Introduction to MLO: A crucial tool for the future of lighting design

The Early Days of MLO

Let’s go down memory lane – in the 1970s when light pollution first emerged as a problem. The astronomers discovered that the night sky was deteriorating due to the increased lighting associated with development and growth and termed it light pollution.

Light pollution knows no bounds; the effects of polluting light can be felt up to 200 kilometers (120 miles) away from the source. The discovery of more environmental impacts of lighting led to the emergence of an international “dark sky” association advocating for a precautionary approach to outdoor lighting design.

Then, the International Dark-Sky Association (IDA) and the Illuminating Engineering Society of North America (IES) developed a Model Lighting Ordinance (MLO) to address the need for outdoor lighting regulation in North America.

MLO Explained

The MLO enables communities to significantly reduce light pollution, glare, and excessive light levels. MLO includes adopting five different lighting zones (LZ0, LZ1, LZ2, LZ3, and LZ4), allowing each governing body to modify the stringency of lighting restrictions based on the area’s sensitivity while also accommodating community intent. Therefore, communities can fine-tune the MLO’s impact without having to customize it.

The MLO also includes a Backlight-Uplight Glare (BUG) rating system for luminaires, allowing better control of unwanted light. Today, many cities and towns have passed the Anti-Light Pollution Legislation and MLOs.

Approach and Purpose

The MLO introduces several novel approaches to outdoor lighting regulation, including:

- The use of five lighting zones to classify land use and assign appropriate lighting levels to each class of land use. For instance, LZ0 zones are designed for pristine natural environments with limited outdoor lighting, while LZ4 zones are intended for limited application in areas of extensive development in the largest cities.

- Placing limits on the amount of light that can be used for each property.

- The use of the IES’s TM-15-11 “BUG” (Backlight, Uplight, and Glare) style to classify outdoor lighting fixtures, ensuring that only well-shielded fixtures are used. In addition, no uplight for an area or street lighting is permitted in any zone.

The purpose of MLO is to establish outdoor lighting restrictions that will:

  1. Allow the use of outdoor lighting that does not exceed the minimum levels established in IES-recommended practices for night-time safety, utility, security, productivity, enjoyment, and commerce.

  2. Reduce the negative offsite effects of lighting, such as light trespass and intrusive lighting.

  3. Diminish light pollution, reduce skyglow, and improve the nocturnal astronomical environment.

  4. Assist in protecting the natural environment from the negative impacts of night lights powered by gas or electricity.

  5. Conserve as much energy and resources as possible.

MLO Applied to a Real Restaurant Project

In adherence to the MLO standards, each project is assigned to a certain lighting zone. The lighting zone a project finds itself will determine the requirements it must fulfill for approval. In this case, the project was less than three miles from a Military Region (MLR1), which makes it a Lighting Zone 2 (LZ-2) project. Therefore, the requirements include:

  1. Lights with consistent color temperature

  2. Lights’ color temperature not exceeding 3000K

  3. Fully shielded lights

  4. No light emitted above 90 degrees of the fixture plane

  5. An appropriate amount of light where needed

  6. Glare restrictions in place

Despite being a location where lighting can be a feature to attract customers, the lighting options for a restaurant are limited, considering its classification as a Lighting Zone 2 (LZ-2).

The next step after establishing the lighting zone is to determine lighting compliance. The MLO provides two methods to achieve that: the perspective method and the performance method. Both methods limit the amount of light usable in the location but do not control the exact use or application of the lighting.

Our engineering team used the MLO’s “Performance Method” to determine compliance since it allows greater flexibility and creativity in meeting the intent of the ordinance. The performance method indicates the Initial Site Lumens and the allowances for each project.

In our case, we used three types of allowances:

1. Building entrances or exits: This allowance is per door, and to use this allowance, luminaires must be within 20 feet of the door = 2,000 lumens per door.

2. Building facades: This allowance is lumens per unit area of the building facade illuminated. To use this allowance, luminaires must be aimed at the facade and capable of illuminating it without obstruction = 8 lumens per square foot area.

3. Outdoor dining: This allowance is lumens per unit area for the total illuminated hardscape of outdoor dining. To use this allowance, luminaires must be within two mounting heights of the hardscape area of outdoor dining = 5 lumens per square foot area.

These allowances plus the Initial Site Lumens gave us a total of 205,301.41 Initial Site Lumens.

Project Outcome

Our team of designers selected the appropriate lighting fixtures that complied with standards and fulfilled the initial design by determining the Site Total Initial Site Lumens.

After proposing the luminaires, we analyzed the entire outdoor lighting design for Offsite Impacts, using industry-standard lighting software. Furthermore, we used our advanced photometric analysis software to create a 3D model of the outdoor lighting environment, including luminaire locations, mounting heights, and aiming directions, while employing the photometric data tested per IES guidelines.

Analysis of the design: We used an enclosure of calculation planes with zero reflectance values around the perimeter of the site to obtain the total lumens on the inside surfaces of the box top and vertical sides and the maximum vertical illuminance on the sides of the enclosure.

From the analysis results, the total lumens on the inside surfaces of the virtual enclosure were less than 15% of the total site lumen limit, indicating compliance with the MLO. Also, the maximum vertical illuminance on any vertical surface was less than the allowed maximum illuminance in table 339.04-3 (3.0 lx).

Not every project needs the same amount of light. However, in specific terms, the area where the project is located will determine the MLO lighting zone to follow and, subsequently, its restrictions/requirements. For this project, the allowed lighting amount showed some limitations due to its location - three miles away from a military facility. But thanks to the MLO, we identified the lighting type needed and allowed it in the zone, construction type, and square ft. area. Finally, we proposed adequate lighting fixtures that complied and achieved the ideal lighting levels.


The adoption of the MLO in a single large city could have a significant impact on a region. However, adoption in suburbs and small towns must be part of a regional effort to achieve substantial improvements in the overall quality of the night sky. Adopting agencies should also keep in mind that the MLO, like all modern codes, is meant to evolve over time.

MLO adjustments will be required every few years since the evolution of lighting technology is continuous. Therefore, we strongly advise continuous renewal cycles as part of any adopting ordinance.

We can help you comply with the MLO lighting standards for your project. Contact us at

Model Lighting Ordinance User's Guide