Foot Candles to Lux Calculator

Understanding Foot-Candles to Lux Conversion

Converting between foot-candles and lux is essential for lighting professionals, architects, engineers, and anyone working with illumination measurements. These units measure illuminance - the amount of light falling on a surface - and understanding their relationship is crucial for proper lighting design, energy efficiency, and compliance with lighting standards.

What are Foot-Candles and Lux?

Foot-candles (fc) are a unit of illuminance commonly used in the United States and some other countries. One foot-candle is defined as the illuminance produced by one lumen of light falling on one square foot of surface area.

$$1 \text{ foot-candle} = \frac{1 \text{ lumen}}{1 \text{ square foot}}$$

Definition of foot-candle

Lux (lx) is the SI (International System of Units) unit of illuminance, used worldwide. One lux is defined as the illuminance produced by one lumen of light falling on one square meter of surface area.

$$1 \text{ lux} = \frac{1 \text{ lumen}}{1 \text{ square meter}}$$

Definition of lux

Conversion Formula

The conversion between foot-candles and lux is based on the relationship between square feet and square meters:

$$1 \text{ foot-candle} = 10.76391 \text{ lux}$$

This means:

To convert foot-candles to lux: lux = foot-candles × 10.76391
To convert lux to foot-candles: foot-candles = lux ÷ 10.76391

Why the Conversion Factor is 10.76391

The conversion factor comes from the relationship between square feet and square meters:

$$1 \text{ square meter} = 10.76391 \text{ square feet}$$

Area conversion factor

Since illuminance is measured in lumens per unit area, and the area units differ by this factor, the illuminance conversion follows the same ratio.

Common Illuminance Levels

Understanding typical illuminance levels helps put these measurements in context:

Application	Foot-Candles	Lux
Moonlight	0.01	0.1
Street lighting	0.1-2	1-20
Residential living room	10-20	100-200
Office workspace	30-50	300-500
Retail store	50-100	500-1000
Surgery room	1000-2000	10,000-20,000
Direct sunlight	10,000	100,000

Practical Applications

Foot-candles to lux conversion is essential in various fields:

Architecture & Interior Design - Ensuring proper lighting levels for different spaces and activities
Electrical Engineering - Designing lighting systems that meet international standards
Photography & Videography - Understanding lighting conditions and equipment requirements
Horticulture - Providing optimal light levels for plant growth in greenhouses
Safety & Security - Meeting minimum illumination requirements for safe environments
Energy Auditing - Assessing lighting efficiency and compliance with energy codes

Lighting Standards and Codes

Various organizations have established lighting standards that specify minimum illuminance levels:

IES (Illuminating Engineering Society) - Provides comprehensive lighting standards for various applications
ANSI/ASHRAE/IES 90.1 - Energy standard for buildings, including lighting requirements
OSHA - Workplace safety standards with minimum lighting requirements
IEC 62471 - International standard for photobiological safety of lamps and lamp systems

Measurement Tools

Illuminance is typically measured using:

Lux Meters - Digital devices that directly measure illuminance in lux
Foot-candle Meters - Similar devices calibrated to display foot-candles
Light Meters - General-purpose devices that can often display both units
Smartphone Apps - Basic illuminance measurement using device sensors

Factors Affecting Illuminance

Several factors influence the illuminance level at a given point:

Distance from Light Source - Illuminance decreases with the square of distance (inverse square law)
Angle of Incidence - Light striking at an angle provides less illuminance than perpendicular light
Surface Reflectance - Light-colored surfaces reflect more light than dark surfaces
Light Source Characteristics - Luminous intensity, beam angle, and color temperature
Environmental Conditions - Dust, humidity, and atmospheric conditions can affect light transmission

Energy Efficiency Considerations

Proper illuminance measurement and conversion are crucial for energy-efficient lighting design:

Over-lighting wastes energy and increases costs
Under-lighting can affect productivity and safety
LED technology has changed efficiency standards significantly
Smart lighting controls can optimize illuminance based on occupancy and daylight

Frequently Asked Questions

Why do some countries use foot-candles while others use lux?

Foot-candles are primarily used in the United States and a few other countries that haven't fully adopted the metric system. Lux is the international standard (SI unit) used worldwide. The choice often depends on local building codes, industry standards, and historical practices. Many professionals need to work with both units when dealing with international projects or equipment specifications.

How accurate is the conversion factor of 10.76391?

The conversion factor 10.76391 is highly accurate and based on the precise relationship between square feet and square meters. This factor is used in all professional lighting calculations and standards. For most practical purposes, you can use 10.76 as a rounded approximation, but for precise calculations, especially in professional applications, the full precision of 10.76391 should be used.

Can I use this calculator for outdoor lighting applications?

Yes, this calculator works for both indoor and outdoor lighting applications. However, outdoor lighting often involves much lower illuminance levels (street lighting, security lighting) compared to indoor applications. The calculator handles the full range from very low levels (moonlight) to very high levels (direct sunlight).

What's the difference between illuminance and luminance?

Illuminance (measured in foot-candles or lux) is the amount of light falling on a surface. Luminance (measured in candelas per square meter) is the amount of light reflected or emitted by a surface. Illuminance describes how much light hits a surface, while luminance describes how bright that surface appears to an observer.

How do I measure illuminance accurately?

For accurate illuminance measurement, use a calibrated lux meter or foot-candle meter. Place the sensor at the work plane or surface where you want to measure. Ensure the sensor is level and not shaded by your body or equipment. Take multiple readings at different points in the space and average them for representative results. Avoid measuring in direct sunlight or near bright light sources that could affect the sensor.

Are there any mobile apps that can measure illuminance?

Yes, there are several mobile apps that can measure illuminance using your phone's ambient light sensor. However, these are generally less accurate than dedicated light meters and should only be used for rough estimates. For professional applications, always use a calibrated light meter. The accuracy of phone sensors can vary significantly between different devices and manufacturers.

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