Introduction
Color temperature of the electromagnetic radiation emitted from an ideal black body is defined as its surface temperature in kelvins. Color temperatures over 5000 K are called Cool colors (bluish), while lower color temperatures (2700–3000 K) are called Warm colors (yellowish). Warm in this context is an analogy to radiated heat flux of traditional incandescent lighting rather than temperature.
As the physical temperature of the object rises, color transitions from red (long wavelengths – low energy) to blue (short wavelengths – high energy) through ROYGBIV (red, orange, yellow, green, blue, indigo, violet). When it comes to light sources, physically, blue is warmer than red. There are also psychological qualities of color linked to temperature. Psychologically, blue is cooler than red. This is due in large part to our physical environment – water is blue, plants are green, sunshine is yellow, fire is red. Warm colors tend to attract more attention than cool hues (which is exactly why street signs with urgent messages are generally either red, yellow, or orange).
Color wheel
Using language of HSL (hue, saturation, luminosity) hue values mark a position measured in degrees on a color wheel. A circle has 360 degrees, so the scale is 0 – 359 as shown in below figure. Think of the color wheel as a clock where every hour marks a new color family i.e.
- 0 red
- 30 orange
- 60 yellow
- 90 yellow green
- 120 green
- 150 blue green
- 180 cyan
- 210 green blue
- 240 blue
- 270 purple
- 300 magenta
- 330 blue red
Absolutely warm and cool colors can be found at 0 (red – no warmer color) and 180 (cyan – no cooler color) degrees. Determining whether one color is warmer or cooler than another can be measured by their proximities to these poles. A line between 90 (green yellow) and 270 degrees can be used to broadly demarcate warm colors from cool colors; colors on the right (towards red) are warm while colors on the left (towards cyan) are cool. While one color can be seen as warmer or cooler than another color, each color also has warm and cool components; there are warm yellows and cool yellows, warm blues and cool blues, etc. Photographic color adjustment strategies rely on adjusting a balance in each of three complements.
- Red (Warm) – Cyan (Cool)
- Green (Cool) – Magenta (Warm)
- Blue (Cool) – Yellow (Warm)
Most hue adjustment tools, like Photoshop’s Color Balance, have these complements built into their interface. You can’t increase one hue without decreasing it’s complement.
Kelvin Scale
Colour temperature is measured in Kelvin and varies from warm, for a cozy atmosphere, to cool for a clean and modern look. Warmer the colour the lower the colour temperature. Candles being around 1500 Kelvin (K) while daylight typically 5200 K.
Example
Camera will measure the colour of light with one of it’s sensors, and then determine what colour the white balance (WB) is. White Balance (WB) indicators on camera are like shade, tungston, daylight, flash etc. These settings over-ride the automatic white balance (AWB) sensor on the camera. Sometimes the camera gets the white balance wrong. If you notice that the camera has screwed up, then override the WB setting with Shade White Balance. White balance (or colour temperature of light) can be fixed in photoshop later on, however it is easier to get it right in-camera. Below image shows the effect of changing WB on a image.
