"Pollution" may seem like an extreme term to apply to stray light from outdoor lighting installations. Even if you contribute to the problem, you may not feel like much of a polluter. But like emissions from automobiles, every little contribution makes the problem just a bit worse.
Types of Light Pollution - Glare is one major type of light pollution. If your lights aren't properly aimed downward, or if they're ineffectively shielded, they can shine directly into the eyes of passersby. For foot traffic, the glare is a nuisance and a hazard. For drivers on the road, the danger is more severe.
Light Trespass - When light spills out from your lamps, it can irritate your neighbor. The same light that shows off your flower bed can shine directly into your neighbor's window.
Skyglow - Some light won't bother people directly because it goes straight up. But hundreds and hundreds of unshielded lamps throughout a city can create a washed out effect, making the stars hard to see. Everyone in the city is robbed of the night sky.
Solutions - A well shielded lamp directs light in the direction it ought to go-toward the ground-while blocking the light in directions where it wouldn't be appreciated.
A full cut-off shield is the best approach. This shield entirely contains the bulb and glass so that no light escapes at any angle parallel to the ground or higher. A standard "cobra head" shield with a flat lens is a basic way to go. A semi-cutoff shield, by contrast, has a lens that drops below the level of the shield or, even worse, a refractor that will spread the light more efficiently in wrong directions as well as right.
The worst is a standard globe lamp mounted on a post. The globe spreads light indiscriminately in all directions. If it's near the house, mounted on a wall arm for example, the eaves may keep the light from drifting up. But you'll still have light going out into the street.
Even lamps that are properly shielded can be improperly aimed. This is especially true if you're going for an uplighting effect. The general rule is, don't uplight. But if you need to, make sure the object you're uplighting takes ALL of the light. Positioning a spotlight at the base of a wall seems easy. And if all you're worried about is lighting the wall, it is. But take a few extra moments to make sure there's no light spilling over the TOP of that wall and wandering off into space.
Lighting types - Incandescent lights and mercury lights are the most likely to produce the glaring light that contributes to light pollution. Halogen lights will produce less glare, but beware of the temptation to over light! Halogens are quite powerful and it's easy to overdo them, rendering your battle against light pollution a draw. LEDs can be even more effective at keeping light pollution down.