1. A method of testing an aircraft or aircraft component comprising composite material, the method comprising: calibrating an infrared spectrometer by selecting a plurality of variables which have the potential to influence the physical characteristics of a composite used in the aerospace industry, the calibration further comprises: selecting a plurality of values for each variable; inputting the variables and values into a design of experiments model, thereby obtaining a sample test matrix; testing a plurality of composite samples according to the test matrix, thereby obtaining a number of infrared spectra; testing a physical characteristic of the composite samples; collating the infrared spectra detected by the infrared spectrometer with the measured physical characteristic of a sample, and correlating of infrared spectra and measured physical characteristics to form a database of measurement results, wherein the database of measurement results represents a calibration of the infrared spectrometer; the method further comprising: taking a measurement of the aircraft or aircraft component comprising composite material using the infrared spectrometer, thereby obtaining an infrared spectrum, comparing the infrared spectrum to the database of infrared spectra and correlated physical characteristics; and based on the comparison, providing a value relating to a measurement of a physical characteristic of the composite material.