What is a rad file and how do I open a rad file?
Files in the .rad format are 3D image files used by an application called VRAD, developed by Valve. This program is integrated with command line functionalities which can be used to create, modify or edit 3D lighting atributes of BSP projects. These BSP projects are more popularly known as map files of digital shooter video games in first person perspective. These RAD files are used to embed lighting options and characteristics of surfaces and other things visible in the video game. These .rad files help improve the display quality of surfaces and items included in the overall design of the shooting video game. XML-based specifications are integrated into .rad files and help users deploy the relevant functionalities of the VRAD program to efficiently define the High Dynamic Range (HDR) and Low Dynamic Range (LDR) configuration settings of the video game. This in turn improves the intensity and vibrancy of the colors used in the game. The "vrad -lights bspmapname.rad" command line, without quotes, will add the 3D lighting properties configured and stored by the VRAD user in the RAD file to the associated BSP map file.
Relevant rad file links:How to open rad file on Youtube
How to open rad file on Wikipedia
How to open rad file on Microsoft.com