The Color of Light Measured in Kelvin
Colour temperature is the method used in physics and filmmaking to quantify the specific color quality of a light source. The measurement is expressed in units of Kelvin (K). The scale is based on the color that a theoretical black-body radiator would emit as it is heated. Counter-intuitively, lower Kelvin temperatures correspond to ‘warmer’ colors on the visible spectrum (reds, oranges, yellows), while higher Kelvin temperatures correspond to ‘cooler’ colors (whites and blues).
Key Benchmarks on the Kelvin Scale
Understanding a few key benchmarks on the Kelvin scale is fundamental for any filmmaker or photographer. The two most important are the industry standards for ‘tungsten’ and ‘daylight’ balanced light.
| Light Source | Approx. Temp. (K) | Color Quality |
|---|---|---|
| Candlelight | 1850K | Very Warm / Orange |
| Tungsten Bulb | 3200K | Warm / Yellow-Orange (Industry Standard ‘Indoor’) |
| Sunrise/Sunset | 4000K | Warm White |
| Direct Midday Sun | 5600K | Neutral White / Slightly Blue (Industry Standard ‘Outdoor’) |
| Overcast Sky | 6500K | Cool / Blueish |
| Blue Sky | 9000K - 12000K | Very Cool / Deep Blue |
Importance in Filmmaking
To create a natural-looking image, a cinematographer typically needs all of the light sources in a scene to have the same color temperature. If a scene is lit with a mix of daylight coming through a window (~5600K) and practical tungsten lamps in the room (~3200K), the camera will capture a distracting clash of blue and orange light. To solve this, filmmakers use two main tools:
- Gels: Thin, colored plastic sheets that are placed over lights to change their color temperature. To match a tungsten light to daylight, a CTB (Color Temperature Blue) gel is used. To match daylight to tungsten, a CTO (Color Temperature Orange) gel is used.
- Bi-Color LEDs: Modern LED lights often have the ability to dial in their color temperature, allowing cinematographers to precisely match any ambient light source on the fly.
Of course, filmmakers can also use mixed color temperatures creatively. The now-ubiquitous ‘orange and teal’ color palette seen in many modern blockbusters is a stylistic choice that leverages the dramatic contrast between warm skin tones and a cool, blue-hued environment.