Lux to Candela Calculator

About the Lux to Candela Calculator

A lux to candela calculator determines the luminous intensity required from a light source to achieve a target illuminance at a specific distance, using the inverse square law in reverse: cd = Lux × d². This is the specification calculation for lighting design — you know what lux level you need at a surface (from a lighting standard, photography requirement, or safety specification) and at what distance the light will be mounted, and you need to find the candela value the fixture must provide. The result tells you what type and size of lamp or LED fixture to specify. This is widely used in automotive lighting (how many candela must a headlamp produce for road visibility?), sports and architectural lighting (what fixture CBCP is needed for a target lux at field/wall level?), photography (what flash candela produces correct exposure at 3m?), and emergency and safety lighting design (what emergency luminaire candela meets the 1 lux at escape route standard?). Our calculator also shows the result in footcandles and millicandela for small LED sources.

How It Works

cd = Lux × d². Example 1 (warehouse high bay): 300 lux required at floor level, 8m mounting height. cd = 300 × 8² = 300 × 64 = 19,200 cd. This is the minimum center beam intensity needed from a high-bay LED fixture. Available fixtures at 100W LED typically produce 12,000-20,000 cd peak — so a 100W high-bay is marginal; specify 150W or use two 100W fixtures. Example 2 (automotive): US FMVSS 108 requires headlamps to illuminate a specific test point (0° vertical, 1.5° left) to at least 12,000 cd. Example 3 (emergency exit): BS EN 1838 requires 1 lux along escape route centerline at 2m width. For emergency luminaire at 3m height covering 1m radius: cd = 1 × 3² = 9 cd minimum needed. Example 4 (camera tracking spotlight): 500 lux needed at 15m: cd = 500 × 225 = 112,500 cd — a very powerful spotlight (equivalent to a 2kW theatrical followspot).

Tips & Best Practices

• ✓The 10× rule for point source assumption: the inverse square law applies accurately when the distance is at least 10× the largest dimension of the light source. For a 500mm × 500mm LED panel, use point source formula only at distances beyond 5m. Closer than 5m, the panel behaves as an area source and illuminance falls off less steeply.
• ✓Cosine correction for angled incidence: the formula Lux = cd / d² assumes the light hits the surface perpendicularly. For a light source at an angle θ from perpendicular: Lux = cd × cos(θ) / d². A spotlight 30° off-axis: actual lux = calculated lux × cos(30°) = × 0.866 (13% less).
• ✓Long-throw architectural lighting: retail window and museum display lighting often uses narrow-beam LEDs 5-10m from the target. At 7m: cd = lux target × 49. To maintain 1,000 lux on a display at 7m: cd = 49,000 cd needed — a very high-output narrow beam. This is why retail lighting uses PAR and AR spotlights with 10-25° beams rather than wide floodlights.

Who Uses This Calculator

Lighting designers specifying minimum candela requirements for fixtures based on mounting height and target lux. Automotive engineers verifying that headlamp designs meet regulatory minimum candela test requirements. Photographers calculating required flash peak intensity for correct exposure at a given subject distance. Security engineers specifying CCTV illumination fixtures based on camera sensitivity and coverage distance.