Illuminance Converter
Convert lux (lx), foot-candle (fc), phot (ph), nox (nx), lm/m², lm/ft².
Result
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Base Unit
Relative Value
*Diagram shows values relative to the selected base unit (Lux).
Unit Information
What is Lux (lx)?
Lux (symbol: lx) is the SI derived unit of illuminance, equal to one lumen per square meter (lm/m²). It measures the intensity of light that strikes or passes through a surface. Higher lux values indicate brighter illumination. Lux is widely used for specifying lighting levels for various environments, from offices and homes to outdoor spaces and specialized applications like plant growth.
What is Lumen per square meter (lm/m²)?
This is not a separate unit but the definition of Lux. It reinforces that illuminance is a measure of luminous flux (lumens) per unit area (square meters).
What is Foot-candle (fc)?
A foot-candle (symbol: fc or ft-c) is an imperial and US customary unit of illuminance. One foot-candle is defined as the illuminance on a one-square-foot surface from a uniform source of one candela. It is equivalent to one lumen per square foot. Foot-candles are commonly used in the United States for lighting specifications in architecture, interior design, and cinematography.
What is Lumen per square foot (lm/ft²)?
This is the definition of a foot-candle, expressing illuminance as the amount of luminous flux (lumens) per square foot.
What is a Phot (ph)?
The phot is the CGS (centimeter-gram-second) unit of illuminance, equal to one lumen per square centimeter. As 1 m² equals 10,000 cm², one phot is equal to 10,000 lux, making it a large unit that is rarely used today.
What is a Milliphot (mph)?
A milliphot is one-thousandth of a phot. 1 mph = 10 lux. This CGS unit is also uncommon in modern practice.
What is a Nox (nx)?
The nox is an obsolete unit of illuminance, equal to one millilux (0.001 lx). It was intended for measuring very low light levels, such as those found under moonlight or starlight.
What is a Kilolux (klx)?
A kilolux is one thousand lux (1 klx = 1000 lx). This unit is used for measuring very high illuminance levels, such as those found in direct sunlight or in specialized industrial lighting applications.
Formulas
1 fc ≈ 10.764 lx
To convert foot-candles to lux, multiply by approximately 10.764.
1 lx ≈ 0.0929 fc
To convert lux to foot-candles, multiply by approximately 0.0929.
1 Phot (ph) = 10,000 lx
The phot is a CGS unit, equal to one lumen per square centimeter.
Illuminance (Lux) = Luminous Flux (lm) / Area (m²)
The fundamental relationship between illuminance, total light output, and area.
Key Reference Points
- Overcast day: ~1,000 - 2,000 lux.
- Well-lit office: ~300 - 500 lux (approx. 30-50 fc).
- Residential living room: ~100 - 200 lux.
- Direct sunlight: ~32,000 - 130,000 lux.
- Full moonlight: ~0.05 - 0.3 lux.
- Sidewalks and pathways: 10 - 20 lux.
- General factory work: 200 - 500 lux.
- Detailed inspection work: 1000 - 2000 lux.
- Hospital operating theater: Can exceed 20,000 lux.
- TV studio lighting: 1000 - 2000 lux on subjects.
Did You Know?
Different activities require different illuminance levels for optimal visibility and comfort. For example, general office work might require 300-500 lux, while detailed technical drawing could need 1000-2000 lux. Outdoor daylight on a sunny day can exceed 100,000 lux, while a full moon might provide only about 0.1 lux.
For a point light source, the illuminance on a surface is inversely proportional to the square of the distance from the source. Doubling the distance reduces the illuminance to one-quarter.
Light meters, or lux meters, are used to measure illuminance. They are essential tools for lighting designers, photographers, and safety officers to ensure appropriate lighting conditions.
While illuminance measures the quantity of light, the color temperature (measured in Kelvin) describes its quality (warm yellow vs. cool blue). Two light sources can have the same lux level but create very different atmospheres due to their color temperature.
For horticulture, while lux is sometimes used, a more accurate measure is Photosynthetically Active Radiation (PAR), which measures the light spectrum that plants actually use for photosynthesis, not just what the human eye perceives.
CRI measures how accurately a light source illuminates colors compared to a reference source like daylight. High illuminance with low CRI can still result in poor color perception, which is critical in art galleries and retail.
Some organisms, like fireflies and deep-sea fish, produce their own light through a chemical reaction. The illuminance from these sources is often very low but highly effective in their dark environments.
SAD is a type of depression related to changes in seasons. Light therapy, which involves exposure to very bright light (often 10,000 lux), is a common treatment used to mimic natural sunlight and alleviate symptoms.
In photography, illuminance on the film or sensor determines the exposure. The camera's settings (aperture, shutter speed, ISO) are adjusted to control the amount of light reaching the sensor to achieve a well-exposed image.
This law states that the illuminance on a surface is proportional to the cosine of the angle between the direction of the incident light and the surface normal. This is why a surface appears brighter when light hits it directly (0° angle) than at a glancing angle.
Skyglow from city lights creates a background illuminance that can interfere with astronomical observations. Even a few lux of skyglow can obscure faint stars and galaxies.
In photography, Exposure Value (EV) is a single number that represents a combination of shutter speed and aperture, corresponding to a specific level of scene illuminance for a given ISO setting. A change of 1 EV corresponds to a doubling or halving of the light.
Phototropism is the growth of a plant in response to a light stimulus. Plants bend towards light to maximize the illuminance on their leaves for photosynthesis. This is controlled by hormones within the plant.
Modern 'smart' buildings use daylight harvesting systems. Sensors measure the amount of natural illuminance (in lux) coming through windows and automatically dim the artificial lights to save energy while maintaining a constant light level.
This is an approach to lighting design that considers the biological and emotional effects of light on people. It involves adjusting the illuminance and color temperature throughout the day to support the natural human circadian rhythm.