The time required for the detector output to go from the initial value to a percentage (e.g. 99%) of the final value. In the case of an exponential behaviour of the detector τR can be related to the time constant τc. The rise time τr is the time required for the detector output to vary between given percentages (e.g. from 10% to 90%) of the final value. Similarly, the fall time τf is the time required for the detector output to vary between given percentages (e.g. from 90% to 10%) of the initial value.