Spotlight distribution casts a focused beam of light like a flashlight, a follow spot in a theater, or a headlight. The light's beam angle controls the main strength of the beam, and the field angle controls the “spill” of light outside the main beam.
Note: Be aware that the scanline renderer and the mental ray renderer treat the beam angle and field angle differently. With the scanline renderer, the beam casts light at full intensity, and the field angle limits the area where the beam is cast. With the mental ray renderer, the center of the beam is at 100 percent, but it fades to 50 per cent at the beam angle, and to a value near 0 at the field angle; however, some light might be cast outside the field angle as well.
Spotlight distribution
Viewport representation of a Target light with spotlight distribution
Spotlight settings appear on the Distribution (Spotlight) rollout.