Lux is a unit of measurement that quantifies the intensity of light as it falls upon a specific surface. This metric provides a standardized way to measure the concentration of light available in a given space. It focuses on how much light actually reaches a designated area, making it the standard metric for assessing lighting quality.
Defining Lux as Illuminance
The lux (symbol: lx) is the International System of Units (SI) derived unit for illuminance, which describes the density of luminous flux on a surface. One lux is precisely defined as one lumen of luminous flux spread uniformly over an area of one square meter. The lux value is inherently a measure of light at a surface, making it the preferred metric for assessing lighting quality in real-world environments.
Lux Versus Lumens
The most common confusion in lighting involves the distinction between lux and the related unit, the lumen. Lumens measure the total quantity of visible light emitted by a source, known as luminous flux, regardless of how that light is distributed. Lux, by contrast, measures the concentration of those lumens across a specific area.
This difference illustrates why distance is important in lux measurement. Imagine a fixed stream of 1,000 lumens directed onto a surface. If that light hits a one square meter area, the illuminance is 1,000 lux. However, if the surface is moved farther away, causing the same 1,000 lumens to spread out over ten square meters, the illuminance drops to only 100 lux. This demonstrates the inverse square law, where light intensity on a surface decreases rapidly as the distance from the source increases.
Practical Applications of Lux Measurement
Measuring lux is a routine practice across several industries because it quantifies the illumination available for a specific task.
Lighting Design
In architectural and interior lighting design, professionals use lux meters to ensure that workspaces meet prescribed standards for visual tasks. Designers confirm that a desk surface receives sufficient lux to prevent eye strain and maintain productivity.
Horticulture
Horticulture and indoor gardening rely on lux to measure the light available for plant photosynthesis and growth. Lux measurement helps growers position supplemental lighting precisely, as different plants have varying light requirements.
Photography
In photography and videography, measuring incident lux helps determine the correct exposure settings for a scene. This measurement ensures the camera sensor receives the appropriate amount of light for a well-exposed image.
Standard Lux Levels for Common Spaces
Recommended lux levels provide practical guidelines for creating environments suitable for various activities. A dark night with a full moon registers an illuminance of about 0.1 to 1 lux. General circulation areas, such as residential hallways and stairwells, are adequately lit between 100 and 200 lux. Office environments, where visual tasks are more demanding, require significantly higher lux levels. For general office work and computer use, the recommended range is 300 to 500 lux. Detailed reading, drafting, or precision task lighting necessitates a minimum of 500 to 750 lux to ensure clarity and reduce visual fatigue.