Lu is a common abbreviation for the element lutetium, which is a rare earth metal with the atomic number 71. It is part of the lanthanide series and is known for its high density and melting point. Lutetium is primarily used in various applications, including catalysts in petroleum refining and in certain types of lasers. In addition to its chemical significance, lu can also refer to a unit of measurement in the field of photometry, specifically the lumen. The lumen quantifies the amount of light emitted by a source, helping to assess brightness in lighting design and applications.