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The Standard Unit of Illuminance

Lumen per square meter (lm/m²), also known as lux, is the standard unit used to measure illuminance, which is the amount of luminous flux (light) falling on a surface per unit area. One lux equals one lumen distributed over one square meter, quantifying how brightly a surface is illuminated.

Lux is widely used in lighting design, architecture, photography, and occupational health to ensure proper lighting conditions for various environments. For instance, office spaces typically require around 300-500 lux to provide comfortable and efficient lighting for work, while outdoor daylight can range from 10,000 to over 100,000 lux.

Understanding illuminance in lux helps in designing lighting systems that optimize visibility, safety, and energy consumption. It allows engineers and designers to calculate how much light a space receives and adjust fixtures or bulbs accordingly.

Lux is preferred globally because it aligns with the metric system and provides a clear, standardized measure of light intensity on surfaces, making it essential for ensuring consistent and effective lighting in both indoor and outdoor settings.

A Unit of Illuminance in Historical Use

Nox is an older, less commonly used unit of illuminance that measures the intensity of light falling on a surface. It is defined as one millilumen per square meter (1 mLm/m²) or equivalently, 0.001 lux. This means 1 nox is equal to one-thousandth of a lux.

The term “nox” comes from the Latin word for “night,” reflecting its use in describing very low levels of light, such as nighttime or dim environments. Although it is rarely used in modern lighting science, the nox still appears in some older literature and niche applications where extremely low light levels are important.

Understanding nox helps provide context for measuring and comparing very faint light intensities. For example, the illumination from a full moon on a clear night is roughly around 0.1 lux, or 100 nox.

While modern standards primarily use lux as the universal unit of illuminance, knowing about nox can be helpful when reading historical documents or dealing with precise measurements in low-light scenarios.