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modern standards primarily use lux as the universal unit of illuminance, knowing about nox can be helpful when reading historical documents or dealing with precise measurements in low-light scenarios.

A Historical Unit of Illuminance

The centimeter-candle (cm·c) is an older, non-SI unit of illuminance that measures the luminous flux falling on a surface area of one square centimeter. It is defined as the illuminance produced at a distance of one centimeter from a uniform point source of one candlepower (now replaced by the candela).

Since the centimeter-candle measures light intensity over a very small area, it represents a much finer scale compared to more common units like the foot-candle or lux. One centimeter-candle is equivalent to 10,000 lux because one square meter contains 10,000 square centimeters.

Historically, the centimeter-candle was used in photometry and lighting sciences before international standardization led to the widespread adoption of the lux as the preferred illuminance unit. Its usage today is rare and mostly of historical interest, but it remains useful for understanding older lighting data and certain precise scientific contexts where small-area measurements matter.

Understanding centimeter-candle helps bridge the gap between historical and modern units of light measurement and offers insight into the evolution of photometric standards.