Since many municipalities across the nation are transitioning to LED street lights, there has been an increase in complaints from local homeowners, pedestrians, and business owners about recently installed LED lights being too bright or intrusive. This has raised the question of how bright should street lights be?
LED Street Light fixtures range anywhere from 125 - 150 lumens per watt. However, the lumen output is not all that matters. The lighting and roadway authorities like the IES and AASHTO have determined that the actual light hitting the road surface should range anywhere from 0.3 FC (foot candles) to 1.2 FC.
The actual Foot candle requirements depend on a variety of factors including the number of pedestrians using the road every hour, the type of road, and whether or not the space has a likelihood of high pedestrian and vehicle interactions.
For example, a major arterial road in an urban environment with a lot of foot traffic will have the highest foot candle requirement of 1.2 FC, especially at a crosswalk where both vehicles and pedestrians are more likely to interact with each other.
With that being said, there is way more to lighting up a roadway than lumen and foot candles. Lighting professionals and Roadway authorities are hardly concerned about lumen and the amount of output from the fixtures themselves. They are more concerned about the total experience of the end users (i.e. pedestrians & drivers) while taking into account local ordinance restrictions.
The following are the 4 factors that go into making roadways not only bright but more importantly visible for both pedestrians and drivers
- Light Illuminance
- Light Uniformity
Illuminance: How it affects Brightness
Illuminance is the measurement of the light level hitting a specific surface, whether it's a horizontal or vertical plane. Not all the lumen output from the fixture actually reaches its target, lenses, reflector, and even the lamp's energy factor all contribute to how much light actually reaches the surface.
Illuminance is measured in lux and foot candles. Lux is the measurement used in most parts of the world while Foot candles (FC) is mostly used in the US. Thankfully it's easy to convert FC into lux as 1 FC = 10.76 lux but most people use the 1FC = 10 Lux formula to make quick calculations
When optimizing for better lighting for a given setting, lighting engineers are concerned about having a bright enough area for the tasks normally performed in that given location.
In the case of roadways and sidewalks, you want to have enough illuminance for pedestrians to walk and see oncoming pedestrians and vehicles when they're at an intersection.
The authorities have determined that this illuminance level is 0.3 to 1.2 FC depending on the type of streets and how busy they can get. Over 100 pedestrians/hour is considered high usage, while low usage is anything under 10 pedestrians/hour.
Luminance: Perceived Brightness is what matters.
While illuminance measures the light intensity hitting a given surface, it's not necessarily the most ideal solution to measure the perceived brightness of the end users. This is where Luminance comes in
Luminance measures the level of light reflected from a given surface. Unlike illuminance which measures the light on a surface, luminance measures what our eyes actually see, which is the light that's reflecting off the said surface.
The luminance level will differ based on your viewing angle or the type of surface on which it is being reflected. This is why the lighting formula to calculate luminance takes into account the type of surface being used in order to come up with a luminance number.
Luminance is measured in candelabra/sq. meters. It's not a unit that can be physically measured but it needs to be calculated with lighting engineering software. As a result, we can make assumptions based on the pavement type.
The lighting authorities have moved to use luminance for street lighting design as that takes into account the visibility of the pedestrians and drivers rather than the streets themselves. For drivers specifically, IES and AASHTO have designated the surface around 270 ft away from the driver as the ideal location to measure the luminance required for that area.
Light Uniformity: More important than Brightness
Light Uniformity refers to the overall evenness of light distributed throughout the roadway. This unit is measured in a ratio of Average illuminance / Minimum illuminance of the desired space.
In the case of roadways and sidewalks, we want to divide the roads in multiple blocks on both sides and measure the illuminance of each targeted area. The ratio from average to a minimum for most roads should be somewhere around 3:1. A ratio slightly higher is fine for less frequented roads but anything significantly higher is undesirable and potentially dangerous for both pedestrians and drivers.
Having an evenly lit road is ideal for the end user and even more important than sheer brightness. Uneven lighting can cause issues with light adaptation for drivers.
Light adaptation is when our eyes adjust to the surrounding environment and allows us to see in both bright and dark areas. Having an even light coverage of the road helps our eyes not have sudden changes, which can lead to glare or worse temporary blindness.
This is one of the reasons why LED Street Lights have replaced traditional HID street lamps. While HID lamps provided an intense beam of light, it was also a very concentrated center beam that caused huge issues with uneven lighting. One of the main advantages of LED street lights is the ability to diffuse the light better and provide even coverage throughout the roadway.
Glare: When Brightness Becomes a Nuisance
Glare is the excessive brightness that causes annoyance, discomfort, and potential safety issues. Glare can be caused by either direct lighting or reflected light from nearby surfaces like walls, buildings, and other vehicles.
Glare is simply the eye not being able to adjust to the bright light since it's much brighter than the light it's currently adapted to.
Glare is a huge issue when it comes to street lighting. Excessively bright lights can cause blinding or temporary blindness when drivers approach an intersection or curves. It can also be a chronic issue as it causes fatigue as the eye has to adapt to various environments of different luminance.
Glare is not something that can be measured physically on the field. In order to make sure you meet recommended requirements for your local area, you would need to have an engineer perform some calculations with industry-standard software.
The unit of measurement for calculating Glare is called "veiling luminance".
Using Assumption and General Rules of Thumb
As you can see, trying to achieve the perfect amount of brightness for any given environment is quite a complex task. While luminance and glare measurements are the best way to go for lighting professionals, the rest of us don't have the access or the luxury of using these complex types of software.
This is why the simplest method that most regular folks use nowadays is the standard guidelines by the IES or local authorities. These guidelines use foot-candles as a measurement which is something that can be measured physically with an illuminance meter. If there is a significant concern about street light brightness in your area, it's something that you can technically inspect yourself as a preliminary measure.
New Installations vs Retrofits
It's important to note that IES recommendations are simply guidelines and not set in stone. In an ideal world, all the streets in the country would be able to meet these guidelines but as with most things, it's not always feasible.
Going forward, local municipalities will be using these recommendations as a guideline and use the proper calculations to make sure all these needs are met. However, it's hard to be as precise when replacing old and existing light fixtures.
Chances are that older light fixtures were designed and installed to meet certain standards that were fine at the time but have become outdated. With better knowledge and better equipment such as LED light fixtures, local authorities are slowly but surely moving in the right direction.
At this moment in time, retrofitting an existing light pole with LED will do wonders for any roadway even if it does not meet all the luminance and glare requirements.