Duration of light is less than duration of darkness.
Symbol: Fl
Example: Fl 10s (Flashing, 10 sec. )
Duration of light is more than the duration of darkness.
Symbol: Oc
Example: Ocl 10s (Occulting, 10 sec. )
Duration of light and darkness are equal.
Symbol: Iso
Example: Iso 10s (Isophase, 10 sec. )