The micrometre (American spelling micrometer) is a unit of length and is a multiple of the SI unit metre. It uses the symbol μm.
It represents one millionth of a metre and is also known by its previously depreciated name micron.
In the US, the term 'micron' helped distinguish the unit name from the device 'micrometer' but the International System of Units (SI) discontinued this term in favour of micrometre as the prefix system required the use of the Greek symbol 'μ' which invoked the factor x10−6.
The angstrom (or ångström) is a unit of length and is a non-SI but metric accepted unit with the symbol Å.
It is equal in length to one ten-billionth of a metre or 0.1 nanometres (0.1nm).
It is approximately the size of an atom and is mostly used to measure very small distances for example wavelengths of light.