The Science of Lighting (Part 2: Color Temperature)

Building on the foundational concepts of lighting measurements and units explored in the first part of this article, Part 2 delves deeper into the technical elements of modern lighting systems. As lighting technology advances, particularly with the rise of LEDs, understanding the nuances of design parameters becomes essential for achieving optimal functionality, aesthetics, and energy efficiency.

This section begins by exploring Color Temperature (CCT), a cornerstone of lighting design that influences ambiance and user perception.

CCT is one of the most critical parameters in lighting design, particularly for LEDs, as it directly impacts ambiance, perception, and functionality in illuminated spaces. This section provides an in-depth analysis of CCT, from its scientific basis to its practical applications, enriched with advanced technical insights, formulas, diagrams, and data.

What is CCT?

Correlated Color Temperature (CCT) measures the color appearance of light emitted by a source, expressed in Kelvin (K). It describes whether the light appears "Warm" (yellowish/reddish tones) or "Cool" (bluish tones) and is based on the principle of a black-body radiator, which emits different colors of light as its temperature changes (Picture 1).

CCT Ranges and Characteristics

In the lighting industry, CCT is a critical parameter that defines light's ambiance, functionality, and application. It is not a measure of brightness but rather the perceived color of the light. The industry typically classifies CCT into three broad ranges:

Warm White Light (2200K–3000K): Produces a yellowish or reddish hue, mimicking traditional incandescent bulbs, natural firelight, and candlelight. It creates a cozy, intimate atmosphere in residential spaces, restaurants, and hospitality environments.

Neutral White Light (3000K–4500K): Balanced white light resembles natural daylight at noon and suits offices, retail spaces, and galleries where balanced visibility is important.

Cool White Light (4500K–6500K): Bluish-white light resembling overcast daylight produces a crisp, bluish light ideal for industrial areas, outdoor lighting, hospitals, and environments requiring high visibility and alertness.

Table 1 shows the details of the CCT range. What is the light color and what are its applications per CCT range?

Modern lighting systems often incorporate tunable CCT LEDs, allowing dynamic adjustment of CCT to suit various tasks and times of day. This innovation supports human-centric lighting by aligning light color with human biological rhythms, enhancing well-being and productivity. CCT standards in the industry, like those from the Illuminating Engineering Society (IES) and International Commission on Illumination (CIE), guide the appropriate use of CCT in diverse applications.

Picture 2: CCT Spectrum

Picture 2 shows the CCT spectrum from a warm color (1900K) to a cool color (12000K).

The CCT spectrum and Table 2 show practical examples of different light sources arranged from the lowest to the highest values.

Table 2: The different CCT values in a spectrum show practical examples of different light sources

Picture 3 shows another example of how different CCT works in the same lighting luminaire. The luminaire shows CCT from yellowish warm (2000K) to blueish cool (8000K).

Picture 3: How different CCT works in the same lighting luminaire

What is CCT relation with different factors?

1. CCT and Human Perception:

• CCT plays a significant role in how we perceive light and the emotions it evokes. Lower CCTs (e.g., 2700K) are often associated with warmth, comfort, and relaxation, while higher CCTs (e.g., 5000K) are linked to energy, alertness, and productivity.
• This is why residential lighting often uses warm CCTs, while office spaces and factories favor cooler CCTs for their stimulating effects2.

2. CCT and Biological Effects:

• Exposure to high CCT light (>5000K) during the day helps regulate the body's circadian rhythm by suppressing melatonin production, making us feel alert and focused.
• Conversely, lower CCT light (<3000K) in the evening promotes melatonin production, signaling the body to prepare for sleep.

3. CCT and Color Appearance:

• CCT affects how objects appear under a light source. For example:
  • A 2700K light may make reds and yellows appear richer, but blues might seem muted.
  • A 5000K light enhances blues and greens but can make warmer colors feel less vibrant.

4. Misconception About Brightness:

• Many people mistakenly believe that cooler CCTs (e.g., 6000K) are brighter than warmer CCTs. In reality, CCT is independent of brightness (luminous flux), and this perception is due to the sensitivity of the human eye to blue light.

5. CCT and Energy Consumption:

• LEDs with higher CCT (cooler light) often exhibit slightly higher luminous efficacy (lumens per watt) due to their spectral composition, making them more energy-efficient in many cases.

6. CCT and Regional Preferences:

• Cultural differences influence CCT preferences:
  • In North America and Europe, warm light (2700K-3000K) is commonly used in homes, while cool light (4000K-5000K) is typical in workspaces.
  • In parts of Asia, higher CCTs (6000K-6500K) are preferred for both residential and commercial applications due to the association of cooler light with modernity and cleanliness.

7. Advances in Tunable White LEDs:

• Modern tunable white LEDs allow users to adjust the CCT dynamically, enabling lighting systems to adapt to specific needs, tasks, or times of day. For instance:
  • Warmer CCTs in the evening for relaxation.
  • Cooler CCTs during the day for productivity.

8. Standards and Guidelines:

• Lighting design standards, such as those by the International Commission on Illumination (CIE) or the Illuminating Engineering Society (IES), recommend specific CCT ranges for different applications:
  • Residential: 2700K-3000K
  • Offices: 3500K-4500K
  • Industrial: 4000K-6000K

9. Historical Evolution of CCT:

• Early light sources, like candles and incandescent bulbs, naturally emitted warm light (~2000K-3000K), which shaped human preferences.
• The development of fluorescent and LED technologies introduced cooler CCT options (~5000K-6500K), expanding the possibilities for lighting design.

10. CCT and Outdoor Lighting:

• Outdoor lighting often employs cooler CCTs (4000K–6000K) to enhance visibility and safety. However, lower CCTs are sometimes preferred to minimize light pollution and reduce the impact on wildlife.

The Science Behind CCT

CCT is expressed in Kelvin (K) and refers to the color appearance of a light source. It is derived from the color of light emitted by a theoretical black-body radiator heated to a specific temperature.

1. Black-Body Radiator: A perfect absorber and emitter of radiation, where its emission spectrum depends solely on its temperature. The emitted light transitions from reddish at lower temperatures to bluish at higher temperatures (Picture 4).
   

Picture 4: Black-Body Radiation Curve 

Explanation of the Black-Body Radiation Curve

Picture 4 illustrates a black-body radiator’s spectral power distribution (SPD) at various temperatures, calculated using Planck's Radiation Law. Each curve represents the intensity of radiation (y-axis) at different wavelengths (x-axis) for specific temperatures in Kelvin (3000K, 4000K, 5500K).

Highlighted factors of the Black-Body Radiation Curve

• A black body is a theoretical object that absorbs all electromagnetic radiation that falls on it.
• When a black body is heated, it emits radiation across a wide range of wavelengths.
• The intensity of this emitted radiation at each wavelength is described by Planck's Law, resulting in a characteristic curve known as the black-body curve.
• The shape of the black-body curve depends on the temperature of the black body

Key Features of the Black-Body Radiation Curve

1.1. Temperature Dependency (3000K, 4000K, 5500K):

Each curve corresponds to a black-body radiator at a specific temperature:

3000K: Emits mostly in the infrared and red part of the visible spectrum, producing warm, reddish light.

4000K: Shifts slightly toward the yellow/green part of the spectrum, with higher energy in visible wavelengths.

5500K: Peaks closer to the middle of the visible spectrum (green-yellow), corresponding to neutral or daylight white.

1.2. Peak Wavelength (Wien's Displacement Law):

The peak of each curve shifts toward shorter wavelengths (left on the x-axis) as temperature increases.

Wien's Displacement Law quantifies this:

Where:

Example: For 5500K CCT:

1.3. Spectral Intensity:

The area under each curve represents the total radiant energy. Higher temperatures increase overall intensity, meaning more energy is emitted across all wavelengths.

1.4. Visible Spectrum and Beyond:

1.5. Ultraviolet and Infrared Regions:

Practical Applications of the Black-Body Radiation Curve

i. Lighting Design:

ii. Color Temperature (CCT):

iii. Astronomy:

iv. Spectroscopy:

2. Chromaticity and Planckian Locus

CCT values are determined by mapping the light source’s chromaticity coordinates (x, y) in the CIE 1931 Chromaticity diagram. The Planckian locus represents the path of ideal black-body radiators in this color space.

For practical light sources, the Correlated Color Temperature (CCT) is the temperature of the black-body radiator whose chromaticity is closest to that of the light source.

Picture 5: Planckian Locus in the chromaticity diagram (copyright: Wikipedia.org)

Explanation of the Planckian Locus Diagram

The Planckian Locus is a curve in the CIE 1931 Chromaticity diagram representing the color of light emitted by an ideal black-body radiator at various temperatures. This curve is essential for understanding the Correlated Color Temperature (CCT) and how the chromaticity of light sources corresponds to their temperature.

Key Components of the Diagram

i. Chromaticity Diagram:

The colored area in the graph represents the visible spectrum of light, with:

Each point in this diagram corresponds to a specific color the human eye perceives.

ii. Planckian Locus (Black Curve):

The black curved line in the middle is the Planckian locus. It shows the trajectory of colors emitted by a black-body radiator at different temperatures (in Kelvin).

The curve spans from reddish hues at low temperatures to bluish hues at high temperatures.

iii. Temperature Labels (CCT in Kelvin):

iv. Infinity and Beyond:

On the far left, the curve approaches infinity (∞), representing extremely high temperatures where the color becomes increasingly blue.

v. Wavelength Labels (Blue Numbers):

Regions and their Significance

i. Below 2000K (Warm Red Light):

ii. 2000K–4000K (Warm to Neutral Light):

iii. 4000K–6500K (Neutral to Cool Light):

iv. Above 6500K (Cool Blue Light):

v. Green and Magenta Shifts (Above and Below the Planckian Locus):

Picture 6: CCT Regions and their usage examples

Planckian Locus and Black-Body Radiator

The black-body radiator emits light purely based on its temperature, and the locus represents its ideal color. Real-world light sources (e.g., LEDs, fluorescent lights) often deviate from this curve.

These deviations are measured by parameters such as Duv:

• Positive Duv: Light shifts toward green (above the locus).
• Negative Duv: Light shifts toward magenta (below the locus).

Applications of the Planckian Locus

i. Lighting Design: The locus helps designers select light sources with desired CCT for specific environments (e.g., warm for residential, cool for offices).

ii. Color Rendering: Ensures light sources closely match the locus to render colors naturally and accurately.

iii. Tuning LEDs: Modern tunable LEDs are engineered to align closely with the Planckian locus for better light quality.

iv. Spectroscopy and Astronomy: Used in fields like astrophysics to determine the temperatures of stars based on their emitted light.

3. McCamy's Approximation Formula

To approximate CCT from chromaticity coordinates:

Where:

• x, y: Chromaticity coordinates
• Accurate for CCT values between 2000K and 10000K
  

4. Calculating and visualizing CCT

4.1. Practical Formula for Designers

For design simulations:

Where:

: Coefficients based on empirical data for specific ranges of n.

4.2. Visualization Tools

Practical Examples:

Example 1. McCamy's Approximation Formula for CCT

Suppose a light source has the following chromaticity coordinates:

• x= 0.38
• y= 0.38

Step 1: Calculate n:

Step 2: Calculate CCT:

Breaking it down:

Result: The light source has a CCT of ~7440K, which corresponds to a cool, bluish light.

Example 2. Visualizing CCT

Where:

: These coefficients depend on the approximation model. These are often used in simulation tools to visualize CCT dynamically.

A dynamic lighting system in an office adjusts the CCT throughout the day. Using simulation software:

• Morning (7 AM): Warm light (3000 K)
• Noon (12 PM): Neutral light (4500 K)
• Evening (7 PM): Cool light (6500 K)

Simulating these transitions allows the system to optimize employee alertness and comfort based on visualized CCT curves in the chromaticity diagram.

Conclusion

• Color Correlated Temperature (CCT) is a fundamental parameter in lighting design, defining the perceived warmth or coolness of light.
• By selecting appropriate CCT values, designers can influence mood, functionality, and visual comfort in various environments.
• Dynamic CCT control further enhances lighting flexibility, aligning illumination with human circadian rhythms and specific application needs.