By Daniel Mozey, LC, MIES

Color Temperature describes the overall color balance of a light source.  For the purpose of lighting, it describes how warm or cool the light is in the spectrum visible to humans.  Color Temperature is expressed in kelvin (K).  

While Color Temperature lets us know the overall balance, this measurement does not tell us how individual colors will look.  To get an understanding of color accuracy you need to look at the Color Rendering Index (CRI), which gives a basic color fidelity “score” of how a particular light source (lamp, etc.) renders a palette of pastel colors.  (Note: CRI is an abbreviation generally used for marketing purposes, but CIE R_a is the full acronym.)  The CRI standard has been around for a long time, but it only uses limited points of measurement to come up with a score.  A new testing standard, known as TM30-2015, gives a significantly more accurate assessment of color fidelity.

Color Temperature

For incandescent lamps, Color Temperature is literally the temperature of the lamp’s filament.  The following are some typical comparisons of light sources. Candles: 2,000K; incandescent lamps: 2,700K; halogen lamps: 3,000K; CFL: 3,000K to 6,500K; fluorescent tubes: 3,500K to 6,500K; CMH: 3,500K to 6,500K; LED: 1,800K to 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 can also be used for general lighting of residential spaces.  3,000K is also good for restaurants, although 2,700K may be better for overall comfort and relaxation.

A Color Temperature of 3,500K is very white.  It adds a degree of sparkle to a room without being harsh. 3,500K can also be used for residential kitchens. Some people like it for their home office, and it’s great for home workshops.  But for residential use outside of a kitchen or workspace, 3,500K is not ideal.  Good uses of 3,500K is lighting commercial offices, car dealerships, and retail.  

4,000K light begins to have a slight blueish tint, yet depending on the application it may be a good choice.  Generally though, this Color Temperature is not great unless it has a high CRI.  Assuming a really good CRI, some typical applications are automotive showrooms, commercial retail, grocery, and factories.

If you’re out shopping for lamps, ignore the hype. Despite the misplaced marketing by lamp manufactures and big-box retail stores, 4000K and above are not “invigorating,” nor do they really feel or look like daylight.  When using in your home at typical lamp lumens (1600L and less), these high Color Temperatures have a bluish tint and look more like bad fluorescent lights or a cloudy day during the winter with snow covered ground.

The 5,000-6,000K range is noticeably light-blue when compared to lighting traditionally used in residential applications.  This temperature range is sometimes 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 general 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 3,000K is preferred, yet anything between 2,700 – 5,000K may likely be chosen for art.  In most cases you will want a very high 90+CRI, and make sure to protect your art from UV to prevent fading.

Color Rendering Index (CRI)

A full Color Rendering Index (CRI) “score” gives you information of how a particular light source renders a palette of 14 pastel colors.  But with consumer products only the basic CRI results are given, which indicate a score from a subset palette of 8 pastel colors.  Nonetheless, it is a way to compare how well light sources will render various colors.  Since the basic CRI score relies on a small sampling of colors, it will give you a good average, but it will not tell the exact performance with all colors.

Sometimes the product labeling or specification sheet will add one more CRI test result labeled as R9, which corresponds to the color red.  This information is great to have when using LED lighting as it will let you know if the light source being considered outputs enough red. (Note: While “white” LED lamps do an adequate job of lighting, they are often very low on red and associated hues.  Having a good R9, along with a good overall CRI rating, will ensure that skin tones look healthy, and in stores, that displayed products have snappy colors and a more natural look.)

It is important to remember that two different light sources (i.e., from different manufactures, or different light source technology), can have different CRI scores, even when they have the same Color Temperature.  For example, one manufacture’s 3,000K LED lamp can have different CRI performance than another’s 3,000K LED lamp. 

Regarding color performance, different types of light source technology perform different as well.  LED’s perform very well, with good color balance and an 80+ CRI that is good for general use.  High-performance LEDs use enhanced phosphorus and other technologies to achieve very good color balance and can have a 90-98 CRI rating.

Incandescent (95 CRI) and halogen (100 CRI) have a very good balance between colors, with blues and greens tapering, and offer great skin tones.  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 where once 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 an 85-95 CRI.  HID lamps are very energy efficient, and in some cases even equal 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.  Before being largely phased out, new generations of fluorescent lamps used tri-phosphorus coatings and achieved 85 CRI to 90 CRI (but typically these lamps still featured strong spikes in the blue, green, and yellow spectrums).  Color rendering performance 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.  Besides heavy metals, one downside of compact fluorescent lamps was that they did not like being frequently switched on and off, leading to early failures in many residential applications.   LEDs, due to their reduced heavy metals, longer life, and lower energy use, have mostly replaced fluorescent in almost all applications.

Deciding Which CRI To Use

For lighting of interior spaces, choose the highest CRI available for your application.  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. Don’t forget to use high CRI lighting for art and in bathrooms (for putting on makeup).

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 benefit from 90+CRI light sources.  Nothing makes a night scene more appealing than good skin tone and vibrant facades.  

Use high CRI when it is needed for visual acuity such as tasks that need to see color differences.  Factories also benefit from good lighting including improved productivity.

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, but 80CRI would improve visual acuity for drivers.

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 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.