Introduction

Outdoor lighting at night is indispensable and critically important. For wildlife-sensitive habitats or residential areas, lighting requirements are particularly stringent. The new DLC SSL Technical Requirements Version 5.1 mandates BUG ratings for outdoor luminaires in SSL products. BUG ratings should be generated according to IES TM-15-11 and Addendum A for IES TM-15-11 using the photometric data of luminaires. For example, a top post light (More ZGSM post top lights) with a BUG rating of BxU1Gx indicates upward light emission—strictly prohibited by the International Dark-Sky Association (IDA)—which contributes to sky glow pollution. Similarly, a streetlight with a BUG rating of B3UxGx indicates it emits over 2500 lumens of light behind the fixture, which will affect residential areas adjacent to the road to varying degrees. Proceed to the main text to better understand the definition of BUG rating, its rating criteria and methodology, and its guiding significance.

What’s BUG rating?

BUG Rating, also known as the Backlight Uplight Glare Rating, is a system for evaluating and specifying the optical performance of outdoor lighting fixtures. Developed by the Illuminating Engineering Society (IES), this rating measures how a fixture’s backlight, uplight, and glare characteristics impact its overall performance and efficiency. B stands for Backlight, referring to light emitted toward the rear of the fixture (away from the road). U denotes uplight, representing light leakage upward that contributes to light pollution (How to reduce light pollution of street lights?). G signifies glare, indicating the proportion of light emitted in relatively distant directions (more illumination in very high or high secondary solid angles, either forward or backward). This includes illumination of buildings, glare into drivers’ eyes, and light spilling onto road peripheries—phenomena often considered undesirable and frequently causing glare. These ratings are based on zonal lumen calculations for secondary solid angles defined in TM-15-11. The zonal lumen thresholds listed in the three tables are based on data from photometric testing procedures approved by the Illuminating Engineering Society. Using data from these approved procedures, we can determine whether zonal luminous flux exceeds corresponding thresholds to confirm the BUG rating. In the next section, there are tables detailing the luminous flux thresholds for B rating, U rating, and G rating.

Three tables about BUG rating

Table for backlight rating (The secondary solid angles labeled BH, BM, and BL, as shown in the figure)

Backlight rating measures light projected behind the light source and potentially beyond the intended illumination area, which may intrude onto adjacent properties. As shown, this light shines toward the back of the fixture, generally away from the target area. When illuminating motor vehicle lanes, such light is typically deemed useless to drivers. Roads or paths near residential communities also often require minimizing backlight, as this light can shine through windows into rooms. This is not only useless but also disruptive to residents, and such light is sometimes referred to as light trespass (What’s light trespass?). When calculating the B rating, light flux within the “backlight low zone,” “backlight medium zone,” and “backlight high zone” areas is categorized. Below is the corresponding table for the Backlight rating. Observant readers may note that the B rating does not involve light within the BVH secondary solid angle.

Table for uplight rating (The secondary solid angles labeled as UH and UL, as shown in the figure)

The uplight rating assesses light emitted at angles exceeding 90 degrees, as illustrated in the uplight high and uplight low (UH and UL) zones. When your city has numerous uplights, it creates sky glow, which is the primary reason you cannot see stars within urban areas. Conversely, you can observe clear road layouts and city nightscapes from an aeroplane. In most cases, LED luminaires (such as street lamps and garden lights) do not emit upward light. However, when luminaires are installed with an upward tilt angle (e.g., the tilt angle of street lights), they also produce upward illumination. Darksky (More about Darksky) therefore requires that uplight ratings be assessed based on the fixture’s actual installation conditions, rather than solely relying on photometric test reports. When calculating the uplight rating, the luminous flux within the ‘uplight high’ and ‘uplight low’ zones is categorised. The table below corresponds to the uplight rating.

Table for glare rating (The secondary solid angles labeled as BVH, BH, FVH and FH, as shown in the figure)

Glare is a condition that must be strictly avoided in good lighting design. The BUG rating identifies forward light (front light) very high, forward light high, backlight very high, and backlight high secondary solid angle as the primary causes of glare. The analysis indicates that light emitted in these directions travels long distances and enters the eyes of pedestrians, users, or drivers. A relatable example: When you turn on your dining room light at home, the bulb is usually enclosed by a shade, so it doesn’t feel blinding. But if the shade doesn’t fully cover the bulb, looking directly at the light source feels extremely harsh. A higher G rating means more light is directed toward your eyes—the more light, the more blinding it feels (similar to staring directly at the bulb in your dining room light). When calculating the G rating, luminous flux within the FVH, FH, BVH, and BH zones is categorized. Below is the corresponding table for glare ratings. Note that different tables apply for luminaires with symmetrical and asymmetrical light distribution. For traditional symmetrical floodlights, refer to the BH threshold value for symmetrical distribution. For asymmetrical floodlights, use the BH threshold value for asymmetrical distribution (What’s asymmetrical distribution?).

How to define the BUG rating?

Below is the photometric test report for the ZGSM Alca series 40W (ZGSM new street light – Alca series).  In the luminaire flux distribution table, we can see the luminous flux within each zone (secondary solid angles). For the B rating, as we can see that the lumens in backlight high is 417.12lm which is lower than that of B1 rating. Then we can also check the lumens of backlight medium and backlight low, and they are all lower than B2 rating but higher than B1 rating. Then we determine the lowest rating is B2 in BM and BL zone and we finally get the result that the B rating is B2. For the U rating, it’s obviously U0 since they are 0 lumens in both UL and UH zone. For the G rating, we can see lumens in FVH zone is 14.224lm, lumens in FH is 918.76lm, lumens in BVH is 7.9125lm and lumens in BH is 417.12lm. And as we check they are lower than lumen limit of G1 so glare rating is G1. So the BUG rating of ZGSM-ST23-40S is B2-U0-G1.

Where to find the BUG rating?

For outdoor LED luminaires such as streetlights, post top lights, and floodlights (ZGSM flood lights), the BUG rating can be obtained through three core data sources. The first is the LM79 report, which primarily tests the photometric parameters of LED products. Beyond electrical parameters, color coordinates, and CRI, it also contains crucial information on the luminaire’s light distribution—with the BUG rating specifically covered within the light distribution testing. Through testing, we can determine the luminaire’s backlight, upward light, and glare performance—key criteria for outdoor application selection. The second is the photometric test report, which is actually a mandatory test for the LM79 report but lacks electrical parameters and light quality metrics. The third is the IES file, primarily used for lighting simulation. However, remember that the BUG rating cannot be directly viewed within lighting simulations. If you only possess an IES file, specialized software is required to open it and view the BUG rating. If you lack such software, contact the supplier to obtain the LM79 or photometric test report corresponding to that IES file. This will reveal the BUG rating, aiding in IES selection and optimizing lighting simulations. Click below photos to find more about ZGSM lighting design solutions.

How to get the ideal BUG rating for your project?

First, the smaller the better? Yes, generally speaking, the lower the corresponding value, the better. The U rating has no exceptions, but for backlight and glare ratings, lighting simulations may necessitate case-by-case selection. For instance, in road lighting, to meet Rei requirements, a certain level of backlight is needed to adequately illuminate both sides of the road. This enables drivers to clearly see pedestrians or animals on either side, preventing accidents caused by their sudden appearance. Similarly, to maximize coverage of longer or wider road sections from a single lamppost, forward light in FH or even FVH may require slightly higher values. This ensures uniform illumination (What’s lighting uniformity and how to improve it?) across different lanes while preventing the emergence of zebra crossings or dark spots. Although the G value may be elevated in such cases, ZGSM maintains that the final illumination (or illumination simulation) Ti can still meet design requirements. If luminaires require a lower backlight rating, ZGSM offers corresponding solutions. As illustrated in the lower left diagram, the B rating is initially B2; however, with the installation of specialized lenses, the B rating is upgraded to B1.

As we know, typically higher wattage correlates with higher B, U, and G values. Understanding this, we recognize that pursuing low BUG values for high-wattage luminaires is often impractical. Take street lighting as an example: the ideal BH, BM, and BL ratios range from 8-10%, with lower values being virtually unattainable. For equivalent lumens, ZGSM maintains that a lower B value is preferable (specifically for expressway illumination). The same principles apply to glare ratings, hence ZGSM’s stance remains aligned with meeting Ti or GR requirements. For instance, in sports field illumination, asymmetrical distribution luminaires may exhibit higher G ratings yet deliver lower glare in practical applications (Click to find how to reduce glare in sports lights).

Summary

This article provides an in-depth analysis of the critical BUG rating system in outdoor lighting. Developed by the Illuminating Engineering Society, this system scientifically evaluates three key optical characteristics of luminaires: backlight, upward light, and glare. Specifically: – “B” denotes light emitted toward the rear of the luminaire, which may intrude onto adjacent properties or cause nuisance to residents. – ‘U’ refers to light directed toward the sky, contributing to light pollution and sky glow. – “G” signifies light emitted within specific high-angle zones, posing a direct glare hazard to pedestrians and drivers. The article elaborates on its rating criteria, which are determined based on the secondary solid angle partitioned luminous flux thresholds defined by IES TM-15-11. Using concrete examples from photometric test reports, it demonstrates how to step-by-step calculate and derive the final rating, such as B2-U0-G1. The article aims to clarify that understanding BUG ratings guides us toward selecting luminaires with more precise optical performance that reduce light pollution and glare. For instance, comparing the B values of two luminaires with identical wattage/lumens (Watts vs lumens) allows us to choose streetlights with lower B values for expressways, as most of their light is concentrated over the roadway. Compared to the B value, the glare rating is more complex. ZGSM recommends using lighting simulations as the standard, balancing glare considerations while meeting illuminance and uniformity requirements. For further information, please contact us to advance the realization of more energy-efficient, people-friendly, and environmentally conscious outdoor nighttime lighting.

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