Light source and color temperature

1. Hot source and cold source:

Light emission (I am talking about incoherent light, not laser light this time) is from two basic types of sources: heat source and non-heat (or cold) source. For the first source, light radiation is only from thermal energy and we call it blackbody radiator (this process is called incandescence). For the latter source, light is not caused by a rise in temperature and we call it luminescence process.

2. Color temperature:

This term is only for blackbody radiators. At 3200K, long wavelength dominates; light appears yellow and is from a typical incandescent light bulb. When temperature rises to 5000K, the blackbody emits relatively flat spectrum and is a very neutral white. At higher temperature, short wavelength starts to dominate and appears blueish.


Figure is from: www.techmind.org/colour/coltemp.html

So when we talk about color temperature, remember it is only for pure thermal source! But color temperature is used very often on all other sources, e.g., fluorescence light tubes. Strictly speaking, this should be called "correlated color temperature" because people are picking the closest blackbody temperature to quantize the color the source appears.

(In addition, color temperature is for white light. When we want to know or quantize how white a source is, we use color temperature so we know if it is neutral, blueish or yellowish. For monochromatic sources, color temperature is no use.)

3. Luminescence:

Luminescence is light that emitted at low temperatures without heat so it is "cold body radiation" (as compared to blackbody radiation). Fluorescence is only one mechanism of luminescence: the material absorb high-energy photons at UV or blue spectrum and re-emit photons (Stokes photon) in the visible spectrum. An example is UV brightener, a material that paper and ink manufacturers use to make an extra-white paper or extra-bright ink.

Atomic physics view of luminescence vs. incandescence:
Luminescence occurs when material absorbs external energy and electrons are pumped to excited states, then the radiative transition back to lower state produces luminescence emission. Incandescence emission is from the thermal vibration of heated atoms themselves. This temperature radiation is in the far IR spectrum region when material is at room temperature and shifts towards visible when material's temperature increases.

References:
1. Bruce Fraser, Chris Murphy, and Fred Bunting, Real World Color Management, 2nd ed. Peachpit Press 2005.
2. Luminescence (physics) -- Britannica Online Encyclopedia. www.britannica.com