Light distribution: it’s an important factor that is often overlooked when we are selecting the proper LED fixture (especially for outdoors). In our industry, we obsess over wattage, efficacy, and colour temperature, but often forget to consider the way light is distributed from the fixture. Will the chosen luminaire adequately cover the area we are trying to illuminate based on the position it will be installed? This is where understanding IESNA Distribution Types will give us an advantage.
Light Distribution: As Classified by the IESNA
To help lighting specifiers choose the proper fixture for their application, the Illuminating Engineering Society of North America (IESNA) introduced a classification system for light distribution patterns. The system consists of five “Types” (Type I – Type V) that makes it easy to identify the distribution pattern of a fixture and correctly match it to the right application.
But I Thought NEMA Beam Angles Did That?
Yes, NEMA Beam Angles is another classification system for light distribution (in fact, we talked about it in this previous blog). However, there is a distinct difference between NEMA Beam Angles and IESNA Distribution Types: while NEMA Beam Angles classify light distribution by a fixture’s horizontal and vertical beam spread (more suited for flood lights and sports lighters), IESNA Distribution Types describe the shape of the area that is illuminated by a fixture (designed to help with area lighting and roadway applications).
Understanding IES Distribution Types
Here are the main differences between each IES Distribution Type and the types of applications they are best suited for:
IES Distribution Type I
Type I distributes a very lineal shape. It is the optimal choice for applications such as one or two-lane roadways, walkways, paths or sidewalks. With a preferred lateral width of 15 degrees, it is suitable for lighting streets or pathways that have a width of up to 2 times the mounting height.
IES Distribution Type II
For Type II, the shape is still linear but is wider in the front. With a lateral width of 25 degrees, it is the ideal choice for wider walkways, entrance roadways, on-ramps, four-lane roadways or any area requiring long, narrow lighting. When considering area width, Type II is best for illuminating areas with a width that are no more than 1.75 times the height the fixtures will be mounted.
IES Distribution Type III
Type III distribution is commonly known as a “bat-wing”. This distribution type is used to provide a larger area of lighting from a position along the perimeter of where lighting is required. At a lateral width of 40 degrees, light will project outward and is optimal for lighting areas with a width no more than 2.75 times the mounting height. It is typically used for driveways, garages, and even sports fields.
IES Distribution Type IV
Type IV has a lateral width of 60 degrees and is known as a “forward throw” distribution. It is the distribution type to choose if you are looking to illuminate sides of building façades, walls or parking areas. Much like Type III, it is best for areas up to 2.75 times the mounting height in width.
IES Distribution Type V
Type V distributes a circular pattern (there is also a Type VS that produces a square distribution with a more defined angle). Having the same distribution at all lateral angles, it is perfect for general area lighting—the ideal choice if you are looking to do parking lots or intersections.
Make Better Fixture Decisions
We hope that this knowledge will help you on your next project; now, you can confidently select a fixture with a distribution type that has better spill light control (if your site is near a residential neighbourhood) or avoid adding more poles and fixtures (and passing those costs onto your customer) because the distribution type you select will most effectively achieve the coverage you’re looking for. By understanding IES Distribution Types, you can now be more confident when you begin selecting the most suitable fixture for your application.