Loosen, P.P.LoosenKnitsch, A.A.Knitsch2022-03-042022-03-042007https://publica.fraunhofer.de/handle/publica/21506210.1007/978-0-387-34729-5_4Beam quality is a measure of how tightly a beam can be focused. The higher the beam quality, the smaller the spot size and the higher the laser intensity. According to the ISO-standard [4.1] this property can be characterized by the beam-parameter product (Q or BPP), multiplying the waist radius (w0) and the far-field divergence (0) of the beam. In the best case, i.e., without any aberrations, this beam-parameter product remains constant, if the beam is transformed by passive optical components such as lenses or mirrors (Eq. 4.1).en621book article