Distribution Type Defined
“Distribution type” refers to how far forward the effective output reaches from the output source. This is not to be confused with “cutoff” classifications, which reference the amount of light that can shine above the luminary.
IES distribution types are an essential part to planning the best lighting solutions for your site. Worst case scenario, lighting is installed without realizing the poor visual design, and costly replacements will be required.
Below are the 6 basic IES roadway beam distributions and explanations of their patterns, ranges and suitable applications:
Type I has a two-way lateral distribution, with two concentrated light beams that illuminate in opposite directions. Type I distributions have a preferred lateral width of 15 degrees, and are best suited for the middle medium of a highway or road that needs illumination on both sides of traffic. In this case, mounting height should be approximately the same as the road’s width, so the light can distribute all the way across. This type can also be used for lighting pathways, sidewalks and other pedestrian walkways.
Type II distribution has one light which distributes a preferred lateral width of 25 degrees on just one side of the luminary. Because this type only has one light and a narrower beam it is best used for long, narrow lighting such as narrow roadways, walkways, and ramps or entrance roadways. Generally, the roadway width should be less than 1.75 times the height of the fixture for effective distribution.
Type III offers a wider 40-degree preferred lateral distribution width from one forward projecting light source. With a wider flood pattern, this distribution type is meant for side, or near side mounting. It best applies to medium-width roadways and general parking areas. For roadside mounting the width of the road should not exceed 2.75 times the height of the luminary.
Type IV distribution provides an even wider flood pattern of 60 degrees lateral width. The semicircular light pattern is great for illuminating perimeters and mounting on the sides of buildings and walls. For roadways, type IV is best for side road mounting where the roadway width does not exceed 3.7 times the luminary’s height. With this distribution, the light maintains the same intensity at every angle from 90º to 270º.
This type has an even, circular 360º symmetry of candlepower at all lateral angles, and is ideal for center roadway, island, and intersection mounting. It provides very efficient illumination all the way around the luminary, making it also a great choice for large parking areas.
Type VS is a special square 360º distribution with light intensity that is virtually the same at every lateral angle. Similar to Type V, Type VS is meant for center roadway mounting and large commercial parking lots. The difference is Type VS produces a more defined light beam edge, rather than a softer faded out beam.
NEMA Beam Spreads
The “NEMA beam spread” of a light refers to the two edges where light intensity spreads horizontal and vertical to 10% of the maximum beam intensity, and correlates to whether the light output is very narrow, very wide, or somewhere in between. The following classifications will help you calculate the horizontal and vertical NEMA rating for your luminary’s particular beam spread, which is also an important aspect in lighting design.
To determine your luminary’s NEMA classifications for an asymmetrical beam spread, complete the following equation:
Asymmetrical Beam Spread = Xº H (Horizontal Spread) by Xº V (Vertical Spread)
Example: 84º H x 110º V = NEMA 5 and NEMA 6
The Impact of Mounting Height on Illumination
As mentioned in the IES distribution types, mounting height plays a role in the effectiveness a luminary has on its intended target. Because LEDs cast a cone of light and are naturally directional, the beam will always dictate the area that will be illuminated.
Determining the ideal beam angle for your luminary, requires knowing the distance between where the light will be mounted, and the area you want it to illuminate. A closer distance will not need a long-range narrow beam, so a wider flood beam will be best. To illuminate a target at a further distance a more narrow, farther-reaching beam is required.
You will also want to consider the distance between light sources and the beam’s radius, so you don’t have extensive overlapping of illumination. If your intent is to have overlapping of wider beam fixtures for ambience, your beam spread diameter (radius x 2) should be equal to, or greater than the distance between fixtures.
Spot vs. Flood Beams
As you can see NEMA types correlate to a description of very narrow to very wide. Narrow beams are further classified as “spot” beams, while wider beam distributions are “flood” beams.
A spotlight beam casts illumination usually no more than 45 degrees wide, and is used to pinpoint a target with more focused light. You will often see spotlights used in art galleries or to brighten architectural landscape or building features.
A flood light is generally any beam distribution above 45 degrees and is used to illuminate larger areas of space with a wider, softer pattern of vertical and horizontal light, such as parking lots and building fronts.
Beam Width Calculation
Light beams are usually written in degrees, but this isn’t always helpful when trying to determine how wide the beam will be from any given distance away. To make this easier you can use the following formula:
Beam Width = (Angle of the Beam) x (0.018) x (Distance from the Light bulb in Feet)
Example: (50º angle) x (0.018) x (13 feet) = 11.7 feet
So, if you have a flood light with a 50º angle and want to know how wide the beam be from a distance of 13 feet, it will be 11.7 feet at the widest point.
Light beam distributions, classifications and ratings can be rather confusing, but hopefully this information has helped you sort everything out enough to make educated calculations and purchases for your project.
Larson Electronics specializes in LED lighting systems for a variety of indoor and outdoor applications.
Feel free to call us today, or go to Larsonelectronics.com for more information. We are happy to help you with your next lighting project.