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A Traditional Unit of Illuminance

The meter-candle (m·c) is a historical unit of illuminance that measures the luminous flux falling on a surface area of one square meter from a uniform point source with an intensity of one candlepower (now replaced by the candela) placed one meter away. It was used primarily before the international adoption of the lux as the standard unit for illuminance.

One meter-candle is roughly equivalent to one lux, since lux is defined as lumens per square meter. However, the meter-candle historically provided a more intuitive measure of illuminance based on the distance from a known light source, which was practical for early lighting calculations and designs.

While the meter-candle is largely obsolete today, replaced by the lux in most scientific and engineering applications, it remains a useful reference when interpreting older lighting standards, manuals, and specifications.

Understanding the meter-candle helps in grasping the evolution of lighting measurement units and appreciating the advances in photometric science that have led to more precise and standardized units.

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.