Probably metrology. I've got a decent start on an electronic metrology home lab, but other sorts are also interesting. Metrology research is particularly intere

Probably metrology. I've got a decent start on an electronic metrology home lab, but other sorts are also interesting. Metrology research is particularly interesting, finding ways to measure things more accurately & precisely than any current equipment can do so, and to quantify how precise & accurate those measurement systems are despite that!