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30 POWDER COATING, March 2016 As the temperature decreases, the en- ergy levels also drop, and the peak en- ergy wavelength shifts to the longer wavelengths. The lowest temperatures from the 250°C, 100°C, and 50°C curves cannot be seen in the graph. When the graph is enlarged to see the lower temperature curves, this shift to the longer wavelengths is more appar- ent. However, the power intensity drops significantly. This shift in power intensity is shown in Figure 2. At 250°C, the blue curve can be seen to have an approximate peak around 6 microns, whereas at 100°C, the peak wavelength is around 7.5 mi- crons. Note also that the extent of wave- length is more evenly distributed and doesn't exhibit the concentrated narrow peak seen at higher temperatures. If we enlarge the same graph again and focus only on the lower temperatures as shown in Figure 3, we see that tempera- tures of 50°C and 25°C have peak wave- lengths of around 9 and 10 microns re- spectively. In the final graph shown in Figure 4, a curve showing the peak wavelength against temperature is shown. This is plotted from Wien's Law, which states that the black body radiation curve for different temperatures peaks at a wave- length inversely proportional to the temperature. The increase in peak wavelength as temperature drops is clearly seen. In summary As we've established, Planck's Law de- scribes the electromagnetic radiation emitted by a black body in thermal equi- librium at a definite temperature. When plotted for various heater (emitter) tem- peratures, the law predicts both the range of frequencies across which IR heating energy will be produced as well as the emissive power for a given wavelength. When selecting an IR emitter for a par- ticular heating task, the target material absorption characteristics are of high importance. Ideally, the emitted IR fre- quencies and the target material absorp- tion frequencies should match to allow the most efficient heat transfer. How- ever, as depicted in the graphs, at longer wavelengths, the amount of energy A closer look at IR distribution for various emitter temperatures from 100°C to 25°C. Figure 3 Figure 2 Planck Distribution, Emissive power and Wavelength with temperature Emissive power (W) Wavelength (µm) 2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 0 5 10 15 20 25 30 35 40 A close up of IR distribution for various emitter temperatures from 350°C to 50°C. Planck Distribution, Emissive power and Wavelength with temperature Emissive power (W) Wavelength (µm) 20 18 16 14 12 10 8 6 4 2 0 0 5 10 15 20 Every material absorbs IR differently. Choosing the correct emitter will ensure the most effective heat work that will, in turn, yield the fastest cycle times.