Isolux – Contour of Equal Illuminance – Photometry

Illuminance mapping, visualized through isolux contours, is a foundational tool in the science and practice of lighting design. From airport aprons and highways to offices and industrial sites, these diagrams convert complex photometric data into actionable, visual information for engineers, architects, and facility managers. Paired with the science of photometry—the measurement of visible light as perceived by the human eye—isolux diagrams ensure lighting systems meet regulatory standards (like EN 12464-1, ICAO Annex 14, or IESNA RP-8) and provide safe, comfortable environments.

1. Photometric Fundamentals

1.1 What is Photometry?

Photometry quantifies the visible portion of electromagnetic radiation (roughly 380–780 nm) using the human eye’s sensitivity curve (V(λ)). This ensures that photometric measurements reflect perceived brightness, not just raw energy.

Key photometric quantities:

• Luminous Flux (Φ): Total visible light output from a source, in lumens (lm).
• Luminous Intensity (I): Light emitted in a specific direction, in candelas (cd).
• Illuminance (E): Luminous flux incident per unit area, in lux (lx), where 1 lx = 1 lm/m².
• Luminance (L): Perceived brightness from a surface, in candelas/m².

Photometric measurements form the basis for lighting standards and are gathered via laboratory instruments like goniophotometers and integrating spheres.

1.2 What is an Isolux Diagram?

An isolux diagram is a plan-view map showing lines (contours) connecting points of equal illuminance (lux) across a surface. Each isolux line represents a threshold (e.g., 10 lx, 20 lx), visualizing how bright or dim different areas will be.

Applications:

• Optimizing luminaire placement and spacing
• Ensuring compliance with standards (EN 12464-1, ICAO Annex 14, IESNA RP-8)
• Identifying dark spots or overlit areas for energy-efficient design

How it works:
A grid overlays the target area. Illuminance at each grid point is calculated or measured, considering luminaire data, mounting height, and environmental factors. Contours are then drawn to connect equal-lux points.

Example:
On an airport apron, a floodlight’s isolux diagram might show 50 lx at the center, dropping to 20 lx at the edge. This guides engineers in arranging additional lights for consistent, compliant coverage.

Isolux diagrams are generated using software (like DIALux, Relux, AGi32) or by hand, using manufacturer photometric data (IES or EULUMDAT files).

1.3 What is an Isocandela Diagram?

An isocandela diagram plots lines of equal luminous intensity (in candelas) from a light source, typically in polar coordinates. Unlike isolux diagrams (which show effects on a surface), isocandela diagrams show how a luminaire emits light in different directions.

Uses:

• Assessing beam shape (spot, flood, wide)
• Classifying luminaires for roadway or area lighting
• Adjusting aiming angles to minimize glare or light spill

Example:
A narrow-beam spotlight’s isocandela plot will show high intensity concentrated within a small angle, while a floodlight’s plot will be broader.

1.4 Related Terms

• Mounting Height (h): Distance from the luminaire to the surface; affects spread and intensity.
• Beam Angle: The width of the light beam, measured between angles where intensity falls to 50% of max.
• Luminaire Spacing: Distance between adjacent fixtures to ensure uniform coverage.
• Room Reflectance: The percentage of light reflected by surfaces, impacting effective illuminance.

2. Structure & Interpretation of Isolux Diagrams

2.1 How Are Isolux Diagrams Constructed?

1. Define area and grid (e.g., 1 m × 1 m).
2. Input photometric data (IES/EULUMDAT files) and mounting height.
3. Calculate illuminance at each grid point: [ E = \frac{I}{d^2} \cdot \cos \theta ] where (I) is intensity, (d) is distance, and (\theta) is the angle of incidence.
4. Draw contours connecting points of equal illuminance.

Rotationally symmetrical luminaires produce circular contours. Asymmetrical fixtures create ellipses or irregular shapes.

Multiple luminaires: Overlapping isolux contours visualize cumulative coverage and uniformity.

2.2 How to Read Isolux Diagrams

1. Find the origin: The luminaire’s mounting point.
2. Read contour labels: Each line is marked with its lux value.
3. Assess coverage: Distance from the center to a contour shows the reach for a given luminance.
4. Check uniformity: Overlap between contours of adjacent luminaires indicates even coverage.
5. Compare to standards: Ensure required minimum or average illuminance contours cover critical areas.

Caveats:
Assumes flat, unobstructed surfaces. Variations in height, obstructions, or reflectance require correction or simulation.

3. Application in Lighting Design

3.1 Using Isolux Diagrams for Luminaire Placement

1. Set targets: Reference standards to determine minimum/average illuminance.
2. Select luminaires: Based on photometric data and application.
3. Consult isolux diagram: Identify the contour for minimum required illuminance.
4. Calculate spacing: Usually twice the radius from the center to the required contour (e.g., if 20 lx contour is at 10 m, space luminaires 20 m apart).
5. Adjust for real conditions: Reflectance, obstructions, and mounting height may require further refinements.

3.2 Correction for Mounting Height

Illuminance decreases as the mounting height increases, following the inverse square law.

Formula: [ E_{new} = E_{original} \times \left( \frac{h_{original}}{h_{new}} \right)^2 ]

Example:
If the isolux diagram is at 4 m, but the installation is at 5 m:

• Original: 10 lx
• Correction factor: (4/5)² = 0.64
• New illuminance: 6.4 lx

For best results, regenerate diagrams at the actual mounting height.

3.3 Uniformity Considerations

Uniformity is vital for safety and comfort. Excessive contrast can cause discomfort and reduce visibility.

Uniformity Ratio: [ U_0 = \frac{E_{min}}{E_{avg}} ]

Design guidelines:

• Maximum spacing-to-height ratio (often ≤1.5)
• Overlap isolux contours of adjacent luminaires for seamless coverage
• Consider room reflectance for interiors

4. Standards & Compliance

Isolux diagrams help designers meet standards such as:

• EN 12464-1: Interior lighting (offices, industry)
• ICAO Annex 14: Aerodrome and apron lighting
• IESNA RP-8: Roadway lighting

These standards set minimum and average illuminance values, uniformity ratios, and sometimes maximum values to avoid glare.

5. Practical Tips & Best Practices

• Always use up-to-date photometric data from certified labs.
• For complex environments, model with professional software.
• Adjust for actual mounting heights and surface conditions.
• Validate designs with on-site measurements post-installation.

6. Visual Example

Sample isolux diagram: Each contour line represents a constant lux value, visualizing how light fades from the center outward.

7. Summary

Isolux diagrams are a cornerstone of modern lighting design, transforming raw photometric data into clear, actionable maps for optimizing luminaire placement, achieving uniformity, and ensuring compliance. Their use spans from interior workspaces to vast airport aprons, supporting both safety and energy efficiency.

Further Reading:

• EN 12464-1: Lighting of Workplaces
• ICAO Annex 14 – Aerodrome Design and Operations
• IESNA Lighting Handbook

For tailored lighting design, isolux diagram consultation, or photometric analysis, contact our lighting experts today!