Transitions between energy levels in hydrogen atom. For weapons systems, the only selective radiator of interest is laser light. It is a characteristic of most lasers to have an extremely narrow bandwidth. A related property is called coherence , which is the degree to which the light has a single frequency and phase. Coherence can be either temporal , meaning the degree to which the frequency and phase is stable in time, or spatial , meaning the degree to which the frequency and phase at one location matches that at another location.
Temporal coherence is measured by a coherence time, which is roughly the inverse of bandwidth.
Spatial coherence is measured by coherence length, which is nothing more than the coherence time multiplied by the speed of light. It is the great coherence length of typical lasers that allow their beams to be narrowly focused. Lasers may operate in a continuous wave mode, just like CW radar, or in a pulsed mode. Helium and Neon gas mixture. A class of noble-gas lasers including Argon, Krypton and Xenon.