What Are Footcandles?
Understanding light intensity is essential for creating well-lit spaces, and footcandles serve as a key measurement in determining the right lighting levels for various environments.
Footcandles measure the intensity of light reaching a surface. One footcandle equals one lumen of light over one square foot. Lux and footcandles measure the amount of light that reaches a surface, with lux indicating lumens per square meter and footcandles indicating lumens per square foot.
Footcandles play a crucial role in lighting design, helping to set the appropriate light levels for visibility, comfort, and safety. Typical levels vary by environment: residential spaces often require 5-40 fc, while commercial and industrial spaces generally need higher intensities, such as 50-100 fc in manufacturing areas for detailed tasks.
For instance, office work areas may need 30-50 fc to reduce eye strain and enhance focus, while general warehouse storage areas need only about 10-20 fc but require higher levels in loading zones.
Overall, footcandle measurements allow lighting designers and facility managers to provide consistent and task-appropriate lighting, meeting safety and productivity standards across different environments.
In practical terms:
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Higher footcandles are generally required for tasks needing clear visibility, like detailed work.
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Lower footcandles are often sufficient for ambient or decorative lighting.
Footcandle measurements are especially relevant in settings where lighting standards are specified, such as commercial or industrial spaces, as they ensure adequate light intensity for safe and effective operations.
How to Measure Footcandles
To measure footcandles, you’ll need a light meter. A footcandle is the amount of light that illuminates a 1-square-foot surface from a source that’s 1 foot away, equivalent to one lumen per square foot. Here’s how you can measure it:
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Obtain a Light Meter: A handheld light meter is ideal for measuring footcandles, as it provides accurate readings in footcandle (fc) units. Many digital light meters have footcandle settings.
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Position the Light Meter: Place the light meter on the surface where you need the reading, ideally at the height of the task area (e.g., on a workbench, desk, or floor). For consistency, take measurements in a horizontal position, as footcandle measurements assume a surface directly receiving light.
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Compare to IES’s Recommended Levels: Once the measure is taken, compare the existing footcandle level to IES’s recommended lighting levels for the space and its purpose. Different tasks and environments require specific footcandle levels. For instance:
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Offices: 30-50 footcandles
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Warehouses general lighting: 10-30 footcandles
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Surgical rooms: 100+ footcandles
Candle Power, Footcandles, Lux, and Lumens
Candle power and footcandles (lux) are both measurements related to light, but they serve different purposes. Candle power measures the intensity of light emitted in a specific direction, while footcandles measure the amount of light that actually reaches a surface.
In other words, candle power (now called Candelas) focuses on the source’s brightness in one direction, whereas footcandles measure the illumination that falls on a given area. A light source with high candle power can result in high footcandle levels on a surface if the beam is focused toward that area, especially over shorter distances.
How Candle Power, Footcandles, Lux, and Lumens Relate
In practical terms, a light source with a high lumen rating emits a lot of light, but whether that light is intense (high candlepower) or effectively illuminates a surface (high footcandles or lux) depends on how the light is directed and the distance from the source.
How Does Lumens Compare to Candle Power?
Lumens measure the total visible light output from a source, indicating overall brightness, while candelas (previously called candle power) measure light intensity in a specific direction, useful for focused beams like flashlights. Lumens are best for general lighting, while candle power is ideal for directional lighting.
Here's how they relate:
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Candela (candle power) describes the brightness of the light source in a particular direction.
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Lumens measure the total light output emitted by the source in all directions.
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Footcandles quantify how much of that light actually reaches a specific area, factoring in distance and any focus from the light source.
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Lux is similar to footcandles but expressed in lumens per square meter, providing an alternative metric for evaluating lighting levels in environments where the metric system is standard.
Footcandles and Lumens
Lumens measure the total light emitted by a fixture, indicating overall brightness. For example, a household LED bulb might emit 800 lumens, while a large commercial fixture may emit thousands. Lumens help compare fixture brightness.
Footcandles, on the other hand, measure the light reaching a surface, expressed as lumens per square foot. The more lumens spread over a smaller area, the higher the footcandles. Footcandles are used in lighting design to meet specific light levels on surfaces like desks or parking lots.
Convert Footcandles to Lumens
To convert footcandles to lumens, use the below formula:
Footcandles = Lumens ÷ Area in Square Feet
Lux and Footcandles
Lux and footcandles are both units of measurement for illuminance, describing the amount of light that hits a surface, but they belong to different measurement systems. Lux is part of the metric system and measures illuminance in lumens per square meter, while footcandles belong to the imperial system and measure lumens per square foot.
Although they measure the same concept, the choice between lux and footcandles often depends on regional standards and industry practices. For example, lux is widely used internationally and in scientific contexts, while footcandles are commonly used in the United States.
Convert Footcandles to Lux
To convert footcandles to lux, multiply the number of footcandles by 10.764 (1 footcandle is equal to 10.764 lux).
Conversely, one lux is equivalent to approximately 0.0929 footcandles.
Conclusion
In summary, understanding and measuring light intensity in footcandles is essential for achieving optimal lighting in various settings. Footcandles provide a precise gauge of how much light reaches a specific surface, helping facility managers, designers, and architects tailor lighting for specific tasks and environments. This metric aids in creating workspaces that enhance visibility, comfort, and productivity while meeting safety standards.
Accurate footcandle levels can be achieved by considering factors such as distance, beam angle, and surface reflectivity and using tools like light meters. Whether you're managing lighting in an industrial facility, an office, or a public space, footcandles offer a practical, actionable approach to effective lighting design.
