Lumen Calculator
Convert between lumens, lux, and candelas with precision photometry calculations
Photometry Unit Conversion
Total amount of light emitted by the source
360° for omnidirectional source, smaller for directional
Distance between light source and receiver
Area of the illuminated surface
Photometry Results
Luminous Flux
Luminous Intensity
Illuminance
Additional Units
Light source type: Bright (e.g., 75W incandescent)
Key formulas: lumens = candelas × Ω, lux = lumens/area, lux = candelas/distance²
Example Calculation
LED Bulb Example
Scenario: 800-lumen LED bulb in a room
Radiation angle: 360° (omnidirectional)
Distance: 2 meters from bulb
Surface area: 1 m² table surface
Calculation Steps
1. Solid angle Ω = 4π = 12.57 steradians (full sphere)
2. Candelas = 800 lm ÷ 12.57 sr = 63.7 cd
3. Illuminance = 63.7 cd ÷ (2 m)² = 15.9 lux
4. Alternative: 800 lm ÷ 1 m² = 800 lux (if all light hits surface)
Photometry Units
Lumens
Luminous flux
Total light output
Candelas
Luminous intensity
Light per direction
Lux
Illuminance
Light received per area
Typical Illuminance Levels
Photometry Tips
Lumens measure total light output from source
Lux measures light received per unit area
Candelas measure light intensity in direction
Distance greatly affects illuminance (inverse square)
1 foot-candle = 10.764 lux
Understanding Photometry Units
What is Photometry?
Photometry is the science of measuring light as perceived by the human eye. Unlike radiometry (which measures electromagnetic radiation), photometry weighs the power at each wavelength according to the luminosity function that models human visual sensitivity.
Key Applications
- •Lighting design and engineering
- •Photography and cinematography
- •Automotive headlight specification
- •Building codes and safety standards
- •Display and screen brightness
Conversion Formulas
Φ = I × Ω
E = Φ / A
E = I / d²
Ω = 2π × [1 - cos(θ/2)]
- Φ: Luminous flux (lumens)
- I: Luminous intensity (candelas)
- E: Illuminance (lux)
- Ω: Solid angle (steradians)
- A: Area (square meters)
- d: Distance (meters)
- θ: Apex angle (radians)
Remember: Light intensity follows the inverse square law - doubling the distance quarters the illuminance.
Solid Angles and Light Distribution
Omnidirectional
4π steradians (360°) - Light bulb, star
Hemisphere
2π steradians (180°) - Downlight, ceiling fixture
Narrow Beam
< π steradians - Spotlight, laser, flashlight