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Lighting Units & Measurements and Their Importance

The first question we are often asked is “How much light do I need?” Our standard response is to ask about the needs and usage of the building or facility. We can get more specific by referencing the number of foot-candles or lux for a specific distance. However, unless familiar with the lighting industry terminology, “lumen”, “lux”, and “foot-candle” often lead to more confusion than solution.  Understanding these terms enables the understanding of a Photometric Layout, which is the most important tool in designing a lighting project.

LED Light Bulbs

In this post we will be defining and breaking down the calculations of each lighting measurement unit. As lighting technology changes, the standard measurement of “brightness” for traditional lighting is not able to be translated to determine how much light you will need with an LED light.

 

 

Watts

Manufacturers of traditional lighting use the term “Watts”, which actually identifies the amount of power consumed, as a means of describing the visible output of light from the bulb. But now with the need for increased energy efficiency through the use of LED lights, the measurement unit of Watts does not work anymore.

So when it comes to changing from traditional incandescent lighting to LED lighting or any other energy saving bulb, it is the number of lumens that represents the brightness, not the wattage. The higher the lumens, the brighter the light.

But what exactly is a lumen?


Lumens

A Lumen (lm) is the most common measurement you will see when shopping for LED lighting. Lumens are scientifically defined as "the International System of Units derived unit of luminous flux, a measure of the total 'amount' of visible light emitted by a source." This means a lumen is a measure of the total visible light output of a light source.  Lumens are the amount of light generated by a light fixture, NOT the amount of light reaching the intended surface.

What is important about this definition is that it focuses on the source of the light. Lumens provides a standard unit of measurement that directly focuses on the light output, which can be very helpful when comparing different types of light sources.

Watts Vs Lumens

All other lighting units of measurements are based on the lumen. The lumen measurement originated from the basic concept of foot-candles. 

So what is a foot-candle?


Foot-Candles

A Foot-Candle (fc) is the non-SI unit of light intensity. A single foot-candle is defined as the amount of light that falls on a surface one foot away from the single candle. This unit of measurement would tell us how much of the light that leaves the fixture arrives at the surface where it is needed. Foot-Candles are the most important unit of measurement when comparing or evaluating lighting.  They can be used to compare different lighting technologies (such as LED vs Metal Halide), as well as comparing how different LED Light fixtures perform in any space.

Foot-candles are directly affected by the distance from the source of lumens and can be expressed in a simple formula. One foot-candle is equivalent to the amount of light needed to saturate a one-foot square with one lumen of light (1 Foot-Candle = 1 lm/ft^2). This means that one foot-candle is equivalent to 10.76 lux. 

How to calculate a foot-candle

 

The idea of using foot-candles is to ensure that areas that require certain levels of light are actually receiving them. For example a foot-candle relates to the amount of light that is on the ground beneath a parking lot light, opposed to the amount of light being produced by the parking lot light. This can be very important for both commercial and industrial spaces.

  • Offices, manufacturing facilities, and workrooms need significantly higher foot-candle measurements in order to make work easier and less stressful on your eyes.

  • Hallways and lobbies need lower foot-candle measurements because the light source simply has to provide enough lighting to prevent injury but is not harsh enough to cause discomfort.

LED Lighting Design in Hallway

Foot-Candles as a unit of measurement are used most commonly in the United States and other places that follow our customary units of measurement. In other parts of the world, such as Europe, the SI-derived measurement is called “lux”.


Lux

A Lux (lx) is the standardized unit of measurement of light intensity. Similar to a foot-candle, a lux is simply another unit used to measure the illuminance or amount of light falling on a surface.

A lux is measured using the number of lumens in the bulb, taking into account the amount of visible light falling on the surface. So one lux is equivalent to the amount of light needed to saturate a one-meter square with one lumen of light (1 Lux = 1 lm/m^2).

So when answering your questions about “How bright is my lighting fixture’s LED light bulb?” or “How much area does one lighting fixture cover?” we will need to properly assess your lighting needs in accordance with these three lighting measurement units. 

One number will not tell the whole story to provide you with the best fixture or LED light. Here at Stouch Lighting we will take into account the area, the application, and any obstructions that may block the light output to properly determine the right lighting fixture for you.