Imaging at the Diffraction Limit

In general, the goal for synchrotron-based FTIR microspectroscopy is to deliver images at the diffraction limit. This usually means setting one (or both) of the microscope s apertures to define a region somewhat smaller than the diffraction limit for the respective objective. For convenience, we use d= XjNA to define this diffraction-limited dimension. This is consistent with the calculated Full Width at Half Maximum (FWHM) of the Schwarzschild diffraction pattern and is also confirmed by experimental resolution studies on test specimens (to be shown later in this section). 

---

Figure 1.4. Degradation of the projected image at the diffraction limit of the lens. Diffraction effects transform an initially square-wave image into a sinusoidal pattern of intensity at the image plane.

Synchrotron radiation can be used as a source of IR for FTIR imaging it is hundreds or thousands of times brighter than the standard benchtop source it and is also pulsed and polarized [309], The extra brightness allows fast data collection at the diffraction-limited resolution... 

To date, a number of chemically selective near-field imaging methods have been demonstrated. Near-field contrast mechanisms that rely on electronic spectroscopy (UV-visible absorption and fluorescence),204 vibrational spectroscopy (IR absorption and Raman spectroscopies), dielectric spectroscopy (microwave dispersion), and nonlinear spectroscopy (second harmonic generation) have all been demonstrated at length scales well below the diffraction limit of light. 

---