Lumens to Candela Calculator

About the Lumens to Candela Calculator

A lumens to candela calculator converts total luminous flux in lumens to luminous intensity in candela by dividing by the solid angle of the light beam. This calculation tells you how intensely a light source shines in a specific direction — critical for applications where the concentration of light matters more than total output. A 1,000 lm spotlight concentrated into a narrow 5° beam (0.006 sr) produces over 166,000 cd — intensely bright in the beam. The same 1,000 lumens from an omnidirectional bulb averaging 4π sr produces only 79.6 cd in any direction. This lumens-to-candela relationship explains why spotlights appear dramatically brighter than reading lamps of the same wattage when viewed from within the beam: they are concentrating the same total light energy into a much smaller solid angle. Candela is the foundational unit for measuring glare, specifying stage and architectural accent lighting, evaluating signal lights and hazard warnings, and complying with automotive headlamp regulations. Our calculator handles standard beam angles from 1° to full sphere, with solid angle presets and a manual input for precise photometric calculations.

How It Works

cd = lm / Ω (steradians). Solid angle Ω = 2π(1 - cos θ) for a cone with half-angle θ. Converting half-angle from beam angle: half-angle = beam angle / 2. Example 1 (LED spotlight): 800 lm spotlight with a 25° beam angle. Half-angle = 12.5°. Ω = 2π(1 - cos 12.5°) = 2π(1 - 0.9763) = 2π × 0.0237 = 0.1490 sr. cd = 800 / 0.149 = 5,369 cd. This bright spotlight has a center beam candlepower of over 5,000 cd. Example 2 (omnidirectional bulb): 1,600 lm "100W equivalent" LED bulb. For full sphere (4π sr): cd = 1,600 / 12.566 = 127.3 cd — relatively low intensity in any direction because light is spread everywhere. Example 3 (LED traffic signal): red traffic LED signal needs to be visible at 300 lux at 100m in direct sunlight. At 100m: cd = lux × d² = 300 × 10,000 = 3,000,000 cd required peak intensity in the viewing direction.

Tips & Best Practices

• ✓Glare control: high candela values at eye level cause discomfort glare. The Unified Glare Rating (UGR) formula uses candela at specific angles from vertical as inputs. Limiting luminaire candela above 65° from nadir (downlight angle) to below 1,000 cd/m² is a typical glare control requirement for office lighting.
• ✓Signal light requirements: ISO 15008 and ANSI Z535 specify minimum candela values for safety signals, emergency lights, and hazard warnings. Emergency exit signs: minimum 50 cd in the viewing direction. Aviation obstruction lights: FAA Advisory Circular AC 70/7460 specifies specific candela requirements by tower height.
• ✓Photography incident light: converting your flash lumens to candela in the beam direction helps predict guide number. Guide number (GN) = distance × f-number = √(candela × 10/ISO_factor). Knowing the candela output of a flash head allows calculating exposure for any subject distance.

Who Uses This Calculator

Lighting engineers converting lumen specifications to candela for photometric reports and glare calculations. LED designers specifying component intensity requirements for directional light sources. Traffic signal engineers verifying that fixture candela meets regulatory visibility requirements. Photographers calculating guide number and expected exposure from flash equipment specifications.