It means that when the distance from the
detector
to the
fire is doubled the fires needs to be four times larger. E.g.
a
detector detects a 0,09 m2 (1 sqft)
petrol fire at 15 meters
(45 ft). In order to see the fire at 30 meters (90 ft) the size of the
fire
needs to be at least 0,36 m2 (4 sqft).