Disclosed is a method analysing plastic preforms, wherein plastic preforms are transported along a predetermined transport path by a transport device and an infrared absorption coefficient is determined in at least one region of the plastic preform, wherein, in order to determine the infrared absorption coefficient of the plastic preform, infrared radiation is applied to the plastic preform by a first radiation device and radiation passing through the plastic preform is received by at least one receiving device and an infrared absorption behaviour of the plastic preform is determined from this received radiation, wherein the radiation device acts upon the plastic preforms with infrared radiation in a predetermined wavelength range and an evaluation device evaluates the radiation as a function of its wavelength in a wavelength range from 1500 nm to 2000 nm.