The plates are equipped with three rings. A plate containing food with a little bit of radiation will light only one ring, whereas a heavily-radiated plate will light three rings (one in red, no less).
Ferber explains on his blog that people perceive radiation risks very differently and "even governments alter the official limiting values in special circumstances." Thus, he allows the consumer to set the levels in which each ring lights up (and Ferber is off the hook for any user error!).