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Figure 3
The concept of resolution in FTIR microscopy. Lateral and axial spatial resolution: as clearly shown by equations both the lateral and axial extent of the confocal point spread functions are reduced by ∼30% if compared with the corresponding widefield illumination. From the equations it is also underlined that the NA of the microscope objective is much more effective on the axial resolution (∼NA2). ATR microscope objective: when a microscope is used in reflection geometry, the incoming radiation illuminates only half the objective aperture and the reflected beam from the specimen returns through the opposite half of the aperture going towards the detector. In the modality of analysis the total magnification of an ATR objective is the product of the magnification of the objective times the refractive index of the internal reflection element (IRE) crystal (M × n) improving the spatial resolution by a factor n. Reflection spectroscopy techniques are also characterized by a small depth of penetration so that a micro-ATR may typically probe depths between ∼0.3 and 3 µm. However, the penetration depends also on the wavelength of the radiation, the refractive indices of both specimen and IRE, and by the angle of incidence of the radiation so that, with a degree of control of the penetration, depth-profile studies are possible.

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