Count per length The length density of a given amount of a substance. This unit group is also used for image and TV resolutions measured in lines. This is distinct from "inverse length" in that there is something specific being counted per unit length, that is, the numerator is not dimensionless. Space and Time Aquarius ODM2 Units Type Controlled Vocabulary ODM2 Working Group A vocabulary for describing the type of the Unit or the more general quantity that the Unit represents.