The light of a white LED consists of the blue light from the semiconductor and orange from the phosphor (that is excited by the blue)
In the semiconductor there is a junction of 2 materials, actually it is the same GaAs but doped with 2 different additives. Additive N have excess electron over the ones allready present in the GaAs, so where ever an N atom is present in place of a GaAs atom in the crystal structure, the extra electron is there and is free to wander around. Additive P lacks an electron, so one of the links in the crystal around the P atom is weakened by lack of an electron, but it still holds
When the LED is connected to DC power - With the N side to Negative and P side to Positive :
- Electrons go from the Negative wire into the LED, and into the N type material
- The connection to the Positive wire pulls out electrons from the allready weakened links in the P side. As "free spaces" for electrons open, the electrons firther away from the wire are pulled into them, then the next electrons pull into the new free spaces and so on
So the electrons flow from the Negative side of the power supply into the crystal, and "holes" move from the positive side of the power supply into the crystal
Where they meet (on the boundary between N and P sides of the diode), the free electrons coming from N side fill the holes from P side. This keeps going as long as power is connected, as new electrons and holes keep coming to the boundary from either side. When the electrons "fall" into the holes they drop in energy level, this amount of energy gets out of the crystal as light. The wavelength of the light depends on the "height" the electrons fall, which in turn depend on the materials
When you switch the power to the LED off, no new electrons and holes are coming, but the ones allready present near the boundary will still come together and reconnect. The time it takes is on the order of nanoseconds, ie. In a few nanoseconds from the moment the power is switched off, the LED stops lighting in the blue part of the spectrum
This is way faster than the afterglow extinguishing in materials like phosphors, where you can see by eye the afterglow so it is on the order of 100's mSec
In short, this means that the blue part of the output spectrum of the LED reacts very fast to switching on/off the power to the LED. No other lamp extinguishes that fast when the power comes off...
If you encode information in the switching operation of the ballast (by altering the switching times a bit), this information will be present and transmit in the light. Assume the time for the LED extinguishing is few nSec, and that the ballast can keep up with the LED at such speeds, that means you can encode "1/few" Gbit/sec of information in the light. Probably on the order of a Mbit/sec
Few proposals were made using this effect, like Li-Fi (internet connection over LED lighting)
The surveillance scheme described in the video, if i get it, is the LED lamp picking up some information accessible to it, and transmitting it invisibly in the light. Then anyone aiming a matching optical reception instrument to your window can pick the information
The lamp can pick up any electronic wireless communications, over the mains communications (HomePlug, ...), speech, video, i think thats it. But for that it must have the matching comonents inside : Antenna and wireless receptor, over the mains receptor, microphone, or camera (and in the latter case, the dome of the lamp better be clear too)
Over the mains receptor - Can be fitted in the lamp
Wireless receptor - Can be fitted in the lamp (and even justifiable, if the lamp is a LiFi lamp)
Microphone - Can be fitted, but you can spot that if you open the lamp...
Camera - You can spot that even without opening the lamp
So yep it can be done, but the latter 2 would not be too hard to catch
The lamp can transmit information in the range in which its light is seen (can be from quite far away, but only in plain view of the lamp, illuminated objects, window of the room etc)
If anyone would really want the information bad, he could have used Wireless or Over the mains communication too. Allthough maybe those are more restricted by FCC and such (need approval, which will uncover the entire story) while the light output is not. Anyway, if the information can be transmitted in means other than light, then any electronic device can be used for that... CFLs, electronic ballasts, anythng... then this would not be a problem specific to LED lights
To your question, the modulator is not a power device. It can control the switching operation of the ballast, where the ballast handle the actual power
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