block metadata also contains a data valid flag which is normally set to 1 for blocks within the Start_block and End block range. In rare cases, the L1B2 software is forced to skip an entire block in order to complete. In these cases, the data valid flag for that block is set to 0. Illustration of Blocks Paths/Blocks Example
Stacked-block background. Example of MISR paths and blocks.