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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.

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.