Illumination Converter
Convert between lux, foot-candle, phot, and other illuminance units
About Illumination (Illuminance)
Illumination, also known as illuminance, is a fundamental photometric quantity that measures the total luminous flux incident on a surface per unit area. It quantifies how much light falls on a surface and is essential for determining adequate lighting levels for various activities, environments, and applications across multiple industries.
What is Illuminance?
Illuminance is defined as the luminous flux per unit area received by a surface. It represents the amount of light energy that strikes a surface and is measured in lumens per square meter (lux) in the International System of Units (SI). The fundamental formula for illuminance is:
E = Φ / A
Where: E = Illuminance (lux), Φ = Luminous flux (lumens), A = Area (m²)
This relationship shows that illuminance decreases with the square of the distance from the light source (inverse square law) and is directly proportional to the luminous flux emitted by the source.
Common Illuminance Units and Conversions
| Unit | Symbol | Definition | Conversion to Lux |
|---|---|---|---|
| Lux | lx | Lumens per square meter | 1 lx |
| Foot-candle | fc | Lumens per square foot | 1 fc = 10.764 lx |
| Phot | ph | Lumens per square centimeter | 1 ph = 10,000 lx |
| Nox | nx | Millilux (0.001 lux) | 1 nx = 0.001 lx |
Types of Illuminance Measurements
| Measurement Type | Typical Range | Applications | Real-World Examples |
|---|---|---|---|
| Horizontal Illuminance | 50-10,000 lx | Work surfaces, floors | Office desks, reading tables |
| Vertical Illuminance | 100-5,000 lx | Walls, faces, displays | Art galleries, retail displays |
| Cylindrical Illuminance | 200-2,000 lx | Facial recognition, modeling | Photography studios, security cameras |
| Semi-cylindrical Illuminance | 150-1,500 lx | Outdoor lighting, facades | Building exteriors, street lighting |
Illuminance Measurement Tools
Accurate illuminance measurement requires specialized instruments designed to respond to light in the same way as the human eye. The most common measurement tools include:
Lux Meters (Illuminance Meters)
- • Silicon photodiode sensors with V(λ) filter
- • Cosine-corrected diffusers for accurate angular response
- • Digital displays with multiple unit support
- • Calibration traceable to national standards
- • Typical accuracy: ±3% to ±5%
Spectroradiometers
- • High-precision spectral measurement
- • Wavelength-by-wavelength analysis
- • Used for research and calibration
- • Can measure color temperature and CRI
- • Typical accuracy: ±1% to ±2%
Illuminance - Luminous Flux - Luminous Intensity Relationship
Understanding the relationship between illuminance, luminous flux, and luminous intensity is crucial for lighting design and calculations. The fundamental relationships are:
E = I / d² (Point source approximation)
E = Φ / A (Surface illuminance)
I = Φ / ω (Luminous intensity)
Where: E = Illuminance (lx), I = Luminous intensity (cd), d = Distance (m), Φ = Luminous flux (lm), A = Area (m²), ω = Solid angle (sr)
Recommended Illuminance Levels by Application
| Application | Illuminance Range | Standard/Code | Notes |
|---|---|---|---|
| General Office Work | 300-500 lx | IES RP-1 | Computer workstations |
| Detailed Office Work | 500-1000 lx | IEC 8995-1 | Reading, writing, drafting |
| Surgical Procedures | 10,000-100,000 lx | IEC 60601-2-41 | Operating room lighting |
| Retail Display | 750-1500 lx | IES DG-18 | Product presentation |
| Outdoor Security | 5-50 lx | IES RP-33 | Parking lots, walkways |
| Horticultural Lighting | 200-800 lx | ASABE S640 | Greenhouse cultivation |
Why Illuminance Measurement is Important
Health and Safety
- • Prevents eye strain and visual fatigue
- • Reduces workplace accidents and errors
- • Ensures compliance with occupational safety standards
- • Supports circadian rhythm regulation
- • Prevents seasonal affective disorder (SAD)
Economic Benefits
- • Optimizes energy consumption
- • Reduces lighting maintenance costs
- • Improves productivity and performance
- • Enhances product quality in manufacturing
- • Increases retail sales through better product visibility
Frequently Asked Questions
What is the difference between illuminance and luminance?
Illuminance measures the amount of light falling on a surface (incident light), while luminance measures the brightness of a surface as perceived by an observer (reflected or emitted light). Illuminance is measured in lux, while luminance is measured in candela per square meter (cd/m²).
How does distance affect illuminance?
For point sources, illuminance follows the inverse square law: E = I/d², where illuminance decreases with the square of the distance from the source. For extended sources, the relationship is more complex and depends on the source geometry.
What factors affect illuminance measurement accuracy?
Key factors include sensor spectral response, cosine correction, temperature effects, calibration accuracy, measurement geometry, and environmental conditions such as humidity and electromagnetic interference.
How often should illuminance meters be calibrated?
Professional illuminance meters should be calibrated annually or after 1,000 hours of use, whichever comes first. Calibration should be performed by accredited laboratories traceable to national standards.
Key Takeaway
Illuminance measurement is fundamental to creating safe, efficient, and comfortable lighting environments. Understanding the principles of illuminance, proper measurement techniques, and recommended levels for different applications enables professionals to design lighting systems that meet both functional requirements and regulatory standards while optimizing energy use and user experience.
Frequently Asked Questions About Illumination Conversion
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