A streetlight or other outdoor fixture sends out light in many directions, but in essence it all goes into one of three zones.

Uplight is any light that shines above horizontal; virtually all of it goes straight into the air, uselessly, and is a direct cause of skyglow.

In the glare zone, light leaves the fixture below horizontal but at a highly oblique angle that can be seen directly by drivers or pedestrians. This creates glare that’s either discomforting or even disabling to our eyes. Glary fixtures are actually a safety hazard.

Useful light illuminates the ground without causing uplight or glare. This is a hallmark of well-designed light fixtures. A small fraction of useful light is reflected off the ground and up into the atmosphere.