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Photoacoustic spectroscopy is the measurement of the effect of absorbed electromagnetic energy (particularly of light) on matter by means of acoustic detection. The discovery of the photoacoustic effect dates to 1880 when Alexander Graham Bell showed that thin discs emitted sound when exposed to a beam of sunlight that was rapidly interrupted with a rotating slotted disk.
Variations in the electrical conductivity and magnetic permeability of the test object, and the presence of defects causes a change in eddy current and a corresponding change in phase and amplitude that can be detected by measuring the impedance changes in the coil, which is a telltale sign of the presence of defects. [5]
Photoacoustic spectroscopy is also useful for the opposite case of opaque samples, where the absorption is essentially complete. In an arrangement where a sensor is placed in a gaseous phase above the sample and the light impinges the sample from above, the photoacoustic signal results from an absorption zone close to the surface.
Spectroscopy is a category of characterization techniques which use a range of principles to reveal the chemical composition, composition variation, crystal structure and photoelectric properties of materials. Some common examples of spectroscopy techniques include:
ISO 9934-2, Non-destructive testing - Magnetic particle testing - Part 2: Detection media; ISO 9934-3, Non-destructive testing - Magnetic particle testing - Part 3: Equipment; ISO 10893-5, Non-destructive testing of steel tubes. Magnetic particle inspection of seamless and welded ferromagnetic steel tubes for the detection of surface imperfections
When the incident light beam is at Bragg angle, a diffraction pattern emerges where an order of diffracted beam occurs at each angle θ that satisfies: [3] = Here, m = ..., −2, −1, 0, +1, +2, ... is the order of diffraction, λ is the wavelength of light in vacuum, and Λ is the wavelength of the sound. [4]
Most fluorochromes are optimized for fluorescence emission and are sub-optimal for optoacoustic detection, because after absorbing the illuminating energy, they tend to emit fluorescence rather than convert it to heat and generate a photoecho. Dyes with higher absorption cross-sections generate stronger optoacoustic signals.
Conversion electron Mössbauer spectroscopy (CEMS) is a Mössbauer spectroscopy technique based on conversion electron.. The CEM spectrum can be obtained either by collecting essentially all the electrons leaving the surface (integral technique), or by selecting the ones in a given energy range by means of a beta ray spectrometer (differential or depth selective CEMS).