Apply barn doors when you want to create the effect of light pouring out of a half-opened door. Barn Doors are shutters applied to a spot light’s beam. Barn Doors are off by default; you must turn them on, then adjust them to get the desired light spill.
Four blue lines appear. These lines represent a rectangular boundary for the spot light. No light reaches the regions outside the blue line-boundaries.
Attributes are in degrees measured from the spot light to the blue line-boundary. For instance, for an 80 degree spot light, the four blue line-boundaries are 40 if the boundaries lie exactly at the spot light border.
When you use the Barn Doors settings, the Light Shape sample changes at the top of the Attribute Editor.
The region between the two circles results in a softer illumination.
You can also verify the illumination by looking at the Light Shape and Intensity Sample at the top of a spot light’s Attribute Editor. These change as you change the values for the spot light.