What is Illuminance?

Illuminance: The Complete Glossary for Aviation, Architecture, and Engineering

Definition

Illuminance (symbol: E) is a photometric quantity that measures the total luminous flux (Φ) incident on a given surface area (A). In simple terms, it quantifies how much visible light falls onto a specific surface, regardless of what the surface does with that light. The SI unit for illuminance is lux (lx):

[ E = \frac{\Phi}{A} ]

• E = Illuminance (lux, lx)
• Φ = Luminous flux (lumens, lm)
• A = Area (square meters, m²)

1 lux (lx) = 1 lumen (lm) per square meter (m²)

Illuminance is strictly photometric, it is weighted by the human eye’s response to visible wavelengths, making it directly relevant to human visual tasks.

Illuminance in Photometry

Photometry deals with measuring visible light as perceived by human vision. Illuminance is one of the core photometric quantities:

Illuminance measures light arriving at a surface, making it the basis for nearly all lighting standards and regulations in workplaces, aviation, and architecture.

Measurement of Illuminance

Instruments

Illuminance is measured using luxmeters or photometers fitted with:

• A photodiode sensor
• Filters matching the human eye’s spectral sensitivity (CIE V(λ) curve)
• Cosine diffusers for accurate angular response

Calibration is essential for accuracy, professional meters are referenced to national or international standards.

Procedure

• Place the sensor parallel to the surface being measured (usually at working height)
• Avoid shadows or reflections from the operator
• Orient the sensor towards the main light source or keep it horizontal for general lighting
• Ensure all relevant lights are on and conditions are stable

Instrument Classes

Class A/B meters are required for professional and regulatory work.

Calculation of Illuminance

General Formula

[ E = \frac{\Phi}{A} ]

Point Source

For a point source emitting flux Φ:

[ E = \frac{\Phi}{4\pi d^2} ]

Where d is the distance from the source.

Using Luminous Intensity

[ E = \frac{I}{d^2} ]

Where I is luminous intensity (candela) and d is distance.

Angle of Incidence (Lambert’s Cosine Law)

[ E = \frac{I \cdot \cos\theta}{d^2} ]

Where θ is the angle between the light and surface normal.

Additivity

Total illuminance from multiple sources is the sum of each source’s contribution:

[ E_{\text{total}} = \sum_{i=1}^n E_i ]

Typical Illuminance Values

Based on EN 12464, ASR A3.4, ICAO, and industry guidelines.

Standards and Recommendations

Office and Workplace (EN 12464-1)

Aviation (ICAO Annex 14, FAA AC 150/5370-2G)

• Apron floodlighting: ≥ 20 lx at pavement
• Runway markings: 50–100 lx on markings
• Maintenance hangars: 500–1,000 lx

Surrounding areas must meet at least half the mean illuminance of task areas. These standards ensure safety and minimize eye strain.

Applications

• Lighting Design: Determines luminaire number/type for compliance and comfort
• Regulatory Compliance: Proof of adequate lighting for audits and certifications
• Quality Assurance: Verifies ongoing performance as lamps age or layouts change
• Scientific Research: Sets standard lighting for experiments
• Architecture/Daylighting: Balances daylight and artificial lighting for efficiency and comfort

Illuminance vs. Related Quantities

• Illuminance is about light received by a surface.
• Luminance is about the brightness of a surface as seen from a direction.
• Irradiance is total power (not eye-weighted).
• Luminous Intensity is a source’s directional output.

Technical Laws

Inverse Square Law

[ E = \frac{I}{d^2} ]

Illuminance decreases with the square of the distance from a point source.

Lambert’s Cosine Law

[ E = \frac{I \cdot \cos\theta}{d^2} ]

Illuminance is highest when light strikes perpendicularly.

Best Practices for Measurement

• Use a calibrated, class A or B luxmeter
• Position the sensor parallel to the surface and facing main light sources
• Minimize operator shadows/reflections
• Measure under representative lighting conditions (include daylight if relevant)
• Use additive principle for multiple light sources
• Rely on objective measurement, visual estimation is unreliable

Common Misconceptions

• Illuminance is not “brightness”: Brightness is subjective, illuminance is objectively measured.
• Luminance ≠ Illuminance: Luminance depends on both illuminance and surface reflectivity.
• Smartphone apps are not accurate for professional use.
• More light is not always better: Excessive illuminance can cause discomfort and glare.

Summary

Illuminance is the key photometric quantity for assessing and designing lighting in aviation, architecture, and engineering. It ensures that environments are safe, comfortable, and meet regulatory requirements. Accurate measurement and understanding of illuminance are critical for quality lighting solutions.

For expert guidance on illuminance assessments and lighting design, contact us or schedule a demo .