1.5.

The "off" time tD means the period of time within which the luminous intensity
of the flashing light is less than 1/100 of the maximum value (peak value) Jm,
but not more than 10 cd.
In the case of groups of several flashes the "off" time shall be measured from the end of the last flash of the group to the beginning of the first flash of the next group.

 a4c0
1.5. The "off" time tD means the period of time within which the luminous intensity of the flashing light is less than 1/100 of the maximum value (peak value) Jm, but not more than 10 cd. In the case of groups of flashes the "off" time shall be evaluated from the last flash of the group to the first flash of the next group; a0c0