"Unless it's readily visible" in which spectrum? Whether X-ray, terahertz, microwave, UHF, or visible light... it's all just electromagnetic radiation. Seems weird to make distinctions. To me (and trust me, IANAL) it seems that any law governing a "camera" should be equally-applicable.
It's actually a pretty easy line to draw; the spectrum is the visible spectrum of the average human. The reason is that this makes it trivial for citizens to know what constitutes "in plain sight", and thus to control what lies in plain sight. If it bothers you that a legal matter is based on fuzzy lines defined by human physiology, then welcome to the entirety of pretty much all law everywhere.
