The world of lighting has undergone many changes in the last century. The up-side of this is that we now enjoy some of the brightest, longest-lasting, and most efficient lighting options the world has ever seen. The down-side is that these technological advances have led to an abundance of terminology that many find confusing. The purpose of this article is to explore the most common terms and explain them in a way that makes them easy to understand…no fancy science degrees required.
Let’s start off with a little quiz. If you have two lights in front of you, one being 75 watts & another 100 watts, which will produce more light? Most people would probably say the 100 watt light, but that’s not necessarily the correct answer. Truth be told, we can’t know the correct answer because watts isn’t a measure of light output. Watts is actually a measure of total power output. You see, not all of the energy emitted by a light source is visible light – heat and invisible light waves (ex. infrared light) are also emitted. Lumens, on the other hand, will tell you the total visible light output of a source. For this reason, lumens (not watts) is the relevant unit of measure when you’re concerned about visibility.
Once the predominant unit of measure used to describe the intensity of a light source in a particular direction, the term “candlepower” has gone the way of the dinosaur. The new standard unit of measure (which happens to be a direct equivalent to candlepower) is the “candela.” One candela is close to the light intensity produced by an ordinary candle as perceived by the human eye and addresses how bright a light source is in a particular direction. The candela is similar to the lumen in that both deal with light output as perceived by the human eye, but the key difference is that the lumen measures the total visible light output, whereas the candela addresses directional intensity (see the illustration below).
Last, but not least, are the foot-candle and lux. Both units measure the same thing – the amount of visible light that falls on a surface. The key difference between the two is that the foot-candle uses the Imperial measuring system (feet, pounds, etc.), while the lux uses the metric system (meters, grams, etc.). To better understand the foot-candle, imagine that you have a one candela light source located in the center of a sphere. If the radius of that sphere is a distance of one foot, a foot-candle is the amount of light that falls on the inside surface of that sphere. For the lux, take that same example and simply change the radius of the sphere to a distance of one meter – the amount of light falling on the inside surface of the sphere would be one lux. See the illustration below for a visual depiction of these measurements and how they relate to one another. If you ever need to convert between foot-candles and lux, the rule of thumb is that one foot-candle is approximately equal to 10.764 lux.
So, to recap the terminology and concepts we’ve covered in the article: