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The folks at BERG developed this neat method for visualizing the sensitivity of an RFID reader. Rather than using an expensive set of test equipment to measure the magnetic field intensity, they just hooked their reader up so that it lit an LED every time their card was detected, and then captured it using a camera. This is pretty similar to the technique used to make the Roomba art. They were also able to show that (due to polarization) the orientation of the card with respect to the sensor changes how it responds. Fascinating!