Candela to Lux Calculator

About the Candela to Lux Calculator

A candela to lux calculator applies the inverse square law to convert luminous intensity in candela to illuminance in lux at a given distance — one of the most practically useful calculations in lighting, photography, security lighting design, and sports facility planning. The inverse square law states that illuminance decreases with the square of the distance from the source: double the distance, quarter the illuminance. This means a 1,000 cd source illuminates at 1,000 lux at 1 m, but only 250 lux at 2 m, 111 lux at 3 m, and just 10 lux at 10 m. Our calculator makes this distance-dependent falloff immediately visible, showing illuminance at the entered distance and at half and double that distance for quick sensitivity analysis. This is used by photographers calculating exposure at different subject distances from a monolight, security engineers sizing floodlights for perimeter surveillance, sports lighting designers verifying adequate illuminance at field level, and theatrical lighting designers predicting fixture output at the stage. The result is shown in both lux and footcandles for international compatibility.

How It Works

Lux = cd / d², where d is the distance in metres. This applies to point sources — for large-area sources (LED panels, softboxes), the formula applies only at distances much greater than the source size. Example 1 (stage lighting): a 5,000 cd Fresnel theatrical fixture at 8m distance. Lux = 5,000 / 8² = 5,000 / 64 = 78.1 lux. Not enough for broadcast TV (which needs 800-1,000 lux). Move to 3m: 5,000 / 9 = 556 lux — better, but still below broadcast standard. Example 2 (photography): A 25,000 cd monolight strobe at 2m: 25,000 / 4 = 6,250 lux incident light. At ISO 100, 1/200s: EV = log₂(6,250 × 0.1 / 0.65) = log₂(961) ≈ 10 → approximately f/11. Example 3 (security): a 3,000 cd PIR floodlight at 20m perimeter: 3,000 / 400 = 7.5 lux — barely adequate for surveillance camera operation (cameras need minimum 3-10 lux for acceptable image).

Tips & Best Practices

• ✓Photographer guide numbers: flash guide numbers (GN) encode the candela-to-lux relationship. GN = √(Lux × d²) = √(cd). At GN 40 (metres, ISO 100): peak candela = 40² = 1,600 cd. At 2m subject distance: lux = 1,600 / 4 = 400 lux. f-number = GN / distance = 40/2 = f/20 at ISO 100.
• ✓Security camera minimum lux: IP cameras typically need: colour mode 0.1-3 lux minimum; low-light mode 0.01-0.1 lux; IR-assisted: can work at 0.001 lux. Calculate required floodlight candela for your perimeter distance: cd = Lux × d². For 3 lux at 30m: cd = 3 × 900 = 2,700 cd.
• ✓Sports lighting design: FIFA minimum for international matches: 1,400 lux horizontal, 1,000 lux vertical. For high-mast floodlights at 20m height: each fixture needs cd = lux × d²/count. Precise sports lighting design uses photometric software (DIALUX, AGI32) rather than point-source approximations.

Who Uses This Calculator

Photographers and cinematographers calculating expected lux at a subject distance from a light source with known candela. Security engineers sizing floodlights for perimeter surveillance at specified distances. Lighting designers verifying architectural accent light levels at target surfaces. Sports facility engineers performing preliminary floodlight specification calculations.