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The Standard Unit of Illuminance

Lux (lx) is the SI unit of illuminance, measuring the amount of luminous flux (light) falling on a surface per unit area. One lux is equal to one lumen per square meter (lm/m²). It quantifies how brightly a surface is illuminated, directly relating to human visual perception of light in an environment.

Lux is widely used in various fields such as architecture, photography, horticulture, and workplace safety to ensure proper lighting conditions. For example, typical office lighting ranges from 300 to 500 lux to promote productivity without causing eye strain. Outdoor daylight varies greatly but can reach up to 100,000 lux on a bright sunny day.

Because lux is based on the metric system, it is the preferred unit globally for lighting measurement and design. It helps engineers, designers, and scientists create environments with appropriate lighting levels, balancing visibility, energy efficiency, and comfort.

Understanding lux is essential for interpreting lighting standards, optimizing artificial illumination, and assessing natural light exposure, making it fundamental in both everyday life and technical applications.

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modern standards primarily use lux as the universal unit of illuminance, knowing about nox can be helpful when reading historical documents or dealing with precise measurements in low-light scenarios.