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Architectural Illumination & Ergonomics, CIE 117 / EN 12464-1 Unified Glare Rating (UGR)

CIE Unified Glare Rating (UGR) Discomfort Formula

Calculate Single-Luminaire Guth Glare Term (L²*omega/p²), Cumulative Multi-Fixture Glare Sum, CIE 117 Unified Glare Rating UGR, and EN 12464-1 <=19 Office Compliance.

Paint an open-office ceiling matte black for "industrial aesthetics" while installing bare-diffuser LED panels and UGR jumps +9.5 points into painful visual glare!

`500` lux on a desk means nothing if high-angle `65°` brightness stabs seated workers’ eyes! **CIE Publication `117` & `EN 12464-1`** quantify psychological indoor discomfort glare on a logarithmic scale (`10` imperceptible, `16` technical drawing limit, **`19` standard office/classroom ceiling**, `22` industrial assembly, `28` intolerable) via **Unified Glare Rating `UGR = 8 * LOG10((0.25 / L_b) * SUM((L^2 * omega) / p^2))`**! For `N` representative visible luminaires of **Projected Emitting Luminance `L`** (`cd/m²`), **Subtended Steradian Solid Angle `omega`** (`sr` at observer eye), and **Guth Off-Axis Position Index `p`** (`1.0` dead ahead to `~10` high overhead), glare grows with **`L^2`** (`6.0` UGR units per doubling of fixture brightness!) AND inversely with **Background Room Adaptation Luminance `L_b = E_{indirect} / PI()`** (`cd/m²`—dark walls/ceilings starve pupil adaptation and dramatically worsen glare!)!

Verify on a `12`-Fixture Micro-Prismatic `UGR < 19` Office Grid—Luminaire_Luminance `L` = 2,800 cd/m² (B2), Bright-Wall_Background `L_b` = 35.0 cd/m² (B3), Representative_Solid_Angle `omega` = 0.0018 sr (B4), Guth_Position_Index `p` = 2.40 (B5), and Visible_Luminaires `N` = 12 (B6): Single-Fixture Glare Term `(L^2 * omega) / p^2` equals `2,450.0` (E2), `12`-Fixture Sum equals `29,400.0` (E3), **`UGR` equals `18.58` (`<= 19`!)** (E4), certifying `"EN 12464-1 OFFICE PASS (<=19)"` (E5)!

CIE 117 UGR Glare KPI (omega=0.0018 sr, Guth p=2.40, N=12 Fixtures)FormulaVerified Output
Single-Fixture Glare Term(Luminaire_L_cd_m2 ^ 2 * Solid_Angle_sr) / (Guth_p ^ 2)2,450.0 (16,200.0 @ bare 7,200 cd/m²)
Multi-Fixture Glare SumVisible_Fixtures_N * Single_Fixture_Term29,400.0 (194,400.0 @ bare lens)
CIE 117 Unified Glare Rating8 * LOG10((0.25 / Background_Lb) * Glare_Sum)18.58 comfortable (28.08 PAINFUL GLARE!)
EN 12464-1 Office Glare GateUGR <= 19.0 Reading / VDT Screen SpecEN 12464-1 OFFICE PASS (<=19)

CIE 117 Logarithmic Luminance-Contrast UGR & Guth Position Formula

Validate `Luminaire_Luminance L` in `[200, 100,000]` cd/m², `Background L_b` in `[2, 500]` cd/m², `Solid_Angle omega` in `[0.0001, 0.1]` sr, `Guth_Position p` in `[1.0, 16.0]`, and `Fixture_Count N` in `[1, 200]` right on `E2` (checking `ISTEXT(E2)` on `E3..E4` so `E5` text stays decoupled)!

=LET(ll_c, B2, lb_c, B3, om_s, B4, gp_i, B5, nf_c, B6, st_g, ((ll_c ^ 2) * om_s) / (gp_i ^ 2), ug_r, 8 * LOG10((0.25 / lb_c) * nf_c * st_g), IF(OR(ll_c < 200, ll_c > 100000, lb_c < 2, lb_c > 500, om_s < 0.0001, om_s > 0.1, gp_i < 1, gp_i > 16, nf_c < 1, nf_c > 200), "Invalid UGR Input", IF(ug_r <= 19, "EN 12464-1 OFFICE PASS (<=19)", "EXCESSIVE DISCOMFORT GLARE (>19)")))

Bare Opal Diffuser (`L = 7,200` cd/m²) Against Dark Exposed Ceiling (`L_b = 15` cd/m²) Check

Substitute un-louvered opal flat panels (`B2 = 7,200` cd/m²) beneath a dark charcoal exposed deck (`B3 = 15.0` cd/m²): Single-Fixture Term E2 jumps `6.6x` to `16,200.0` and **`UGR` E4 rockets `+9.50` points to `28.08`**—triggering `"EXCESSIVE DISCOMFORT GLARE (>19)"` (`> 28` intolerable CIE ceiling) in E5!

=IF(ISTEXT(E2), E2, LET(st_g, ((B2 ^ 2) * B4) / (B5 ^ 2), ROUND(8 * LOG10((0.25 / B3) * B6 * st_g), 2)))

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