Since both sensitivity and range are related to fire size, if the detector is placed further away from the fire source the detectable fire size will vary according to the Inverse Square Law. Therefore, doubling the detection distance results in only 25% of the radiant energy reaching the detector. As a result, to retain the same response time the surface area of the fire then needs to be four times larger.

For example, if a standard UV/IR detector, capable of detecting a 1 foot 2 inch (0.3m2) fire at 50 feet (15m), is located at 100 feet (30m) detection distance, the minimum size fire then needs to be 4 feet 2 inches (1.2 m2).