Lumens vs Footcandles : What’s the Difference?

Comparing two units of light output can be confusing when they are measured in differing units. Here we look at a common photometric comparison of lumens with footcandles, to understand the difference and relationship between the two.

What are Lumens?

The lumen (lm) is a unit of measurement for luminous flux, that is used to show how strong or intense a light source is. It measures the total quantity of visible light emitted from a light source regardless of its direction.

So lumens measure the amount of light that is produced by a light bulb or lighting fixture. The more lumens, the brighter the light will be.

What is Footcandle?

The footcandle (fcd or fc) is the unit of measurement for luminous intensity. It is typically used to measure illuminance, how much light is shining on a surface.

A foot-candle measures the amount of light that reaches a one-foot square with one lumen of light.

1 footcandle is equal to the amount of light produced by 1 candle (which is 1 lumen) / 1 ft square

1 fcd = 1 lm/ft²

1 footcandle is the imperial unit of illuminance measurement equivalent to 10.76 lux, the metric unit of illuminance measurement.

1 fcd = 10.76 lx

What is the Difference?

Lumens and footcandles are both units of measurement used to describe light intensity, but they differ.

Lumens measure the total light output of a light source, regardless of direction. Footcandles measure the amount of light that reaches a surface.

So lumens tell you how bright a light is overall, while footcandles tell you the illuminance on a surface.

For example,

Let’s look at recessed ceiling light or known as a downlight in Europe, and see how the footcandles (intensity on a surface) change when the beam angle widens.

In this instance, the lumens initially created from the internal LED chip begin at 1138lm.

• With a Medium beam angle of 26 degrees, the luminaire delivers 685 lm as it leaves the lens and the footcandles are 191.9 fcd at a 3.3ft (1 metre) distance.
• With a Flood beam angle of 50 degrees, the luminaire delivers 610 lm as it leaves the lens and the footcandles are 79.1 fcd at a 3.3ft (1 metre) distance.
• With a Wash beam angle of 58 degrees, the luminaire delivers 651 lm as it leaves the lens and the footcandles are 79.1 fcd at a 3.3ft (1 metre) distance.

So the results show that as the beam angle of the light widens from 26 to 58 degrees, the intensity of the light on a surface, the footcandles, diminishes significantly compared to the lumens being delivered.

Note: the delivered lumens in this example only vary a little between each beam angle because of the type of glass lens used to create each angle.

How to Convert Lumens to Footcandles

The relationship between lumens and footcandles is:

1 footcandle = 1 lumen / 1 ft²

or

1 lumen = 1 footcandle x 1 ft²

For example, a light source that emits 1000 lumens spread evenly over a surface will produce 1000 lumens / 1 ft² = 1000 footcandles on a surface one foot away.

The same light source will produce 250 footcandles on a surface two feet away, because the light is now being spread over a larger area twice the distance away.

(This is calculated as 1000 lm / 2ft x 2ft, equivalent to 1000 / 4 = 250 fcd)

At 4 feet away it will produce 62.5 fcd

This demonstrates the inverse square law which states that “the illuminance from a point source is inversely proportional to the square of the distance from the source”.

So, doubling the distance from a light source creating the lumens will result in ¼ of the original foot candles.

Verdict

Footcandles do not define how bright a light source generally is, but rather how bright it will be within given distances and beam angles.

So foot-candles are therefore a more specific measurement and less useful for a quick comparison between light sources, such as when selecting a flashlight or torch.

Lumens are more commonly used because they are easier to measure, understand and compare.

For more quick to read easy to understand photometric comparisons of light, take a look at lumens vs kelvins, lumens vs nits, lumens vs candela and What are Lumens?

Andrew Orange, the owner of Orange Lighting qualified and worked as an interior designer in 1993 before specialising in lighting working on high profile projects based in London. Since starting Orange Lighting Ltd in 2003 he has been sharing his knowledge and unique teaching style mostly to his designer clients, offering practical real life advice born from running a busy consultancy and lighting supply business. Launching in 2020, his blog has evolved into Quick & Easy Lighting, curating some 25 years design experience into making the lighting choice and design process achievable and easy to understand for all.