Virtual aperture

The final beam-limiting aperture was originally in the gap of the final lens, but only the design in Fig. 3.17A has sufficient room. The apertures shown are virtual apertures at an optically equivalent position, and their actual diameter must be divided by the demagnification of the final lens to give the effective aperture diameter D. 

The problem is overcome in practice by operating with a virtual objective aperture. Whilst it is, of course, impossible to eliminate such sources of radiation, they can be... 

Figure 6. Triangular aperture SNOM probe excited with two different polarization states. With polarization shown on the left side two maxima of the electric field occur. Rotating the polarization by 90 degrees virtually only one maximum occurs [7]. 

Abscisic acid supplied to leaves causes a shift in the distribution of stomatal apertures toward narrower pores and an increase in the fraction of closed stomata. The increase of the fractions of completely closed and very narrow stomata leads to a virtual reduction of the photosynthetically active area of a leaf. Stomatal... 

Open channels with virtual walls have appeared recently in the literature. In such geometries, the liquid is partly in contact with the surrounding air, and partly in contact with different solid walls. Some open channels are simply rails, i.e. a channel wall with air gaps on both sides [29]. Other open channels are more complex, such as open rectangular U-grooves with circular apertures in the bottom plate [30]. 

The first experimental proof of virtual linearity of response, up to some 80% of the aperture diameter, came from Barfield and Knight and Barfield, Wharton and Lines.27 They used spherical particles of polymer latex and the COULTER COUNTER model ZM. The experiment involved the use of different sizes of "mono-sized" latex particles measured by a range of different apertures, and therefore required no assumed or measured "real" sizes for the particles. The experiment has not been repeated for other particle shapes as no other series of suitable model particles appear to exist, so the linearity of response for other particle shapes has not yet been verified experimentally. It is reasonable to assume however, from all existing theory that no significant extra alinearity will exist for non-spherical shapes. 

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