Understanding CRI, CCT, and Foot Candles in High Quality LED Lighting
June 27, 2018
LED’s are taking the lighting landscape by storm, with a longer lifespan and drastically lower energy expenses, it seems like everyone is considering an LED retrofit. However, one important thing to know is that not all LED lights are created equal. There are a few key metrics that you want to keep your eye on to ensure that you’re getting high-quality LED’s. These key metrics are: color rendering index (CRI), correlated color temperature (CCT), and finally foot candle (FC) readings. Below we will look at each one of the terms in more detail so that you can ensure you are getting the lighting solution needed for your facility.
Color Rendering Index (CRI)
The Color Rendering Index (CRI) is a scale from 0 to 100% indicating how accurate a “given” light source is at rendering color when compared to a “reference” light source. In layman’s terms (CRI) is a measurement of a light’s ability to reveal the actual color of objects. Generally speaking the higher the color rendering index the better. There are very few applications where CRI is not a relevant variable for your facility. CRI is still by far the most widely published index by which everyday consumers can make a judgment on the color rendering ability of a particular light.
Color Temperature is a method to describe the characteristics of visible light from different emitters. The color temperature scale works by comparing the visible light emissions from a given light bulb to those of a “blackbody” emitter.
Incandescent Bulb Example
As the filament of an incandescent bulb is heated it will eventually begin to glow. The glow, of course, has color characteristics. As the surface temperature of the filament itself becomes hotter, its glow will change color. The glow will begin at relatively low temperatures (around 1500K) as a deep red color. As the filament’s surface temperature continues to rise the glow will transition from red to orange, orange to yellow, yellow to white, and if it were to get hot enough from white to a light blue.
The deep red glow occurs at a relatively lower surface temperature, it is typically referred to as “warm” color temperature. Conversely, the white or light blue glow occurs at a relatively higher filament surface temperature.
The chart above shows the color temperature of different lights when they are operated at their typical operating power, but it’s important to realize that the actual color temperature will be a range depending on the amount of power that is applied to the light (more power will result in a hotter filament temperature and thus a “cooler” color temperature).
Correlated Color Temperature (CCT)
Correlated color temperature is a measurement used in lieu of color temperature for lights that do not approximate a black body radiator (that is, they emit light through processes other than thermal radiation). Both fluorescent lights and LED lights fall under this category and thus are evaluated using CCT.
Correlated Color Temperature is a specification used to describe the dominant color tone for non-blackbody light emitters such that they can be accurately compared and contrasted with those light emitters that do approximate blackbody radiation (like incandescent bulbs). Ratings at the lower end of the scale (2000K) are generally referred to as “warm” (typically red and yellow colors) while those at the higher end of the scale (5000K+) are typically referred to as “cool” (typically white to light blue)…the same as for color temperature scales.
The foot-candle is a measure that describes the amount of light reaching a specified surface area as opposed to the total amount of light coming from a source. Foot-candles are measured in lumens per square foot as opposed to simply lumens.
Simply measuring lumens is a deceiving metric because light that is illuminating an irrelevant area (such as the ceiling) is not important for your application. In fact, it’s a waste. What you really care about is the amount of light actually illuminating the desired surface area. Therefore, from a user’s perspective, foot-candle is a much more important measurement than total luminous flux.
In addition to the difference in units, some of the light coming from a source does not ever make it to the desired aim point. A certain amount of light is always lost to inefficiencies like light absorption, reflection, and/or dissipation. Foot-candle takes this into account while luminous flux does not. So forget luminous flux for now and focus on foot-candles as they are a much more relevant measure of lighting in the real world.
In conclusion, we can see the importance of evaluating the: color rendering index, correlated color temperature, and foot candle of LED lighting before making a purchase decision. If you need help in evaluating LED lighting for your facility reach out to us here at Action Services Group, we could be happy to put together a free no-obligation proposal for your organization. We look forward to working with you.