Illuminance Conversion Formulas
Complete formulas for converting between different illuminance units including lux, foot-candles, phots, and specialized lighting measurements.
💡 Basic Illuminance Unit Conversions
Primary Illuminance Units
Lux ↔ Foot-candles
Example: 1,000 lux
1,000 lux = 92.9 foot-candles
Foot-candles ↔ Lux
Example: 50 foot-candles
50 fc = 538.2 lux
Phot ↔ Lux
Example: 0.1 phot
0.1 phot = 1,000 lux
Illuminance-Luminance Relationships
Lambert's Law
1,000 candela at 2m, 30° angle
Illuminance: 216.5 lux
Reflected Light
500 lux on 80% reflective surface
Luminance: 127.3 cd/m²
🏢 Common Illuminance Levels Reference
| Environment/Task | Lux | Foot-candles | Application |
|---|---|---|---|
| Starlight | 0.0001 | 0.00001 | Natural minimum |
| Moonlight (full) | 0.25 | 0.023 | Night vision |
| Street Lighting | 5-15 | 0.5-1.4 | Pedestrian safety |
| Living Room | 50-200 | 5-18 | General activities |
| Office General | 300-500 | 28-46 | Computer work |
| Office Task | 500-1000 | 46-93 | Reading, writing |
| Retail General | 500-1000 | 46-93 | Merchandise display |
| Precision Work | 1000-2000 | 93-186 | Detailed assembly |
| Surgery | 10,000-25,000 | 929-2323 | Medical procedures |
| Bright Sunlight | 100,000 | 9,290 | Natural maximum |
🎨 Lighting Design Calculations
Lumen Method (Zonal Cavity)
Average Illuminance Formula
Office: 20×30 ft, 24 fixtures, 3200 lm each
CU = 0.65, LLF = 0.85
Average: 70.7 fc (761 lux)
Required Number of Fixtures
Target: 500 lux (46.5 fc), 400 m²
Fixture: 4,000 lm, CU = 0.7, LLF = 0.8
Required: 90 fixtures
Point Source Calculations
Single Point Source
1000 cd fixture, 3m high, 2m horizontal
Distance: d = √(H² + r²) = √(9 + 4) = 3.6m
cos α = H/d = 3/3.6 = 0.833
Horizontal illuminance: 64.2 lux
Multiple Point Sources
4 fixtures in 2×2 grid, 6m spacing
Center point illuminance from each:
Corner fixtures: 4 × 32.1 = 128.4 lux
Total at center: 128.4 lux
Point under fixture: 64.2 + (3 × 18.7) = 120.3 lux
Uniformity ratio: 0.94 (excellent)
⚡ Energy Efficiency in Lighting
Luminous Efficacy Comparisons
Incandescent
Efficacy: 10-17 lm/W
60W bulb: 800 lumens
Power density: 15 W/m² for 300 lux
Annual cost: $52 (10¢/kWh, 8hr/day)
Least efficient, high heat
Fluorescent
Efficacy: 50-100 lm/W
32W T8: 2,850 lumens
Power density: 8 W/m² for 300 lux
Annual cost: $23 (10¢/kWh, 8hr/day)
Good efficiency, contains mercury
LED
Efficacy: 80-200 lm/W
15W LED: 1,600 lumens
Power density: 4 W/m² for 300 lux
Annual cost: $11 (10¢/kWh, 8hr/day)
Highest efficiency, long life
Lighting Power Density Calculations
Target LPD Formula
Office: 500 lux, LED 150 lm/W
CU = 0.7, LLF = 0.85
Target LPD: 5.6 W/m²
Energy Code Compliance
ASHRAE 90.1 Office: 11 W/m² max
California T24: 7 W/m² typical
EU Standard: 8-12 W/m² office
LEED Points: 30% below code
Daylight controls: 10-30% savings
Occupancy sensors: 15-25% savings