UGR quantifies discomfort glare from luminaires in a space, typically on a 10–30 scale. Lower values mean less glare and more visual comfort.
High glare fatigues the eyes during sustained tasks like reading or lab work. Most environments here target UGR ≤ 19 for comfortable, detailed viewing.
The Science
UGR = 8·log₁₀(0.25/Lb · Σ(Ls²·ω/p²))
Lb = background luminance, Ls = luminance of the glowing part of the fixture, ω = solid angle it subtends at the eye, p = Guth position index (adjusts for how far the fixture sits from the direct line of sight).
Glare comes from stray light scattering inside the eye's own optics (cornea, lens, vitreous humour), which lays a veil of extra luminance over the retina and reduces image contrast — that's disability glare. Discomfort glare is milder: high brightness contrast that's simply annoying without impairing vision. On the hardware side, overdriven chips, cheap constant-voltage drivers, overly narrow beam angles, and degraded reflectors are the usual causes; constant-current drivers, proper TIR optics, adequate heatsinking, and physical baffles or louvers are the fix.
Squinting against glare contracts the muscles around the eyes and forehead, leading to facial soreness and tension headaches, and it reduces normal blink rate — which dries the tear film faster than usual.
Bright sources in the field of view pull attention away from the actual task involuntarily, which measurably slows task completion and adds to mental fatigue over a session.
See this parameter in action, across every stage and environment, in the full interactive simulator.
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