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About Illuminance Conversion

Illuminance measures the amount of light falling on a surface—the total luminous flux per unit area. It tells you how well-lit a surface is, regardless of what color it is or how much it reflects. This distinction is important: a black surface and white surface under the same lamp have identical illuminance, but very different luminance (perceived brightness). Illuminance is what a light meter measures and what lighting engineers specify when designing spaces—it's the starting point for ensuring adequate visibility and visual comfort.

The SI unit is lux (lx), equal to one lumen per square meter. Illuminance follows the inverse square law: doubling distance from a point source reduces illuminance to one-quarter. Illuminance standards define minimum lighting levels for safety, productivity, and comfort in different environments—offices typically require 300-500 lux, while surgical operating rooms need 10,000-100,000 lux. Building codes, occupational health regulations, and energy codes all reference illuminance requirements.

Our converter handles all standard illuminance units used in lighting design, photometry, and regulatory compliance.

Common Illuminance Conversions

Illuminance Unit Reference

Lux (lx) – The SI unit of illuminance, equal to one lumen per square meter. The worldwide standard for lighting specifications and recommendations. Typical indoor values: home living room 50-200 lux, office workspace 300-500 lux, detailed assembly work 750-1000 lux, surgical operating table 10,000+ lux. Outdoor: overcast day ~1000 lux, direct sunlight 30,000-100,000 lux. Photography light meters read in lux or EV (related by EV = log₂(lux/2.5)).

Foot-candle (fc) – The US customary unit, defined as one lumen per square foot. 1 fc ≈ 10.764 lux. Remains prevalent in American lighting industry, building codes (IESNA), and HVAC specifications. US workplace standards often specify foot-candles: general office 30-50 fc, detailed work 50-100 fc. Converting mentally: divide lux by 10 for approximate foot-candles.

Phot (ph) – The CGS unit of illuminance. 1 phot = 10,000 lux = 1 lumen/cm². Too large for most practical applications—even direct sunlight is only about 10 phots. Occasionally encountered in older scientific literature and high-intensity applications like solar concentrators.

Nox – A unit for very low light levels, defined as 1 millilux (0.001 lux). Used in astronomy, night vision research, and scotopic (rod-mediated) vision studies. Full moonlight provides about 250 nox; starlight about 0.1 nox. The threshold of human vision is approximately 0.01 nox.

Meter-candle – An older term equivalent to lux (1 meter-candle = 1 lux). The name emphasizes the relationship: illuminance from 1 candela at 1 meter distance. Rarely used in modern literature but may appear in historical documents.

Learn More

Explore our in-depth guides on this topic:

• Understanding Lumens and Lux
• LED vs Traditional Lighting
• Lighting Standards and Guidelines