Glare Index Calculator

Calculate

Luminance of the glare source

Adaptation luminance of the surrounding field

Apparent angular size of the glare source in steradians

Guth position index for the source location in the field of view

Overview

The Glare Index Calculator estimates how visually uncomfortable a lighting condition is and classifies the result from very low glare to very high glare. It is a screening tool for discomfort glare, intended to help you quickly judge whether a space is likely to feel visually comfortable or visually disturbing under the stated lighting condition.

This calculator uses one fixed rule: the displayed glare index is the primary engineering result, and that exact number is used for interpretation. It does not convert between glare metrics. CIE describes Unified Glare Rating, or UGR, as a method developed to predict discomfort glare for indoor lighting systems, and IES likewise defines UGR as a measure of the discomfort produced by a lighting system.

Lower glare values generally mean better visual comfort, while higher values indicate a greater probability of discomfort glare. Industry guidance commonly treats glare assessment as a comfort and usability issue, not just a brightness issue.

This calculator is intentionally simple. It is useful for early design review, comparison between options, and quick comfort screening. It is not a replacement for full luminance mapping, detailed scene-based analysis, or project-specific glare simulation.

How to Use This Calculator

  1. Enter the source luminance — brightness of the glare source in cd/m².

  2. Enter the background luminance — adaptation luminance of the surrounding field in cd/m².

  3. Enter the solid angle of the source — apparent size of the glare source in steradians (sr).

  4. Enter the position index — Guth position index for the source location in the field of view.

  5. Click "Calculate" — get the Glare Index result.

  6. Review the result — use the glare classification to judge whether the condition is very low, low, moderate, high, or very high.

  7. If the result is moderate or worse, review source brightness, contrast, shielding, and viewing direction.

This calculator uses a simplified single-source BGI/UGR-style formula. For multi-source scenes, each source should be evaluated separately or summed before applying the logarithm.

Inputs & Outputs

Inputs

  • •Source Luminance (cd/m²)
  • •Background Luminance (cd/m²)
  • •Solid Angle of Source (sr)
  • •Position Index

Outputs

  • •Glare Index

Formula

Glare Index Formula (Simplified Single-Source)

This calculator uses a simplified BGI/UGR-style formula based on established glare research methods.

Glare Index = 8 × log₁₀( 0.25 / Lb × (Ls² × ω / p²) )

Where:

  • Ls = Source luminance (cd/m²)
  • Lb = Background luminance (cd/m²)
  • ω = Solid angle of the source (sr)
  • p = Position index (Guth position index)

Interpretation Against EN 12464-1 UGR Limits

Results are classified against EN 12464-1 and IES UGR reference thresholds. Key normative limits: UGR ≤ 19 (office acceptability limit), UGR ≤ 22 (industrial and manufacturing limit).

Glare Index Range Classification EN 12464-1 Reference
> 0 and < 10 Very Low Glare – Excellent Visual Comfort Well below all standard UGR limits
10 to < 16 Low Glare – Good Visual Comfort Within UGR ≤ 16 (detailed and technical work)
16 to < 19 Acceptable Glare – Within Office Limit Within UGR ≤ 19 (office acceptability limit)
19 to < 22 Elevated Glare – Exceeds Office Limit Exceeds UGR 19; within UGR ≤ 22 (industrial limit)
22 to < 28 High Glare – Exceeds Industrial Limit Exceeds UGR 22; discomfort is likely
≥ 28 Very High Glare – Poor Visual Comfort Exceeds all standard EN 12464-1 UGR limits

Supporting Contrast Logic

If both source luminance and background luminance are provided:

Luminance Ratio = Source Luminance / Background Luminance

When Luminance Ratio ≥ 10, a supporting contrast flag is triggered as a practical screening threshold for contrast-related glare concern.

Calculator Variables

Variable Meaning Units
sourceLuminance (Ls) Luminance of the glare source cd/m²
backgroundLuminance (Lb) Adaptation luminance of surrounding field cd/m²
solidAngle (ω) Apparent angular size of glare source sr
positionIndex (p) Guth position index dimensionless
glareIndex Calculated glare index dimensionless

What is Glare Index?

Glare Index is a numerical indicator of how likely a lighting condition is to cause visual discomfort. In lighting engineering, discomfort glare depends on more than just how bright something is. It is affected by source luminance, background luminance, source position, and the observer's visual direction. IES notes that UGR is used as a glare metric in many parts of the world, while CIE materials emphasize that discomfort glare evaluation is a distinct technical subject with established methodologies.

EN 12464-1 defines normative UGR limits for different space types: UGR ≤ 19 is the office acceptability limit, and UGR ≤ 22 applies to industrial and manufacturing environments. Results from this calculator are classified against those thresholds.

How This Calculator Works

This calculator applies a simplified single-source BGI/UGR-style formula using source luminance, background luminance, solid angle, and Guth position index. The result is classified into six bands aligned with EN 12464-1 and IES UGR reference thresholds. Lower glare values indicate better visual comfort.

Units

  • Glare Index is dimensionless
  • Source and background luminance: cd/m²
  • Solid angle: sr (steradians)
  • Position index: dimensionless

When to Use This Calculator

Use this calculator for preliminary screening of glare conditions in office, industrial, and general interior spaces. It is not a substitute for full luminance mapping, detailed scene-based analysis, or project-specific glare simulation.

Key Facts

  • CIE developed UGR to predict discomfort glare for indoor lighting systems.
  • IES defines UGR as a measure of the discomfort produced by a lighting system.
  • The perception of discomfort glare is influenced by source luminance, source size, source position, and background or adaptation luminance.
  • In practical lighting design, glare control is treated as an important part of visual comfort, not just a secondary aesthetic issue.
  • Daylight and electric lighting can both create glare problems, and scene context still matters even when a glare metric is available.
  • A glare calculator is useful for screening, but final design may still require simulation, shielding review, fixture changes, shading strategy, or view-direction changes.

Applications

  • Office lighting comfort checks
  • Workstation glare screening
  • Daylight glare review
  • Electric-lighting comparison
  • Interior visual-comfort screening
  • Early design review before detailed simulation

Example Calculation

Given:

  • Source Luminance (Ls) = 5,000 cd/m²
  • Background Luminance (Lb) = 200 cd/m²
  • Solid Angle of Source (ω) = 0.01 sr
  • Position Index (p) = 1.5

Step 1 — Calculate the argument inside the logarithm

Ls² = 5,000² = 25,000,000
Ls² × ω = 25,000,000 × 0.01 = 250,000
p² = 1.5² = 2.25
Ls² × ω / p² = 250,000 / 2.25 = 111,111.11
0.25 / Lb = 0.25 / 200 = 0.00125
0.00125 × 111,111.11 = 138.89

Step 2 — Apply logarithm

log₁₀(138.89) = 2.1427

Step 3 — Calculate Glare Index

Glare Index = 8 × 2.1427 = 17.14

Interpretation: A glare index of 17.14 falls into Acceptable Glare – Within EN 12464-1 Office Limit. The result is above 16 but below the EN 12464-1 office acceptability limit of UGR 19, so the condition is within the standard office threshold. Occupants may notice the glare during sustained visual tasks, and the result should be confirmed against task requirements and space type.

The luminance ratio is 5,000 / 200 = 25, which exceeds the contrast screening threshold of 10, indicating high brightness contrast that can contribute to uncomfortable glare.

Standards & References

  • CIE — International Commission on Illumination — developed and maintains internationally recognized discomfort-glare methods, including the Unified Glare Rating (UGR) methodology.
  • IES definition of UGR — identifies UGR as a measure of discomfort produced by a lighting system.
  • CIBSE Factfile on glare and UGR — explains that discomfort glare can be calculated with the CIE UGR method.
  • EN 12464-1 context — indoor workplace lighting standards commonly reference UGR in practical glare-control workflows.

Limitations

  • This calculator is a screening tool, not a full glare-simulation platform.
  • It does not replace full luminance mapping, observer-position analysis, or detailed daylight / electric-lighting simulation. Discomfort glare depends on scene conditions, not only one number.
  • It does not automatically prove that a space is acceptable for every task type.
  • It should not be used to convert one glare metric into another. CIE and IES treat glare metrics as specific assessment systems.
  • It does not replace project-specific lighting design judgment.

Common Mistakes to Avoid

  • Assuming one glare number fully describes the whole visual scene.
  • Confusing glare with illuminance alone.
  • Ignoring source-to-background contrast.
  • Ignoring observer direction.
  • Assuming low glare means every task will feel comfortable.
  • Treating a screening result as final design approval.
  • Mixing different glare metrics as if they were interchangeable.
  • Forgetting that daylight and electric-lighting glare may need different design responses.

Frequently Asked Questions

What does this calculator actually calculate?
It calculates a Glare Index and interprets it as a visual-comfort screening result.
What is the main result I should focus on?
The main result is the Glare Index itself.
Is glare index usually unitless?
Yes. Glare ratings are generally treated as dimensionless indices rather than physical units.
Does a low glare index guarantee perfect comfort?
No. It indicates lower discomfort-glare risk, but final comfort still depends on source brightness, view direction, background luminance, and scene layout.
Why does contrast matter so much?
Because strong contrast between a bright source and a darker background is one of the classic drivers of discomfort glare.
Can this be used for daylight and electric lighting?
As a screening tool, yes. But complex daylight glare often needs fuller scene analysis and view-specific evaluation.
Is this the same as UGR?
Not always. UGR is a specific glare metric. If your calculator displays a different glare index, do not assume it is automatically identical to UGR.
Is this enough for final lighting design sign-off?
No. Final design may still need detailed simulation, shielding changes, layout changes, or formal glare review.

Frequently Used Together

Engineers often use these calculators in combination for complete project workflows:

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Office Open Plan Cooling Load Calculator

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