Color temperature, expressed in degrees Kelvin (K), refers to the numerical measurement of the color appearance of light emitted by a black body radiator being heated. A black body radiator is a theoretical reference source that can completely absorb all radiant energy falling upon it and produce light through thermal radiation. As a black body gets heated and the temperature increases, it glows red, orange, yellow, white, and finally blue. At any point during this heating process, the black body's color appearance could be measured by a color temperature scale. This reference source is also called a Planckian black body radiator because in 1900 German physicist Max Planck discovered that the energy density at different wavelengths of black body radiation corresponds to the temperature of the body. Hence, thermal radiators are also termed Planckian radiators by academics.

Where Comes Correlated Color Temperature (CCT)?​

As mentioned above the term "color temperature" was originally coined to visualize the color of a Planckian light source based on the comparison of the light source to that of a black body radiator. The sun is a Planckian radiator. An incandescent lamp is also a Planckian source because it uses heated tungsten filaments to emit light, similar to a black body radiator. Fluorescent, HID and LED lamps do not make use of heated elements. The numerical measurements of their light colors have to be approximated through correlation, and therefore the term "correlated color temperature (CCT)" was created. However, this term appears primarily in technical documents. General consumers and most lighting professionals use the term "color temperature" as a shorthand way to depict the color appearance of both Planckian and non-Planckian light sources.

Color Temperature References​

The Warm-to-Cool Curve​

The color temperatures of sunlight that we see throughout the day vary from low to high. The early morning and late afternoon sunlight has a more significant red component or a "warmer feel," while sunlight that we see in the middle of the day has a more significant blue component or a "cooler feel." Color temperatures between 2700K and 6500K are typically useful for general illumination. Within this range, light with tones extending from reddish to yellowish is called warm white light. Light that has more blue wavelengths in their spectral power distribution (SPD) is known as cool white light. The whites that cover the middle range of the Kelvin scale (between warm and cool whites) have a more balanced appearance and are thus called neutral whites. In the United States, blue-enriched high CCT white light (5000 K and above) is referred to as daylight.

Typical CCTs for Artificial Lighting​

There're no standard CCT ranges for warm white (WW), neutral white (CW), and daylight white (DW). The CCT ranges of WW and CW, in particular, vary a lot in different markets. In the U.S., the 4000 K CCT is cool white, and the 5000 K CCT is daylight white. Whereas in China, both 4000 K and 5000 K CCTs are categorized into neutral whites. Most commercial applications use in the U.S. use light source of 3500 K to 4100 K, and the residential lighting market in this country overwhelmingly chooses warm white (2700 - 3000 K) light sources. In China, 6000 - 6500 K is not only the dominant CCT range for commercial and industrial applications, but also finds its significant presence in residential applications.

Typical CCTs labeled on lamps and luminaires sold in the U.S. market

• Warm white: 2700 K, 3000 K
• Neutral white: 3500 K
• Cool white: 4000 K, 4100 K
• Daylight white: 5000 K

Typical CCT ranges in China and developing countries

• Warm white: 2700 - 3200 K
• Neutral white: 4000 - 5500 K
• Cool white: 6000 - 6500 K

The Science behind Color Temperatures​

Why the hell the CCT preferences differ so hugely in the world? No offense to people and markets who stubbornly prefer extremely high CCT (6000 K and above) lighting day and night, but the culture and science of artificial lighting have definitely not been populated in these markets. Artificial lighting directly relates to our interaction with our surroundings and enhances our ability to see and interpret the world around us. The color characteristics of a light source have profound effects on human productivity, comfort, and even health. The human eye is very sensitive to the tones of white light and associates different color temperatures with distinct visual perceptions, emotional feelings, biological responses, and psychological reactions.

The critical importance of color temperature selection can be related to the human evolution. In the long history of humankind, our ancestors once spent 90% of their time outdoors and relied on sunlight, moonlight, and firelight that were given from nature. Over time, people have evolved in harmony with nature's light-dark cycle and registered the subtle changes of the moving sun in the biological clock. The circadian rhythm established by this clock synchronizes our physiological and psychological rhythms to the Earth's natural lighting cycle. The color and intensity variation of natural daylight that occurs from sunrise to sunset is at the heart of a human being's 24-hour circadian rhythm. Apparently, the color temperature of sunlight is inherently tied to the dynamic aspects of circadian response. People today spend approximately 90% of their time indoors and thus rely heavily on artificial lighting. Accordingly, the color temperature of artificial light sources should be adapted to the natural human biological rhythm.

White light, from warm white light with a high percentage of red to cool white light with a high percentage of blue, regulates various biological processes throughout the body. Exposure to artificial light with an inappropriate spectral composition can interfere with the synchronization of our biological clock and result in circadian rhythm disruption. The human circadian system is more sensitive to short wavelength blue light. A small subset of retinal cells called intrinsically sensitive retinal ganglion cells (ipRGCs) located within the retinal area of the eye relays light information to parts of the brain controlling the biological clock and mediates photobiologic responses that include circadian rhythms of metabolism and physiology. The ipRGCs contain melanopsin, a blue-light absorbing photopigment that has a peak photosensitivity at 480 nm.

When the sun rises in the morning and the tone of sunlight shifts gradually from warm to cool, the human eye naturally receives a dose of light in which the blue content increases as the color temperature of light goes up. The relatively higher amounts of blue light activate the ipRGC photoreceptors which promote the release of serotonin and cortisol while at the same time suppressing the production of melatonin. Cortisol a key stress hormone which makes us feel alert and alive. Serotonin is a natural mood regulator that makes us feel emotionally stable, less anxious, and even more focused and energetic. Melatonin is a natural soporific that regulates our sleep-wake cycle. To put it simple, it induces us to sleep. Therefore, higher amount of blue radiation at higher CCT can make us more alert, active and productive. When the sun is at its high point (from the middle morning to the middle afternoon), the human beings are exposed to high intensity daylight rich in blue. This is the period when we have the highest concentration and productivity in the 24-hour cycle.

When the sun sets in the evening, we receive light that is blue-depleted, and stronger in the red part of the spectrum. Low intensity light with low quantity of blue stimulates the secretion of melatonin. The sleep hormone signals to the human body to prepare for a regenerative sleep. Meanwhile, red enriched warm light promotes relaxation and creates a sense of comfort.

Stay Aligned with Circadian Rhythms​

The natural course of light-dark cycle places emphasis on the intensity and color temperature of light. Since most of people now spend 90% of their time indoors with electric lighting, it is critical to adapt artificial lighting to the patterns of natural daylight. Professional lighting designers make use of high intensity, cool white light in offices, industrial and educational facilities to foster more productive learning or working environments during the day. However, in hospitality and residential applications soft ambiances with warm tone light are typically provided to encourage relaxation and regeneration. Nighttime exposure to bright, high CCT light causes the body to simulate a daytime physiological response and negatively impacts the release of melatonin and other hormones. Hormonal imbalance not just affects sleep as a consequence of the disturbed physiological rhythms, but almost the entire metabolism, including immune responses.

At the biological level, we can conclude that electric lighting should not be designed against the physical, mental and behavioral changes that follow the 24-hour solar cycle. Artificial light with high blue content is a double-edged sword. We need high CCT lighting to improve task performance, raise concentration levels and support mental acuity. On the other side, extending high intensity, high CCT lighting from daytime to nighttime will have a negative effect on human psychology and physiology. Aside from health problems such as sleep and mood disorders as a result of improper lighting, excessively high CCT radiation may pose photobiological hazards.

Photobiological Hazards of High CCT Lighting​

The European Union (EU) specifically mandates that the photobiological hazards of all LEDs and products using LEDs must assessed. The IEC/EN 62471 standard establishes exposure limit risk groups (RGs) relating to the effects of optical radiation on the eye.

• Exempt Group (RG0: no risk) — Lamp/LED does not pose any photobiological hazard
• Risk Group 1 (RG1: low risk) — Lamp/LED does not pose a hazard due to normal behavioral limitations on exposure
• Risk Group 2 (RG2: moderate risk) — Lamp/LED does not pose a hazard due to the aversion response or thermal discomfort
• Risk Group 3 (RG3: high risk) — Lamp/LED may pose a hazard even for momentary or brief exposure

Light emitted by LED lights does not contain any energy in the UV part of the spectrum and LEDs hardly emit any infrared (IR). The photobiological risk associated with LED lighting is blue light hazard (BLH), a photochemical damage of the retina resulting from radiation exposure at wavelengths primarily between 400 nm and 500 nm (the blue portion of a light spectrum). However, BLH isn't unique to LED light sources. The United States Department of Energy (DOE) reported that all light sources at the same color temperature have the same risk potential of blue light hazard, and there is a strong linear correlation between BLE and CCT. This means high color temperature lighting is the primary contributor to blue light hazard. At an illuminance of 1000 lux, a 6000 K white light source is very likely to fall into the blue light hazard classification of RG2 or even RG3. Whereas a 2000 K white light source can be very safe unless the illuminance exceeds 8000 lux. That's why light sources with a CCT of 6000 K or above should not be used. In particular, infants and children are more susceptible to blue light hazard because the lenses of their eyes filter blue light less efficiently than adults' lenses. High CCT radiation in close proximity to the human eye, e.g. task lighting with a desk lamp, is high dangerous for this age group because they have not yet developed natural photophobic response to bright light. In fact, a CCT between 4000 - 5000 K is high enough to help people stay alert and productive if the lighting is intended for use in learning or working environments.

Psychological Preference​

CCT selection sometimes is driven by psychological considerations that are primarily cultural and possibly climate-related. For example, many Americans and Europeans favor warm white light sources because they grew up in the classic, warm ambience of incandescent lighting. In these regions, warm white artificial lighting has been a cultural element. Long before electric lighting was invented, people used candlelit light fixtures such as chandeliers to create a cozy ambience and make a visual statement. The regional CCT preference can possibly be the result of psychological compensation for the predominant weather in a specific latitude. People in northern latitudes are more likely to choose warmer light sources, while people in southern latitudes tends to use cooler light sources.

Uneducated Consumers​

In China and developing countries, consumers have a strong preference over high CCT (6000 - 6500 K) lighting primarily because they think cool white light is brighter than warm light. This "theory" was established since the introduction of fluorescent lighting. Even with the widespread applications of LED technology which is highly flexible in CCT manipulation, lighting manufacturers in China never care to educate consumers to use light sources with photobiologically safe CCTs. In fact, most of these manufacturers even don't know the science of color temperatures. For LEDs in the 2700 - 6000 K CCT range, the lower the color temperature, the lower the luminous efficacy due to the Stokes energy loss and low eye sensitivity over saturated colors in warm white light. The addition of red phosphor which is used to produce warmer white light further adds to the cost of LEDs. Lighting manufacturers obviously won't compromise on brightness and sacrifice profit margin just to encourage consumers to use low CCT light sources.

How to Choose the Right Color Temperature​

Warm white (2700 - 3200 K): This color temperature range gives residences, hotels, and restaurants a signature hue that distinguishes these spaces from public spaces by imparting a feeling of comfort, coziness and intimacy. The warm glow from a 2700 K lamp is reminiscent of the incandescent lamp at full brightness. It naturally creates a relaxed atmosphere and makes a space feel smaller. This CCT is suitable for bedrooms, dining rooms, and living rooms. 3000 K is comparable to the warmth of halogen lamps. It warms up the ambience and creates an inviting sensibility while enhancing the visual dimension and visibility of a space such as a bathroom, kitchens, and hallway.

Neutral white (around 3500 K): Neutral white is in the pure white portion of the spectrum. The neutral tone neither renders intense warmth to activate melatonin secretion that makes people less desired to concentrate, nor does it appear bluish to make people excessively alert. Neutral white light is a popular choice for conference rooms, retail spaces, and environments where the atmosphere should be friendly and inviting, while also encouraging interaction and engagement.

Cool white: (4000 - 4500 K): This CCT range simulates the natural daylight in mid morning and afternoon which are the most productive periods over the course of a day. A moderately high percentage of blue in the spectrum keeps people alert, active and ready for working and learning while reducing fatigue and daytime sleepiness. Cool white is the standard CCT of choice for office buildings, institutional facilities, commercial environments, and other applications where productivity, efficiency, and concentration are of paramount importance.

Daylight white (5000 - 5500 K): High CCT lighting finds its popularity in outdoor, industrial, sports, and public facilities. The appeal of blue-enriched lighting in high output lighting applications is its high energy efficiency and invigorating effect. The high scotopic/photopic (S/P) ratio of the blue-enriched white light contributes to improved mesopic vision which takes place in the luminance range 0.001 cd/m2 and about 5 cd/m2. However, in 2016 the American Medical Association (AMA) expressed its concern over high CCT LED lighting in outdoor environments. AMA encourages the use of 3000 K or even lower CCT illumination for outdoor installations such roadways.

Dynamic CCT Control​

Human centric lighting (HCL) which brings the dynamics of natural light to interior is a fresh trend that drives the latest LED technology developments. HCL imitates the continuous spectrum of sunlight during the course of the day to keep circadian rhythm hardwired with the the Earth's natural lighting cycle. At the core of this technology is IoT-based dynamic CCT control. Tunable white lighting allows for continuous adjustment of color temperature along the Planckian curve from warm white to cool white (2700 - 6500 K). Dim-to-Warm technology enables variable color temperature controls (e.g. from 2700 to 2200 K) with proportional intensity variations to replicate the familiar, intimate feeling of dimmed incandescent bulbs.

What is LED Binning​

LED binning refers to the sorting of LEDs based on chromaticity, lumen output, and sometimes forward voltage. The phosphor-converted LED manufacturing process involves chemical interactions (e.g. crystal growth, phosphor mixing). This may create variations in color appearance and light intensity. LED binning is designed to maintain a narrow window of tolerance so that visually perceptible color inconsistency will not occur in lighting installations. Tight color control ensures LED-to-LED, lamp-to-lamp, and fixture-to-fixture color uniformity that is critical in architectural lighting applications. In practice, MacAdam ellipses are used to establish the variability in color a light source can exhibit.