Question 1

What is illuminance and how does it differ from luminance?

Accepted Answer

Illuminance measures how much luminous flux (light) is incident on a surface per unit area (measured in lux or foot-candles), while luminance measures how much light is emitted or reflected from a surface in a particular direction (measured in candela per square meter or nits). Illuminance is about light falling on a surface (incident light), whereas luminance is about light coming from a surface (emitted or reflected light). For example, illuminance would measure how much light hits your desk, while luminance would measure how bright your computer screen appears.

Question 2

When would I use foot-candles versus lux?

Accepted Answer

Foot-candles (fc) are commonly used in the United States for architectural lighting design, while lux (lx) is the international standard (SI unit) used in most other countries. 1 foot-candle equals approximately 10.764 lux. While the lighting industry in the U.S. primarily uses foot-candles, scientific and international projects typically use lux. This converter makes it easy to switch between these units when working with international standards or collaborating across regions.

Question 3

What are typical illuminance values for common situations?

Accepted Answer

Illuminance values vary widely:
• Direct sunlight: 32,000-100,000 lx
• Overcast daylight: 1,000-10,000 lx
• Office lighting: 300-500 lx
• Home lighting: 100-300 lx
• Street lighting: 5-30 lx
• Full moon: ~0.25 lx
• Moonless night: ~0.001 lx
Photographic light meters often measure in lux or foot-candles, with typical exposure values ranging from about 10 lx (dim interior) to 100,000 lx (bright sunlight).

Question 4

Why are there so many different units for measuring light?

Accepted Answer

The variety of light measurement units exists because of historical development, different measurement systems (metric vs. imperial), and the need to measure different aspects of light. The phot (ph) comes from the older CGS system, while lux is from the modern SI system. Foot-candles persist in the U.S. due to customary units. Lumen-based units relate to total luminous flux, while candela-based units relate to luminous intensity. Each unit has specific applications where it's most convenient - for example, phots for very high illuminance (like in scientific instruments), lux for general use, and foot-candles for architectural applications in the U.S.

Question 5

How does illuminance relate to exposure in photography?

Accepted Answer

In photography, illuminance (measured in lux or foot-candles) directly affects exposure. The exposure value (EV) system is based on illuminance levels, with each EV representing a doubling or halving of light. Typical conversions:
• EV 15 = 32,768 lx (bright sunlight)
• EV 12 = 4,096 lx (cloudy bright)
• EV 9 = 512 lx (overcast)
• EV 6 = 64 lx (indoor lighting)
• EV 3 = 8 lx (dim interior)
Photographers often use light meters that measure in lux or foot-candles to determine proper camera settings for a given lighting condition. This converter helps translate between different light measurement units used in photography equipment and standards.

Illumination Measurement Converter

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