Illuminance Converter – Convert Between All Illuminance Units
The Illuminance Converter is a free online tool that instantly converts illuminance values between ten standard photometric units used in lighting design, photography, horticulture, workplace safety, and scientific research. Whether you need to convert lux to foot-candles for a US lighting specification, translate phots to lux for a scientific publication, or compare light levels across measurement systems, this tool delivers accurate results in real time with a configurable all-units table.
All conversions are computed client-side using lux (lx) as the normalized base unit. Enter any value and the result updates instantly — no button press needed.
What Is Illuminance?
Illuminance is a photometric measure of the total luminous flux (light energy weighted by human eye sensitivity) received by a surface per unit area. It quantifies how much light falls onto a surface — such as a desk, a plant canopy, or a photographic subject — regardless of the direction or brightness of the light source.
The SI unit of illuminance is the lux (lx), defined as one lumen per square metre (lm/m²). Unlike luminance (which measures how bright a surface appears to an observer), illuminance describes only the incoming light. A matte black surface and a mirror-finish surface may receive identical illuminance while appearing completely different to the human eye.
Supported Illuminance Units
SI / Metric and Scaled Units
- Lux (lx) — The SI unit of illuminance and the global standard. Equal to 1 lumen per square metre. Used universally in lighting design, safety standards, environmental monitoring, and scientific research.
- Millilux (mlx) — One thousandth of a lux (0.001 lx). Useful for characterising very dim environments such as a moonlit night (~1 mlx) or deep indoor shadows.
- Kilolux (klx) — One thousand lux (1,000 lx). Convenient for expressing bright outdoor conditions: overcast daylight is approximately 10 klx; direct sunlight reaches 100 klx or more.
Imperial / US Units
- Foot-candle (fc) — The imperial unit of illuminance, equal to one lumen per square foot. 1 fc ≈ 10.764 lx. Widely used in US architectural lighting, photography, film production, and workplace illumination standards. The IESNA (Illuminating Engineering Society of North America) publishes many recommendations in foot-candles.
CGS Units
- Phot (ph) — The CGS unit of illuminance, equal to 1 lumen per square centimetre. 1 ph = 10,000 lx. Appears in older European scientific literature and some legacy industrial standards where CGS units remain in use.
- Nox (nx) — An obsolete unit for dim-light illuminance. 1 nox = 0.001 lx = 1 millilux. Defined specifically to describe night-time illumination levels and occasionally referenced in older photometry texts.
Lumen-Based Units
- Lumen per square metre (lm/m²) — Exactly equivalent to lux. Explicitly states the photometric definition of illuminance: luminous flux per unit area.
- Lumen per square centimetre (lm/cm²) — Equivalent to the phot. 1 lm/cm² = 10,000 lx. Used in CGS photometric contexts.
- Lumen per square foot (lm/ft²) — Exactly equivalent to the foot-candle (fc). Common in US architectural lighting specifications.
- Lumen per square inch (lm/in²) — 1 lm/in² ≈ 1,550 lx. Occasionally used in optical engineering and close-up photometric measurements where small-area illumination is relevant.
How Illuminance Conversion Works
Every conversion normalizes through lux in two steps. First, the input value is multiplied by the source unit's conversion factor to obtain the equivalent in lux. Then that value is divided by the target unit's conversion factor to produce the final result.
Example — converting 500 lux to foot-candles:
- 500 lx ÷ 10.7639 lx/fc = 46.45 fc
Example — converting 1 phot to lux:
- 1 ph × 10,000 lx/ph = 10,000 lx
Common Illuminance Reference Values
- Starlit night (no moon): ~0.001 lx
- Full moon: ~1 lx
- Streetlighting: ~10 lx
- Stairwell / corridor minimum: ~50 lx
- Office lighting (ISO standard): 300–500 lx
- Operating theatre: 10,000–40,000 lx
- Overcast daylight: ~10,000 lx
- Direct sunlight (midday): 80,000–120,000 lx
Practical Applications
- Architectural and workplace lighting: International and national standards (ISO, EN, IESNA) specify minimum illuminance levels for different tasks in lux or foot-candles. This converter lets designers verify compliance across both unit systems quickly.
- Photography and cinematography: Exposure calculations and light meter readings are often expressed in foot-candles (US) or lux (international). Converting between the two is essential when working across different production environments.
- Horticulture and plant growth: Greenhouse lighting and grow-light specifications use lux, kilolux, or foot-candles to define the light intensity needed for photosynthesis and plant development at various growth stages.
- Workplace safety: Occupational health regulations prescribe minimum illuminance for hazardous tasks. Engineers often need to convert between lux (ISO/EN standard) and foot-candles (OSHA/US standard) when comparing international guidelines.
- Scientific research: Photobiology, vision science, and environmental monitoring use various illuminance units depending on equipment and publication conventions. The phot and nox units appear in legacy literature.
Tips for Using the Illuminance Converter
- Enable Show all unit conversions to display your input value in all ten units simultaneously, grouped by measurement system. This is ideal for quickly cross-referencing values across different industry standards.
- Use the Swap button to immediately reverse the conversion direction — for example, verify that a lux-to-foot-candle conversion round-trips back to the original value.
- Adjust the Decimal places setting for very small conversions (such as millilux to phots) where extra precision matters. The tool automatically switches to scientific notation for extreme values.
- The conversion history panel records your last ten conversions. Click any entry to restore those values, making it easy to compare multiple illuminance levels without re-entering values manually.