"The roughness of a surface is measured in micro inches (uin), or micrometers (um). One micro inch is equal to one one-millionth of an inch, as one micrometer equals one one-millionth of a meter. The translation from standard to metric is: one micro inch =.025 micrometer; from metric to standard, one micrometer= 40 micro inches. Within this scale every surface has roughness."
A google search provided this result. hope it helps.
