How Bright is a Watt?
Written by John Higo
As a manufacturer of LED lamps, we are asked regularly if we sell a 3 Watt lamp, or a 9 Watt MR16. Which begs the question, what information is the customer actually looking for?
As you can probably guess, most people are not looking for a lamp of a particular wattage, what they are trying to determine is the brightness of a lamp. And like most people who’ve used incandescent or halogen lamps most of their life, they associate overall lamp brightness with wattage.
But, just as not all 20W halogen lamps are created equal, nor are all 6W LED lamps created equal.
What is wattage?
To answer this question, let’s begin with some basic electrical theory. A watt is a measurement of the amount of electrical power consumed by a device while in operation. W = V x A: One Watt = one Volt multiplied by one Amp. In the lighting industry, we often use words like watts and brightness interchangeably, but as new light sources are introduced, it is important to distinguish between power consumption (watts) and total energy output or lumens (lm).
In actuality, a watt has no brightness, it is simply a measure of the power consumed, and a lumen measures the brightness. What we are really measuring is luminous efficacy, or how well we use the energy consumed to produce light.
So, what are lumens?
The amount of light a device actually produces is best measured in lumens. A typical 60W incandescent light bulb produces between 650-850 lumens. If you average that out to 750 lumens, you could surmise that 1 Watt = 12.5 lumens (750 lumens divided by 60W). And if you measured a typical 20W halogen MR16, at 263 lumens, you would come close to the same conclusion (263 lumens divided by 20W = 13.15lpw).
If instead, however, you measured an Illumicare LED MR16 at 6W, with an average output of 260 lumens, you would believe that 1 Watt = 43.3 lumens. You could conclude that one of our Watts is more than 3 times as bright as one of their Watts!
Does this mean that the more lumens a bulb has, the more it will light up your landscape?
Now this is where things get a little complicated, because lumens don’t give us a clear indication of how much usable light is available either. A 60W light bulb produces more light than one of our 6W LED MR16 lamps, but how much of that light is used?
While an incandescent light bulb throws light in all directions, which can be difficult to harness and direct, our LED MR16, and LED chips in general, are specifically designed to be directional.
This is where the difference between lumens and footcandles becomes important.
Whereas a lumen is a measure of the total light output in all directions, a candela measures light output along a solid angle. So for a light source with a directional beam, like an MR16, candelas or candlepower is a more relevant means of measuring light.
In lighting design, our biggest concern is how much light is hitting our subject. Interior and exterior lighting are similar in this aspect. The amount of light our dining room chandelier produces is not as important as the amount of light that actually falls on our dining room table.
Why? Because we need to see in order to eat.
In landscape lighting, it is important to ensure the lamps we choose produce enough light to highlight a tree, accent a statue, or illuminate a pathway sufficiently for safe passage. No matter what the wattage or lumen output of the lamp you have chosen is, what you want to measure is footcandles. Footcandles are the illuminance at certain points on a surface, which are a short distance and perpendicular to your light source, or the amount of light that lands squarely on our subject at measured distances from your lamp.
How do you measure footcandles?
Illumicare provides the standard footcandle ‘V’ charts that most of us are familiar with for just this reason. If you are not familiar with footcandle ‘V’ charts, they are very simple to use.
On the left hand side of the chart, you have your distance from the light source to your subject. Down the center of the chart, you have the beam spread diameter at that distance, and down the right hand side, you have the footcandles in the center of that beam.
How you use the chart is up to you.
Figuring out how bright you want the light at your subject, you can determine the optimum distance, or correct lamp for your application. Similarly, if you know the mounting distance from your subject, you can determine the amount of light and spread of light falling on your subject, and either add more lights, or select your lamp accordingly.
Lighting purists would argue that footcandles are not as important as footlamberts, which is the amount of light refelected by an object, but we’ll save that topic for another day.
For more detailed information about the comparison between lumens, candelas and footcandles, please feel free to contact our sales team at firstname.lastname@example.org