Foot-Candles vs Lumen Output
The modernization of lighting systems has generated a significant improvement in the energy efficiency and lighting performance of buildings and facilities.
One lumen per square foot equals one footcandle, while one lumen per square meter equals one lux. Both of these units measure an area’s illuminance, just as both miles and kilometers measure distance.
LED lighting is rapidly taking over the market for a variety of commercial, industrial, and stadium applications. Many building and facility managers who are new to LED lighting find themselves confused over the terminology that describes LED lighting performance, particularly with respect to the lumen output of LEDs. Before we get too in the weeds on the technical stuff, let’s quickly review some key terms we’ll be discussing through this post:
- Luminous Flux – Commonly referred to as “Lumen Output”. Measures the amount of light radiated by a lamp, regardless of direction.
- Foot-Candles – A measurement of light intensity, describes the amount of light reaching a specified surface area. Measured in lumens per square foot.
- Lumens per watt – refers to the energy efficacy (not efficiency) of lighting: how much visible light you get for a given amount of wattage consumption
- HID Equivalency – A manufacturers claimed HID lamp wattage equivalency for a given LED lamp or fixture.
While it may seem like the easiest way to determine if a lamp or fixture is going to be “bright enough,” simply measuring luminous flux is deceiving because light that is illuminating an irrelevant area (such as the ceiling) or getting trapped within a fixture housing is not productively used. In fact, it’s a waste of money.
Further, lumen output doesn’t indicate how focused the light output may or may not be. Different optics and casings are required for HID lamps, as well as screw-in LED “HID replacement” lamps to focus the omni-directional lighting in order to direct or redirect the light in the intended direction. In the case of screw-in LED “HID Replacement” lamps, the lumen output is an even less relevant measurement compared to that of an actual HID lamp like Metal Halide.
The reason for this is that although a manufacturer may state that the LED lamp has, for example, 10,000 lumens, and an HID equivalency/replacement of 250 watts, there is absolutely no way to be certain that this HID equivalency claim is accurate.
The stated lumen output of an LED screw-in retrofit is that of the bulb not the fixture in which it is housed. Unless a manufacturer has tested a specific lamp within a specific HID fixture’s housing (a common parking lot shoebox fixture for example) there is no way to determine the amount of light actually leaving the fixture. Lumens generated by the LED lamp become trapped in the fixture and are limited by the fixture housing and lensing in regards to directional light distribution.
So what does this mean? How do you know what LED lighting product is the right choice? What you really care about is the amount of light actually illuminating the desired surface area – NOT the amount of light leaving a light source (Lumen Output).
Therefore, from a user’s perspective, Foot-Candle is a much more important measurement than total luminous flux. As we just mentioned, a certain amount of light is always lost to inefficiencies like absorption, reflection, and/or dissipation. That is, no light is 100% efficient, especially when you install a new LED screw-in retrofit to and old Metal Halide fixture housing. The foot-candle measurement takes this into account while lumen output does not. So let’s focus on foot-candles for now, as they are a much more relevant measure of light in the real world.
Now we’ve figured out half of the equation, we know how to measure the output of light in the most relevant way (foot-candle), but how do we know which LED lighting product is the best for your application?
Here’s what you should do: Ask your LED lighting supplier to do a PHOTOMETRIC LAYOUT. A photometric layout will show the foot-candle levels specific to your site, accounting for pole/fixture locations, mounting height, and fixture spacing. It’s not just about the lighting output, but the amount of light reaching the intended surfaces. If your LED Supplier or Contractor cannot provide a Photometric Layout, that means the fixtures they are recommending have not gone through necessary testing to claim an HID equivalency.
How bright is your space? What metrics matter?
That’s a lot of information. How does it all work together? Good question. I’ll try to explain it.
(Note: all of the terms defined above will be boldfaced and italicized when mentioned below to help in understanding how the above terms work together.)
Delivered lumens (illuminance) is influenced by the beam angle of a particular lamp. If you have two lamps producing the same amount of lumens but different beam angles, the center beam candlepower (CBCP) will be higher for the more narrow beam angle. But how do you know how much light reaches your product or work surface? There are two general ways to measure delivered lumens, or illuminance. Those two measurements are footcandles and lux. The former, footcandles, are the unit used in the U.S. Footcandles are defined as lumens per square foot. Lux is the lighting equivalent of a metric system measurement, as it’s used most everywhere except the U.S. Lux is defined as lumens per square meter. As a rule of thumb, one footcandle is equivalent to 10 lux.As a side note, people sometimes perceive warm color lighting products (with low correlated color temperatures) as less bright than products with cooler temperature products.
So if you’re wanting to brighten your space, when it comes to choosing a product, think about more than just the lumens the product carries. Consider all of the above. How will the product’s beam angle affect the illuminance of the space? Will it brighten the specific area you’re looking to light? Will a product with a warm CCT make your space feel less bright than you wish? To measure illuminance in footcandles or lux, you’ll need a light meter.
So we get it, nothing new right? We’ve discussed several times here how and why LED lighting is better in every way than traditional lighting.
Hopefully this article has helped with your LED selection process as you contemplate your conversion from HID to LED Lighting. But what about LED vs. LED? How do you know if you should go with an HID “Equivalent” Screw-in lamp, or a new LED fixture? No, not all LEDs are created equal, return for part 2 and we’ll tell you why.
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