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The purpose of street lights is obvious, to illuminate the roadway and surrounding areas so that drivers, cyclists, and pedestrians can function safely in the same space. Street lights should allow you to clearly see any obstructions or other potential hazards with enough time to avoid them. In fact, several studies have shown that installing street lights can reduce car crashes by up to 40%. 

Installing street lights help prevents accidents and injuries, and makes both pedestrians and drivers feel safer. It only seems natural that the brighter a light is, the better it is for street lights, but that’s not the case at all. If a light is too bright it can cast dark shadows or even prevent drivers from seeing. There are many things to keep in mind when on the hunt for the ideal street light, and that’s just one of them. We’ll cover all of them below, but here’s a quick overview of what to keep an eye out for. 

  • Light output by the lamp
  • Light reflection from the road and other surfaces
  • Color rendering ability
  • Mesopic vision is determined by the S/ P ratio of light
  • The ratio between maximum lumens, average lumens, and minimum lumens<
  • Uniform distribution of light
  • Cut off the angle of the fixture
  • The height of the mounting pole – Optimum pole height is important for the correct lighting density. As the height of the mounting pole increases the footprint of the light increases but light density decreases.

Luminosity of Roadway Lights


The term luminosity refers to the light output from a lamp. Oftentimes, you see metal halide bulbs used to light streets and roadways. These present numerous problems, all of which we will address, but one of the most challenging problems they create for roadway lighting is a harsh glare when looked at head on. What’s the best way around this? LED street lights.

Light & The angle Above the Nadir

Nadir Street Lights

In the world of lighting, nadir is the angle pointing directly downward from the center of the light. This point should be perpendicular to the ground. The Illuminating Engineering Society (IES), a non-profit that sets lighting standards, uses the nadir to classify different street lights a ‘full cut off’, ‘semi cut off’ and ‘non cut off.” 

 The table below shows the percentage light levels and their relation to the classification of luminaires.


Fixture Classification

Percent of Total light output at 90° from Nadir

Percent of Total Candlepower at 80° from Nadir

Full Cutoff


<= 10%


<= 2.5%

<= 10%

Semi Cutoff

<= 5%

<= 20%

Non Cutoff

No limitation

No limitation


As you can see, full cut off lights direct more of their light downward. This is important for Dark Sky regulations (we’ll talk about these in more detail later), but it means that light spillage in the sky can be minimized. And, another advantage to this is that lamps with lower electricity usage can be used as there are very few instances of wasted light. 

Simply adding a light shield allows more light to be focused on the intended area thus providing better illumination with fewer lumens and reducing the possibility of irritating neighbors with bright lights. LED lights do away with the need to add light shields because they are naturally directional. Better light angles and finer quality optics make for great outdoor light.

Recently, IES, or the Illuminating Engineering Society, has put more emphasis on the BUG rating system for street lamps. This system is more comprehensive than others and rates light on three different parameters – Backlight, Up-light, and Glare. Since most municipalities still work with the earlier classification we will only be discussing that one in detail.

What is the importance of full cutoff lights?

Full cut off lights minimize environmental light pollution and help you save money.  Reduction of ambient light levels in the sky makes the skies ‘night friendly’ allowing astronomers to observe heavenly bodies and minimizes the impact of anthropogenic lighting on the circadian rhythm of nocturnal animals and plants. This is what it means to be Dark Sky compliment. Minimizing the amount of light pollution that a fixture contributes to the environment. You’ll see the strictest regulations in coastal areas, but any facility with natural surroundings should take steps to reduce their light pollution.


This is a picture of San Jose taken at night, and as you can see, the leakage of light from city fixtures, illuminating the sky is clearly visible. This night sky light pollution is a major challenge for both astronomers and wildlife. You may not think of a city as a hotspot for wildlife, but having this much light pollution can even negatively affect trees. Full cut off Lights installed properly could counter this problem.

The single most important reason for installing full city off light, however, is that in most areas around the world anything but a full cutoff light may be illegal. For example the lighting code of Douglas County, Colorado requires that all light fixtures for outdoor lighting (except those specified) must be full cut off as defined by the IES. The aim of roadway lighting is not just to provide a safe nighttime environment but also to protect the average Americans right to enjoy the beauty of a star studded night sky. In fact, even tilting full cut off lights can lead to the violation of the outdoor lighting code.

The light footprint of different types of Luminaries

light distributon

The above figure illustrates some of the most common types of luminaries, according to IES’s classification and their typical position along roads. It shows how different luminaries work better in different situations. For example, type 5 are suitable for installation for a 360 degree illumination footprint, like a 4-way stop or roundabout, while Type 1 is most suitable for installation along a road median.

The problem with measuring light output of lamps in Lumens

The method of calculating lumens output from lamps assumes zero light reflection and similar light quality. In theory, depending on the surface color, different amounts of light may be reflected resulting in different brightness. If a yellow-colored light illuminates a green colored object, for example, all the light will be absorbed and the object will appear black in color. And so, if the green object is placed on a black surface – it will not be visible at all. 

In reality, a green object – grass, for example, consists of several pigments – chlorophyll, carotenoids, and xanthophylls, etc and reflects a small amount of yellow light and appears grayish in color. This discrepancy is one of the reasons why a high luminous output is needed when LPS lamps are used for roadway light applications. LPS lamps produce only one color on the light spectrum. LED Lights, on the other hand, produce a much fuller spectrum of light. Therefore, the color differences between objects are highlighted and contrasted, which results in better visual acuity and greater feeling of safety with far fewer Lumens.

Another contributing factor to this better perception of colors and objects is CRI, or the color rendering index. You can read more about it here, but for now, just know that CRI is a measurement of a lights quality. A light with higher CRI stimulates the rod cells of the eye more effectively, which in turn, allows for a better illumination with less lumens. 


Last Updated: 09/21/2020
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