The present paper is the first of a series reporting on a comprehensive study of the hydrodynamics, kinetics, and spectroscopy of the transient species formed following the detonation of lead azide (LA). The spatial and temporal behavior of the detonation products expanding into vacuum is obtained via high-speed framing photography, transmission of a HeNe laser beam, and chemiluminescence from excited Pb atoms. The photography reveals that following the initiation of LA the products form an expanding, bell-shaped cloud. The HeNe beam is attenuated when the cloud of products traverses its route. The attenuation starts 4-15 μs after initiation and depends on the height of the beam above the LA sample. The chemiluminescence consists of two components: the first, appearing 1-2 μs after initiation, is obtained from excited products formed by the detonation near the surface of the sample, while the second, starting 2-14 μs after initiation, originates from the expanding cloud of products. The intensity and the temporal behavior of the second component of chemiluminescence depend on the distance to a barrier placed above the LA sample. The cloud contains gaseous products and solid particles which propagate perpendicular to the LA surface with a maximum velocity of 4.48±0.10 km/s and 3.78±0.18 km/s, respectively. To reproduce the experimental results, two alternative hydrodynamic models are applied: Stanyukovich's model [K. P. Stanyukovich, Unsteady Motion of Continuous Media (Pergamon, London, 1960), pp. 498-501] for isentropic expansion and London and Rosen's model [R. A. London and M. D. Rosen, Phys. Fluids 29, 3813 (1986)] for exploding foil. The latter model is preferred and when incorporating Beer's law and Mie's theory [C. F. Bohren and D. R. Huffman, Absorption and Scattering of Light in Small Particles (Wiley, New York, 1983), p. 77] it reproduces very well both the temporal behavior of the second component of chemiluminescence and the attenuation of the HeNe beam and suggests that lead particles with radius of 0.05-0.15 μm are involved in the attenuation. The model also provides an estimate of the composition of the product cloud and of the density of the gaseous and solid species as a function of time and distance from the LA sample.