Select the correct luminaire. For example, avoid post-top lanterns that allow lots of light to spill upward, such as those with clear or opal spheres. Be wary of street lights with prismatic bowls. Luminaires with clear bowls and mirror reflectors can afford more accurate optical control without additional cost. Floodlights aimed at high angles to the vertical are the worst offenders.
Some light is bound to spill outside the area.
Luminaires with reflector baffles mounted above the lamp reduce spill light significantly. Avoid high vertical aiming angles whenever possible. This does mean higher masts or closer spacing between masts. However, excessively high aiming angles cause glare and long, dark shadows. This can override a mistaken economic benefit of using high aiming angles.