A foot-candle (fc) is the unit used to measure the amount of light hitting a surface from 30 degrees above the horizontal plane of the surface. It is equal to the amount of light 1 foot away from the surface being lit by a candle. Lumens are the units for measuring the amount of light emitted by a source. Foot-candles in turn are the measurement of lumens per square foot. The Illuminating Engineering Society of North America (IESNA) sets suggested standards of foot-candle illumination in the United States.
While IESNA sets standards for sufficient foot-candle lighting, the only legal requirements for foot-candle lighting are determined by lighting codes. The standards are set by the IESNA because it has professionally determined that a given amount of foot-candle illumination is sufficient for tasks in specified spaces and in turn are generally required to perform the tasks efficiently.
The amount of foot-candle illumination used in spaces that are a means for emergency egress is bound by lighting code. The requirement for this is set by the NFPA 101: Life Safety Code at an average of 1 fc. The same amount is also required at the interior and exterior of an emergency exit.
The IESNA categorizes its standards for the proper amount of foot-candles sufficient to perform tasks by considering how complex the task being performed in a particular area is. These are Categories A through G and can be referenced in IESNA's Lighting Handbook. The more complex the task, the higher the amount of foot-candles required to light the space effectively.
How foot candles effectively function in a space is in relation to the category of the space. Public Spaces or Category A according to the IESNA should gain sufficient lighting from 3 fc. This is because very complex tasks are not usually done in public spaces such as a building lobby or a parking lot. According to "Lighting Design Basics" by Mark Karlen and James Benya, most office spaces fall under Category D: Tasks of high contrast and large size. These spaces usually require 30 fc for users to effectively perform everyday office tasks such as reading and typing. More detailed-oriented tasks such as accounting may need 50-100 fc.
The benefits of having standards and requirements for illumination are that lamps, bulbs and light fixtures are designed with these standards in mind. Light sources are designed with sufficient ability to light the proper amount of foot-candles for tasks in categorized spaces. They are readily available to consumers. For instance, for areas of the home that require actions with attention to detail such as the stove for cooking or the bathroom for grooming, 75 fc would offer sufficient light. Task lighting is readily available for purchase that can provide this amount of suggested foot-candle illumination.