LED Insights: Color Temperature vs. Color Rendering Index

White LED modules are commonplace for backlighting illuminated signs and are being used in applications from channel letters to cabinets and graphic advertising boxes. However, not all white LEDs are created equal. LEDs are generally monochromatic, meaning that they emit a very narrow spectrum (band) of light.

In order to make white light, LED lamp manufacturers cover a blue LED chip with a phosphor. This phosphor converts the blue light to a broad spectrum. This process adds some art to LED manufacturing, as there can be both variations in the phosphor thickness and distribution.

Since this is a batch process, it also can create variations from batch to batch. Depending upon the manufacturing parameters, there are a range of hues of white that are created from a yellow-red to a blue-white coloration. Depending on the application, these color variations or hues can have a significant impact on the effectiveness, perceived “brightness” and quality of your sign.

In this article we will provide an overview of color temperature and color rendering and how to choose the correct “color” of white to optimize your sign application. First, we need to define some key variables.

What is CCT?

Correlated color temperature (CCT) is a description of the warmth or coolness of a light source. When a piece of metal is heated, the color appears red in appearance and gradually changes color to orange, yellow, white, blue-white, and finally a deeper blue color. The temperature of this metal is a thermal measurement in degrees Kelvin or absolute temperature.

While LED lighting does not perfectly match the output of this piece of metal, we use the CCT convention to describe the appearance of light as it relates to this type of black body radiator. The sun is an ideal blackbody radiator and above atmosphere has a CCT of about 5900K. Depending upon the time of day and conditions, the CCT we see varies between 5000K at horizon daylight to 6500K during overcast conditions. In general lighting, warmer CCTs that mimic candlelight are used to create relaxation, and cooler CCTs are used to enhance concentration in office environments (see Figure 1).

What is CRI?

Color rendering index (CRI) is a quantitative measure of how well a light source renders colors versus an ideal blackbody radiator. Without getting too technical, it is calculated by taking eight CIE (International Commission on Illumination) standard color data points and comparing them to a standard.

A general CRI (Ra) is calculated by taking the arithmetic mean of these points. The smaller the average difference in chromaticities versus the ideal standard, the higher the CRI. A CRI of 100 represents the maximum value. A table of typical CRI values of various light sources is shown in Figure 2.

Let me make an important point. Higher CRI does not mean “better.” CRI is important when you are back-illuminating graphics. Typically, a CRI value greater than 80 is more than acceptable. However, one should consider the graphic as well. Sometimes color appearance (typically determined by CCT) is more important than color fidelity (CRI).

For instance, say you are trying to highlight a burger and fries, it may be more important to highlight warmer tones, even at the expense of a higher CRI. You may be surprised how well an 80 CRI product performs and may be disappointed with a 90+ CRI product depending on the graphic. As a general rule, when looking at LED backlighting for graphics, CCTs in the 4000-5000K range and above 80 CRI are the most versatile and will do a good job of lighting both warmer and cooler colors in the same sign at high fidelity. An example would be the American flag, where a higher CRI will render all three colors accurately.

CCTs and Cabinet Signs/Channel Letters

So what about cabinet lighting and channel letters? Here the decision is in some ways simpler. First, CRI values are of less importance. The only general caveat is that warmer text and print (red, orange, brown, etc.) render better at lower CCTs (4000-5000K) and cooler colors (blue, green, etc.) render better with higher CCTs (6000-7500K).

Temperatures below 3500K and above 7500K are typically not recommended, as they can cause significant color copy shift. For example, a 9000K LED can give red copy a purple hue and a 3500K LED can make blue copy look greenish. If there are colors other than white on the sign, it is a good practice to use LEDs between 6000-7000K, which is the part of the color spectrum with the least amount of hue variation.

Choosing the Right White

White channel letters are completely a matter of preference. Often, I see many sign companies and end users leaning toward the 6500-7500K CCT range. The reason is usually perceived brightness and getting a “white” white.

If you take a 4000K LED and a 7000K LED at the exact same lumen brightness and backlight a piece of white acrylic and ask someone which is “brighter,” almost everyone will point to the higher CCT LED. This has to do with the human eye and the sensitivity differences of the rods and cones in our eyes. Most people view signs at night and peak scotopic vision is around 510 nm (in the blue-green region). So even though the total lumens count is the same, the apparent or “scotopic” lumens that the eye sees is about 25 percent to 30 percent higher at 7000K versus 4000K. So, this factor should be considered when brightness is key.

The higher the CCT, the higher the scotopic brightness. Like anything, however, this can be taken too far. Some “bright” imported LEDs are often in the 9000-12000K range. This very strong bluish-purple hue can create a very harsh lighting effect. While this may seem like the best choice in the shop, when it goes into the field, it can look very purple, especially if other lower CCT signs are in the area or nearby.

The human eye is very sensitive to shade variation (we can actually see about 300 shades of white). Therefore, I recommend the use of LED modules for generic channel letters in the 6000-75000K range to give an attractive eye-popping effect. Warmer colors (3000-4000K) should be used when the client is trying to achieve a less intrusive, softer and more inviting effect. A neutral effect can be achieved in the 4500-5500K range.

Many tier-1 LED manufacturers offer their LED modules in a range of color temperatures. Having a variety of options of white can help you in the prototyping process and eventually secure business by being able to meet the color effect the customer wants to achieve.

Principal LED’s Street Fighter Family addresses the need for a variety of CCTs. The module comes in four of the most commonly needed color temps:

• 3500K (Warm White)
• 5000K (Natural White)
• 7000K (True White)
• 9000K (Cool White)

Figure 3 shows the difference between the various color temperatures of Street Fighter on a white acrylic face. Figure 4 features a sign that was retrofitted using Street Fighter Heavyweight in 7000K.

Importance of CCT and CRI

Understanding both color temperature and color rendering are important when helping your customer achieve the look they desire. In the case of graphics, color rendering and overall fidelity (driven by CRI) is key; however, color tint or hue (driven by CCT) is really a matter of customer preference when it comes to channel letters. Knowing the difference will help you make an informed decision when you build your next LED backlit sign.