IR Illuminator without visible glow

In the mid 70's and into the 90's, 3M designed and built Xenon-strobe emitters called 'Opticom' that used a visible light, flashing Xenon strobe of narrow frequency and specific rise time and duration. It was mounted generally atop the cab of fire trucks and in some cases, ambulances and police cars.

An optical detector mounted atop the traffic signal heads or mast arms pointed in the direction of travel of emergency vehicles. When those equipped with the Opticom emitter placed it into service the detector would pass that signal on to a receiver in the traffic signal cabinet. The receiver would analyze the signal from the strobe, decode and validate the signal. If valid, a command was issued to preempt the traffic signal and provide a green signal to the emergency vehicle as soon as possible. The flashing strobe was visible, very bright but was rapid enough (8 to 10 pulses per second, I believe) to be displayed legally by emergency vehicles, according to local jurisdiction regulations.

Later it was decided by a transportation agency in Northern California to use the Opticom system on public buses to encourage ridership, lower smog, etc. The Opticom system would insure the green signal would be awarded a little sooner to the bus but not at the expense of serving at least a minimum vehicle green time to vehicles that were preempted, and also after allowing a full pedestrian clearance time (the flashing 'Don't Walk' or orange 'upraised hand') so as to not endanger pedestrians that had received the 'Walk' or white 'walking man' symbol.

The Opticom emitter for a bus modulated at a lower frequency so when decoded by the receiver it would determine that the priority for the preemption of the traffic signal would be low for a bus, and not an emergency vehicle (highest priority).

Because the bus is not considered an emergency vehicle, the state's motor vehicle code required that a dark filter be placed over the emitter so the light would not be visible to people and motorists. I do not recall the wavelength if the IR that made it through the filter (it's been about 35 years for me) but the detectors on the traffic signal mast arms would pick up IR.

Over time, the UV light emitted by the Xenon tube would cause a reaction with the Lexan IR filter making it totally opaque dead-center of the filter after about 2 years and the bus could not preempt the traffic signal. I would remove the filter and polish the inside center with automotive rubbing compound; a huge amount of green stuff would be left on my rag from the IR-passing Lexan. I'd re-install it and all was well with the Opticom preemption on that bus for another couple of years.

That's it! Thanks for indulging me.

P.S. -

First, a little history and my only experience with IR... sorry if I'm verbose or if off topic.In the mid 70's and into the 90's, 3M designed and built Xenon-strobe emitters called 'Opticom' that used a visible light, flashing Xenon strobe of narrow frequency and specific rise time and duration. It was mounted generally atop the cab of fire trucks and in some cases, ambulances and police cars.An optical detector mounted atop the traffic signal heads or mast arms pointed in the direction of travel of emergency vehicles. When those equipped with the Opticom emitter placed it into service the detector would pass that signal on to a receiver in the traffic signal cabinet. The receiver would analyze the signal from the strobe, decode and validate the signal. If valid, a command was issued to preempt the traffic signal and provide a green signal to the emergency vehicle as soon as possible. The flashing strobe was visible, very bright but was rapid enough (8 to 10 pulses per second, I believe) to be displayed legally by emergency vehicles, according to local jurisdiction regulations.Later it was decided by a transportation agency in Northern California to use the Opticom system on public buses to encourage ridership, lower smog, etc. The Opticom system would insure the green signal would be awarded a little sooner to the bus but not at the expense of serving at least a minimum vehicle green time to vehicles that were preempted, and also after allowing a full pedestrian clearance time (the flashing 'Don't Walk' or orange 'upraised hand') so as to not endanger pedestrians that had received the 'Walk' or white 'walking man' symbol.The Opticom emitter for a bus modulated at a lower frequency so when decoded by the receiver it would determine that the priority for the preemption of the traffic signal would be low for a bus, and not an emergency vehicle (highest priority).Because the bus is not considered an emergency vehicle, the state's motor vehicle code required that a dark filter be placed over the emitter so the light would not be visible to people and motorists. I do not recall the wavelength if the IR that made it through the filter (it's been about 35 years for me) but the detectors on the traffic signal mast arms would pick up IR.Over time, the UV light emitted by the Xenon tube would cause a reaction with the Lexan IR filter making it totally opaque dead-center of the filter after about 2 years and the bus could not preempt the traffic signal. I would remove the filter and polish the inside center with automotive rubbing compound; a huge amount of green stuff would be left on my rag from the IR-passing Lexan. I'd re-install it and all was well with the Opticom preemption on that bus for another couple of years.That's it! Thanks for indulging me.P.S. - @Fastb , I'm an old fart now, too old to carry ANY baggage!