1. A method of determining whether the composition of a test material is the same as the composition of a standard material, said test material and said standard material each having a reflectivity, comprising the steps of measuring the reflectivity of the standard material throughout a range of the near infrared spectrum to produce a measurement of reflectivity, determining from the measurement of the reflectivity of the standard material a first set of n values representing a derivative of any variation of reflectivity with wavelength throughout said range, measuring the reflectivity of said test material throughout said range to produce a measurement of reflectivity, determining from the measurement of the reflectivity of said test material a second set of n values representing a derivative of any variation of the reflectivity of said test material with wavelength throughout said range, defining a first vector having an end point in n-dimensional space represented mathematically by said first set of n values as coordinates of the end point of said first vector, defining a second vector having an end point in n-dimensional space represented mathematically by said second set of n values as the coordinates of the end point of said second vector, and determining the cosine of any angle between first and second vectors as an index of similarity of the composition of said test material and the composition of said standard material.