Lux vs Lumens: Understanding the Difference

Navigating the world of lighting can be tricky, especially when you encounter terms like “lux” and “lumens.” These two measurements are crucial for understanding light, but they represent different aspects of it. Let’s break down the difference.

Introduction to Light Measurement

Light level, or illuminance, refers to the amount of light falling on a surface. We measure this using units like foot candles or lux, the metric SI unit. Think of it this way: a foot candle represents one lumen of light density per square foot.

Lux: Measuring Light Intensity

• What it is: Lux measures the intensity of light, specifically the amount of light output within a given area.
• What it tells you: Lux helps us understand not just the total visible light, but also the intensity of illumination on a surface. For example, a surface directly under a strong light source will have a higher lux reading than a surface in a dimly lit corner.
• How it’s calculated: One lux is equal to one lumen per square meter.

Lumens: Measuring Total Light Output

• What it is: Lumens measure the total amount of light emitted by a light source, such as a flashlight or lamp.
• What it tells you: The higher the lumen value, the brighter the overall light output. Think of it as the total “amount” of light the source is capable of producing.
• Relationship to candela: Lumens can be expressed in candela. An isotropic (radiating equally in all directions) one-candela light source emits approximately 4π lumens of luminous flux.

Key Differences Summarized

Conversion Formulas

Here are some handy conversion formulas to help you switch between units:

• 1 Lux = 1 Lumen / meter²
• 1 foot candle (fc or ftcd) = 1 lumen / sq. ft = 10.752 lux
• 1 lux = 1 lumen / sq. meter = 0.0001 phot = 0.0929 foot candle
• 1 phot = 1 lumen / sq. cm = 10,000 lumens / sq. cm = 10,000 lux

Further Reading

For more information, consider looking into these topics:

• Recommended Light Levels in Indoors and Outdoors
• LED Test Equipments
• Lux Light Meters (used for testing lighting devices)