There is only one file type that uses the .LT2 extension:
Poser Light Set File
Contains lighting information used by Poser, a 3D character modeling and animation program; may include lights, such as spotlights and distant lights; light set files created with Poser 6 and later may also contain point lights and image-based lights.
Poser was originally developed by Smith Micro, but was sold to Bondware in June 2019.
Updated: April 10, 2020