Watts versus Lumens versus Footcandles

What is the difference between Lumens vs Foot Candles vs Lux?

We often find there is a lot of confusion between Lumens and Foot Candles and Lux. And even more confusing is how some represent conversion from traditional light sources like Metal Halide and High Pressure Sodium in terms of watts, and not lumens. So why all the confusion. Well, it may be because those who represent this conversion are trying to avoid letting you know about the efficiency of the LED lights, so they claim you can replace X watts of Metal Halide with X watts of LED. What they should really say is that you can replace a 400W Metal Halide with X lumens, and their lights only need Y watts to produce these lumens. We’ll dive into this a little later, but lets first talk about lumens vs foot candles vs lux.

What is a lumen? A lumen is a measurement of light related to (1) fixture or bulb. So when we talk about lumens, we say that a fixture or bulb creates (X) lumens. These lumens are determined by sophisticated measurement equipment.

What is a foot candle? A foot candle is a unit of measurement at a specific location in a room or outdoor location that is created by 1 to many lights. It is a cumulative reading of 1 to many light sources at a specific location. It is both real and calculated. Real in the sense that you can take a light meter and walk to any location in a room or outdoor location and take a reading that represents the amount of light recorded from all contributing lights or fixtures. Calculated in the sense that you can use software to predict the foot candles of a space based on the placement of lights within the simulation.

What is lux? Lux is identical to foot candles. It is a measurement of real or calculated light, but it is done on a different scale. As a rough estimate, 1 foot candle = 10 lux.

So, simply stated, lights produces lumens, and the cumulative lumens of all lights are then determine the foot candle or lux measurement of light at a specific point in an indoor or outdoor space.

 

So how does Watts represent the amount of light?

It really doesn’t, but some have decided that it is easier to say that X watts can replace a specific Metal Halide, High Pressure Sodium or Fluorescent fixture. In reality, this fixture produces lumens, and the amount of energy that bulb or fixture uses to create those lumens is a measurement of watts. While it is true that the fixture they are recommending does consume those number of watts, it is better to represent this fixture in terms of the number of lumens it creates.

 

What about Light Efficiency?

All lights create lumens. They all consume electricity to make those lumens. How efficient they produce those lumens with the least amount of watts consumed is what converting to LED is all about. If you look at a typical Metal Halide bulb, they consume 455 Watts to produce 36,000 lumens. This calculates out to 79 lumens per watt. Compare that to some of the most efficient LED Retrofit Kits on the market today, which are rated at 165 lumens/watt. This efficiency, along with other factors such as higher quality of light, directionality of light, allows you to replace 455 Watts with as little as 100 watts. And we understand that we should be talking about replacing lights with lumens, but for purpose of this discussion, we are equating this in terms of energy savings which then translates into dollars saved month after month.

 

Lumens per Watt Matters. Not all LED efficiencies are the same…

As mentioned above, some of the most efficient LED retrofit kits on the market are working at 165 lumens/watt. Yet, there are other retrofit options available, and their efficiencies are only at 97 lumens per watt? Why?

You have to think of LED as a technology. And with any technology, scientists are always trying to make the technology better. Better in terms of LED is producing light more efficiently. So as more new products are introduced in the market today, some use the latest technology and some use the older technology.

So why is this important? Why should you care? Well chances are, the reason you are converting over to LED is because you want to save money. So we all know you will save money when you convert to LED. But lets take it one step further, shouldn’t you want the most efficient LED?

To put it another way, lets say you got tired of paying $4 a gallon for gasoline, and you are tired of your existing car that only gets 17 mile per gallon. So you are in the market to save money by buying a more efficient car. So it is true that any car that you purchase that gets better than 17 miles per gallon will save you money, wouldn’t you want to find that car that gets 54 mpg instead of one that only get 27 mpg? The same applies to LED.

And since high quality LED Lights and fixtures will last a long time, over the life of the product, it makes sense to get the most efficient LED light source? The difference between efficient and less efficient LED products in terms of dollars spent over the life of the product can be pretty significant. Multiply those savings over 100’s of lights, and we are talking about significant dollars not being spent.

Here is a simple way you can identify the most efficient LED light products. Look for the DesignLights Consortium Premium Qualified symbol. This is not to be confused with the standard DesignLights Consortium Standard Symbol. Look for the words Premium in the symbol. This signifies that these lights are among the more efficient lights that have been certified by DLC.

 

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