Figure 2(a) depicts characteristic spectral features for C4 (91% RDX [1,3,5-trinitroperhydro-1,3,5-triazine] and 9% plasticizer, binder, and oil), TNT, penta-erythritol tetranitrate (PETN), and ANFO (94% ammonium nitrate and 6% diesel fuel). It can be observed that each type of explosive has a different critical energy range for optical absorption, that is, PETN, ANFO, C4, and TNT have absorption edges at 290, 335, 340, and 420 nm, respectively. These absorption edges have different center wavelengths and different full widths from one another, therefore, the differential reflectograms allow us to distinguish each explosive (see below differentiation algorithm). The origin of the absorption edges stem from electronic transitions which are observed due to hydrogen bonds between explosive molecules in condensed state.5 Without hydrogen bonds, the energy levels are different and the absorption edges listed above are not observed. Figure 2(b) displays DR spectra of white powder materials (flour, Splenda, and salt) which are similar in physical appearance to explosives but have substantial different spectral features which allows one to distinguish them from one another. Additionally, Fig. 2(b) displays the DR spectra of a red jacket and denim jeans as examples of background materials on which explosives residue can be found. The explosives spectral fingerprints are also clearly different than these background clothes.