Is there an electronic device that detects EM waves while allowing them to pass through? Kind of like a window that can detect a measure the qualities/quantities of a light ray but lets it pass through to other devices or similar? I have no idea of what of keywords/phrase to use to even search this kind of device. I made a diagram in case for better understanding.
Yes and no.
Yes, there are devices that will allow some, or most, of the electromagnetic energy impinging upon it to go through while measuring the overall flux.
No, there's nothing that will allow all of the radiation through while detecting it. All electromagnetic radiation is composed of photons*. It's a basic truth of quantum mechanics that you can't detect a particle without disturbing it. In the case of photons, this means consuming it or at least** reducing its energy level and thus changing its wavelength.
* Even RF and below -- they're just bigger, less energetic photons.
** Maybe there's a way to detect it by reflecting it -- I'm not that much of a quantum mechanic. But you're still not passing the light through.
