By Robert Hawkins

Hi, it’s Robert Hawkins, TLA Applications Engineering Manager and resident lumen counter, and I’m here to talk about Unified Glare Rating (UGR). Read on for what you need to know about this important but highly misunderstood lighting metric.

What is UGR?

UGR is a measure of potential discomfort glare experienced by an occupant in interior, electrically lit spaces. High-angle glare can cause occupants to continually need to adjust their eyes, which can contribute to headaches, fatigue, and other physiological issues.

UGR is defined by document CIE 117-1995, Discomfort Glare in Interior Lighting, and clarified in terms of the tabular display in CIE 190:2010, Calculation and Presentation of Unified Glare Rating Tables for Indoor Lighting Luminaires.

Why is this becoming important to the lighting community? A couple of organizations are referencing UGR values in their documents as criteria for achieving certain goals. The WELL Building Standard and the DLC (DesignLights Consortium) both reference UGR values in their documents.

A lot of what you’ll see below is pulled from a NEMA document (NEMA LS 20001-2021 white paper on UGR), with many references to how UGR may be being misused and misunderstood, including:

CIE 190 is one of the most referenced and misunderstood documents in recent publications. This document was created specifically to support the requirements of ISO 8995-1 (this is similar to the IES Illuminance Selection Procedure) but has recently been used to evaluate luminaires without consideration of the actual application.

All luminaires are evaluated using a single room/luminaire layout from the CIE 190 tabular method, resulting in a UGR_LUM value.

While it may be appealing because of it’s simplicity, UGR_LUM has extremely limited value because it does not include application factors and can be applied only as a relative comparison in one specific situation.

Defining UGR

UGR has three different definitions:

1. UGR_{Appl :} Application UGR is the value obtained by full use of the International Commission on Illumination (CIE) method on the particular application (room shape, luminaire layout, luminaire parameters, tasks). (UGR_Appl includes correct “averaging” of the relevant UGR_Point values – see below.)
2. UGR_LUM : Luminaire UGR is the use of UGR to evaluate/compare luminaires in a single defined application (room shape, luminaire layout) when the luminaire may actually be used in many different applications.
3. UGR_Point : Point UGR is UGR calculated at a single point in a room – by simulation software, for instance.

Both UGR_Point  and UGR_LUM  are inappropriate uses of UGR for luminaire evaluation, as will be detailed below. UGR_LUM does not consider the application and UGR_Point  is subject to error because no averaging is done.

How is UGR calculated?

Unified Glare Rating can be calculated in the following way:

 UGR = 8 log [0.25/L_b *  L_S ^2*ω/ρ²] that factors in*:

1. Background luminance (cd/m²)
2. Luminance of the apertures of each luminaire in the direction of the observer’s eye (cd/m²)
3. Solid angle of the luminous parts of each luminaire at the observer’s eye (sr)
4. Displacement of luminaire from line of sight.

 *1 is L_b, 2 is L_S 3 is ω, and 4 is ρ, the Guth position index of fixture to viewer.

CIE 117 does limit the use of the formula. UGR is for indoor lighting only, uniform background Luminance, not for small light sources (<.0003 steradian or point sources), not for large light sources (>0.1 steradian) and not for indirect lighting.

SO, UGR is not a single value for a fixture, but a value that changes depending on your total environment and observer position. Individual points of UGR can be calculated in lighting applications programs, which then consider the actual room conditions and layout, and observer position.

The table of UGR values generated by using utilities and the fixture ies file, are for a specific room size and fixture layout. Deviation from this then results in differences in real application UGR vs pulling a max value from a table.

The tables follow these assumptions:

1. Luminaire mounting height above observer’s eye (H) = 2m
2. Observer’s eye is at 1.2m.
3. Luminaire spacing: 1H, or 2m on center.
4. Observer’s position is centered on back and side walls, eyes towards the midpoint of wall opposite.
5. Room dimensions expressed in terms of mounting height (H).

The discussion above is based on CIE 117. CIE 190 limited the parameters for the UGR_Appl calculation to facilitate comparison of luminaires.

H was fixed at 2m and S was fixed at 1.0H
The height of the observer’s eye above the floor is 1.2 m.

DesignLights Consortium

DesignLights Consortium (DLC) technical requirements v5.1 establishes individual luminaire UGRlum values in four “general Applications”: troffers, linear ambient, low bay, and high bay indoor fixtures. No consideration is given for actual use, the definition is based on single room shape, 4H x 8H with 70.50.20 reflectances and S/H =1

We can use the fixture below as an example.

Here we have an actual light fixture with the Table generated by file utility program. We’ve highlighted the values used by DLC in the table. Notice the max UGRlum is different and occurs for a room with really dark reflectances. (30/30/20). This is, however, not likely to be the case, so max UGR is likely not a value to utilize to pick fixtures.

EPANL Flat Panel
Highly Configurable LED Flat Panel
By Lithonia Lighting

ISF 192813P10 Visual Photometric Tool

Max UGR is 19.8
DLC UGR would be 17.2

WELL Building Standard

The WELL Building Standard under Light L04 Electric Light Glare Control has this:

Choose between the following:

Option 1: Luminaire considerations

Each luminaire meets one of the following requirements for regularly occupied spaces at light output representative of regular use conditions. Wall wash fixtures and concealed fixtures, installed as specified by manufacturer’s data, as well as decorative fixtures may be excluded from meeting these requirements:

1. 100% of light is emitted above the horizon plane.
2. Classified with Unified Glare Rating (UGR) of 16 or lower.
3. Luminance that does not exceed 6,000 cd/m² at any angle between 45 and 90 degrees from nadir.

OR

Option 2: Space considerations

 The following requirement is met in all regularly occupied spaces:

1. Unified Glare Rating (UGR) of 16 or lower.

Unfortunately, this does not really specify what particular UGR value they are referencing. If “max” you see the difficulty in using a rating in from our chart above as a 30/30/20 room as criteria for fixture selection.

You can also reference this Unified Glare Rating (UGR) FAQ document from Acuity Brands for more information.

TLA can help

So, in short, UGR is a now referenced, but highly misunderstood, metric by which lighting designers and others may choose to investigate the photometric characteristics of luminaires.

Fortunately, Team TLA is ready to help with your next commercial lighting project. Whether you’re designing a flexible office space or a “Zoom Room,” we have the expertise you need to support your client’s end-use goals.

Drop me an email, or reach out to your TLA Sales Rep for more information.

Thanks for reading along and see you next time!
— Robert Hawkins