What is a collimator?

A collimator (Latin: collineare: to lead in a straight line) is a lens or mirror system in an optical setup according to which divergent radiation emanating from a point light source is aligned parallel and imaged to infinity. The optical setup can also consist of a single spherical or aspherical lens.
In contrast to camera lenses, collimators often only image correctly near to their optical axis and are in such cases not suitable for image generation.