Colour 104 : Colour Temperature
- sharmistha
- Feb 19
- 9 min read
In the 20th century, Albert Einstein showed that light has a dual nature:
Particle-like behavior (photons) in the photoelectric effect.
Wave-like behavior (interference, diffraction).
A light wave is a type of electromagnetic radiation that can travel through the vacuum of space at the speed of 299,792 km/s or 186,282 miles/s. Our eyes can only detect a tiny portion of the electromagnetic spectrum (visible light). Many animals, like bees, can see ultraviolet light, and snakes can detect infrared heat!
🌟 Types of Light in the Electromagnetic Spectrum:
Light is part of the electromagnetic spectrum, which includes:
Gamma Rays: Shortest wavelength, highest energy.
X-Rays: Used in medical imaging.
Ultraviolet (UV): Causes sunburn.
Visible Light: The only part we can see (400–700 nm).
Infrared (IR): Felt as heat.
Microwaves: Used in cooking and communication.
Radio Waves: Longest wavelength, used for broadcasting.
🌈 Properties of Light Waves:
Wavelength (λ): The distance between two consecutive peaks or troughs.
Measured in nanometers (nm).
Determines the color of visible light.
Red light: Long wavelength (~700 nm).
Violet light: Short wavelength (~400 nm).
Frequency (f): The number of wave cycles per second.
Measured in Hertz (Hz).
Higher frequency means more energy.
Amplitude: The height of the wave.
Determines the brightness or intensity of light.
Speed (c): All electromagnetic waves travel at the same speed in a vacuum:
c=299,792 km/sc = 299,792 \, km/sc=299,792km/s
Polarization: The direction in which the electric field oscillates.
🔑 Fun Fact : Key elements of wave
🌊 Wavelength, Frequency, Amplitude, and Their Relationship
📌 1. Wavelength (λ)
The distance between two consecutive crests or troughs in a wave.
Measured in meters (m), nanometers (nm) for light.
Symbol: λ (lambda)
📌 2. Frequency (f)
The number of wave cycles passing a point per second.
Measured in Hertz (Hz) (1 Hz = 1 cycle per second).
Symbol: f
📌 3. Amplitude (A)
The height of the wave from its midpoint (rest position).
Related to the intensity or brightness of light.
Symbol: A
📌 4. Wave Speed (v or c for light)
The speed at which the wave travels.
For light in a vacuum, it’s c = 299,792 km/s (approx. 3 × 10⁸ m/s).
🧮 Formula Connecting Wavelength, Frequency, and Speed:
v=f×λv = f \times \lambdav=f×λ
v: Speed of the wave (m/s)
f: Frequency (Hz)
λ: Wavelength (m)
⚡ For Light Waves:
Since light travels at speed c=3×108 m/sc = 3 \times 10^8 \, m/sc=3×108m/s in a vacuum:
c=f×λc = f \times \lambdac=f×λ
If you know the frequency, you can find the wavelength and vice versa
📡 Why Are Wavelength and Frequency Inversely Proportional?
The speed of light is constant.
When wavelength decreases, the wave cycles fit into a shorter space, meaning more cycles pass per second (higher frequency).
Conversely, when wavelength increases, fewer cycles pass per second, resulting in lower frequency.
⚡ Higher Frequency and Energy
The energy of a wave is directly proportional to its frequency. This is given by Planck’s equation:
E=h×fE = h \times fE=h×f
EEE: Energy (joules)
hhh: Planck's constant (6.626×10−34 Js6.626 \times 10^{-34} \, Js6.626×10−34Js)
fff: Frequency (Hz)
Implication:
Higher frequency means higher energy (e.g., gamma rays).
Lower frequency means lower energy (e.g., radio waves).
💡 Higher Frequency and Refraction
When light enters a different medium (like glass or water), its speed changes, causing the wave to bend, a phenomenon known as refraction.
The refractive index (nnn) is:
n=cvn = \frac{c}{v}n=vc
nnn: Refractive index
ccc: Speed of light in vacuum
vvv: Speed of light in the medium
Effect of Frequency:
Higher-frequency light (like violet) interacts more with the particles of the medium, slowing down more than lower-frequency light (red).
This causes higher-frequency waves to bend more during refraction, which is why a prism splits white light into a rainbow (dispersion).
🌈 Summary:
Inversely Proportional: Fixed speed means shorter wavelengths lead to higher frequency.
Energy: Higher frequency means more energy.
Refraction: Higher frequency bends more in a medium.
🌊 Types of Waves: Longitudinal and Transverse
Waves are disturbances that transfer energy from one point to another. They are classified based on how the particles of the medium move relative to the direction of wave travel.
📈 1. Longitudinal Waves
In longitudinal waves, particles in the medium move parallel to the direction of wave propagation.
Key Features:
Areas of compression (particles are close together)
Areas of rarefaction (particles are spread out)
Examples:
Sound waves in air
Seismic P-waves (primary waves during earthquakes)
Ultrasound
📝 Analogy: Imagine a slinky compressed and released—coils move back and forth along the slinky’s length.
📊 2. Transverse Waves
In transverse waves, particles move perpendicular to the direction of wave propagation.
Key Features:
Crest: The highest point
Trough: The lowest point
Examples:
Light waves (electromagnetic)
Water waves
Seismic S-waves (secondary waves)
📝 Analogy: Imagine shaking a rope up and down—the wave travels horizontally while the rope moves vertically.
💥 Interference of Waves
When two or more waves meet, they combine to form a new wave pattern. This is called interference, which can be either constructive or destructive:
🔆 1. Constructive Interference
Occurs when waves add together because their crests and troughs align.
Result: A wave with a larger amplitude (brighter light, louder sound).
📝 Example: Two speakers playing the same sound wave in phase produce a louder sound.
🌑 2. Destructive Interference
Occurs when waves cancel each other out because their crests align with troughs.
Result: A wave with a smaller amplitude (dimmer light, softer sound, or silence).
📝 Example: Noise-canceling headphones use destructive interference to reduce background noise.
Different wavelengths manifest as different colors. The spectrum of light visible to the human eye (visible spectrum) ranges approximately from 380 nm (violet) to 700 nm (red).
Some digital cameras can detect wavelengths greater than red, known as infrared. Thermal images are captured using infrared photography techniques.
Wavelengths shorter than violet are called ultraviolet (UV). UV light is used in blacklights. UV light can cause a purple hue in photos taken in bright sunlight, especially if the camera sensor picks up some UV wavelengths (common with older or cheaper camera sensors without proper UV filters).
Spectral Power
Spectral Power refers to the distribution of power (intensity) of light across different wavelengths in the electromagnetic spectrum. It is commonly represented by a Spectral Power Distribution (SPD) curve, which shows how much energy a light source emits at each wavelength.
📊 Spectral Power Distribution (SPD)
X-axis: Wavelength (typically in nanometers, nm)
Y-axis: Power (intensity) at each wavelength
The SPD is crucial because it determines how we perceive color from a light source. Different light sources with the same color temperature can have very different SPDs.
💡 Examples of SPD in Light Sources:
Sunlight: Smooth, continuous spectrum (all colors of the rainbow).
Incandescent Bulbs: More energy in longer wavelengths (red, orange) with a warm appearance.
LED Bulbs: Often have distinct peaks, especially if they use a blue LED with a phosphor coating.
Fluorescent Lights: Sharp peaks at specific wavelengths due to gas excitation.
🌈 Spectral Power and Color Perception:
The SPD of a light source affects how objects' colors appear.
Metamerism: When two objects appear the same under one light but different under another due to differences in their spectral reflectance and the light’s SPD.
📌 Relation to Color Temperature and CRI (Color Rendering Index):
Color Temperature: Describes the overall hue (warm/cool), but SPD reveals the actual distribution of colors.
CRI: Measures how well a light source reveals the colors of objects compared to natural light, based on its SPD
Colour temperature
Colour temperature refers to the hue of light that a source emits when heated to a specific temperature. The concept originates from black-body radiation, where a theoretical object (called a black body) emits different colors of light depending on its temperature. It is measured in Kelvin (K).
Low colour temperature (2,000K – 4,000K): Warm, reddish or yellowish light (e.g., candlelight, sunrise).
High colour temperature (5,000K – 10,000K): Cool, bluish light (e.g., overcast sky, twilight)
A fascinating irony in colour temperature is that hotter objects emit light that appears cooler (bluish-white), while cooler objects emit warmer (reddish) light. This contradiction can be confusing because the terms “warm” and “cool” in color perception don’t align with the actual physics of temperature. Our brain associates red with warmth and blue with coolness due to natural experiences (e.g., fire vs. ice).
Higher Temperature (Hotter): Bluish-white light (perceived as cool)
Lower Temperature (Cooler): Reddish-yellow light (perceived as warm)
This reversal of terms highlights the difference between physical reality (temperature) and human perception (color psychology).
🔥 The Black-Body Radiation Concept
A black body is an idealized object that absorbs all electromagnetic radiation and emits light based on its temperature. As it heats up, it changes color:
At 1,000K, it glows red (like heated iron).
At 3,000K, it glows orange-yellow (like a tungsten bulb).
At 5,500K, it appears white (like midday sunlight).
At 10,000K, it glows with a blue hue (like a clear blue sky).
This phenomenon is described by Planck’s law.
💡 Color Temperature Scale (Kelvin Scale)
Temperature (K) | Light Source | Appearance |
1,800K | Candlelight | Deep orange-red |
3,000K | Incandescent bulb | Warm yellow |
4,000K | Sunrise/sunset | Soft white |
5,500K | Midday sunlight | Neutral white |
6,500K | Daylight (D65) | Cool white |
8,000K | Overcast sky | Pale blue |
10,000K | Clear blue sky | Deep blue |
📌 Quasi-Planckian Body:
Real light sources, such as incandescent bulbs, behave like approximate black-body radiators but deviate from the ideal spectrum. These are called quasi-Planckian sources because their emission patterns are close to, but not perfectly, Planckian.
🌡️Colour Temperature and Correlated Colour Temperature (CCT):
Color Temperature: Defined only for black-body radiators (e.g., a tungsten filament at 3200K).
Correlated Color Temperature (CCT): Used for non-black-body sources (like LEDs and fluorescents). It is the temperature of a black-body radiator that produces the closest perceived color to the light source
📏 Measuring Colour Temperature: Micro Reciprocal Degree (mired):
Mired (Micro Reciprocal Degree): A unit to express color temperature more practically, especially for small adjustments.
Mired + 1,000,000 / Kelvin (k)
Lowering the mired value increases the color temperature (cooler light), and raising it decreases the color temperature (warmer light).
📌 Example:
5,000K = 200 Mired (1,000,000 divided by 5000 = 200)
10,000K = 100 Mired
Mired shift value is a measure of the change in color temperature. It is used in photography, cinematography, and lighting to adjust white balance precisely. Mired shifts are more intuitive than Kelvin adjustments because they represent changes proportionally rather than linearly.
Mired Shift Formula:
Mired shift is the difference between two color temperatures:
Mired Shift = Mired Target − Mired Current
Example: To convert from 5,500K (daylight) to 3,200K (tungsten):
Mired Value of Tungsten (3200 K) = 1000000 / 3200 = 312.5
Mired Value of Daylight (5500 K) = 1000000 / 5500 = 181.82
Now, Mired shift value is = Mired Value of Tungsten (target) - Mired value of daylight (current) = 312.5 - 181.82 = 130.68 Mired
🎛️ Why Use Mired Shift?
Mired values give a linear scale for adjusting color temperature.
A 10 Mired shift has the same visible effect whether at 3,000K or 6,000K, unlike Kelvin adjustments.
Filters and gels are rated in Mired values for precise corrections.
📊 Mired Shift Examples for Common Temperature Conversions:
From (K) | To (K) | Mired Shift | Correction Filter |
5,500K (Daylight) | 3,200K (Tungsten) | +130 | CTO (Color Temperature Orange) |
3,200K (Tungsten) | 5,500K (Daylight) | -130 | CTB (Color Temperature Blue) |
5,500K (Daylight) | 4,200K (Fluorescent) | +58 | 1/2 CTO |
4,200K (Fluorescent) | 3,200K (Tungsten) | +71 | 1/2 CTO + 1/8 CTO |
🎨 White Balance:
White balance (WB) is a camera setting that adjusts the colors in an image to make them appear more natural. It ensures that white objects appear white under different lighting conditions. Without proper white balance, images may have unnatural color casts (e.g., too blue or too yellow).
Light sources emit different color temperatures. Human eyes automatically adjust to these variations, but cameras need assistance to interpret colors correctly. White balance compensates for these differences by neutralizing the color temperature, ensuring that whites and grays remain accurate.
🎛️ How White Balance Works:
White balance works by adjusting the camera’s color sensitivity based on the light source’s color temperature. The goal is to produce a neutral image where whites appear truly white and other colors are accurately represented.
The camera achieves this by:
Adding complementary colors to cancel out the light's tint (e.g., adding blue to balance a warm, orange-tinted image).
Shifting color channels (RGB) to maintain a natural look.
📷 White Balance Settings:
Most cameras have several white balance presets, including:
Preset | Color Temperature (K) | Typical Lighting Condition |
Auto (AWB) | Adjusts automatically | Mixed lighting conditions |
Daylight | ~5,500K | Direct sunlight |
Cloudy | ~6,500K | Overcast skies (adds warmth) |
Shade | ~7,000K | Shaded areas (adds warmth) |
Tungsten (Incandescent) | ~3,000K | Indoor lighting (adds cool tones) |
Fluorescent | ~4,000K | Office lighting (neutralizes green cast) |
Flash | ~5,500K | Flash lighting (similar to daylight) |
Custom/Manual | User-defined | For precise control using a reference |
🛠️ Manual White Balance Adjustment:
Use a White or Gray Card: Place a white or gray card under the same lighting as your subject.
Capture a Reference Image: Take a photo of the card.
Set Custom White Balance: In the camera’s settings, select this reference image to calibrate the white balance.
🧠 The Science Behind White Balance:
White balance relies on human color perception principles:
Chromatic Adaptation: The ability of human vision to perceive white consistently under varying lighting conditions.
Color Constancy: Our brain adjusts to light variations automatically, while cameras require calibration.
🚨 Common White Balance Mistakes:
Auto White Balance (AWB) Overcorrection: AWB can neutralize creative warm or cool lighting.
Mixed Lighting Issues: Different light sources with different temperatures cause color imbalances (e.g., fluorescent vs. daylight).
Ignoring Color Temperature in Editing: Failing to adjust white balance in post-processing can ruin the image’s mood.
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