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A Unit of Illuminance

The foot-candle (ft·c or fc) is a unit of illuminance used primarily in the United States and some other countries that follow imperial measurements. It measures the amount of luminous flux (light) falling on a surface of one square foot. One foot-candle is defined as one lumen per square foot.

Foot-candles are commonly used in lighting design, architecture, photography, and workplace safety to ensure spaces have appropriate lighting levels. For example, an office workspace typically requires around 30 to 50 foot-candles to provide sufficient brightness for tasks without causing eye strain. Retail stores and hospitals may require higher levels, while hallways or staircases might have lower standards.

Foot-candles are related to the metric unit lux, where 1 foot-candle equals approximately 10.76 lux (lumens per square meter). Understanding foot-candle measurements allows designers and engineers to plan lighting layouts effectively, balancing energy use with visual comfort.

Despite the growing global adoption of the lux, foot-candles remain prevalent in industries and regions that use imperial units, making it important for professionals to be familiar with both.

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.