Color Temperature & CRI
By Daniel Mozey, LC, MIES
Color Temperature describes the overall color balance of a white light source. The Color Rendering Index (CRI) gives a rough color fidelity “score” of how a particular light source renders a palette of pastel colors, while a new method, know as TM30-2015, gives a significantly more accurate assessment of color fidelity.
Color Temperature is expressed in kelvin (K). For incandescent lamps, Color Temperature is literally the temperature of the lamp’s filament. Here are some typical comparisons of light sources: Candles = 2000K; incandescent lamps = 2700K; halogen lamps = 3000K; CFL = 3000-6500K; fluorescent tubes = 3500-6500K; CMH = 3500-6500K; LED = 1800-6500K.
In practical terms, Color Temperature can be used to set the mood of a space. For example, 2,700K, with its warm tone, feels comfortable and inviting, and is ideal for dining rooms, living rooms, and bedrooms. A 3,000K light source is still very comfortable but is somewhat “whiter” and ups the saturation of the cooler colors like blue and green. Its crisp light is ideal for task lighting in kitchens and office space, and good for general lighting of hospitality and residential spaces. The 3,500 – 4000K range is ideal for general lighting of commercial offices, commercial retail and grocery, and task lighting. The 5000 – 6000K range is similar to full sun at noon, but do become noticeably blueish. This temperature range is used for automotive headlights, aquariums, horticulture, and by the film and television industry. Color Temperatures at 6,000K and above have their applications, yet for lighting purposes their use is limited due to the very strong blue content.
In regards to the lighting of art, color temperature selection is based on the art work being lit, and what attributes the user wants to draw out. Typically 3000 – 3500K is preferred, yet anything between 2700 – 5000K may likely be chosen for art.
Color Rendering Index (CRI)
The Color Rendering Index (CRI) gives a “score” of how a particular light source renders a palette of eight pastel colors. It is a way to compare how well light sources will render various colors. Since the CRI score relies on a small sampling of colors, it will give you a good average, but it will not tell you the exact performance with all colors.
It is important to remember that two different light sources (i.e., from different manufactures, or different light source technology), will likely have different CRI scores, even when they have the same Color Temperature. For example, one manufacture’s 3,000K LED lamp will have different CRI performance than another’s 3,000K LED lamp.
In regards to color performance, different types of light source technology perform different as well. These days, LED’s perform very well, with good color balance and a 80+ CRI that is good for every day use. High-performance LEDs use enhanced phosphorus and other technologies to achieve very good color balance, and have a 90-98 CRI. Incandescent (95 CRI) and halogen (100 CRI) have excellent CRI with a good balance between colors, great skin tones, with blues and greens tapering. The high CRI rating of these lamps is not surprising, as the halogen lamp was used as the “benchmark” for the CRI rating system
High Intensity Discharge (HID) lamps are widely used for the illumination of large indoor and outdoor spaces. The CRI range varies quite a bit for HID, with 16-25 CRI for HPS lamps, 60-65 CRI for standard Metal Halide lamps, and 75-80 CRI for CMH lamps. Low wattage CMH lamps, typically used for retail lighting and other high-color applications, feature a 85-95 CRI. HID lamps are very energy efficient, even equal to, in may cases, to that of the LED. But gains in LED lumen output and life span have made LED a logical replacement for HID.
Older CFL and fluorescent lamps had marginal CRI, with very weak red and skin color tones. Better fluorescent lamps using triphosphorus coatings can achieve 85 CRI to 90 CRI. Typically though, these lamps feature strong spikes in the blue, green, and yellow spectrums. Performance of individual colors is very uneven with most fluorescent lamps. In practice though, the uneven performance of modern “high CRI” fluorescent lamps isn’t particularly distracting when used for general illumination applications, which explains their wide spread use throughout the world. LEDs, due to their reduced heavy metals, longer life, and lower energy use, are quickly replacing fluorescent in almost all applications.
TM30 – A Better Color Measurement System
The old CRI color assessment for light sources had numerous issues, not to mention that the “score” was based on only 8 pastel color swatches. CRI gave a decent estimate, perhaps acceptable for consumer products and incandescent lamps, but did not meet the needs of lighting professionals and did poorly at rating LED sources.
After many years of discussion, the Illuminating Engineering Society (IES) formulated and published a new standard for rating the color fidelity of light sources. Know as TM30-2015, the standard measures a greatly expanded pallet of 99 colors called the Color Evaluation Samples (CES). TM30 was also adopted by the International Commission on Illumination (CIE) and, with some minor revisions, the CIE developed their own color fidelity metric called Rf. Over the coming years, an increasing number of manufacturers of LEDs and luminaires will begin publishing the new TM30 performance scores for their products.
A full TM30 report for a light source includes three important graphs: the Color Fidelity score, Hue Angle, and Color Shift. The Color Fidelity bar graph shows how close the light source came to matching the Color Evaluation Samples. Each color is scored between 0 – 100, with 100 being a perfect match. Either oversaturation or under saturation of a color will reduce a score. The Hue Angle bar graph indicates whether colors were spot on, oversaturated or under saturated. In many cases, the Color Fidelity Score and Hue Angle will be sufficient to make an informed decision.
But sometimes more information is desired. That’s when the TM30 Color Shift vector graph is useful. This graph compares the score of the light source being evaluated with the score of an “ideal” reference of that same Color Temperature.
Comparing the three graphs will inform you how a light source or luminaire will “feel” in a particular application. In other words, what colors are being drawn out and what the ambience is being suggested. It’s important to remember that, different light sources, with varying TM30 performance, lend themselves to different applications. Therefore, a “lower” score does not necessarily mean that one is better than the other.
The International Commission on Illumination’s slight tweek of TM30 gives a color fidelity metric called Rf. The Rf score is an average of all 99 TM30 Color Fidelity measurements.
It’s important to remember though, that Rf, just like the older CRI score, gives an averaged metric based on color fidelity. Rf does not tell you how individual colors performed. As an example, let’s compare two make believe LED consumer lamps from different manufacturers. For this example’s sake, both lamps scored perfect on the Color Fidelity test, accept for one color, “CES #5” (one of the red sample colors), where they both scored 80. One lamp is over saturated by 20% for CES #5 and the other is under saturated by 20% for CES #5. So, both lamps have an Rf score of 80. To find the overall color balance differences between the two lamps you would need to view the Color Fidelity scores (Which for this example would show that for both lamps, the CES #5 color scored lower than other colors). For more detail, the Hue Angle graph would show why each lamp had an Rf 80 score by indicating over or under saturation.