Illuminance Converter
Convert illuminance units — lux, foot-candles, phot, nox, and more. Free online illumination converter for lighting design, photography, and workplace safety compliance.
Understanding Illuminance
Illuminance measures the amount of light falling on a surface. You express illuminance in lux (lx), where one lux equals one lumen per square meter. This photometric quantity determines how well you can see tasks and objects.
Illuminance differs from luminance. Illuminance measures incoming light. Luminance measures reflected or emitted light. Lighting design specifies illuminance levels for different activities.
Illuminance Formulas
E = Φ / A
Definition: Illuminance equals luminous flux divided by area. Units are lux (lm/m²) or foot-candles (lm/ft²).
E = I / d² × cos(θ)
Inverse square law: Illuminance equals intensity divided by distance squared, times cosine of incidence angle.
E = L × π × ρ
From luminance: Illuminance equals luminance times pi times reflectance for uniform diffuse surfaces.
Example: A 1,000 lumen lamp illuminating 10 m² produces E = 1,000 / 10 = 100 lux average illuminance.
Common Illuminance Conversions
| Lux (lx) | Foot-candles (fc) | Phot (ph) |
|---|---|---|
| 0.0929 | 1 | 0.00000929 |
| 1 | 0.0929 | 0.0001 |
| 10 | 0.929 | 0.001 |
| 100 | 9.29 | 0.01 |
| 1,000 | 92.9 | 0.1 |
| 10,000 | 929 | 1 |
Recommended Illuminance Levels
| Space/Task | Illuminance (lux) | Illuminance (fc) |
|---|---|---|
| Moonlight (full) | 0.1-0.3 | 0.01-0.03 |
| Living Room | 100-150 | 10-15 |
| Office (general) | 300-500 | 30-50 |
| Office (desk task) | 500-750 | 50-75 |
| Surgical Lighting | 10,000-50,000 | 1,000-5,000 |
| Direct Sunlight | 32,000-100,000 | 3,000-10,000 |
Practical Applications
Office Lighting Design
EN 12464-1 specifies 500 lux for office workstations. Uniformity ratio should exceed 0.6. Glare control requires proper luminaire selection and placement.
Photography Lighting
Portrait photography uses 500-1,000 lux key light. Product photography may need 2,000-5,000 lux. Light meters measure illuminance for exposure settings.
Workplace Safety
OSHA specifies minimum illuminance for different tasks. Assembly work requires 100 fc (1,000 lux). Warehouses need 5-10 fc (50-100 lux).
Plant Growth Lighting
Photosynthesis requires specific illuminance levels. Leafy greens need 10,000-20,000 lux. Fruiting plants require 20,000-40,000 lux for optimal growth.
Frequently Asked Questions
What is the difference between lux and lumens?
Lumens measure total light output from a source. Lux measures light arriving at a surface. One lumen spread over one square meter equals one lux.
What is a foot-candle?
Foot-candle is the Imperial unit of illuminance. One foot-candle equals one lumen per square foot. One foot-candle equals 10.76 lux.
How do you calculate illuminance from a light bulb?
Use the inverse square law. E = I/d² where I is luminous intensity in candelas and d is distance in meters. For omnidirectional bulbs, I ≈ lumens/4π.
How do you measure illuminance?
Use a lux meter or light meter. Place the sensor at the work surface height. Take multiple readings to assess uniformity. Smartphone apps provide approximate readings.
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