Scattering of light from particles significantly smaller than the wavelength of the light is known as Rayleigh scattering. The scattering process is elastic, meaning that the scattered light has the same wavelength as the incident light. The scattering of light on molecules in a gas is an example of this phenomenon.
Rayleigh scattering depends on the number density of molecules in the gas. All molecular species in the gas contribute to the scattering process, and thus the global gas density can be measured.
In general, a gas will expand with increasing temperature, meaning that the number density and thus the intensity of scattered light will decrease. Rayleigh Thermometry strives to determine temperature distribution in flames by measuring Rayleigh scattering from the gas molecules, provided that the gas composition and pressure are known. Measuring the intensity of scattered light at a known temperature and monitoring how that intensity changes allows the user to determine corresponding changes in temperature. By using a laser light sheet to illuminate a thin plane, planar measurements can be made providing 2D temperature maps of the flow. An intensified CCD camera placed perpendicular to the light sheet records the scattered light from the gas.
Rayleigh scattering is orders of magnitude weaker than Mie scattering and generally require a clean environment free from soot, dust particles, and similar contaminants to minimize interference with the measurement.