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A footcandle is defined as one lumen per square foot. This measurement is used in the field of lighting to quantify illuminance, which is the amount of light that illuminates a surface area. In practical terms, this means that if a surface receives one lumen of light from a light source uniformly distributed over an area of one square foot, it is said to have an illuminance of one footcandle.

Understanding illuminance is crucial for ensuring adequate lighting in various environments, such as offices, studios, and public spaces. By using footcandles, professionals can design and evaluate lighting systems to meet specific requirements for visibility and comfort.

The other definitions in the options refer to different units:

- One watt per square meter is a measure of power per unit area, not illuminance.

- One candela per square foot is related to the intensity of light but does not directly represent illuminance.

- One lux per square meter is a metric measurement similar to footcandle, where one lux is equal to one lumen per square meter, but it is an entirely different unit used in the International System of Units (SI).

Thus, the appropriate choice that accurately defines a footcandle is the option that states it is one lumen per square