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A Historical Unit of Luminous Intensity

The flame is a traditional, non-SI unit used historically to measure luminous intensity, based on the light emitted by a standard flame, typically from burning a specific fuel under controlled conditions. It originated before the invention of modern light measurement devices and provided a practical way to describe light brightness using everyday sources.

One “standard flame” was defined by the intensity of light from a candle burning colza oil or other specified fuels. Although exact values varied by region and standardization efforts, the flame served as a common reference for brightness in industries like lighting, photography, and early scientific research.

With the advent of electric lighting and precise photometric units like the candela, the flame unit fell out of use. The candela, now the SI base unit for luminous intensity, provides a reproducible and universal standard far more accurate than flame-based measures.

Today, the flame unit is mainly of historical interest, helping us understand how people quantified light before modern technology. It also reflects the evolution of photometry from subjective, practical measures to objective, scientific standards.

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modern standards primarily use lux as the universal unit of illuminance, knowing about nox can be helpful when reading historical documents or dealing with precise measurements in low-light scenarios.