What Does BUG Rating Mean?

 

To the untrained ear, hearing the words “BUG rating” may bring to mind swarms of insects or bad reality TV. However, the BUG rating has nothing to do with either, and everything to do with lighting. BUG is simply an acronym coined by the Illuminating Engineering Society (IES) and the International Dark Sky Association to better explain how light trespass can be measured. The BUG rating of a luminaire determines how much light trespass that a light fixture produces. The BUG rating replaced the old measuring system known as the “cutoff system” around 2005 and is more comprehensive, taking Backlight, Uplight, and Glare into account (the B, U, and G, of BUG).

  • Backlight – This category takes into account the light that is spilled from behind the fixture into areas where it is unwanted. This isthe opposite area to the area where light is intended to be. Backlight is mostly a problem regarding light trespass on adjacent sites and areas.
  • Uplight – Uplight is the resulting light spill above the top of the fixture. Uplight contributes greatly to light pollution, sky glow, and is generally not “dark-sky friendly.” Minimizing uplight in commercial lighting fixtures can make the stars more visible at night.
  • Glare – Have you ever driven past a really bright streetlight that almost seemed to blind you for a moment? That’s glare. Light glare is the amount of front light in the forward zones but happens when the light is too strong or concentrated. Glare is a safety issue as well as a light trespass issue near adjacent properties.

 

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Why Does it Matter?

BUG rating is a system that allows luminaires with photometric data to be measured. It works in tandem with the International Dark Sky Association’s light zones, which are accepted levels of light, or light limitations, in certain outdoor areas. The following light zones represent all the possible degrees of ambient light in an outdoor area, ranging from complete darkness (LZ0) to very bright municipal areas (LZ4). 

 

  • LZ0: No Ambient Lighting 
  • LZ1: Low Ambient Lighting 
  • LZ2: Moderate Ambient Lighting

    A diagram depicting backlight, uplight, and glare zones of a light fixture.

  • LZ3: Moderately High Ambient Lighting
  • LZ4: High Ambient Lighting

 

Different luminaires have different BUG ratings. Obviously, the BUG rating for an outdoor luminaire in a LZ0 no ambient lighting zone will need to be much lower than that of a fixture in a LZ4 high ambient lighting zone. BUG rating and light zone requirements work together to ensure that local and municipal codes are met when installing outdoor light fixtures. Certain levels of glare are more permissible in some locations over others. 

Overall, the lower the BUG rating, the fewer light trespass problems the fixture will cause. It is best to aim for the least amount of light trespass possible while still maintaining the required amount of footcandles for your project. BUG rating can be a helpful determinant in both commercial and residential outdoor lighting projects to reduce light trespass and uplight glow.

In order to determine the BUG rating of your outdoor lighting project, request a photometric analysis from Access Fixtures. Contact your local building code department to find out what the light trespass requirements are in your jurisdiction and we can design a lighting plan to fit those parameters.