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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.

Understanding Radiance

Candela steradian per square meter (cd·sr/m²) is a unit of radiance, which measures the amount of light power emitted, reflected, or transmitted from a surface in a specific direction per unit area. It combines the candela (cd), which quantifies luminous intensity, with the steradian (sr), the unit of solid angle, and divides by the surface area in square meters.

Radiance describes how bright a surface appears when viewed from a particular direction and is critical in fields such as optics, imaging, computer graphics, and remote sensing. Unlike illuminance (which measures light falling on a surface), radiance tells us about the directional brightness coming off or through a surface.

This unit helps in characterizing light sources and reflective surfaces, allowing scientists and engineers to model how light behaves in complex environments. It is essential for designing lighting systems, calibrating cameras, and simulating realistic visual scenes.

Understanding candela steradian per square meter allows precise control over brightness and directional light properties, supporting advancements in display technology, vision science, and photometric measurements.