The IESNA (Illuminating Engineering Society of North America) has defined various light distribution types. Light fixtures are designed to fall under one of the types defined, and lighting designers use this a guideline when selecting lighting for a particular area. Incorrectly selecting light fixtures can result in glare and light trespass into unwanted areas, such as onto the property of someone else.
Type 1 distribution has a very narrow and long lighting area. This is useful when selecting a light fixture to light up a narrow area, such as a pathway or very narrow roadway. Type 1 fixtures would normally be placed along the center of the pathway.
Type 2 distribution is used for wider pathways and narrow roadways (single lane),. Type 2 fixtures are usually placed off to the side of the pathway or roadway, since they are able to provide a small degree of forward light throw.
Type 3 distribution is used primarily for roadway lighting, and is placed off to the side of the road. The forward throw on these fixtures allow light to product outwards, illuminating multi lane roadways.
Type 4 distribution is used to light up parking lots as well as very wide roadways. Type 4 fixtures throw more light forward than they do to the sides, thus is not practical for lighting long roadways, as fixtures will need to be spaced closer together. Wall mounted fixtures such as wallpacks also commonly use type 4 distribution to throw light in front of an exterior entrance door to a building.
Type V / Type VS
Type 5 fixtures throw an equal amount of light in all directions around the fixture. This is usefull for lighting up parking lots, where the light fixtures are placed away from perimeter of the lot. Type V fixtures provide light in a circular pattern, where as Type VS provides light in a square pattern around the fixture. Both however have very similar distribution patterns.