Comment by photonic277

In my intern days some time around 10 years ago, a PI at the NASA GRC facility told me about a problem of this flavor an old grad student of his had.

The guy was working on an optical sensor in a light-tight lab. Every morning, he came in, calibrated the sensor, and performed measurements. All morning, it held calibration with negligible drift. But when he came back from lunch, each time, the calibration had drifted off.

Could it be related to the time of day? He tried taking his lunch an hour earlier and an hour later. Each time, the calibration was rock solid until right after lunch.

In spite of protocol, he tried eating lunch in the lab, no one else in or out. Before lunch: good calibration. After lunch: bad calibration.

He tried not eating lunch at all. That day, the calibration held all day.

How could an optical sensor have any concept of whether its user had eaten lunch? It turned out, it only had to do with the lunch box. The sensor was fiber coupled, and it was sensitive to changes in transmission losses generated by changes to local radii of the patch chord. Every morning, the grad student set his lunch box down on the lab bench, nudging the fiber into some path. After eating, he’d replace his lunch box on the bench, nudging the fiber into a different path.

After that, the fiber was secured with fixed conduit, and lunch boxes no longer entered the lab.