Saving Money With LEDs: Improved Performance For Less

Most of the focus on LED lights is on energy efficiency. Unfortunately that leaves two of the most relevant improvements (lighting quality and reduced maintenance liability) severely underappreciated. The focus of this discussion is on lighting quality. For a full discussion of the maintenance efficiencies achieved with LED lighting read here .

Lighting Quality With LED Lights: The three most important characteristics regarding the lighting quality of a bulb and its luminaire are the Color Temperature (often called the Correlated Color Temperature ( CCT ), the Color Rendering Index ( CRI ), and the Foot Candle level. For a full discussion of all three characteristics you can read here .

The evolution of lighting: (left to right): vintage incandescent, modern incandescent, compact fluorescent light (CFL), LED. Image From: portlandoregon.gov

Color Temperature: Color Temperature is a method to describe the characteristics of visible light from different emitters. The relevant color temperature scale runs from approximately 1500K to 8000K. For a more technical discussion of this metric read here . Otherwise, the simple way to understand color temperature is to think of it as “glow.” Lights that emit a deep red glow are referred to as “warm” lights and they are on one end of the color temperature spectrum (closer to 1500K). Those that emit a bright white to blue glow are referred to as “cold” color temperatures and they are on the other side of the spectrum (closer to 8000K). There isn’t a good or bad color temperature. It just depends on the desired characteristics for the particular situation. A good way to understand color temperature without looking at any lights is to think of the sun. At sunrise and sunset it emits a deep orange to reddish glow (this is a warm color temperature). During the day it is much closer to yellow/white (a moderate to cool color temperature around 5000K). If the sky is overcast then daylight is even cooler (around 6500K). Unlike the sun, light bulbs have a fixed color temperature that doesn’t change with the weather or time of day. When choosing between the available options you simply need to evaluate the intended use and your perception of the objects you are trying to evaluate under the given light.

Color Rendering Index: CRI is a measurement of a light’s ability to reveal the actual color of objects as compared to an ideal light source (natural light). The rating goes from 0 to 100 where a light with a rating of 100 (the highest possible) would reveal objects in exactly the same color as natural light and 0 would essentially display them as shadows in grayscale. CRI ratings greater than 90 are generally considered excellent. Additionally, it’s important to recognize that CRI is independent of color temperature. You can have a warm color temperature light (e.g. 2500K) with a high CRI rating (e.g. 95) or a cool color temperature light (e.g. 5000K) with a low CRI rating (e.g. 65). It depends on the actual light. Generally speaking, LED lights render color very well. This is readily apparent when you compare the the lighting from a project’s before and after photos. Consider the following image:

On the left you see a parking lot illuminated by the traditional lighting solution for outdoor applications: high pressure sodium (HPS) lights. HPS lights have a warm color temperature (as is evidenced by the yellow glow they emit and a notoriously low CRI value (as is evidenced by the fact that the grass in the image to the left does not appear to be green). On the right you can see the same parking lot illuminated by modern LED lights. Two things are readily noticeable: first, the yellow glow is gone because the LED used in the right image has a much higher color temperature (i.e. it is a white light). Second, the grass (and the trees for that matter) appear to be green (indicative of the higher CRI of LED lights). The thing you might not notice as easily is the better “illumination footprint” on the ground from the new lighting. You can see this best by comparing the shadows in the two images (see the red boxed areas in the two images below). Notice how the highlighted section on the left is almost entirely shadow while the image to the right is getting significantly more light.

Another way of saying this is that the LEDs (on the right) have a much improved foot candle level (more on this below). A higher foot candle level means that the LEDs are distributing more light over the same target area. An added benefit is that the light emissions in general from the LEDs are distributed over a wider area without compromising the quality in the primary target area.

While the motivation for the new lighting above was largely economic (i.e. the customer was principally interested in energy savings and reduced maintenance costs), the most noticeable benefit is the immensely improved lighting quality throughout the parking lot.

Foot Candles: A foot-candle, mentioned above, is a measure that describes the amount of light reaching a specified surface area (measured in lumens per square foot) as opposed to the total amount of light coming from a source (a metric known as luminous flux). It is much more important to consider foot candles than luminous flux because, to use the parking lot example above, light that is emitted by a bulb towards the sky does not help illuminate the parking lot (the target area that matters). Luminous flux ratings count the emissions going towards the sky while foot candle ratings do not.

Left Above: LED light with 180 degree emissions (zero light emitted behind the bulb); Right Above: Fluorescent tube with 360 degree emissions (50% of the light emitted behind the bulb and onto the ceiling). Image source: unknown

LEDs are uniquely advantageous in the world of lighting when it comes to foot candle levels because they emit light directionally (only for 180 degrees). Every other relevant lighting technology on the market (incandescent, high intensity discharge bulbs such as LPS, HPS, mercury vapor, halogen, fluorescent, and metal halide) emits light omnidirectionally (a full 360 degrees). 360 degree emissions require reflectors to redirect the light away from the ceiling or sky. Reflectors mean losses. That is, the bulb’s luminous flux rating might look good on paper but the foot candle levels (what you see in reality) will be significantly reduced after the losses from light not reflected are taken into account.

Conclusion: Light emitted from LEDs is likely to be much higher quality than traditional lighting solutions for two principal reasons. First, the way that objects are revealed when subject to LED emissions is very close to natural light. This means that objects will generally appear in color like you are used to seeing them during the day. Second, the amount of light that hits the target area is likely to be much higher for a given power rating than it will be with virtually any other light. In other words, the area you care about will get all of the light!

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