The interesting thing is the chart can be extended to include a much broader range of frequencies.
So what is/was the application?
Now we jump up on the roof and find an interesting antenna. Observe the helix is very similar to out bulb filament. Much larger and straight. It should be noted that helix as this one will radiate or receive from the end. Because it is designed to send or receive satcom. It could be used for omni directional com also but would be designed at different dimensions.
Yes the rubber ducky is a helix sized for broadside or omni directional com. So let's look closer at the pen light bulb.
1. As the wire heats it will radiate visible and invisible electromagnetic signals.
2. If the power to the light is varied at an audio rate it will cause the frequency to swing at an audio rate. This is what we call a Frequency Modulated wave.
3. Consider grandads chart, 100 degrees produces a different color. Less than 100 degrees will produce enough frequency swing to detect. BUT that's not all of the story.
4. There is a thing called thermal ionic emission. As electrons are emitted into the void in the bulb that can create EM radiation.
5. Another character at play here is Skin Effect. True as the filament heats it produce changes in the wire resistance. As the frequency changes it produces changes in AC impedance.
6. Remember the rubber ducky antenna? Looking at the filament as a rubber ducky can produce an image of it radiating RF.
You can see from this chart the RF spectrum is as broad as a football field and as small as an atom. The amazement here for me is not that the bulb can radiate a signal but that someone developed a receiver that was able to receive it!
The plan was to modulate a light beam but the bulb response was to slow because of its mass. The receiver was able to distinguish frequency changes in the RF spectrum.
That's my story.