1. A method of assessing the occurrence of an unknown substance in a sample that comprises multiple entities, the method comprising: providing a reference library comprising a plurality of reference data sets representative of at least one known substance; assessing a first feature of the entities wherein the first feature is characteristic of the unknown substance; selecting a region of interest of the sample wherein said region of interest of the sample comprises at least one entity exhibiting said first feature; obtaining a spatially accurate wavelength resolved. Raman image of said region of interest wherein each pixel in said image is the Raman spectrum of the sample at the corresponding location; and assessing said spatially accurate wavelength resolved image to thereby identify said unknown substance.