Illuminance Converter – Lux, Foot-candles and Phot

What illuminance measures

Illuminance is the amount of light falling on a surface. The SI unit is the lux (lx): one lumen spread over one square metre. The converter supports:

• 1 kilolux (klx) = 1,000 lx, used for daylight
• 1 foot-candle (fc) = 1 lumen per square foot = 10.764 lx
• 1 phot (ph) = 1 lumen per square centimetre = 10,000 lx

Foot-candles are common in US building codes and lighting guides, while the rest of the world uses lux.

Typical light levels

• Full moon on a clear night: about 0.1–0.3 lx
• Living room in the evening: 50–150 lx
• Office workspace: 300–500 lx (30–50 fc)
• Detailed work, such as drawing or electronics: 750–1,000 lx
• Overcast day outdoors: 1,000–10,000 lx
• Direct sunlight: up to about 100,000 lx (10,000 fc)

Lux versus lumens

Lumens describe the total light a lamp emits; lux describes how much of it lands on a surface. A 1,000-lumen lamp lighting 10 m² evenly gives about 100 lx, but the same lamp focused on 1 m² gives 1,000 lx. That is why lighting plans start from the required lux on a desk or floor and work back to the number of lamps.

Plants and photography

Indoor plants and grow lights are sometimes described in lux or foot-candles, although plant scientists prefer photosynthetic photon flux (PPFD), which counts only the light plants use. Light-loving houseplants typically need several thousand lux. In photography, handheld light meters may show lux or foot-candles, and the reading can be converted to an exposure value.

Measuring illuminance

A lux meter, or a phone app using the front light sensor, gives a quick reading. Phone sensors are not calibrated, so use them for comparisons rather than for checking compliance with a standard.

Sources

Conversion factors follow the SI definitions in the BIPM SI Brochure and the exact factors for US customary and imperial units in NIST Special Publication 811.