As you know, the metal heated in a fire soon starts to shine. First the metal takes on a dull red glow, then cherry red. As the temperature of the metal increases, the color becomes warm white, then blue-white. It is these color changes observed with heating that inspired the color temperature system.
Also, you know that most of the world uses the Celsius system. This sets water freezing to zero (0) and boiling water to 100. The unit degree translates to "step". Early experimenters discovered that a hydrogen thermometer is super accurate. It is a hollow tube, filled with hydrogen with a float at the top of the column. The float rises and falls uniformly with changes in temperature. Other substances like mercury and alcohol don't have this uniformity. In addition, when the environment cools, the float falls near the bottom of the tube. The lowest possible temperature has been calculated to be an absolute zero and, if reached, the float would bottom out. This is how the absolute temperature scale was born. This scale has been favored by many, because all temperatures are positive, without confusing +20 with -20. This temperature scale was renamed the Kelvin scale after the document of the scientist Lord Kelvin of 1848, on absolute zero.
Now, many disciplines use the color of glowing hot substances to measure temperature. To name just a few: blacksmiths, metallurgists, steel, ceramics, glass blowing, etc. Experiments have shown that the glowing color and its associated temperature are approximately the same for all materials. The key here is the lighting industry which was initially the carbon arc and glowing tungsten, adopted the Kelvin scale to connect the color output of the lamps.
Selected Kelvin temperatures:
Candle flame 1850K
7520 2820K Domestic Tungsten Light Bulb
200 watt General Service Electric Blub 2980K
500-watt 3200K photo-flood light bulb
500 Watt 3400K Photo-Flood Film Light Bulb
Flash bulb 3800K – 4200K
Caron 5000K arc lamp
Photographic Daylight 5500K
Sunlight standard US Bureau of Standards Noon 5500K
Blue sky 12000K -18000K at various times of the day
Color films were manufactured to operate under specialized conditions.
Daylight color balance
Color balance Tungsten cinema lamps
Color balance Tungsten photo flood
Color films for scientific work – other temperatures Kelvin
Note: the custom is to write the word kelvin scale in lowercase k and omit the sign of the degree °.
The manufacturers of digital cameras have logically adapted the photo film industry using their color balance ratings.