Making Sense of the IES Ratings Systems—“Cutoff” versus “TM-15-07 / BUG”

Light emitted at “high” angles immediately below the horizontal plane of a luminaire can cause glare among street-level onlookers, such as pedestrians or motorists.  Light emitted above the luminaire is undesirable because it does not illuminate a targeted area on a roadway, creates light pollution problems, and is believed to harm wildlife and the adjacent environment.  

For years, the Illuminating Engineering Society of North America (IESNA) used the “cutoff” classification system—full cutoff, cutoff, semi-cutoff, non-cutoff—to rate the amount of light emitted from a luminaire in unwanted directions.  However, in 2005, IESNA began its efforts to replace the cutoff classification system, ultimately resulting in the new TM-15-07 methods applicable to rating outdoor luminaires.  The resulting BUG (Backlight, Uplight, Glare) rating methodology represents a comprehensive system that limits lamp lumens to values appropriate for the lighting zone.  For more information, download the BUG specification bulletin here.

In summary, while the outdated “cutoff” classification may be applied to EcoFit’s products (and other competing outdoor LED products), lighting designers, specifiers, engineers and operators are rapidly recognizing and adopting the new BUG ratings system and TM-15-07 standards to evaluate LED luminaires.

To determine the BUG rating, the flux distribution by solid angle reporting per IES TM-15-07 measurement procedures are compared to published BUG ratings criteria provided in the specification bulletin. The sphere surrounding a luminaire is divided into zones:

Backlight, which creates light trespass onto adjacent sites. The B rating takes into account the amount of light in the BL, BM, BH and BVH zones, which are in the direction of the luminaire OPPOSITE from the area intended to be lighted.

Uplight, which causes artificial sky glow. The U rating takes into account the amount of light into the upper hemisphere with greater concern for the lower uplight angles in UL. Lower uplight (zone UL) causes the most sky glow and negatively affects professional and academic astronomy. Upper uplight (UH) is mostly energy waste.

Glare, which can be annoying or visually disabling. The G rating takes into account the amount of frontlight in the FH and FVH zones as well as BH and BVH zones.

EcoFit’s DuraStreet Series luminaires exhibit one of the lowest (i.e., most restrictive) BUG ratings possible.  The best BUG rating is B0,U0,G0, with the worst being B5, U5, G5:

 

	Drive Current	Distribution	BUG Rating
DuraStreet 30 Series	350mA	2/3	B1-U1-G1
	350mA	5	B1-U0-G1
	525mA	2/3	B1-U1-G1
	525mA	5	B1-U0-G1
DuraStreet 42 Series	350mA	2/3	B1-U1-G1
	350mA	5	B2-U1-G2
	525mA	2/3	B1-U1-G1
	525mA	5	B2-U1-G2
DuraStreet 63 Series	350mA	2/3	B1-U1-G1
	350mA	5	B2-U1-G2
	525mA	2/3	B1-U1-G1
	525mA	5	B2-U1-G2