Foot-Candle

Overview

The foot-candle (abbreviation fc, also lm/ft² or ft-c) is a unit of illuminance, i.e., the amount of light falling on a surface. It is defined as one lumen per square foot. The foot-candle is a non-SI unit and is primarily used in the USA, where the Anglo-American system of measurement is common. The SI equivalent is the lux (lumen per square meter).

Important: The foot-candle does not measure the light output of a fixture, but rather how much light actually reaches the measured surface. Thus, it is independent of camera settings such as aperture or ISO and purely describes the physical lighting situation on set.

Definition and Conversion

Visually, one foot-candle corresponds to the illuminance on the inner surface of a sphere with a radius of one foot, at the center of which is a point source of light of one candela. Equivalently, it is the uniform illumination of an area of one square foot by a luminous flux of one lumen.

For practical purposes on set, the rule of thumb 1 fc ≈ 10 lux is often sufficient, as long as an accuracy of approximately ±10% is adequate.

Use on Set

In American film and television production, cinematographers (DoP) and gaffers work with light meters that display foot-candles to quantify the amount of light on the subject and location. Since the measurement is independent of the camera, it is particularly suitable for location scouting and lighting planning during pre-production: the necessary equipment can be determined before the camera is configured.

Typical applications:

• Exposure: From the light value in foot-candles, the appropriate aperture (f-stop) is derived in conjunction with sensitivity (ISO/ASA), shutter speed or shutter angle, and the effect of any filters.
• Lighting Ratios: By taking measurements at multiple points, contrast ratios between key light and fill light can be set and checked.
• Consistency: Recurring foot-candle values help to keep lighting moods reproducible across multiple shots or shooting days.