Color temperature

Colour temperature is a parameter used to describe the perceived colour of a visible light source by comparing its hue to that of an ideal blackbody radiator. The temperature, expressed in kelvins (K), corresponds to the blackbody whose emitted light most closely matches the colour of the source being described. Although actual physical temperature may differ greatly—especially for artificial light sources—the colour temperature scale remains an important tool across lighting, imaging, astronomy and manufacturing.

Principles and Measurement

An ideal blackbody radiator emits light whose spectrum depends solely on its surface temperature, shifting from red to orange, yellow, white and eventually blue as temperature increases. The colour temperature of such radiation is therefore directly linked to the radiator’s actual temperature. Incandescent lamps approximate blackbody behaviour, so their colour temperature reflects the filament temperature.
Many modern light sources, such as fluorescent lamps and LEDs, do not emit light via thermal radiation and therefore do not follow blackbody spectral curves. For these sources, a correlated colour temperature (CCT) is assigned: the temperature of the blackbody radiator whose colour most closely resembles the source’s colour to human vision.
Colour temperature can also be expressed in mireds (micro reciprocal degrees), a unit useful for calculating light filtration and adjustments.

Warm and Cool Colour Temperatures

Colour temperatures below approximately 2700–3000 K appear as warm hues—red, orange or yellow—while temperatures above 5000 K appear cool—white or bluish. The descriptors “warm” and “cool” refer to psychological colour associations rather than physical heat. The hue–heat hypothesis proposes that lower colour temperatures feel warmer because their spectral peaks lie closer to infrared wavelengths, even though the corresponding blackbody temperatures are lower than those of bluish light.
The perception of warm and cool lighting can lead to confusion: lamps marketed as “warm white” have lower colour temperatures, while “cool white” lamps have higher values.

The Sun and Daylight

The Sun behaves closely like a blackbody radiator. Its effective temperature, derived from its total energy output, is about 5772 K. Sunlight outside Earth’s atmosphere has a colour temperature around 5900 K. However, the Sun’s appearance from Earth changes throughout the day due to atmospheric scattering, not because of temperature variations.

• Rayleigh scattering causes the blue colour of the sky by preferentially scattering shorter wavelengths.
• At low solar elevation, sunlight passes through more atmosphere, shifting its apparent colour towards warmer hues—an effect underlying the “golden hour”.
• Standard daylight for imaging applications typically uses a correlated colour temperature of 6500 K (CIE Standard Illuminant D65) or 5500 K for daylight-balanced photographic film.

Applications in Lighting

In architectural lighting, colour temperature strongly influences ambience and functionality:

• Warm light (approx. 2700–3000 K) promotes relaxation and is common in homes, hotels and hospitality spaces.
• Cool light (4000–6000 K) enhances alertness and concentration and is often used in offices, schools and commercial environments.

LED lighting complicates colour control due to manufacturing variations (“binning”), ageing and temperature-related shifts in output. To maintain consistent colour, advanced LED systems may incorporate feedback loops using sensors that monitor and adjust the colour mix.

Applications in Aquaculture

Colour temperature plays a role in aquarium lighting. In freshwater aquaria, the goal is often aesthetic, with lights chosen to enhance the appearance of fish and plants. In saltwater and reef aquaria, colour temperature becomes critical because light in the shorter-wavelength range penetrates water more effectively and supports the photosynthetic algae that live in coral tissues. As water depth increases, higher (bluer) colour temperatures become more representative of natural illumination.

Digital Photography and White Balance

Digital cameras use the concept of colour temperature to adjust white balance, ensuring that white objects appear white under various lighting conditions. Presets such as “sunny,” “cloudy” and “tungsten” approximate common ambient conditions, while many cameras allow manual entry of a Kelvin value.
Colour adjustments usually operate along a blue–yellow axis, with additional magenta–green adjustments available in raw processing software. Since these adjustments depend on artistic interpretation, colour temperature settings in digital photography are not always literal representations of physical temperature.

Photographic Film

Photographic film has fixed spectral sensitivities that vary from those of the human eye. As a result:

• A scene appearing neutral to the observer may appear tinted on film.
• Correction during printing is limited because film has discrete layers for different colours, each responding differently under mismatched lighting.
• Light sources with discontinuous spectra, such as fluorescent tubes, may produce colour casts impossible to correct fully.

Film is manufactured for specific lighting conditions:

• Daylight film is balanced for approximately 5500–6500 K.
• Tungsten film is balanced for around 3200 K and yields neutral images under incandescent lighting.