Optothermal Beam Deflection is a special means of optothermal detection.
Absorption of the pump laser beam (green) by the sample leads to local warming. Temperature dependency of density and refractive index result in a deflection of a second laser beam, the probe beam (red). The measured beam position is a measure for the absorption. OT Beam Deflection or the Mirage effect can be employed for instance for absorption measurements on single aerosol particles.