The optical detection of an explosion or fire event is considered quantitatively in terms of the source radiance, background or stray irradiances, and the spectral responsitivities of the available sensors. The infrared spectral source radiances from spherical methane-air “ignitions” were measured and the data analyzed. They served as a basis for the development of a new detector which uses wavelength selection about the 4.4-μm CO 2 band to detect fires and explosions rapidly and reliably; and to discriminate effectively against false sources. The data are also of fundamental interest, yielding consistent temperatures and spectral growth patterns. An equation is derived fro the fraction of combustion power radiating to free space which seems to approach a natural limit for slow explosion of large size. Typical radiance data from hydrocarbon pool flames are also considered. An earlier, empirical, linear correlation of large pool burning rate with the ratio, H c /H v , is revised and related to radiative transport factors and the limit burning velocity for quenching by natural convection at the flammability threshold.