Glare

Glare is the visual discomfort or loss of visibility caused when a bright source sits in the field of view against a darker background. A bare LED, a badly aimed flood light, low sun on a wet road: all three do the same thing, which is present the eye with a luminance far above what it has adapted to.

The key point for anyone specifying fixtures: glare is caused by luminance contrast, not by lumen output. A fixture can be uncomfortable in a space that measures perfectly well on lux, and spreading the same output over a larger emitting surface reduces glare without losing a single lumen.

The Types of Glare

Type	What happens	Where it bites
Discomfort glare	Causes strain and fatigue without necessarily hiding anything	Offices, picking aisles, assembly benches, anywhere people work a full shift
Disability glare	Scatters light inside the eye and genuinely hides detail	Roads, crane and forklift areas, ramps, any safety-critical movement
Direct glare	The source itself is in the field of view	Low-mounted floods, unshielded high bays, mast heads seen from a bridge wing
Reflected (veiling) glare	The source reflects off the task surface into the eye	Screens, glossy paper, polished machinery, wet floors

Discomfort and disability glare are not the same problem and they are not measured the same way. A scheme can be safe and still exhausting to work under, which is why standards limit both.

What Actually Drives It

Four variables, and only one of them is the fixture’s output:

1. Source luminance. How bright the emitting surface is, in cd/m². A bare LED die reaches millions of cd/m². A diffuser over the same LEDs cuts that by orders of magnitude at a small efficiency cost.
2. Background luminance. The same fixture is far more glaring at night, or in a dark warehouse, than in a daylit space, because the eye’s adaptation level is lower.
3. Position in the field of view. A source near the line of sight is much worse than one well above it. This is why mounting height and cut-off angle matter as much as the optic.
4. Apparent size. A large, moderately bright surface is more comfortable than a small, intensely bright one at the same total output.

How Glare Is Quantified

Indoors: UGR. The Unified Glare Rating runs roughly 13 to 28, lower being better, and standards set a limit per task. Common limits are UGR ≤ 19 for offices and precision work, ≤ 22 for general industrial tasks, and ≤ 25 to 28 for rough work and circulation. UGR is calculated for a defined observer position and viewing direction, so a UGR figure without those conditions stated is not a real number.

Outdoors and sport: glare rating and threshold increment. Outdoor area and sports lighting use a glare rating scale, while road lighting limits threshold increment, the percentage rise in contrast a driver needs in order to see the same object because of disability glare. Both are limits on the design, not properties of a fixture in isolation.

In luminaire terms: intensity classes and cut-off. Outdoor luminaires are classified by how much intensity they emit at high angles. This is also the mechanism that controls light trespass and sky glow, which is why glare control and spill control are usually the same conversation.

Controlling It

In rough order of effectiveness:

• Raise the mounting height. Glare scales inversely with height. The same lumens from a 6 m pole sit in everyone’s eyeline; from 15 m they do not.
• Aim properly and limit tilt. A fixture tilted to reach further throws intensity toward the horizon, which is where eyes are. This is the most common cause of complaints on flood and mast installations.
• Choose the right beam angle. A narrow beam that puts light on the target needs less spill to do the job.
• Increase the emitting area. Diffusers, deeper regressed LEDs and larger optical surfaces lower source luminance directly.
• Add louvres or shields where the source is unavoidably in view, accepting a few percent efficiency loss.
• Lift the background. Lighter surfaces and a modest general level reduce the contrast the eye has to absorb, which is why very high-contrast spot lighting in a dark space feels harsh.

Where It Matters Most in Industrial Work

Picking aisles, because staff look up at rack faces for a full shift. Sports halls, where players track a ball against the ceiling. Roads and yards, where disability glare is a safety matter rather than a comfort one. And any port or waterside installation, where a mast head can sit level with a ship’s bridge window, discussed in the port and terminal lighting guide.

For the high bay case specifically, linear fixtures spread the same output across a larger surface and are generally more comfortable than round point sources, a trade covered in UFO vs linear high bay.