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A Unit of Illuminance in Photometry

The phot (ph) is a unit of illuminance formerly used in photometry, representing the amount of luminous flux falling on a surface per unit area. One phot is equal to 10,000 lux (lm/m²), meaning it measures a very high level of illumination.

The phot was commonly used before the widespread adoption of the lux in the International System of Units (SI). It is derived from the Greek word "photos," meaning light, and was particularly useful for expressing strong light intensities, such as direct sunlight, which typically measures about 100,000 lux or 10 ph.

Though largely replaced by lux, the phot is still occasionally referenced in older scientific literature and certain industrial contexts. Because 1 phot equals 10,000 lux, it provides a convenient scale for quantifying intense lighting conditions without resorting to large numbers.

Understanding the phot unit helps in interpreting historical data and in fields where very bright light levels are relevant, such as outdoor lighting design, solar energy studies, and some photographic applications.

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.