Light Intensity Converter

Convert between lux, foot-candles, phot, nox and other light intensity units

Options
Preset Scenarios
Light Intensity Ranges
0 lx (Dark)
100 lx
1,000 lx
10,000 lx
50,000 lx
120,000 lx
Converter
Conversion Formula: 1 lx = 0.092903 fc
About Light Intensity

Light intensity, also known as illuminance, is a measure of how much luminous flux (light) is incident on a surface per unit area. It's typically measured in lux (lumens per square meter) in the SI system or foot-candles (lumens per square foot) in the imperial system.

Illuminance decreases with the square of the distance from the light source (inverse square law) and is affected by factors like reflectance, absorption, and scattering in the medium.

  • Moonlight 0.1-1 lx
  • Night (street lighting) 10-20 lx
  • Living room 100-500 lx
  • Office lighting 500-1,000 lx
  • Overcast day 1,000-10,000 lx
  • Full daylight 10,000-25,000 lx
  • Direct sunlight 32,000-130,000 lx

Unit Symbol Definition Conversion to Lux
Lux lx SI unit (lumens per square meter) 1 lx = 1 lx
Foot-candle fc Imperial unit (lumens per square foot) 1 fc ≈ 10.764 lx
Phot ph CGS unit 1 ph = 10,000 lx
Nox nx Historical unit 1 nx = 0.001 lx
Lumen/m² lm/m² Equivalent to lux 1 lm/m² = 1 lx
Lumen/cm² lm/cm² Higher intensity unit 1 lm/cm² = 10,000 lx
Watt/m² (photopic) W/m² Energy-based intensity 1 W/m² ≈ 683 lx (at 555 nm)
Lumen/ft² lm/ft² Alternate form of foot-candle 1 lm/ft² ≈ 10.764 lx
Candela/sr•m² cd/sr•m² Used for luminance Depends on geometry
Kilolux klx Higher intensity unit 1 klx = 1,000 lx
Recent Conversions
  • 1000 lx → 92.90 fc Just now
Quick Reference
Common Conversions:
  • 1 lx 0.0929 fc
  • 1 fc 10.764 lx
  • 1 ph 10,000 lx
  • 1 klx 1,000 lx
Applications:
  • Photography lighting
  • Architectural lighting design
  • Greenhouse lighting
  • Workplace illumination standards
  • Scientific research

Learning Center: Light Intensity & Illuminance

What This Converter Teaches You: This tool helps you understand illuminance - how we measure the amount of light falling on surfaces. You'll learn to convert between different measurement systems used worldwide.

Understanding Each Unit

lx Lux (SI Unit)

Simple meaning: 1 lux = 1 lumen of light spread over 1 square meter. Imagine one candle's light evenly distributed over a 1m x 1m surface.

When to use: Worldwide standard for lighting design, photography, workplace safety standards.

fc Foot-candle (Imperial)

Simple meaning: 1 foot-candle = 1 lumen of light spread over 1 square foot. Used in United States lighting standards.

When to use: American architecture, film/TV lighting, older lighting specifications.

ph Phot (CGS Unit)

Simple meaning: 1 phot = 10,000 lux. Used in older scientific literature. "Phot" comes from Greek "phos" meaning light.

When to use: Reading older physics papers, some European technical documents.

nx Nox (Historical)

Simple meaning: 1 nox = 0.001 lux. Used for very low light measurements. Latin for "night."

When to use: Measuring moonlight, starlight, or very dim lighting conditions.

Real-World Examples

Step-by-Step Conversion Understanding: The converter works in two steps: (1) Convert your input to lux (the common reference), then (2) Convert from lux to your desired output unit. This "lux bridge" method prevents errors.

How to Use This Tool Effectively

Input Value Interpretation:
Understanding Your Results:

Common Student Mistakes to Avoid

  1. Confusing lux with lumens: Lux measures light ON a surface, lumens measure light FROM a source
  2. Area confusion: Forgetting lux uses square meters while foot-candles use square feet. For help with area calculations, our modern area converter can clarify these differences.
  3. Inverse square law: Doubling distance from light source reduces illuminance to ¼, not ½
  4. Unit mixing: Never mix SI and imperial units in calculations without conversion
  5. Photopic vs. scotopic: These conversions assume photopic (daytime) vision, not scotopic (nighttime)

Exam and Study Tips

Memorization Shortcuts:
  • "10-10 Rule": 10 lux ≈ 1 foot-candle (actually 10.76, but 10 is close enough for estimates)
  • "Thousand Rule": 1 phot = 10,000 lux = 10 kilolux
  • "Night Rule": 1 nox = 0.001 lux (thousand times smaller)
  • Remember: lux = lumen/m² (SI), foot-candle = lumen/ft² (Imperial)
Visual Understanding Suggestions:
  • Visualize 1 lux as one candle lighting a 1m x 1m table
  • Imagine 100 lux as 100 candles on that same table
  • Think of foot-candles literally: candles per square foot
  • Use the preset scenarios to build intuition for different light levels
Exam Strategy: In physics exams, always show your conversion steps. Write "Converting to lux first..." to demonstrate understanding. Use scientific notation for very large/small numbers.

Frequently Asked Questions

Q: Why do we need so many different light units?

A: Different fields developed their own systems over time. Lux is international standard, foot-candles are American, phot is older scientific, and nox is for very dim light. It's similar to how we have separate tools for length conversion and measuring weight with our mass-weight converter tool.

Q: How accurate are these conversions?

A: The mathematical conversions are precise. Real-world measurements have ±5% variation due to measurement conditions, sensor calibration, and environmental factors.

Q: Can I use this for plant grow lights?

A: Yes, but plants respond to specific wavelengths. Lux measures visible light to humans. For plants, consider PAR (Photosynthetically Active Radiation) measurements too.

Q: Why does Watt/m² conversion mention "at 555 nm"?

A: The conversion from watts (energy) to lux (perceived brightness) depends on wavelength. 555 nm is where human eyes are most sensitive in daylight.

Q: What's the difference between illuminance and luminance?

A: Illuminance (measured here) is light falling ON a surface. Luminance is light coming FROM a surface (like a screen or sign).

Important Notes

Accuracy Disclaimer

This tool provides mathematically precise conversions between unit definitions. Real-world measurements may vary due to:

  • Measurement instrument calibration differences
  • Environmental conditions (temperature, humidity)
  • Angle of light incidence (not always perpendicular)
  • Surface reflectance properties
  • Spectral sensitivity of measurement devices

For critical applications (safety standards, legal requirements, medical use), consult official standards and use calibrated measurement equipment.

Update Notice (November 2025)

This educational edition was enhanced with learning materials while preserving all original conversion algorithms. The calculation engine remains unchanged from the original version. All conversion factors are based on internationally accepted standards.

Learning tip: Try converting the same value through different units to build intuition. Notice patterns in how the numbers change!