I just watched a Cosmos episode about Hershel's experiment, of which i think i was not aware. According to it, red light temperature is higher than blue (measured in celsius). As far as I know, however, color temperature is measured in kelvins, and blue has higher temperature than red. Are these two separate concepts?

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    \$\begingroup\$ Note also that Celsius and Kelvins are very much the same thing - Celsius is just offset by 273.15. \$\endgroup\$
    – Philip Kendall
    May 26, 2017 at 10:34
  • \$\begingroup\$ Philip, I know, but the two facts contradicts each other. So i suppose they refer to different concepts.. but i am not sure, that is why i asked ( : \$\endgroup\$ May 26, 2017 at 10:55
  • \$\begingroup\$ @PhilipKendall to be precise, a difference of one degree Celsius has the same magnitude as a difference of one Kelvin . \$\endgroup\$ May 26, 2017 at 11:21
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    \$\begingroup\$ I'm voting to close this question as off-topic because without an application or connection to photography, this question is pure science and belongs on Physics.SE. \$\endgroup\$
    – scottbb
    May 26, 2017 at 13:12
  • \$\begingroup\$ Related Physics.SE question: Why is a degree Celsius exactly the same as a Kelvin? \$\endgroup\$
    – scottbb
    May 26, 2017 at 13:15

1 Answer 1


As objects are heated they reach a temperature where they begin to glow. First dull-red-hot, then cherry-red, then white-hot. Seems the temperature and color outputted are the same regardless of material. For centuries, iron workers, glass blowers and ceramic potters, have used the glowing color as a marker. The Celsius temperature scale is favored by scientists because it blends in with the metric system.

There seems to be no limit as to how high a temperature is possible, however the lower limit is -273.16°C. It is presumed that if a material is cooled to this absolute lowest possible temperature, molecular motion ceases. The Absolute temperature scales sets -273.16°C as the zero point. Thus all readings will be positive, no confusing negative temperatures with positive. The Absolute scale was renamed the Kelvin scale to honor William Thomas, 1st Baron Kelvin.

K = C + 273.16

C = K – 273.16

All objects above
-268°C --- 5°K Infrared radiation (electromagnetic radiation) invisible to human.

400°C --- 673°K Red heat, visible in the dark

474°C --- 747°K -- Red heat, visible in the twilight

525°C --- 798°K -- Red heat, visible in the daylight

581°C --- 854°K - Red heat, visible in the sunlight

700°C ---973°K - Dark red

800°C --- 1073°K - Dull cherry-red

900°C --- 1173°K - Cherry-red

1000°C -- 1273°K - Bright cherry-red

1100°C --- 1373°K - Orange-red

2525°C --- 2800°K - Yellow

3225°C --- 3500°K - Yellow-White

4225°C --- 4500°K - Warm-White

5225°C --- 5500°K - White

6125°C --- 6400°K - Sky Blue

  • \$\begingroup\$ So, to actually answer the question could you include in your answer: What is the temperature, in the Kelvin scale, for black body objects that radiate in the infrared spectrum before they are warm enough to begin radiating light visible to human vision? \$\endgroup\$
    – Michael C
    May 26, 2017 at 23:40

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