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, known 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 = 2,000K; incandescent lamps = 2,700K; halogen lamps = 3,000K; CFL = 3,000-6,500K; fluorescent tubes = 3,500-6,500K; CMH = 3,500-6,500K; LED = 1,800-6,500K.
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. 2,700K was the most common for residential for many decades (remember the ubiquitous 60-watt incandescent lamps?). A 3,000K light source is still wonderfully comfortable, but is somewhat “whiter” and ups the saturation of the cooler colors. 3,000K is ideal for lighting in kitchens, offices, bathrooms, and the general lighting of residential spaces. The 3,500K Color Temperature is very white. It adds a degree of sparkle to a room without being harsh. 3,500K can also be used for use in residential kitchens, and some people use it for general lighting of residential spaces. Good uses of 3,500K is for lighting commercial offices and hospitality spaces. For dining rooms and bedrooms, 3,500K would not be the best choice.
4,000K lighting begins to look somewhat blueish, yet depending on the application it can be a good choice. Commercial offices, automotive showrooms, commercial retail and grocery, and factories are spaces where 4,000K lighting is often found. The 5,000-6,000K range is noticeably blue when compared to lighting traditionally used in residential applications. 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 strong blue content.
Regarding the lighting of art, color temperature selection is based on the artwork being lit, and what attributes the user wants to draw out. Often 3,000 – 3,500K is preferred, yet anything between 2,700 – 5,000K 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.
Regarding color performance, different types of light source technology perform different as well. These days, LED’s perform very well, with good color balance and an 80+ CRI that is good for everyday 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 many 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 tri-phosphorus coatings can achieve 85 CRI to 90 CRI, but typically 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 is not particularly distracting when used for general illumination applications, which explains their widespread 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.
Deciding Which CRI To Use
Ideally, simply choosing the highest CRI available would be logical. But, there are both price differences and luminous efficacy differences to consider. First price: Although the prices between light sources in the 80CRI range versus 90+CRI range have come closer together, higher CRI often costs more. Second comes luminous efficacy (i.e. lumens per watt). High CRI LEDs require more sophisticated, and in simple terms, a thicker coating of phosphorus. You get better overall color, but slightly less light output.
So, one simple rule of thumb is to use high CRI when it is needed for visual acuity (tasks that need to see fine color differences), lighting art, and even in a bathroom for putting on makeup. Ideally, your home should be lit with at least 90CRI. Simply put, high CRI makes for better skin tones, better looking food, and a more inviting atmosphere.
High CRI is also especially important for showing merchandise. Automotive showrooms, clothing stores, restaurants, bakeries, and meat departments all benefit tremendously from good lighting. Even hotel lobbies are more inviting when lit with good quality light. Commercial sidewalk lighting in popular shopping districts definitely benefits from 90+CRI light sources. Nothing makes a night scene more appealing than good skin tone and vibrant facades.
But high CRI is not needed everywhere. Good applications for CRI in the 80’s are exterior lighting (facade, pathway, garden), general lighting inside retail stores, hallway lighting, utility rooms, factories (except where fine visual acuity is needed). For street and highway lighting a 70CRI range certainly seems sufficient, or at least for now, it is a good compromise between lumen output and energy use.
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. Known 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 is 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. Yet 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 tweak of TM30 gives a color fidelity metric called Rf. The Rf score is an average of all 99 TM30 Color Fidelity measurements.
It is 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 us compare two make-believe LED consumer lamps from different manufacturers. For this example’s sake, both lamps scored perfect on the Color Fidelity test, except 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 a 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 a Rf 80 score by indicating over or under saturation.