Window lighting
By default, light will only shine in through a window if the light_environment angle is set so that sunlight shines directly through it; however, real-world windows will often stream in refracted (indirect) sunlight, independent of the suns angle.
Valve commonly uses the following technique to simulate this:
1. Create the opening that you want light to refract through (such as a window).
2. Cover the opening with a brush using the toolsnodraw texture.
3. Texture the inward-facing side of this brush with lights/white001 or lights/white001_nochop. This particular texture emits light in-game. You can adjust the strength of this light with the "Texture scale" settings. Values between 0.5-1 tend to work well. The closer to zero, the brighter the light. If using the version without nochop, adjusting the "lightmap scale" will adjust the quality of the light.
Important:Setting the brightness too high will cause an undesired effect where dynamically lit models such as player viewmodels glow when right next to the windowsill.
4. Tie the brush to a func_illusionary or func_brush entity. In its properties, set Render Mode to Don't Render to will prevent the non-transparent brush from blocking the view as a white sheet. (It will still emit its light.) If using func_brush, also set Solidity to Never Solid (unless you want the brush to simulate unbreakable glass).
