This file is only used by tactical saves.
Length of file is map size (overall number of tiles) + some 4 bytes at the end.
File contains data about light from mobile sources (soldiers' personal lights). It is recalculated every time a unit moves (does action requiring energy). A value of 0 is perfect light, while 15 is perfect dark. Note that soldiers can only see in tiles with values of no more then 9, while aliens ignore all lighting.
Light sources produce values of 0, and each tile away gets darker by one point (in a circular pattern) up to the maximum darkness value (depending on the time of day, or in some cases, terrain - alien bases are always dark).
Together with TERMP.DAT it is full info about current lighting of the map. Actual lighting of tile is lower value from both files. Exact meaning of values is a number that palette is shifted towards darker colours, though doors will never be shifted by more then three or SCANG.DAT images by more then seven.