Found it on a quiz worded this way. No other details unfortunately. I can only assume it has to do with the heated element materials of tungsten or molybdenum reaction with film layer materials. Perhaps it is a coating to reflect nothing but infrared. Hope to explain to students taking aviation maintenance course on ice and rain systems.

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    $\begingroup$ Welcome to the site. If they are transparent how could they have a color? Please edit and add more detail. $\endgroup$ – GdD Aug 13 '20 at 16:31
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    $\begingroup$ This is borderline off-topic for me as it isn't clear what's being asked from the way it is worded. It would improve your question if you edited to explain the source of the quiz and explain you don't know what it means, otherwise I could see it getting closed. $\endgroup$ – GdD Aug 13 '20 at 16:49
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    $\begingroup$ It's not a problem, what was the quiz from, online or a magazine? $\endgroup$ – GdD Aug 13 '20 at 17:16
  • $\begingroup$ We found it in a folder of quizzes for this system. Do not know it's origin. $\endgroup$ – JbAIM Aug 13 '20 at 17:30
  • $\begingroup$ Well, fundamentally, different density materials refract light differently. So it can be loosely explained at a high level by prisms, rainbows, etc. In other words, Junior High level science. What are your expectations for an acceptable answer? $\endgroup$ – Michael Hall Aug 13 '20 at 18:27

The heating element in cockpit windows is typically a thin film comprised of a transparent conducting oxide - indium tin oxide is common. In thin layers, it is transparent to the visible light spectrum but acts as a mirror in the infrared. As with all transparent conductive films, there will be some tradeoff between transparency and conductivity.

Thin-film interference is a natural phenomenon in which light waves reflected by the upper and lower boundaries of a thin film interfere with one another, either enhancing or reducing the reflected light. When the thickness of the film is an odd multiple of one quarter-wavelength of the light on it, the reflected waves from both surfaces interfere to cancel each other. Since the wave cannot be reflected, it is completely transmitted instead. Thus when white light, which consists of a range of wavelengths, is incident on the film, certain colors are intensified while others are attenuated. Thin-film interference explains the multiple colors seen in light reflected from soap bubbles and oil films on water. It is also the mechanism behind the action of antireflection coatings used on glasses and camera lenses.

For any certain thickness, the color will shift from a shorter to a longer wavelength as the angle changes from normal to oblique. This interference produces narrow reflection/transmission bandwidths, so the observed colors are rarely separate wavelengths, such as produced by a diffraction grating or prism, but a mixture of various wavelengths absent of others in the spectrum. Therefore, the colors observed are rarely those of the rainbow, but browns, golds, turquoises, teals, bright blues, purples, and magentas. 

Additionally, keep in mind that in its thicker, bulk state, indium tin oxide is naturally yellowish-grey. Therefore, the thicker the film, the more it's natural color will influence the light passing through it. I don't know the natural colors for every transparent conductive film material, but it stands to reason that there will be some variation in natural coloration there as well, owing to the different elements included in the composite.

  • $\begingroup$ In cars, there are (rather older) examples of window heaters that are thin wires and not a conductive layer. Are these used in aviation? $\endgroup$ – fraxinus Aug 14 '20 at 15:01
  • $\begingroup$ @fraxinus: Yes. They have seen some use. For example, certain DC aircraft incorporated embedded wiring in their windows. The problem is visibility. For the same reason that cars dont use wires all across the front windscreen, this type of heating element obstructs a pilot's vision. Further, the uneven nature of the heating produced by this kind of system can cause additional optical distortion. $\endgroup$ – Aaron Holmes Aug 14 '20 at 15:27

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