Kelvin, Lumens, Foot-candles: Watt Do They Mean?

The lighting industry is full of many confusing terms, but few are more technical than some of the units of measurement involved with light. Kelvins, lumens, foot-candles, and even watts can be difficult to understand. But it’s important to be able to speak with authority on these terms to your clients or customers. This article will try to provide a background and summary of each term without making it feel like a science textbook. We’ll tackle these terms roughly in order of difficulty.


Measuring Light’s Power

In the lighting world, you see lights rated in watts. A flat-panel LED light, for example, might operate at 30 or 50 watts (or have selectable wattage). This number represents the electrical power that the light fixture uses, in other words how much energy per second. The symbol of the watt is “W,” and the name comes from the 18th-century engineer James Watt who was credited with the invention of the steam engine (though he did not invent the concept). Often the word “watt” is capitalized, because it is based on a proper name, but that rule is no longer a strict one; capitalizing the symbol (W), however, still seems like good form.

The watt is an important measure of lighting because it is shorthand for how much electricity a light is using. It is also one of the key numbers on an electricity bill: the kilowatt is 1,000 watts.

Like most of the units of measurement in this article, the watt is an SI unit (something you might remember from high school). SI units are internationally accepted forms of measurement defined by the International System of Units. Also, like many topics in science, if you look at the watt more closely, it gets a bit complicated. You can define a watt in a couple different ways. Because it is a unit of power, it can be defined as one joule (J) of energy happening per second (s). Or, more appropriate for the lighting world, it can also be called the power created by one ampere (A) crossing an electrical potential difference of one volt (V).

A simpler way to think of a watt is the rate at which electricity is flowing. Like miles per hour. A 50W LED light needs a flow of 50W at all times. (A kilowatt-hour (or kWh), on the other hand, is how much electricity will flow through the light in one hour. The kilowatt-hour also appears on electricity bills.)


Measuring Light’s Color Temperature

Some light is warm and yellowish and cozy, while some is bluish and cool. These are lights’ color temperatures, or, more specifically, their color corrected temperature (CCT). The unit of measurement of CCT is degrees Kelvin (K).

Color temperature is important in lighting design because the temperature greatly impacts a space. An office or warehouse needs cool, bluish light for good visibility, whereas a living room or lounge space needs warm, cozy light for comfort. If you switched the two, it would be obvious that something was wrong.

The key thing to know about this unit of measurement is that its numbers are a bit counterintuitive. The lower numbers are the “warm” colors, and the higher numbers are the “cool” colors (which is obviously the opposite of most other temperatures, where higher numbers are the warmest).

To use a specific example, let’s compare a flat-panel LED light to a high bay LED light. The first fixture will be used in a school library, while the second will be used in a warehouse. The flat-panel light, like an IntrinsiX Vista Series LED Light Panel, offers adjustable color temperatures of 3500, 4000, or 5000K. This will produce a variety of light. The 3500K setting will be significantly yellower and warmer than the 5000K light. A powerful LED high bay light, like the IntrinsiX CIRCA round high bay light, produces 5000K light that is perfect for warehouses or big box stores. This light is white-ish and cool and produces great visibility.


Measuring Light’s Brightness

Brightness also has an SI unit of measurement, and it’s called the lumen (lm) (also called lumen output). A lumen is a quantity of visible light coming from a source. The lumen output of a light usually roughly correlates to the light’s wattage, but not exactly. A higher wattage light will no doubt produce more lumens than a lower one. But light fixtures differ in quality and design, so there is not a direct correlation between the two numbers.

Light fixtures often offer adjustable lumen outputs. The IntrinsiX LED flat panel mentioned above offers lumen outputs of 2500, 3750, 5000, and 6250lm (and also include the option of dimming). This allows users to customize their lights to whatever application they need as well as to natural light sources.

The sub-units around brightness can grow confusing if one digs in very deeply. One could get lost in the differences between luminous flux and radiant flux and the angles of light that can be measured (called steradians). But the only other unit you’re likely to encounter (and probably not, actually) is the candela. The candela is another SI unit that measures light intensity rather than light quantity (the lumen). But knowing about the candela helps explain our final unit of measurement, the foot-candle.


Measuring Light’s Brightness in Another Way

If a “foot-candle” sounds old fashioned, you’re not wrong. It comes from a time when the main source of indoor light was the humble candle. The unit of the foot-candle (fc) is not part of the SI system of units. (The nearest SI unit is the lumen.) In fact, today the foot-candle is rarely used outside of the United States. But a foot-candle is still employed now and then to describe a quantity of light, especially in certain fields (museum galleries, cinematography, horticulture, and so on).

A foot-candle is defined as the light of one candela cast upon the inside of a sphere with a radius of one foot. In other words, it is the quantity of light cast upon that area. Another way to define it is one lumen per square foot (at even light distribution).

While the unit is slowly going out of fashion, you might still encounter it. The unit is useful in describing an amount of light, and that is always useful for designing a space.

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