Arnold "light filters" are arbitrary shaders that can modify a light's output based on distance, position or other factors. This makes it easy to extend the built-in light sources with additional effects. There are 4 light filters:
The light_decay filter specifies the attenuation ranges of where the light starts and ends.
The light_blocker filter will block out an arbitrary geometrically defined area of a connected light.
The gobo filter is used to break up the light beam into an irregular, more natural pattern such as tree leaves etc.
Barndoors are opaque moving panels attached to the sides of the light's opening used for additional control over the shape of the light beam.
brandoor light filter -> spotlight