Candela to Lux Converter & Inverse-Square Law Calculator

Candela to Lux, Inverse-Square Law & Lambert's Cosine Law Metrology

Illuminance (E) measures the luminous flux incident on a surface per unit area:

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• Inverse-Square Law for Point Sources**:

E = (I_v α) / (d^2)

Where I_v = Luminous Intensity (cd), d = Distance (m), α = Angle of Incidence

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• Foot-Candles (Imperial) & Metric Conversion**:

E_fc = (I_v) / (d_feet)^2 = E_lux10.76391

1 Foot-Candle (fc) = 10.7639104 Lux (lx) ≈ 10.764 lx

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• Standard Illuminance Requirements**:

• Emergency Stairwells / Corridors: 50 to 100 Lux = 5 to 10 fc

• General Office Workstations: 500 Lux = 46.5 fc Standard computer/reading desk

• Fine Mechanical Assembly / Drawing: 1,000 to 1,500 Lux = 93 to 140 fc

• Hospital Operating Theater: 100,000 to 160,000 Lux Maximum shadowless cavity luminaire

• Full Direct Sunlight: 32,000 to 100,000 Lux = 3,000 to 10,000 fc

How to Use This Calculator

Enter your Luminous Intensity (Candela / cd) (e.g. 1500 in cd), From Luminous Intensity Unit — choose from Candela (cd = lm/sr - SI Standard), Millicandela (mcd = 10⁻³ cd), Kilocandela (kcd = 1,000 cd), Distance from Light Source (d) (e.g. 1.5 in distance), Distance Unit — choose from Meters (m - Standard SI), Feet (ft - Imperial), Centimeters (cm), Angle of Incidence (α in degrees relative to normal) (e.g. 0 for perpendicular in deg (°)), and Photometric & Lighting Design Benchmark — choose from Custom Intensity & Distance, Standard Candle at 1.0 m (1.0 cd @ 1.0 m) → 1.00 Lux = 0.0929 fc, Office Desk Task Spotlight (1,500.0 cd @ 1.5 m) → 666.67 Lux = 61.93 fc. Results update instantly as you type — adjust any value to compare scenarios.

The Formula

Lambert's Cosine Law & Inverse-Square Law: E = (I_v * cos(alpha)) / d^2

Worked Example

An electrical engineer calculates that a 1,500 cd spotlight mounted 1.5 meters above a desk delivers 666.7 Lux, confirming it satisfies OSHA and EN 12464-1 workspace lighting requirements.

Understanding Your Result

Unit conversion precision: round to the same number of significant figures as your original measurement. Converting 6 feet to meters gives 1.8288 m — but if your measurement was approximate (±1 inch), expressing more than 2 decimal places is false precision. For international trade and technical specifications, SI units (meters, kilograms, liters) are the standard. US customary units (feet, pounds, gallons) differ from Imperial UK units: 1 US gallon = 3.785 L vs. 1 UK gallon = 4.546 L.