Glare significantly impacts visual comfort and productivity in indoor environments, necessitating precise measurement methods. Unified Glare Rating (UGR) is a standardized metric quantifying discomfort glare from lighting installations.
This article explores the UGR calculation process, essential formulas, practical tables, and real-world applications for lighting designers and engineers. Understanding UGR ensures optimal lighting design minimizing visual discomfort.
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- Calculate UGR for a 4×4 office ceiling grid with luminaires at 3m height.
- Determine UGR for a classroom with 12 luminaires, each 500 cd luminous intensity.
- Find UGR in a retail store with mixed direct and indirect lighting at 2.5m.
- Evaluate UGR for a hospital ward with 8 luminaires, 4000 cd intensity, 3.2m height.
Comprehensive Tables of Common Unified Glare Rating (UGR) Values
Below are detailed tables presenting typical UGR values for various lighting scenarios, aiding quick reference and design decisions.
| Environment Type | Ceiling Height (m) | Luminaire Type | Typical UGR Range | Recommended UGR Limit |
|---|---|---|---|---|
| Office (Open Plan) | 2.7 – 3.0 | Recessed Fluorescent | 16 – 19 | ≤ 19 |
| Classroom | 2.7 – 3.0 | Surface Mounted LED | 15 – 18 | ≤ 19 |
| Retail Store | 3.0 – 4.0 | Track Lighting | 18 – 22 | ≤ 22 |
| Hospital Ward | 2.8 – 3.2 | Indirect LED | 13 – 16 | ≤ 16 |
| Conference Room | 2.7 – 3.0 | Downlights | 14 – 17 | ≤ 19 |
| Luminaire Luminance (cd/m²) | Typical Room Surface Reflectance | UGR Impact | Design Consideration |
|---|---|---|---|
| < 2000 | Walls: 0.5, Ceiling: 0.7, Floor: 0.2 | Low glare risk | Standard luminaire placement |
| 2000 – 4000 | Walls: 0.4, Ceiling: 0.6, Floor: 0.3 | Moderate glare risk | Consider shielding or diffusers |
| > 4000 | Walls: 0.3, Ceiling: 0.5, Floor: 0.2 | High glare risk | Use baffles, louvers, or indirect lighting |
Fundamental Formulas for Unified Glare Rating (UGR) Calculation
The Unified Glare Rating (UGR) is calculated using the formula standardized in EN 12464-1 and CIE 117:1995. The formula quantifies discomfort glare by considering the luminance of each luminaire, its position relative to the observer, and the background luminance.
- UGR: Unified Glare Rating (dimensionless)
- Lb: Background luminance in cd/m² (typically room surface luminance)
- Ln: Luminance of the nth luminaire in cd/m²
- ωn: Solid angle of the nth luminaire from the observer’s eye in steradians (sr)
- pn: Guth position index for the nth luminaire (dimensionless)
- Σ: Summation over all luminaires visible to the observer
Explanation of Variables and Typical Values
- Background luminance (Lb): Usually the average luminance of the room surfaces visible to the observer, often between 20 and 50 cd/m² in office environments.
- Luminance of luminaire (Ln): Depends on the luminaire type and its luminous intensity distribution; typical values range from 1000 to 6000 cd/m².
- Solid angle (ωn): Calculated based on the luminaire’s apparent size from the observer’s viewpoint; smaller angles reduce glare contribution.
- Guth position index (pn): Accounts for the position of the luminaire relative to the line of sight; values range from 1 (directly in line) to 10 (peripheral).
Calculating Solid Angle (ωn)
The solid angle ωn subtended by a luminaire is calculated as:
- A: Apparent area of the luminaire as seen by the observer (m²)
- dn: Distance from the observer’s eye to the luminaire (m)
For rectangular luminaires, A = width × height projected onto the observer’s line of sight.
Guth Position Index (pn)
The Guth position index is a dimensionless factor that adjusts glare contribution based on the luminaire’s angular position relative to the observer’s line of sight. It is derived from the angle θ between the observer’s line of sight and the luminaire:
- θ = 0° (directly in line) → pn = 1 (maximum glare)
- θ increases → pn increases, reducing glare contribution
Detailed Real-World Examples of UGR Calculation
Example 1: UGR Calculation in an Open-Plan Office
An open-plan office has a ceiling height of 3 meters with 16 recessed luminaires arranged in a 4×4 grid. Each luminaire has a luminance of 3500 cd/m², an apparent area of 0.1 m², and the observer is seated 2 meters from the nearest luminaire. The background luminance is 30 cd/m². Calculate the UGR for the observer.
Step 1: Calculate the solid angle ωn for one luminaire
Assuming the observer views the luminaire head-on, the distance to the nearest luminaire is 2 m. For others, distances vary, but for simplicity, calculate for the nearest luminaire first:
Step 2: Calculate Guth position index pn
Assuming the luminaire is directly in the line of sight (θ = 0°):
Step 3: Calculate the glare contribution for one luminaire
Step 4: Sum contributions for all 16 luminaires
Assuming all luminaires have similar contributions (simplified):
Step 5: Calculate UGR
Calculate log10(163,333.33): approximately 5.213
This UGR value is very high, indicating significant glare discomfort. In practice, the distance and angular positions vary, reducing the sum. This example highlights the importance of accurate geometry and luminaire positioning.
Example 2: UGR Calculation in a Classroom with Mixed Lighting
A classroom has 12 surface-mounted LED luminaires, each with luminance 2500 cd/m² and apparent area 0.08 m². The observer is seated 4 meters from the nearest luminaire. Background luminance is 25 cd/m². Calculate the UGR considering the Guth position index for an angle θ = 30°.
Step 1: Calculate solid angle ωn
Step 2: Calculate Guth position index pn
For θ = 30°:
Step 3: Calculate glare contribution for one luminaire
Step 4: Sum contributions for all 12 luminaires
Step 5: Calculate UGR
Calculate log10(9,740.16): approximately 3.989
This UGR value suggests moderate glare, which may be uncomfortable for prolonged periods. Adjustments such as diffusers or repositioning luminaires can reduce glare.
Additional Technical Considerations for UGR Calculation
- Observer Position: UGR varies significantly with observer location; multiple positions should be evaluated for comprehensive assessment.
- Luminaire Luminance Distribution: Non-uniform luminaires require detailed photometric data for accurate Ln values.
- Room Surface Reflectance: Influences background luminance Lb; higher reflectance increases Lb, reducing UGR.
- Multiple Light Sources: Summation over all visible luminaires is essential; partial occlusion or shielding affects contributions.
- Standards Compliance: EN 12464-1 and CIE 117:1995 provide guidelines for acceptable UGR limits in various environments.
Practical Tips for Reducing UGR in Lighting Design
- Use luminaires with lower luminance or indirect lighting to minimize direct glare.
- Increase background luminance by selecting lighter wall and ceiling finishes.
- Optimize luminaire placement to avoid direct line-of-sight glare.
- Incorporate shielding elements such as louvers, baffles, or diffusers.
- Consider dimming controls to adjust luminance based on occupancy and daylight.
Authoritative References and Further Reading
- CIE 117:1995 – Guide on Unified Glare Rating
- EN 12464-1: Light and lighting – Lighting of work places – Part 1: Indoor work places
- IES Lighting Handbook
- Philips UGR Calculator Tool
Mastering the Unified Glare Rating calculation empowers lighting professionals to design visually comfortable spaces. Accurate UGR assessment ensures compliance with standards and enhances occupant well-being.