Glare: why UGR<19 on a datasheet proves nothing

Discomfort glare is the most common complaint about a finished lighting installation, and UGR is the most commonly misused number in the industry.

What UGR actually computes

The Unified Glare Rating aggregates every luminaire in the field of view:

UGR = 8 · log₁₀ [ (0.25 / Lb) · Σ ( L² · ω / p² ) ]

• L — luminance of each luminaire in the observer’s direction
• ω — solid angle it subtends at the eye
• p — Guth position index, weighting how far off-axis it sits
• Lb — background luminance

Read the inputs. Three of the four depend on the room and the observer, not on the luminaire. Room size, surface reflectance, mounting height, where the desk sits and which way the chair faces all change the result.

So when a datasheet says “UGR < 19”, it means: in the standard reference room, with reference reflectances, viewed crosswise from a reference position. It is a comparison tool between products. It is not a compliance statement about your project. Compliance is demonstrated by calculating UGR in your geometry, and every mainstream calculation package does it.

The scale

UGR	Perception	Typical application
16	Imperceptible	Technical drawing, precision inspection
19	Just perceptible	Offices, classrooms, meeting rooms
22	Perceptible, acceptable	Reception, retail, canteens
25	Noticeable	Circulation, storage, plant
28	Uncomfortable	Corridors, rough industrial

Each step of 3 is roughly a doubling of perceived glare. The jump from 19 to 22 is not a rounding difference; it is a different experience of the room.

Where the number stops helping

UGR is defined for reasonably small, reasonably uniform luminaires in a reasonably rectangular room. It is not valid for:

• Large-area sources — luminous ceilings, backlit panels above ~1.5 m
• Indirect and cove schemes — the ceiling becomes the source; use ceiling luminance directly
• Linear runs viewed end-on — the geometry falls outside the model
• Exterior lighting — use GR (Glare Rating) instead
• Non-uniform luminaires — a fitting with visible bright LED dots behind a clear lens can compute a good UGR and still be unbearable

That last case is worth dwelling on. UGR uses average luminance across the emitting surface. A diffused panel at 3000 cd/m² average and a clear-lensed fitting whose individual dies hit 2,000,000 cd/m² can return the same UGR. One is comfortable and the other causes afterimages. This is the single biggest gap between the metric and the experience.

Controlling glare when the number cannot

Cut-off. The most reliable tool. Deep recess, honeycomb louvre, or a snoot so the source is not visible within the normal 45–85° viewing cone.

Surface luminance, not just cut-off. Ask for the luminance table at 45°, 55°, 65°, 75° and 85° from nadir. EN 12464-1 gives limiting values for screen-based work. A fitting that hides its source but still puts 8000 cd/m² at 75° will reflect in every monitor in the room.

Raise the background. Look again at the formula — Lb is in the denominator. Putting light on the walls and ceiling lowers UGR without touching a single luminaire. This is why a scheme of bare downlights in a dark room feels harsh at 300 lx while the same lux with washed walls feels calm. It is also the cheapest fix available on a project already in construction.

Reduce source count in view. Fewer, better fittings beat many small ones. The Σ in the formula is a sum.

Check the actual seat. Run the UGR calculation from where people will genuinely sit and face — not from the software’s default observer grid marching down the room centreline.

The site test that beats every calculation

Stand where the occupant will stand. Look where they will look — at a screen, at a colleague’s face, down the corridor. If you have to squint or your eye is pulled toward a fitting rather than toward the space, the number does not matter. Something needs to change.