Non-Intrusive Measurement

A measurement should not influence the measured quantity. As an example, let us look at measuring the temperature of a hot medium in a closed container. We could measure the temperature by opening the container and holding a thermometer in the medium. But this cools the medium. So by measuring, we influence the measured quantity, the medium temperature. Not a good idea.

Instead, we had better use a battery-powered thermometer installed in the container. It transmits the measured temperature to the outside, e.g. via Bluetooth, where it is displayed. This gives us a measurement without feedback on the measured quantity.