As you know, metal heated in a fire soon begins to glow. First the metal takes on a dull red glow, then cherry red. As the metal’s temperature rises, the color changes to white hot, then blue-white. It’s these observed color changes with heating that inspired the color temperature system.
Also, you know that most of the world uses the Celsius system. This establishes water freezing at zero (0) and water boiling at 100. The unit degree translates to “step”. Early experimenters discovered that a hydrogen thermometer is super accurate. This is a hollow tube, filled with hydrogen with a float atop the column. The float uniformly moves up and down with temperature changes. Other substances like mercury and alcohol lack this uniformity. Further, as the surroundings cool, the float drops to near the bottom of the tube. It was calculated that the lowest possible temperature is absolute zero, and if achieved, the float would hit bottom. Thus the Absolute temperature scale was born. This scale was favored by many, as all temperatures are positive, no confusing +20 with -20. This temperature scale was renamed the kelvin scale after the scientist Lord Kelvin’s paper of 1848, on absolute zero.
Now many disciplines use the color of glowing hot substances to gage temperature. To name a few: blacksmiths, iron workers, steel making, ceramics, glass blowing etc. Experiments proved that the glowing color and its related temperature was approximately the same for all materials. Key here is the lighting industry that initially was carbon arc and glowing tungsten, adopted the kelvin scale to relate the color output of lamps.
Some selected kelvin temperatures:
Candle flame 1850K
75 watt household tungsten electric bulb 2820K
200 watt general service electric blub 2980K
500 watt photo-flood electric bulb 3200K
500 watt movie photo-flood electric bulb 3400K
Flash bulb 3800K – 4200K
Caron arc lamp 5000K
Photographic Daylight 5500K
Sunlight standard US Bureau of Standards Noon 5500K
Blue sky 12000K -18000K various times of day
Color films were manufactured to operate under specialized conditions.
Color balance Daylight
Color balance Tungsten movie lights
Color balance Tungsten photo flood
Color films for scientific work – other kelvin temperatures
Note: custom is to write the word kelvin scale as lower case k and omit the degree sign °.
Digital camera makers logically adapted the photo film industry using their color balance notations.