Michael S. answered • 13d
B.S. in Chemistry, Indiana University; Organic Chem Teaching Intern
The rule that decides every line
Color temperature is the temperature of the blackbody whose emission curve best matches the source. So it is only meaningful for sources that glow because they are hot - thermal, incandescent, continuous-spectrum emitters. Any source whose color comes from something other than temperature has no color temperature at all, no matter what color it looks. That covers gas discharges (line spectra), LEDs, lasers, fluorescence, chemiluminescence and phosphorescence.
Ten of your twenty sources are thermal and ten are not.
N/A - color temperature does not apply
Orange neon sign. Neon emits a set of discrete atomic lines, not a continuum. The orange is neon's spectral signature, unrelated to the gas temperature.
Green traffic light. Either an LED or an incandescent bulb behind a narrow green filter. In both cases the light reaching you is a narrow band, which no blackbody curve resembles.
Mercury arc lamp. Line spectrum again - the greenish-blue is the Hg 436 and 546 nm lines.
Sodium arc lamp and sodium flame. Both are essentially the 589 nm sodium doublet. A sodium flame is a particularly clean example: the flame is only about 2000 K, but the light is one wavelength, so it carries no temperature information.
Long-wave fluorescent black light. Mercury lines plus a phosphor, and most of the output is outside the visible range entirely.
Red LED indicator and red laser pointer. Electroluminescence and stimulated emission. The laser is the extreme case - essentially a single wavelength, and its actual temperature is room temperature.
Kitchen gas burner flame. The blue of a well-adjusted premixed flame comes from CH and C2 molecular band emission in the reaction zone, not from incandescence. Note the contrast with the candle below.
Phosphorescent glow stick. Chemiluminescence - a chemical reaction populating an excited state. A glow stick is at room temperature and can even be held in your hand.
Ranked in ascending color temperature
1. Red-hot iron bar, dull cherry red - roughly 1000 K. The lowest temperature at which a solid glows visibly.
2. Charcoal glowing yellow-orange in a barbecue - roughly 1400 K.
3. Yellow-white candle flame - roughly 1900 K. Unlike the gas burner, a candle glows because soot particles in the flame are incandescent, which is exactly why it is thermal and the blue burner is not.
4. White sparks from the grinding wheel, and the white-hot molten steel from the cutting torch - both roughly 2000 to 2200 K. These are essentially equal; both are incandescent iron near its melting point.
5. 100 W tungsten household bulb - roughly 2850 K. This one is a genuine standard; tungsten lamps are used as calibrated color-temperature references.
6. The daylight-filtered tungsten microscope lamp - roughly 5000 K. Worth pausing on: this is a filtered source that DOES have a color temperature, unlike the filtered traffic light. The difference is that a daylight correction filter reshapes a broad continuum into another broad continuum, so a blackbody curve still fits it. A narrow green filter leaves nothing for a blackbody curve to fit.
7. Electronic flash bulb, and noon sunlight in clear weather - roughly 5500 and 5800 K. Treat these as equal; photographic flashes are deliberately engineered to match daylight so that film and sensors balanced for one work with the other.
8. Blue dwarf star - 10,000 K or more, the hottest on the list by a wide margin. Stellar color is the purest example of the whole concept, since a star really is close to an ideal blackbody.
Two things to notice
First, color and color temperature run opposite to everyday language. Red is "warm" and blue is "cool" in decorating, but physically red-hot is the coolest thing on this list and blue-white is the hottest.
Second, the two sodium entries and the candle make the whole point of the exercise. The sodium flame and the candle flame are at almost the same physical temperature, yet only the candle has a color temperature - because only the candle emits a thermal continuum. The question is never how hot the source is; it is whether the light was produced by heat.
The numbers above are the standard approximate values; if your instructor supplied a table, use theirs, since references vary by a few hundred kelvin.
