Classifying Luminaire Cutoff and Glare Metrics

The transition from traditional luminaire cutoff classifications to the modern Backlight, Uplight, and Glare (BUG) rating system marks a significant evolution in exterior lighting design. For decades, the industry relied on the simple classifications of full cutoff, cutoff, semi-cutoff, and non-cutoff to evaluate how much light a fixture emitted above certain vertical angles. However, as the focus on precise light control, dark sky preservation, and the mitigation of light trespass has intensified, the limitations of the older system became apparent.

This article provides a deep dive into the BUG rating system as defined by ANSI/IES TM-15-20 (Luminaire Classification System for Outdoor Luminaires). It explores the specific mechanics of the system, the rationale for moving away from legacy cutoff metrics, and how to specify modern LED luminaires for optimal glare control and dark sky compliance.

The shift to LED technology fundamentally altered the way fixtures are manufactured and specified. With older High-Intensity Discharge (HID) sources, the lamp was a separate component inserted into a reflector housing. Photometrics were heavily dependent on the positioning of the lamp within that reflector. LEDs, however, integrate the light source and the optical control (the lens) directly on the circuit board. This unified approach demands a more granular, precise method of evaluating luminous intensity distributions.

Understanding the BUG system is no longer optional for lighting practitioners; it is a fundamental requirement for designing compliant, efficient, and visually comfortable exterior environments. Whether specifying a sprawling retail parking lot, a specialized sports complex, or a subtle pedestrian pathway, mastering these metrics ensures the precise application of light while protecting neighboring properties and preserving the natural nocturnal ecosystem.

The Limitations of Traditional Cutoff Classifications

The legacy cutoff classification system was primarily concerned with restricting light emitted at high angles (typically above 80 degrees and 90 degrees vertical). It evaluated the total luminous flux directed upward into the night sky, which is a key contributor to sky glow. The categories were defined by the luminous intensity in candelas per 1,000 lamp lumens:

• Full Cutoff: Zero candela at or above 90 degrees vertical. The candela per 1,000 lamp lumens does not exceed 100 (10%) at or above a vertical angle of 80 degrees.
• Cutoff: The candela per 1,000 lamp lumens does not numerically exceed 25 (2.5%) at or above an angle of 90 degrees vertical, and 100 (10%) at or above 80 degrees vertical.
• Semi-Cutoff: The candela per 1,000 lamp lumens does not numerically exceed 50 (5.0%) at or above an angle of 90 degrees vertical, and 200 (20%) at or above 80 degrees vertical.
• Non-Cutoff: No restrictions on the light distribution at any angle.

While simple to understand, this system had critical flaws. First, it only looked at the total amount of light pushing upwards; it did not differentiate between light pushing forward (toward the street) and light pushing backward (toward a property line or residential window). A fixture could qualify as “Full Cutoff” while still blasting intense, uncontrolled light into a neighboring bedroom, creating severe light trespass.

Second, the old system was tied to the concept of “lamp lumens.” With the advent of integrated LED luminaires—where the diodes and optics are a single sealed unit—the concept of a separate “lamp” with its own lumen rating became obsolete. The industry needed a metric based on absolute luminaire output, evaluated within specific spatial zones.

Furthermore, the traditional system struggled to account for asymmetrical distributions. As lighting designers pushed for highly customized photometric footprints to meet strict uniformity ratios while minimizing pole counts, manufacturers developed complex optics. The old cutoff metrics were simply too blunt an instrument to accurately characterize these highly engineered optical distributions, leading to unpredictable results in the field.

Enter the BUG Rating System (ANSI/IES TM-15-20)

To address these deficiencies, the Illuminating Engineering Society (IES) developed TM-15, establishing the Luminaire Classification System (LCS) and the resulting BUG rating. The system evaluates the absolute lumens emitted by a luminaire into a highly specific 3D geometry of solid angles (zones) surrounding the fixture.

Instead of just looking “up,” the BUG system evaluates light distribution across three critical areas of concern:

1. Backlight (B)

Backlight evaluates the light directed behind the luminaire, typically toward the “house side” or a property line. It is the primary metric used to control light trespass. The B rating calculates the total lumens falling into four specific zones behind the pole:

• Backlight Low (BL): 0 to 30 degrees vertical.
• Backlight Mid (BM): 30 to 60 degrees vertical.
• Backlight High (BH): 60 to 80 degrees vertical.
• Backlight Very High (BVH): 80 to 90 degrees vertical.

Controlling backlight is essential when fixtures are mounted near the perimeter of a site. By utilizing specific LED optics (such as a Type II or Type III distribution with integrated shielding), manufacturers can sharply cut the light behind the pole, achieving a low B rating (e.g., B1 or B0) and preventing illumination from spilling onto adjacent parcels.

2. Uplight (U)

Uplight evaluates the light directed above the horizontal plane (90 degrees and higher). This is the metric responsible for mitigating sky glow and protecting the natural nighttime environment. It is evaluated in two zones:

• Uplight Low (UL): 90 to 100 degrees vertical.
• Uplight High (UH): 100 to 180 degrees vertical.

The U rating is critical for dark sky compliance. A U0 rating guarantees absolute zero light emission above the 90-degree plane, ensuring maximum protection for astronomical observation and nocturnal wildlife. It is the most rigorous standard for environmental lighting design.

3. Glare (G)

Glare evaluates the light emitted at high angles, specifically in the zones where it is most likely to directly intersect with the human eye and cause disability glare or visual discomfort. It evaluates light in both the forward and backward directions at high angles:

• Forward Light High (FH): 60 to 80 degrees vertical.
• Forward Light Very High (FVH): 80 to 90 degrees vertical.
• Backlight High (BH): 60 to 80 degrees vertical.
• Backlight Very High (BVH): 80 to 90 degrees vertical.

Notice the overlap: The BH and BVH zones are evaluated for both Backlight and Glare. This reflects the reality that light pushing backward at high angles is simultaneously a trespass issue and a glare issue. High glare ratings can significantly impair a driver’s vision, making strict G rating specifications vital for roadway and high-speed traffic applications.

Calculating the Final BUG Rating

To determine the final BUG rating (e.g., B2-U0-G1), software calculates the absolute lumens falling into each of the specific zones outlined above. ANSI/IES TM-15-20 provides a complex reference table. The total lumens in each specific zone are compared against absolute maximum limits.

The luminaire receives a sub-rating (0 through 5) for each zone based on the highest threshold it crosses. The final B, U, and G ratings are simply the highest sub-rating achieved across all the zones within that category. A rating of 0 represents the strictest control (lowest output in those zones), while a rating of 5 represents the least control.

This zone-by-zone analysis provides a highly detailed profile of the fixture’s performance. Unlike the older cutoff system, which provided a single, overarching categorization, the BUG system delivers granular data that empowers specifiers to make highly informed decisions based on precise site conditions and stringent municipal ordinances.

Standard Lighting Zones and Typical BUG Ratings

The IDA/IES Model Lighting Ordinance (MLO) integrates the BUG system by establishing Lighting Zones (LZ), which dictate the maximum allowable BUG ratings for a given area.

Lighting Zone	Description	Typical Applications	Common BUG Requirement
LZ0	No Ambient Lighting	National Parks, wilderness, pristine coastal habitats	B1-U0-G0
LZ1	Low Ambient Lighting	Rural areas, residential neighborhoods, neighborhood parks	B1-U0-G1
LZ2	Moderate Ambient Lighting	Light commercial, mixed-use zoning, church parking lots	B2-U0-G2
LZ3	Moderately High Ambient Lighting	Large commercial districts, industrial areas, shopping centers	B3-U0-G3
LZ4	High Ambient Lighting	Dense urban centers, entertainment districts	B4-U1-G4

The Role of Optical Distributions

The resulting BUG rating of a luminaire is inextricably linked to its optical distribution type (Type I through Type V).

Asymmetric, forward-throw optics (like Type III or Type IV) are engineered specifically to push light forward while minimizing backward spill. Consequently, a high-quality Type III LED shoebox will typically achieve a very low Backlight rating (e.g., B1). This makes them ideal for perimeter mounting.

Conversely, a Type V distribution is completely symmetrical, pushing light equally in 360 degrees. Therefore, by definition, half of the light is considered “Backlight.” A Type V fixture will naturally have a much higher B rating (e.g., B4 or B5). This does not indicate a poor fixture; it simply dictates that a Type V fixture must be placed in the center of an area, far from sensitive property lines.

Understanding the synergy between the IES distribution type and the resulting BUG rating is critical. Specifying a strict B1 rating while simultaneously demanding a Type V distribution represents a fundamental contradiction in photometric geometry and will inevitably lead to project delays and costly revisions during the submittal phase.

DarkSky International Certification

DarkSky International (formerly the International Dark-Sky Association, IDA) heavily relies on the principles of the BUG system for their Fixture Seal of Approval and DarkSky Approved programs. To earn DarkSky certification, exterior luminaires must strictly limit both the spectrum of the light (minimizing blue light content, typically capping CCT at 3000K or lower) and the physical distribution of the light.

A fundamental requirement for DarkSky approval is that the luminaire emits absolutely zero uplight. In the context of the BUG system, this mandates a strict U0 rating. By completely eliminating direct upward emission, these certified fixtures drastically reduce sky glow, protecting nocturnal ecosystems and preserving the visibility of the night sky.

The DarkSky Approved program goes further, ensuring that the luminaires provide objective, third-party certification for products, designs, and completed projects that minimize glare, reduce light trespass, and don’t pollute the night sky. High-quality outdoor lighting protects and restores the natural night environment by controlling light pollution effectively. The certification processes review the technical specifications, photometric files, and physical designs of the fixtures to confirm compliance with these rigorous standards.

Best Practices for Specifying Glare Control

When specifying exterior lighting using the BUG system, consider the following best practices:

1. Do not use traditional cutoff terminology: Remove “Full Cutoff” from specification language and master standard details. Replace it with explicit BUG rating requirements tailored to the specific application (e.g., “Luminaire shall possess a maximum BUG rating of B2-U0-G2”). Continuing to use legacy terminology creates legal ambiguity and technical confusion.
2. Target U0 whenever possible: For the vast majority of area and roadway lighting, zero uplight (U0) should be the absolute standard to minimize sky glow. Architectural uplighting should be restricted to highly specific, controlled applications, and strictly managed via automated dimming and shut-off controls.
3. Contextualize Backlight: Evaluate the B rating based on the pole’s proximity to a property line. A B3 rating is perfectly acceptable in the center of a massive commercial parking lot, but unacceptable 10 feet from a residential property boundary. A rigid “one-size-fits-all” BUG specification across an entire large site often leads to over-designed, inefficient layouts.
4. Utilize House-Side Shields only when necessary: While the BUG system evaluates the native performance of the optics, aftermarket shields can be installed to further reduce the B rating. However, these shields reduce overall efficacy and should only be specified as a mitigation tactic for exceptionally tight boundaries. Designing the layout using the native optics is always preferable to bolting on aftermarket modifications.
5. Coordinate with CCT and Output: Remember that the BUG rating solely evaluates distribution, not spectral content or absolute intensity. A highly controlled U0 fixture can still be ecologically damaging if the CCT is 5000K. Always pair strict BUG requirements with appropriate CCT limits (3000K maximum) and precise lumen output targets.

By mastering the nuances of the BUG rating system, lighting professionals can design exterior environments that are safe, visually comfortable, and highly respectful of neighboring properties and the night sky. The precision offered by ANSI/IES TM-15-20 represents a significant advancement in the pursuit of responsible, sustainable illumination.

Related Resources

• Point-by-Point Lighting Calculations: A Technical Designer’s Guide
• Cosine Law of Illuminance Explained
• Inverse Square Law in Lighting

Frequently Asked Questions

What replaced the traditional cutoff classification system for luminaires?

The traditional full cutoff, cutoff, semi-cutoff, and non-cutoff classifications were replaced by the ANSI/IES TM-15-20 Luminaire Classification System, which uses the BUG (Backlight, Uplight, Glare) rating.

How does the BUG rating evaluate light trespass?

Light trespass is primarily evaluated using the Backlight (B) rating, which measures absolute lumens directed behind the luminaire into four specific vertical zones.

What BUG rating is required for a fixture to have zero uplight?

A fixture must achieve an Uplight rating of U0 to guarantee zero direct uplight, which is a common requirement for dark sky preservation and DarkSky International certification.

Does a Type V optic have a low Backlight rating?

No, a Type V optic has a 360-degree symmetrical distribution, meaning half of its output is considered backlight. It will naturally have a high B rating (e.g., B4 or B5).