Hounsfield scale

The Hounsfield scale /ˈhaʊnzˌfiːld/ or CT numbers, named after Sir Godfrey Newbold Hounsfield, is a quantitative scale for describing radiodensity.

The Hounsfield unit (HU) scale is a linear transformation of the original linear attenuation coefficient measurement into one in which the radiodensity of distilled water at standard pressure and temperature (STP) is defined as zero Hounsfield units (HU), while the radiodensity of air at STP is defined as -1000 HU. In a voxel with average linear attenuation coefficient ${\displaystyle \mu }$, the corresponding HU value is therefore given by:

Where ${\displaystyle \mu _{water}}$ and ${\displaystyle \mu _{air}}$ are respectively the linear attenuation coefficients of water and air.

...
Wikipedia