It’s difficult to be comfortable when your eyes have to strain just so you can see what’s in front of you. Reading in the dark or stepping into a dimly lit space both cause you to squint and struggle for focus and clarity. These instances may make you wish for more light, but if a luminaire shines too brightly, it can actually be harder to see than a dark space.
That’s because some fixtures, if not properly positioned and aimed, give off an intense illumination that impedes your ability to see. This is known as glare, and whether it’s reflected or it comes directly from a source, it can reduce your focus and be uncomfortable. Glare is bad enough indoors, but encountering it outside is even more frustrating. Outdoor areas are much more comfortable with diffuse and indirect lighting, but this is not feasible with street lighting. LED streetlight luminaire manufacturers are developing new ways of controlling glare.
Designers need to be proactive if they want to prevent glare, but this means that they have to understand its causes and effects. This guide will show you how fixtures create glare and what you can do to prevent it.
How Discomfort Glare Differs from Disability Glare
Not all glare is created equal. Before you can eliminate it, you need to know why it occurs. In many cases, environmental factors are at least as responsible for glare as light-related issues are.
Two types of uncomfortable brightness can make an environment unbearable. Discomfort glare is the kind with which most people are familiar. It’s the result of an overly bright or poorly placed fixture. A heavy contrast between the source of the light and its background can also create this irritation. Disability glare doesn’t produce the same type of pain, but it can inhibit normal functions. Environmental factors are usually responsible for this aggravation, so it can be difficult to manage. The latter type occurs more often in outdoor settings, but designers must try to reduce both as much as possible.
How Can You Reduce Glare?
As with many other problems, it’s easier to plan for glare and prevent it from happening than it is to fix an existing issue. Use lighting design software to predict the amount of glare that your fixtures and other environmental factors will create. Some lighting software is specifically made for street lighting design, evaluating veiling luminance and visual comfort probability (VCP), a measure of glare.
To control glare, consider the following when selecting street lights and determining their locations: luminaire light output, light distribution, mounting height, and pole spacing.
First, be mindful of the lumen output or wattage of your luminaires. The brighter the luminaire, the more likely it will cause glare. Of course, if high illuminance levels are needed, brighter luminaires might be necessary. Optionally, using more, but less bright luminaires can reduce glare.
Second, evaluate the lighting distribution of the luminaire carefully. If much of the luminaire’s light is lighting sideways or up to the sky, it’s probably producing glare in your eyes and not getting the light down to where you need it—the road.
Lastly, luminaire mounting height and pole spacing go hand in hand. Higher mounting heights allow the spread of light across a larger area. Poles can then be spaced further apart, but the lights need to be brighter since they are higher in the air. Having luminaires mounting high may reduce glare by getting them up out of view, but the added brightness and shadows they produce could be unsightly.
A good rule of thumb is spacing street lights apart about five times their mounting height. If street lights are mounted 15 feet in the air, space them 75 feet apart. If they are 30 feet in the air, they can be spaced 150 feet apart. Usually lower mounting heights are best used in areas with pedestrians and slower, neighborhood traffic. Use taller heights where traffic is moving faster on main roads.