How to Convert Lux to Footcandles? [The Formula] 

Dealing with different lighting specifications? Perhaps you need to convert lux to footcandles for a project referencing international standards, or vice versa. Understanding this illuminance unit conversion is straightforward with the right approach. This guide provides the clear formulas and steps required to accurately convert lux to footcandles, ensuring your lighting calculations are precise and your projects meet the correct specifications. Let's clarify this essential lighting calculation.

What Is Lux?

Lux (lx) serves as the standard international unit for measuring illuminance, which is the total luminous flux incident on a surface, per unit area. It quantifies the intensity of light that actually lands on a given surface.

Defining Lux Illuminance

Specifically, one lux is defined as one lumen of light evenly distributed over an area of one square meter (1 lx = 1 lm/m²). This unit is a fundamental part of the International System of Units (SI) and is widely adopted globally for lighting calculations and specifications, especially in scientific, architectural, and photographic contexts.

Common Applications of Lux

Lux measurements are commonly utilized in various fields:

• Architectural and commercial lighting design to ensure spaces meet visual comfort and task performance requirements, particularly in Europe and Asia.
• Photography and videography to determine appropriate exposure settings.
• Workplace safety and health regulations to specify minimum lighting levels for different environments.
• Horticulture for optimizing light conditions for plant growth.
• Museums and galleries to control light exposure on sensitive artifacts.

What Are Footcandles?

The footcandle (fc or ft-c) is an illuminance unit predominantly used in the United States and some other countries that still employ Imperial or US customary units. Similar to lux, it measures the amount of light falling onto a specific surface area.

Defining Footcandle Illuminance

One footcandle is equivalent to the illuminance produced on a one-square-foot surface by a uniform light source of one candela, located one foot away from the surface. More practically, one footcandle equals one lumen of light distributed over an area of one square foot (1 fc = 1 lm/ft²).

Common Applications of Footcandles

Footcandle measurements are frequently applied in:

• Building design and construction in the U.S. for specifying and verifying lighting levels in commercial, industrial, and residential spaces.
• Compliance with U.S. occupational safety standards (e.g., OSHA recommendations) for workplace lighting.
• Sports lighting design for athletic fields and arenas in North America.
• Theatrical and stage lighting design to quantify light intensity on performance areas.
• General lighting audits and assessments within regions using Imperial units.

Why Is Lux to Footcandle Conversion Needed?

Working with lighting often means encountering illuminance values in either lux or footcandles. For clarity and accuracy across different lighting tasks, it becomes helpful to know when and why a conversion between these units is necessary.

Bridging Global Lighting Standards

Project specifications from Europe, for instance, will typically use lux. In contrast, North American guidelines frequently rely on footcandles. If you are involved in international projects or sourcing luminaires from various global suppliers, a clear conversion ensures everyone is working towards the same required light levels for a particular space.

Reading Different Equipment Specifications

Imagine reviewing a light fixture's technical data where performance is listed in lux, while your own light meter primarily displays footcandles. In such cases, converting the units is essential to accurately assess if that fixture meets your needs or to correctly evaluate existing lighting installations.

Meeting Project Illuminance Targets

Designs might specify 500 lux for an office environment or 30 footcandles for a workshop area. To ensure these targets for safety and visual performance are met without error, a confident handling of both units from the outset helps prevent costly mistakes in either the design phase or during final implementation.

Converting Lux to Footcandles Step by Step

The process to convert lux to footcandles is straightforward once you understand the relationship between these two units of illuminance. The key is a simple division.

The Lux to Footcandles Formula

To convert an illuminance value from lux (lx) to footcandles (fc), you use the following formula:

Footcandles (fc) = Lux (lx) / 10.764

This conversion factor, 10.764, arises from the fact that one footcandle is approximately equal to 10.764 lux. Therefore, to find the equivalent footcandle value, divide the lux value by this number. Some may use a rounded factor of 10.76 for simplicity.

Lux Conversion Calculation Examples

Let's look at a couple of practical examples:

• Example 1: A specification requires an area to be lit to 500 lux. To find the equivalent in footcandles:
  Footcandles = 500 lx / 10.764 ≈ 46.45 fc
• Example 2: A light meter measures an illuminance of 1200 lux. The footcandle equivalent would be:
  Footcandles = 1200 lx / 10.764 ≈ 111.48 fc

Lux to Footcandles Quick Reference

For quick estimations, here are some common lux values converted to footcandles (using the 10.764 factor, rounded to two decimal places):

• 100 lux ≈ 9.29 fc
• 200 lux ≈ 18.58 fc
• 300 lux ≈ 27.87 fc
• 500 lux ≈ 46.45 fc
• 750 lux ≈ 69.68 fc
• 1000 lux ≈ 92.90 fc

Converting Footcandles to Lux Step by Step

Similarly, converting footcandles back to lux involves a simple multiplication using the same conversion factor.

The Footcandles to Lux Formula

To convert an illuminance value from footcandles (fc) to lux (lx), apply this formula:

Lux (lx) = Footcandles (fc) * 10.764

Since one footcandle is approximately 10.764 lux, multiplying the footcandle value by this factor gives you the equivalent illuminance in lux.

Footcandle Conversion Calculation Examples

Here are a couple of examples demonstrating this conversion:

• Example 1: A lighting plan specifies a target of 30 footcandles for a workspace. To convert this to lux:
  Lux = 30 fc * 10.764 ≈ 322.92 lx
• Example 2: An existing area is measured to have an illuminance of 70 footcandles. The lux equivalent is:
  Lux = 70 fc * 10.764 ≈ 753.48 lx

Are Lumens the Same as Lux or Footcandles?

It is a common point of confusion, but lumens, lux, and footcandles measure different, though related, aspects of light. They are not interchangeable.

Understanding Luminous Flux (Lumens)

Luminous flux, measured in lumens (lm), quantifies the total amount of visible light produced by a light source. Think of it as the raw "output" or "quantity" of light emitted in all directions by a lamp or luminaire. A higher lumen rating generally means a brighter light source.

How Surface Area Influences Light Level

Lux and footcandles, on the other hand, are units of illuminance – they measure the amount of light that actually falls on a specific surface area. The same number of lumens spread over a larger area will result in a lower lux or footcandle reading (less illuminance). Conversely, concentrating the same lumens onto a smaller area increases the illuminance.

Lumens per Square Meter Equals Lux

The relationship is direct: one lux is defined as one lumen of light distributed evenly over an area of one square meter.

So, Lux (lx) = Lumens (lm) / Area (m²).

Lumens per Square Foot Equals Footcandle

Similarly, one footcandle is one lumen of light distributed evenly over an area of one square foot.

So, Footcandles (fc) = Lumens (lm) / Area (ft²).

This clarifies that while lumens indicate the total light available from a source, lux and footcandles describe the intensity of that light upon a surface.

Typical Building Illuminance Levels

While precise illuminance requirements vary based on specific tasks, occupant age, room surface reflectances, and local regulations, certain target ranges are generally established in lighting design for common building spaces. These serve as initial reference points.

General Office Lighting Suggestions

For general office environments, including open-plan areas and individual cubicles, illuminance levels between 300 and 500 lux (approximately 28 to 46 footcandles) on primary task surfaces are common. Conference rooms often require similar levels, with provisions for dimming.

Retail Space Illumination Ideas

Retail lighting is designed to attract customers and highlight merchandise. General circulation areas in department stores or malls usually target 200 to 500 lux (around 19 to 46 fc). Specific display areas can utilize significantly higher accent lighting, sometimes exceeding 1000 to 2000 lux (93 to 186 fc) locally.

Educational Facility Light Level Guidance

Classrooms and lecture halls typically require illuminance levels from 300 to 750 lux (approximately 28 to 70 fc) on desks and whiteboards for tasks like reading and writing. Corridors and non-task areas will have lower illumination needs. Specialized areas such as laboratories or art studios possess distinct, often higher, lighting requirements.

(Note: Always consult local codes and IES standards)

The values mentioned above are illustrative. It is imperative to consult current local building codes, national standards (such as those published by the Illuminating Engineering Society - IES in North America, or similar bodies like CIE internationally), and specific project documentation for mandatory and recommended illuminance levels. These official sources provide the authoritative guidance necessary for compliant and effective lighting design.

How Is Light Measured Practically?

Verifying or assessing illuminance levels in a space is achieved using a specialized instrument: the light meter, also known as a lux meter or footcandle meter. Correct usage ensures reliable data acquisition.

Using Your Light Meter Correctly

When taking measurements, position the light meter's sensor (photocell) at the plane of interest – typically the task surface (e.g., a desk, countertop, or the floor). Hold the meter steady, allowing the reading to stabilize. For uniformity assessment, take multiple readings across an area and calculate the average. Crucially, avoid casting a shadow over the sensor with your body or hand during measurement.

Tips for Single Unit Light Meters

If a light meter displays readings in only one unit (lux or footcandles), manual conversion using the formulas (fc = lx / 10.764 or lx = fc * 10.764) is necessary if the target or reference is in the other unit. Keeping these conversion factors readily available simplifies this on-site process.

Important Points for Lighting Conversion

To effectively manage illuminance unit conversions in lighting projects, consider these fundamental principles:

• Utilize the correct conversion factor: Employing 10.764 (or its inverse, approximately 0.0929) ensures accuracy in calculations between lux and footcandles.
• Distinguish illuminance from luminous flux: Remember that lux and footcandles quantify light incident on a surface, whereas lumens measure the total light emitted by a source. This distinction prevents common errors.
• Reference authoritative standards: Always contextualize illuminance measurements and targets by consulting relevant lighting standards (e.g., IES, CIE) and project-specific requirements for appropriate and compliant designs.

الأسئلة الشائعة (FAQ)

Q1: What is the quickest way to estimate footcandles from lux?

A1: For a quick mental estimation, you can roughly divide the lux value by 10. Since 1 footcandle is approximately 10.764 lux, dividing by 10 gives a close, though slightly higher, footcandle value. For accuracy, always use the 10.764 factor.

Q2: Is there a simpler conversion factor than 10.764?

A2: While 10.764 is the more precise factor, some people use 10.76 or even 10 for very rough approximations. However, for professional calculations or compliance, using 10.764 (or its inverse for fc to lx) is recommended for better accuracy.

Q3: If I increase the lumens of a light, will both lux and footcandles increase?

A3: Yes, generally. If the area being illuminated remains the same, increasing the lumens (total light output from the source) will result in higher illuminance values, meaning both lux and footcandle readings on that surface will increase.

Q4: Does the distance from the light source affect lux and footcandle readings?

A4: Absolutely. For a given light source, as the distance to the measurement surface increases, the illuminance (lux or footcandles) on that surface will decrease due to the inverse square law (for point sources) and light spread.

Q5: Why do some light meters have settings for both lux and footcandles?

A5: Many professional light meters include settings for both units to cater to users working with different standards or project requirements. This allows for direct measurement in the desired unit without needing manual conversion.