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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.

Measuring Illuminance

Lumen per square foot (lm/ft²) is a unit of illuminance, representing the amount of luminous flux (light) falling on a surface area of one square foot. It measures how brightly an area is illuminated, helping to quantify the perceived brightness for human vision.

This unit is commonly used in lighting design, architecture, and workplace safety to ensure adequate and comfortable lighting conditions. For example, offices, classrooms, and public spaces have recommended illuminance levels expressed in lm/ft² to optimize visibility and reduce eye strain.

Illuminance measured in lm/ft² is closely related to the SI unit lux, which is lumens per square meter (lm/m²). Since one square foot equals approximately 0.0929 square meters, 1 lm/ft² is about 10.76 lux.

Designers use lm/ft² to plan lighting layouts, select appropriate fixtures, and evaluate energy efficiency. Proper illuminance levels contribute to productivity, safety, and aesthetics by ensuring spaces are neither too dim nor overly bright.

Understanding lumen per square foot helps balance lighting quality and energy consumption, making it a vital concept in modern lighting engineering.