A detailed study on the visible light emission from multiple granular HMX reactions has been completed. New techniques for measuring the reaction properties were devised, both directly and indirectly from the results gained from using optical spectroscopy. Two different impact initiated deflagration reactions were performed. Improvements were made to the fallhammer sensitivity test to allow for more quantitative measurements, and the Split Hopkinson Pressure Bar (SHPB) was used for the first time to purposely initiate deflagration, providing better measurements of the energy present during initiation. Using the greybody emission, the temperatures of these reactions were measured as 3900 ± 400 K and 2900 ± 200 K respectively. Building a time-dependent pyrometer showed a constant temperature throughout the main reaction period. The significant difference between the two temperatures inspired the addition of a mass spectrometer to investigate the products from each reaction. The results showed the two reactions did not resemble variants of an ‘ideal’ deflagration reaction, and so identical chemistry should not be assumed to be present in both. Optical spectroscopy also showed the existence of a spectral peak in deflagration due to emissive sodium impurities. The peak was red-shifted under the pressures of the reaction, and the calibration curve of the red-shift was calculated. The functional form of the shift has a dependence on pressure and temperature in agreement with Lindholm-Foley collisional theory, allowing the pressure to be calculated optically from position of the shift. This was achieved with a streak spectrometer, where time-dependent measurements of the temperature (using greybody emission) and pressure (using the sodium peak) from the same source of light were made. Finally, detonation of HMX was examined. The detonation pressure and velocity were measured as a function of initial density, and found consistent with models of shock conservation and Group Interaction Modelling. High temperatures of 7000 K were recorded from the greybody emission, with the light emitted from mechanical high pressure void collapse dominating over the heat produced from the chemical reaction. These temperatures were not affected by the density of the HMX bed, but instead the size of the voids present. Larger particles increased the size of the voids, leading to higher temperatures and decreased light scattering, with the inclusion of sodium absorption features.