UGR - Unified Glare Rating

Unified Glare Rating (UGR) is a relative number intended to help predict the presence of discomfort glare when viewing a lit environment containing light sources. Higher numbers mean a higher chance to perceive discomfort glare. The UGR formula is derived from a combination of other major glare formulae. UGR is defined in CIE 117.

Since UGR is a glare metric, it is calculated for a specific observer position within a space and is inherently an application metric, not a luminaire level metric. Application specific UGR can be calculated in programs like Dialux, Relux, AGi, and Visual.

In order to provide a consistent way to compare luminaires, the CIE provides a tabular format where UGR values can be presented for typical room dimensions, reflectances, and viewing directions. The rooms are based on the assumption of a single observer position in the center of one wall, 1.2m above the floor. The room aspect ratios are in multiples of “H,” the height of the room above the observer position. The rooms are assumed to be 3.2m tall, so H = 2.0m. Tables are shown for specific Spacing to Height Ratio (SHR) values. Tables for SHR = 1.0 and SHR = 0.25 are commonly used for different standards in different markets. Photopia Reports shows tables at both SHR values.

UGR tables generally show 2 sets of values for viewing the luminaire in 2 orientations. These are often labeled as "Viewed Crosswise and “Viewed Endwise," as suggested in the CIE UGR standards. These terms assume a rectangular shaped luminaire photometered in the "standard" orientation for quadrilaterally symmetric luminaires in Europe. In Europe, it's most common to orient quadrilaterally symmetric luminaires so that the 0-180 deg. plane is across the luminaire's short dimension. In North America it's most common to orient the 0-180 deg. plane along the luminaire's long dimension. Quad symmetry photometry can be done either way, but different regions ended up with different industry conventions. The authors of the CIE standards for UGR were biased toward the European conventions. As a result, this has always been a point of confusion to those following North American conventions trying to understand which plane of their distribution is producing the higher set of UGR values. To avoid this confusion, Photopia’s reports are explicit about the plane of the photometry for each luminaire view direction, “Viewing C0-180” & “Viewing C90-270.” The data is in the same order as tables using the Crosswise/Endwise descriptions, it's just labeled differently.

Viewing Directions

Photopia Reports makes no special accommodations for bilaterally symmetric or fully asymmetric photometry. It simply uses the data provided and shows 2 sets of UGR values. One with the luminaires oriented so that their 0-180 degree plane is parallel to the viewing direction. The other with the luminaires oriented so that their 90-270 degree plane is parallel to the view direction. While it would be ideal to show more luminaire viewing directions in the UGR table for asymmetric luminaires, Photopia Reports does not do this. A point can also be made that UGR tables aren't meaningful for asymmetric luminaires since those types of luminaires are not used in uniform arrays for general lighting in a room. Since Photopia does show tables for 2 views, it's useful to know what is being shown. The C0-180 view direction UGR values depend on whether the strong side of the beam is facing toward or away from the observer position. In Photopia UGR tables, the 0 degree plane is facing away. In other words, the 0 degree horizontal plane is in line with the observer's viewing direction.

UGR table values may vary in different photometric reporting software. There are 2 CIE standards that describe how to compute UGR tables, CIE 190 and CIE 117. CIE 190 is limited to SHR=1.0 and uses a “table based” computational method, with many of the factors used pre-computed in look-up tables. The method is simpler, but strictly limited. CIE 117 is more generalized but is limited in that it only describes the method for computing the direct light contribution to UGR onto the observer position. The indirect/interreflected light contribution and background luminance calculation are not covered by the standard and are left open to any method the developer chooses. This inherently introduces variability in UGR tables from various photometric reporting software. There is no single “standard” reference for UGR values since no standard exists that fully defines the complete computation of the values. UGR values from various reporting software generally show similar trends, but the absolute values can vary by as much as 1.0 or more in some cases.

LTI Optics worked with Kenton Wong at Fluxwerx Illumination, a Division of LMPG Inc., to complete a review of 19 different IES files for various distributions and luminous shapes, and have presented the computed UGRs at 4H / 8H, 70/50/20 for each of them.

There are files which have strong agreement between programs and others that vary more widely, but no consistent trends based on anything we could identify. Generally SHR = 0.25 is more consistent than SHR = 1.0 which may be because several programs don't compute SHR = 1.0 directly but instead rely on a single correction factor for the entire table.

SHR = 0.25

SHR 0.25	Dialux	Relux	Toolbox*	Photoview	Photopia	Avg	Range
File 1	22.6	22.6		22.7	22.7	22.7	0.1
File 2	NA**	NA**		12.3	12	12.2	0.3
File 3	NA**	NA**		18.3	18.2	18.3	0.1
File 4	NA**	NA**		15.7	15.5	15.6	0.2
File 5	16.5	16.3		16.6	16.3	16.4	0.3
File 6	22.8	22.7		23.3	22.6	22.9	0.7
File 7	NA**	NA**		21.9	23.1	22.5	1.2
File 8	21.9	22.2		22.7	21.9	22.2	0.8
File 9	24	23.9		24.4	23.9	24.1	0.5
File 10	17.3	17.6		17.4	17.6	17.5	0.3
File 11	11	11		11.1	11	11.0	0.1
File 12	NA**	NA**		9.8	9.7	9.8	0.1
File 13	NA**	NA**		16.3	16.3	16.3	0
File 14	14.2	14.2		14.2	14.3	14.2	0.1
File 15	15.9	16.2		16	16.2	16.1	0.3
File 16	20.1	20.2		20.2	20.3	20.2	0.2
File 17	19.5	19.6		19.6	19.8	19.6	0.3
File 18	12	11.5		12.2	11.8	11.9	0.7
File 19	19.1	18.6		19.2	18.9	19.0	0.6

* Photometric Toolbox doesn't compute UGR for SHR values of 0.25

** Dialux and Relux don't compute UGR for asymmetric distributions, and how other software handles these is unknown. They may be averaged, or their full distribution may be used.

SHR = 1.0

	Dialux	Relux	Toolbox***	Photoview	Photopia	Avg	Range
File 1	22.1	23.2		20.5	22.3	22.0	2.7
File 2	NA**	NA**	9.2	9.2	11.4	9.9	2.2
File 3	NA**	NA**	14.7	15.7	17.9	16.1	3.2
File 4	NA**	NA**	13.5	13.4	15.5	14.1	2.1
File 5	16.4	14.6	14.7	14.4	16.3	15.3	2
File 6	22.6	18	20.4	21.9	22.9	21.2	4.9
File 7	NA**	NA**	20.1	19	24	21.0	5
File 8	21.1	18.7	20.4	20.6	22.6	20.7	3.9
File 9	23.8	19.1	21.3	23.1	24.1	22.3	5
File 10	16.8	17.6	16	12.8	17	16.0	4.8
File 11	9.9	11	8.8	2.2	9.7	8.3	8.8
File 12	NA**	NA**	7.1	-0.4	8.8	5.2	9.2
File 13	NA**	NA**	11.3	11.9	12.9	12.0	1.6
File 14	11.1	14.2		8.8	10.6	11.2	5.4
File 15	15.5	16.2		11	15.9	14.7	5.2
File 16	20.1	20.2		16.9	17.6	18.7	3.3
File 17	17.3	19.6	15.9	16.2	17	17.2	3.7
File 18	11.9	11.8	8.7	8.6	11.8	10.6	3.3
File 19	19	18.7		17	15.8	17.6	3.2

** Dialux and Relux don't compute UGR for asymmetric distributions, and how other software handles these is unknown. They may be averaged, or their full distribution may be used.

*** Photometric Toolbox values were not obtained for some files.