The colour of heat

What’s incandescence?

When a solid object is heated up to a certain temperature, it glows or radiates light. This reaction, a heated solid emitting light, is known as incandescence. Incandescence results from the vibration of atoms when the object is heated and thermal vibration energy is created. Some of this heat energy gets transformed into light energy (i.e. incandescent light) as the heated object releases some of its thermal vibration energy as photons.

Applications of incandescence

Incandescence can be use relate colour to temperature. When an object is heated and glows, infrared is emitted initially, followed by red, orange, yellow, and white light as it becomes increasingly hotter [1]. Color temperature refers to the temperature one would have to heat a black body source to produce light of the same color [2].

 Table 1. Typical colour temperatures from various sources [2]

Temperature (K) Source
1500 Candlelight
2680 40 W incandescent lamp
3000 200 W incandescent lamp
3200 Sunrise/sunset
3400 Tungsten lamp
4500-5000 Xenon lamp/light arc
5500-5600 Electronic photo flash
6500-7500 Overcast sky
9000-12000 Blue sky

 Figure 1. Colour temperature scale [3]

When the temperature exceeds 1000 K the peak wavelength lies in the visible spectrum, therefore the colour of hot objects above 1000 K can be used to estimate their temperatures

From Figure 1 we can see that when liquid metal is said to be “red hot” is actually colder than a lightbulb which is “white hot”. Similarly, photographs taken under tungsten light is said to be warmer (i.e. appears more reddish) than those taken under daylight [4].

 What is black body radiation?

Black body source (i.e. radiator) is an abstract object which absorbs all incident energy then re-radiates the energy absorbed. Black body radiation relates the temperature of an object to the light it emits. Figure 2 shows the relationships between the equations that describe black body radiation [6]. For any given temperature the energy distribution can be estimated using Planck’s Law [1]. Stefan-Boltzmann Law is used to calculate the total emitted power [1]. Wien’s displacement law determines the wavelength of the peak emission (i.e. dominate colour at a particular temperature) [1]. Wien’s displacement law tells us that the wavelength of the peak of the black body radiation curve decreases linearly as the temperature is increased—colour shifts from red to blue (See Figure 3). This can be used to determine the temperatures of hot radiant objects such as stars, as well as determining the temperature of any radiant object whose temperature is far above that of its surroundings [5].

Figure 2. The relationship between Planck Law, Stefan-Boltzmann Law and Wien Displacement Law [6]

Figure 3. A graphical representation of Wien’s Displacement Law [5]

References

[1] Douma, M., curator. (2008). Seeing Heat. In Cause of Color. [Online Document].Available: http://www.webexhibits.org/causesofcolor/3.html

[2] schorsch.com (2004-2011). Lighting Design Glossary [Online Document]. Available: http://www.schorsch.com/en/kbase/glossary/cct.html

[3] W.A. Steer. (2008). Colour-temperature [Online Document]. Available http://www.techmind.org/colour/coltemp.html

[4]C. Salvaggio, “Photometry, Radiometry, and Measurement,” In Focal Encyclopedia fo Photography, 4th ed. USA: Elsevier, 2007, ch. 7, pp. 741.

[5] HyperPhysics Department of Physics and Astronomy, Georgia State University. Wien’s Displacement Law [Online Document]. Available: http://hyperphysics.phy-astr.gsu.edu/hbase/wien.html#c2

[6] HyperPhysics Department of Physics and Astronomy, Georgia State University. Applications of the Planck Radiation Formula [Online Document]. Available http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/planckapp.html#c1

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