When shopping for Outdoor commercial lighting products, you will often see the term "distribution" used. This term refers to how the light is dispersed from the fixture. Understanding these distributions can help you make the best choice for a particular project or application.
There are two different classifications of beam spread types and they are used for different types of lighting fixtures
1. The IES Luminaire Classification is used for area lighting. This include Street lights, Parking lot lights and LED Wall packs
2. The NEMA Beam Spread classification is used for outdoor flood lights, spot lights, & stadium lighting.
This article will focus on the IES distribution types that are specifically aimed towards area lights and street lighting.
The 5 Types of Lighting Distribution for Area Lights
Area lights are most often installed in open spaces like sidewalks, parking lots and roadways. The goal is to illuminate the targeted areas without intruding on adjacent areas where lighting is not desired.
Lighting Engineers are tasked with the job of installing fixtures for well lit areas, but they have to be careful so the lights don't bother nearby neighbors, pollute the night sky or introduce too much glare to the space.
As a result, the IES (Illuminating Engineering Society) have come up with some guidelines to help classify the different types of distributions for area lights.
These classifications are not only useful to know the projection of the light but it also aids in spacing and placement of the light poles.
Type I - Narrow Spread
The Type I distribution type is narrow and projects light far out side to side but barely projects out in front of it. It is used in very narrow pathways or on islands in the middle of the roadway. Type I fixtures are rarely found in the US. It is very much a niche case.
Type II - Narrow spread with a more forward throw
Fixtures with a type II distribution type are used in narrow roadways and larger center islands. This distribution type has a more of a forward throw than type I but with less projection side to side.For Spacing Type II fixtures next to each other, it's important to note that the rule of thumb is that they project out around 3x the mounting height side to side and 1x the mounting height out front. For example, two fixtures mounted 15 ft high will provide adequate lighting if they are spaced 90ft apart.
Here is how we it works
Side to Side projection of each fixture at 3x = 3 x 15ft = 45 ft for each fixture
Fixture 1 covers 45 ft + 45 ft from fixture 2 = 90ft spacing.
Type III - Most Common distribution type
Type III distribution type is the most common as it can be found on most standard size streets or throughout open parking lot spaces. Type III fixtures are most commonly installed along the side of an open space.These types of fixtures have more of a forward throw while the side to side projections are more narrow.
General rule of thumb for type III fixtures is that they project out 1.5x out in front and about 2.5 side to side.
In our previous example of two 15ft high fixtures, type III lights would have to be spaced closer to each other at 75 ft.
15 ft x 2.5 = 37.5 ft for each fixture
37.5x2 = 75 ft.
The forward projection also comes into play in areas like parking lots where two fixtures can be installed across from each other. at 1.5x the mounting height , a 45 ft wide parking lot would have adequate light uniformity with the fixtures installed at said height. For wider parking lots, the installer would need to mount the fixture at a little higher.
Type IV - Highest Forward Projection
The type IV distribution type has the most forward projection. Type IV fixtures are typically used on the perimeter of parking areas, especially in lots that do not have any poles in the interior. Since the Type IV fixtures project out so far, they can cover a lot of ground but that's at the expense of the side to side projection. This means that the installer would need more fixtures installed closer to each other on the perimeter compared to Type III fixtures.Type IV distribution typically projects out front up to 4x the mounting height. Side to side projections are about 1.75x the mounting height.
In our 15ft pole example, a type IV light will project out about 60ft forward, while the space between fixtures would only be about 53.5 ft (26.25 ft for each fixture).
Type V & Type VS - Even Projection all around
A type V distribution type provides the same amount of projection in all directions around the fixture. The surface covered by one type V picture results in a perfectly round shape as it's symmetrical all around.A type VS on the other hand stands for Type V square. In other words, it projects out in equal distances around the fixture just like the Type V but in the area it covers is square shaped.
Type V fixtures are typically used in the center of a parking lot where real estate is limited and the installer is trying to cover as much ground with one fixture. A type V fixture can be placed in the center instead of two type III installed back to back without losing much coverage.
A type VS on the other hand has a similar application type as the Type V, however its projection is more defined and is ideally used in square areas where light coverage is desired at the corners.
The Role Of Mounting Height on Distribution Types
As previously mentioned, the height at which an area light is mounted is critical to how far it projects. In the examples above, we use the mounting height as a multiplier in our projection formulas.; the higher the mounting height, the farther apart we can theoretically place the light poles.
However, there is an obvious tradeoff. As we start mounting these fixtures higher and higher, we start losing lumen output to our desired areas. This is why one must always consider upgrading to a higher wattage fixture when increasing the mounting height. The higher the wattage of the fixture, the more lumen we have at our disposal.
Below is a good rule of thumb to follow when picking higher mounting heights.
up to 75 Watts (up to 5000 lumens) - up to 15 ft Mounting Height
75-150 Watts (5000-20000 lumens) - 15-25 ft Mounting Height
300 Watts Fixtures (40000 lumens - 30 ft + Mounting Height
It's important to note that having too much light can also be a problem. While delivering too little light is an obvious issue, having too much lighting can also be an issue as it introduces too much glare.
It can also cause an issue in terms of light adaptation. Having a high wattage fixture in a low mounting height make the bright areas really bright and can be an issue as there is less uniformity and the eye has to do a lot more work to adapt from the darker spots to the bright spots.
The purpose of Light Distribution
Now that you've learned so much about distribution types and mounting height, you might wonder why it's so important that an independent body issues a set of rules for installers to abide by.The reason comes down to safe navigation, security and light uniformity.
These three aspects go hand in hand.
Safe Navigation
Ideal outdoor lighting conditions allow the users of the space to safely navigate the area and see potential pitfalls, cracks and obstruction along the way. In order for the area to be safe to navigate, you need the lights to be bright
Security
Well lit spaces deter potential intruders from entering the space. While lighting alone won't prevent someone from trespassing, having great lighting with the right security measures like fences and barriers will provide a secure space for users.
Uniformity
Selecting the right distribution types for your area light fixtures will ideally result in the correct uniformity. Ideal uniformity is when there is a low ratio of dark spots to light spots.We achieve this by measuring the foot candle of multiple spots in the space and identifying the highest foot candle spot and the lowest. The ratio from Maximum to minimum will give you the overall uniformity of the space. An ideal ratio should be somewhere between 15:1 or lower.
To have a well lit space with uniform lighting, you need to make sure that the light fixtures are spaced at an ideal location that provides enough lights throughout the space. In other words, you need to select the fixture with the correct distribution type and use the guidelines above to space them in a way that minimizes dark spots.
You also don't want to put them too close to each other and have too much light intensity in the space. As we mentioned before, too much light is not just wasted energy but can also cause glare, and light adaptation issues.
