Light pollution is no longer a topic restricted to astronomers. Today, it is related to lighting design, safety, visual comfort, biodiversity, and the preservation of the night sky. With the revision of ABNT NBR 5101:2024, street lighting has more explicitly incorporated criteria related to the control of light emission and the impacts of artificial lighting on the night environment.
This guide brings together what designers, engineers, and managers of public lighting need to consider in order to specify technically adequate, efficient systems that are aligned with current standards and best practices.
What is light pollution?
Light pollution is the set of adverse effects caused by excessive, poorly directed, or inappropriately used artificial light in nighttime environments.
From a technical standpoint, different effects can be analyzed separately, as each has its own characteristics, impacts, and control methods.
O skyglow It is the diffuse brightening of the night sky caused by the scattering of light in the atmosphere. Direct light emission above the horizon contributes to this phenomenon, as does light reflected from illuminated surfaces. Controlling light emission to the upper hemisphere is therefore one of the important measures to reduce this impact.
A intrusive light (light trespass) This occurs when lighting extends beyond the area that should be illuminated and reaches unwanted locations—such as a facade or window of a neighboring property illuminated by a street light or parking lot fixture. One way to assess this is by measuring the vertical illuminance incident on surfaces and property boundaries.
O glare Glare occurs when the presence of a light source or an inadequate distribution of luminance impairs visual capacity, potentially compromising comfort and, in certain situations, safety. In road lighting, ABNT NBR 5101:2024 establishes criteria for glare control through threshold increments (fYOU).
A light clutter It is associated with the excessive or disorganized grouping of light sources, especially in urban environments with a large amount of signage, facades, advertisements, and other light sources. This excess can impair visual legibility and increase the complexity of the nighttime environment.
Why light pollution matters
Human health
Exposure to artificial light at night, especially when it involves shorter wavelength spectral components, can interfere with the regulation of the circadian rhythm and melatonin production.
Therefore, controlling the intensity, operating hours, spectral distribution, and direction of light is increasingly considered in discussions about responsible outdoor lighting.
It is important, however, to avoid generalizations: the health effects depend on factors such as intensity, duration, time of day, spectrum, and individual exposure. Outdoor lighting should be designed to provide only the light necessary for the intended purpose.
Environment and biodiversity
Artificial light at night can alter the behavior of different species, affecting activities such as feeding, reproduction, migration, and orientation.
Insects can be attracted to light sources and remain concentrated around the lights. Migratory birds may experience changes in their orientation patterns, while certain nocturnal species may have their behaviors modified by the presence of artificial lighting.
The spectrum is also relevant: sources with a higher proportion of shorter wavelength components may have different impacts on certain species.
Therefore, in environmentally sensitive areas, the project must consider not only the amount of light, but also its spatial distribution, operating hours, and spectral characteristics.
Astronomy and the preservation of the night sky
Artificial skylights reduce the contrast between celestial objects and the background, hindering astronomical observation.
Studies conducted using citizen observations have indicated a significant increase in the brightness of the night sky in various regions of the world. A study published in 2023 estimated an average increase of approximately 9.6% per year between 2011 and 2022 in the regions analyzed.
For observatories, astronomical sites, and areas designated for sky observation, controlling light emission is especially important.
Energy and economy
Any light directed towards an area that does not need to be illuminated potentially represents energy that is not being used for the project's original purpose.
Controlling light pollution and improving energy efficiency are, in many cases, complementary objectives: using the right amount of light, directing it correctly, and operating the system only when necessary can simultaneously reduce energy consumption, operating costs, and environmental impacts.
What does the technical regulation require?
ABNT NBR 5101:2024 — the Brazilian technical framework
ABNT NBR 5101:2024 — Road lighting — Procedures brought a more comprehensive approach to the impacts of lighting on the night environment, including aspects related to light pollution control, glare, light emission, and color temperature.
An important point is to distinguish between regulatory requirements, design recommendations, and best practices. Not every criterion presented by the standard has the same mandatory character in all situations.
Among the relevant aspects for the specification are:
Correlated color temperature (CCT)
Correlated color temperature (CCT) is one of the parameters considered in street lighting design.
NBR 5101:2024 adopts a more restrictive approach for certain applications, recommending lower color temperatures on roads and, especially, in environmentally relevant areas.
For local roads, the standard recommends 2200 K, while for other roads it recommends 2700 K. In areas of significant environmental importance, 1800 K is recommended, and the TCC (Terminal Composition Charge) should not exceed 2200 K.
Lighting for pedestrian crossings or lanes located in the middle of a block (or not situated at a road intersection) should use light sources with a Tcp higher than the Tcp used on the road. For example, a road with a Tcp of 2700 K should have the pedestrian crossing located in the block (or not situated at a road intersection) illuminated with a source with a Tcp of 3000 K.
These values should be analyzed in conjunction with the road classification, site characteristics, and other project requirements.
It is important not to confuse the NBR 5101 standard with Inmetro's product regulations. Currently, street lighting fixtures are regulated by Inmetro Ordinance No. 62/2022, and Inmetro has been conducting studies to revise these regulations, including on color temperature limits.
Therefore, any specification must take into account the regulations in force on the date of contracting, manufacturing, importing, or marketing of the product.
Control of light emission for the upper hemisphere
One of the most important strategies for reducing light pollution is controlling the amount of light emitted to regions above the horizontal plane.
NBR 5101:2024 uses luminous distribution criteria and the BUG classification — Backlight, Uplight and Glare — to characterize the photometric behavior of luminaires.
In practice, the project should aim for a light distribution compatible with the area to be illuminated, avoiding unnecessary emissions into the sky and into regions that are not part of the area of interest.
In addition to the photometric characteristics of the luminaire, installation is also crucial. A correctly specified luminaire can exhibit undesirable emission if installed at an improper angle.
Glare and intrusive light
Controlling glare is an important part of street lighting design.
NBR 5101:2024 uses the threshold increment (fTI) as a parameter for evaluating glare, with criteria associated with road classification.
Intrusive light should be analyzed considering the surroundings of the installation, especially when the lighting could reach neighboring properties, residential areas, environmentally sensitive locations, or other regions where light is not desired.
In specific situations, international references, such as CIE 150, can be used as a complement to assess intrusive light and obstructive illumination.
CIE 150:2017 — International reference for intrusive light
CIE 150:2017 — Guide on the Limitation of the Effects of Obtrusive Light from Outdoor Lighting Installations is an important international reference for the assessment and control of intrusive light.
The publication uses environmental zones, identified from E1 to E4, which represent different levels of sensitivity to artificial lighting:
Reference table — CIE 150:2017
Zone | Feature | Maximum vertical illuminance | Maximum ULR |
E1 | Dark natural areas | 2 lx | 0% |
E2 | Rural areas with low brightness | 5 lx | 2,5% |
E3 | Urban/suburban areas of medium brightness | 10 lx | 5% |
E4 | High-brightness urban centers | 25 lx | 15% |
After the so-called curfew, the applicable limits may be reduced, according to the environmental zone and the parameter being evaluated.
O ULR — Upward Light Ratio This represents the portion of the installation's luminous flux emitted above the horizon and is one of the parameters used to assess the installation's direct contribution to sky brightness.
The definition of the environmental zone and applicable boundaries should always take into account the specific context of the project.
IES TM-15 and the BUG classification
In luminaire specification, the BUG system — Backlight, Uplight, Glare — combined with the IES TM-15 methodology, allows for the characterization of light distribution in three aspects:
B — Backlight: It characterizes the light emitted to the rear region of the luminaire;
U — Uplight: It characterizes the light emitted to the upper hemisphere;
G — Glare: It characterizes light emission at angles associated with glare potential.
Classifications are expressed by codes such as B0, U0, and G0, among other possible levels.
The lower the classification, the lower the amount of luminous flux present in the corresponding zones.
The BUG classification is therefore an important tool for comparing luminaires and transforming light distribution control objectives into technical specification requirements.
But note: U0 should not be treated as a universal requirement for all outdoor lighting fixtures. The necessary rating depends on the application, design, photometric distribution, and specific site requirements.
DarkSky and the five principles of responsible lighting.
DarkSky International, in conjunction with IES, uses five principles to guide responsible outdoor lighting projects:
- Light should have a clear purpose;
- Light should only be directed where it is needed;
- The light should not be brighter than necessary;
- Light should only be used when necessary;
- Light sources with warmer tones should be used whenever possible.
These principles do not replace Brazilian technical standards or regulations. They serve as an international benchmark for best practices in reducing the impacts of artificial lighting at night.
DarkSky's approval criteria also establish specific technical requirements for different categories of luminaires.
Compliance checklist for project and management.
When specifying or auditing an outdoor lighting system, verify:
- The photometric distribution of the luminaire is suitable for the area that needs to be illuminated;
- The BUG classification is compatible with the application;
- Light emission to the upper hemisphere is controlled;
- The TCC (Technical Competency Certificate) was defined according to the application, the project, and the applicable regulatory requirements;
- In environmentally sensitive areas, specific criteria for color temperature and lighting control were considered;
- The illuminance in adjacent properties meets the criteria applicable to the project;
- o fTI meets the road classification requirements established by NBR 5101:2024;
- The light fixture is installed with the correct orientation and angle;
- Control measures such as dimming, time programming, and sensors were evaluated, where applicable;
- The current Inmetro regulations were taken into consideration for the specified product;
- Where applicable, international references such as CIE 150 and the DarkSky criteria were used to complement the project.
Conclusion
Light pollution has definitively ceased to be merely an environmental concern and has become part of the technical decisions related to outdoor lighting design.
ABNT NBR 5101:2024 establishes a more comprehensive approach to the control of street lighting, while references such as CIE 150:2017, the BUG/IES classification, and the DarkSky principles offer complementary tools to assess and reduce the impacts of artificial lighting.
In practice, a good project aims to provide the right light, in the right quantity, in the right place, and for the necessary amount of time.
This means directing the light appropriately, controlling glare, avoiding unnecessary emissions into the sky and onto neighboring properties, selecting a color temperature compatible with the application, and using controls that allow you to reduce the lighting when it is not needed.
More than just meeting technical requirements, designing in this way means seeking more efficient, comfortable, safe, and responsible public lighting for the nighttime environment.
Do you need to adapt your lighting design to NBR 5101:2024?
Our team designs and adapts public and outdoor lighting systems considering the requirements of NBR 5101:2024, international references, and best practices in responsible lighting.
We analyzed aspects such as photometric distribution, color temperature, glare, intrusive light, emission to the upper hemisphere, and energy efficiency.
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