1. A method of detecting an image anomaly within a scene represented by image data, the method comprising: defining a test window within the scene; using a hyperspectral sensor to obtain test data relating to the test window; obtaining reference data; wherein the test data are derived from a test spectrum obtained from the test window, and the reference data are derived from a reference spectrum; generating combined data by combining the test data and the reference data; detecting the image anomaly by comparing the combined data with the test data or the reference data; obtaining variability spectra, the variability spectra being used to enhance the statistical independence of the test data and the reference data; wherein the test data comprise a test feature set derived from the test spectrum and the variability spectra; the reference data comprise a reference feature set derived from the reference spectrum and the variability spectra; and the combined data comprise a combined feature set, the combined feature set being a combination of the test feature set and the reference feature set; wherein the test feature set is generated by: applying a high-pass filter to the test spectrum to obtain a filtered test spectrum; applying a high pass filter to each of the variability spectra to obtain filtered variability spectra; and generating each component of the test feature set using the filtered test spectrum and one of the filtered variability spectra, the test feature set having a number of components equal to the number of variability spectra.