A micrometer (µm) is a unit of length in the metric system equal to one millionth of a meter.

A light-year is the distance that light travels in a vacuum in one Julian year (365.25 days), approximately 9.46 trillion kilometers.

This is because a micrometer is an incredibly small unit, while a light-year represents an enormous cosmic distance.

While direct practical engineering use is rare due to the extreme difference, it's valuable for educational purposes and scientific thought experiments about scale.