What does a "foot-candle" measure?

A "foot-candle" is a unit of measurement that quantifies the illuminance, or the amount of light that falls on a given surface area. Specifically, it describes the light produced by a single candle when it is one foot away from that surface, illuminating one square foot. This definition is essential for understanding how light is measured in environments such as workplaces, public areas, and security settings, as it allows for the evaluation of adequacy in lighting for tasks or safety.

This concept contrasts with other options provided, such as measuring the total light in a room or the intensity of sunlight, which do not specifically relate to the definition of a foot-candle. Additionally, while brightness might be informally thought of in terms of a streetlight's output, it does not provide the precise scientific basis that the term foot-candle embodies.