Lighting

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Guides

Adding Light - A beginners tutorial on how to set up a basic light entity.
Intermediate Lighting - A tutorial dealing with all the light sources in more detail.
Advanced Lighting
Constant-Linear-Quadratic Falloff - An easy-to-read explanation of the old C-L-Q light attenuation system.
HDR - High Dynamic Range Lighting.

Tip:Static lighting is compiled into Lightmaps (which illuminate world brushes) and Cubemaps (which illuminate models).

light: A static, omni-directional point light source. (e.g. a naked light bulb or flame.)
light_spot: A static, uni-directional point light source. (e.g. a torch) The conical beam angle cannot exceed 90°.
light_environment: Provides two linked light sources; a "Diffuse light" reflected from overhead clouds/sky and a "Parallel light" which represents sun- or moon- light. Both are emitted from the toolsskybox material wherever it is used on a map. See also Skybox.
env_particlelight: Illuminates particles produced by env_smokestack. Particles are not affected by normal lighting. Use (no more than) two of these to make (only) env_smokestack particles appear to reflect ambient lighting.
luminous materials: Brush surfaces with a light-emitting material (as defined in game_dir/lights.rad) applied will cast volumetric light in a 180° cone.

Tip:Dynamic lighting is calculated at runtime, which makes it expensive to render.

env_projectedtexture shines from a doorway.

light_dynamic: A simple, uni-directional dynamic light that can be moved, turned, and adjusted.
point_spotlight: A dynamic spotlight beam effect, with an optional dynamic light source where it hits a surface.
npc_spotlight: A spotlight NPC, similar to point_spotlight but with the capability of tracking targets.
env_projectedtexture: A dynamic light that lights using a texture rather than a solid colour, and which also casts realistic dynamic shadows. Affects all surfaces that can accept lighting, and is used for the player flashlight in Episode Two.

Dynamic Shadows are cast only by World Models, and only onto Brush surfaces (from both Static and Dynamic light sources). They are calculated at runtime, so they are quite crude and relatively expensive to render.
Static Shadows are cast only by the combination of static light sources and static geometry (Brushes and Models); they are calculated and baked into the Lightmaps by VRAD, so they look much better and are relatively cheap to render.

shadow_control: Controls dynamically projected shadows (produced by player models, NPCs, and physics props) in the entire map.
info_no_dynamic_shadow: Point entity used to list Brush surfaces that shouldn't receive dynamic shadows.; Useful to apply to walls and floors where shadows are drawn improperly, giving away the location of enemies.; Dynamic shadows do not coagulate with each other or with Lightmapped shadows when they overlap. As light_environment casts both lightmap and dynamic shadows, use info_no_dynamic_shadow to prevent 'shadows within shadows' from a single light source.
Lightmap Scale: Used to define sharp/diffuse shadows on Brush surfaces. See also shadow_control "distance" parameter for diffusion of Dynamic shadows.

Tyndall effects are caused by light scattering on suspended (colloid) particles in a transparent medium. Eg car headlights in fog.

env_sprite - entity used to create omni-directional glow or flare effects.
env_lightglow - an asymmetrical glow (for areas of contrasting light).
env_sun - adds a bright haloed spot to the Skybox to represent the position of the Sun or Moon.

For values to accompany the stock skybox materials, see Sky List.

Naming a light makes it more expensive. Only name lights when you need to. Multiple lights sharing the same name are cheaper than lights with separate names.
light_dynamic is especially expensive, and will not even show up on some systems
The basic light entities do not come with a visible representation (e.g. a lightbulb). For that you need a prop. See HL2 light props for a list.
The basic light entities do not come with a "glow" as you'd expect to see in a foggy or misty area. To provide this, either compile and run the map with HDR enabled, or add a point_spotlight (with dynamic lighting off unless needed) or env_lightglow entity.
Maps will not be lit unless you run vrad (or equivalent). Vrad will not run properly and will not calculate realistic light bounces unless the level is free of leaks.
Several textures provide their own light & can do so using lights.rad in a mod's directory or the one in sourcesdk/bin.
Moving brushed-based objects will not change the way they are lit. Their lighting will be calculated only according to their positions in Hammer. (e.g. a brush in a dark room will not become bright if it is brought into a bright room.)