Tyler Brooks
Foot candles are a measurement of light intensity, or illuminance, on a given surface. They are used to ensure that areas that require certain light levels are receiving those light levels. This is important for electrical construction and creating comfortable, efficient, and practical lighting solutions. The number of foot candles needed depends on the nature of tasks being performed, the age of people in the space, and other variables. To determine the number of foot candles, you can use a sensitive light meter, or for simple math, multiply the room square footage by the foot-candle requirement.
| Characteristics | Values |
|---|---|
| Definition | Foot-candle (fc, lm/ft2, or ft-c) is a measurement of light intensity |
| Purpose | Foot-candles are used to ensure that areas requiring certain light levels are receiving those light levels |
| Calculation | To determine the number of foot-candles, multiply the room square footage by the foot-candle requirement |
| Measurement Tools | Sensitive light meters, lighting plan software, smartphone apps |
| Considerations | Nature of tasks, age of people using the space, room type, room purpose, physical security requirements, etc. |
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What You'll Learn
Using a light meter
To measure foot-candles using a light meter, you would position the light meter at the point where you want to assess the light level, with its sensor facing the illumination source directly. For example, to measure the light falling on a work desk, you would place the device on the desk surface. The sensor should have a clear, unobstructed view of the light source.
It is important to note that light meters record the cumulative light intensity, so when the meter is exposed to multiple light sources, such as daylight from a window and interior artificial lighting, it can give inaccurate readings. In such cases, it is recommended to block off or turn off additional lights and repeat the measurement.
Additionally, light intensity changes throughout the day, so to get a reliable reading, you should take multiple measurements at different times and calculate an average.
Light meters can be especially useful in fields such as cinematography, photography, and lighting design, where understanding light intensity and its impact on a subject is crucial. By using a light meter to measure foot-candles, you can gain insight into how lighting affects a space, allowing you to optimise light usage and create comfortable environments.
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Calculating room square footage
A foot-candle is a measurement of light intensity, or how bright the light is one foot away from the source. The amount of light needed depends on the type of room being lit. For example, a bathroom or kitchen will require more foot-candles than a living room or bedroom.
To calculate the number of foot-candles needed for a room, first determine the square footage of the room. If the room is rectangular, measure the length and width of the room, then multiply those numbers together to get the area in square feet. If the room is an unusual shape, it may be easier to divide it into two or three separate sections and calculate the square footage of each section, then add them together for the total.
Once you have the room's square footage, you can determine the foot-candle requirement for your application. Browse the IES-recommended foot-candle requirements for different types of rooms. Multiply the room's square footage by the foot-candle requirement to get the total number of foot-candles needed.
For example, let's say you have a rectangular kitchen that is 10 feet wide and 12 feet long. The room's square footage is 120 square feet (10 x 12 = 120). From the IES-recommended foot-candle requirements, you determine that a kitchen needs 70-80 foot-candles. Multiplying the room's square footage by the foot-candle requirement, you get a range of 8,400 to 9,600 (120 x 70 = 8,400; 120 x 80 = 9,600). So, for this kitchen, you would need between 8,400 and 9,600 foot-candles.
You can also convert foot-candles to lumens by multiplying the number of foot-candles by 10.76. So, in the above example, you would need between 89,760 and 103,680 lumens (8,400 x 10.76 = 90,264; 9,600 x 10.76 = 103,296).
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Understanding lux
Lux is a standardised unit of measurement of light level intensity, also referred to as "illuminance" or "illumination". It is a measure of how much light falls on a surface. Lux is derived from the Latin word "lux" meaning light.
One lux is the amount of illumination provided when one lumen is evenly distributed over an area of one square metre. This is also equivalent to the illumination that would exist on a surface where all points are one metre from a point source of one international candle (candela). One lux is equal to 0.0929 foot-candles.
Lux is important for knowing how bright a particular surface appears. This is a crucial metric if you want to know how bright a surface will appear, such as a tabletop, reading material or photography subjects. Without a sufficient lux level, it can be difficult to read or take good photographs.
The number of lux can vary depending on the weather conditions and time of day. Sunlight can provide between a few hundred to a few thousand lux. The lux of artificial indoor lighting is typically 1000 lux or below.
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OSHA requirements
OSHA's foot-candle requirements are as follows: first-aid stations, offices, and infirmaries require 30 foot-candles; general construction plants and shops require 10 foot-candles; concrete placement, waste areas, loading platforms, active storage areas, field maintenance areas, refueling areas, and excavation sites require 3 foot-candles; forklift operating areas require a minimum of 2 foot-candles; warehouses, corridors, hallways, and exit-ways require 5 foot-candles; and tunnels, shafts, and general underground work areas require 10 foot-candles.
It is important to note that these requirements may overlap, as certain areas may be classified as both indoor and outdoor work areas. For example, a convenience store with LED lights would have different lighting requirements for the indoor area compared to the outdoor gas pump area.
In terms of installation and maintenance, OSHA standard 1910.305 outlines the requirements for both permanent and temporary lighting. All lighting fixtures must be installed securely to prevent falls or electrical hazards, and light fixtures in high-risk areas must be protected by strong barriers to prevent breakage or exposure to live electrical parts. Temporary lighting systems, such as those used during construction, must be under 600 volts and grounded to prevent shock hazards.
OSHA also requires that emergency lighting is implemented effectively to ensure the safety of employees in situations where regular power sources fail. This includes during routine power outages or emergencies that cause power shutdowns, such as fires. Emergency light sources provide illumination to critical areas such as walkways and exits, allowing workers to exit the premises safely.
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Lighting design
To design lighting for a space, it is essential to first establish the intended use of the area and the specific tasks that will be performed there. Different tasks require different levels of lighting. For example, higher precision activities like drafting or intricate handwork often demand 50-75 foot-candles, while less visually intensive tasks like casual reading or walking through a hallway require considerably fewer foot-candles, usually around 10-20. The age of the people using the space and other variables can also impact the lighting requirements.
Once the lighting requirements are understood, lighting designers can use lighting plan software to calculate the light levels within the space and create a lighting plan that meets those requirements. This software allows designers to simulate foot-candle readings on the ground and provides detailed reports on light levels and light balance.
To convert foot-candles to lumens, which are also used to measure light, multiply the number of foot-candles by 10.76. For example, 50-foot candles would equate to 50 x 10.76, which is 538 lumens. To determine the required lumens for a space, multiply the room's square footage by the foot-candle requirement. For instance, a 100-square-foot room with a 20-foot candle requirement would need 2,000 lumens.
By understanding foot-candles and using lighting plan software, lighting designers can create accurate, effective, and user-friendly lighting designs that provide the optimal lighting conditions for the intended use of the space.
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Frequently asked questions
A foot candle is a measurement of light intensity. One foot candle is defined as enough light to saturate a one-foot square with one lumen of light.
The number of foot candles you need depends on the nature of the tasks being performed, the age of the people using the space, and other variables. For example, an office desk is often lit at about 50 foot candles, while a hallway might require only 10.
To convert foot candles to lumens, multiply the number of foot candles by 10.76. For example, 50 foot candles would equate to 538 lumens.
