Aperture, ideal microscopes

Figure 41. Image formation in an ideal microscope a) Specimen b) Objective lens c) Back-focal plane, objective aperture d) Image plane...

This is the same as (3.1) except that the numerical constant is smaller. In practice, in an acoustic microscope the ideal of a numerical aperture is never even approximately realized. It is one of the experimental frustrations of acoustic microscopy that no satisfactory method exists for measuring the pupil function of the lens (i.e. the amplitude of the waves as a function of radial position on the lens surface) but, whatever else it is, it is certainly not uniform everywhere on the lens surface and zero elsewhere. 

However, electron microscope lenses are far from ideal, and the probe is broadened by spherical aberration (proportional to the cube of the aperture size) and by diffraction (inversely proportional to the aperture size). Balancing the two effects by finding an optimal aperture size (ao) gives the theoretical minimum probe sizes at the specimen plane. However, the current within the probe size may be too small for useful signal to be collected within an acceptable time. Thus, while many manufacturers will claim smaller probe sizes, in reality the useful minimum probe size, one that yields enough... 

The ideal collection optic would collect fluorescence from a Ansr solid angle around the detection zone with 100% efficiency, while being completely inefficient at collecting scattered laser light. Obviously, there exists no such optic, but there are various approaches reported in the literature taken to emulate this ideal. In most cases, either a high numerical aperture microscope objective or a fiber-optic system is used to collect the fluorescence and transmit it to the signal transducer. 

FTIR microspectroscopy is a microanalytical technique, which interfaces an FTIR spectrometer to an optical microscope. Regions of interest in the sample are spatially isolated using the microscope s apertures. It enables the IR spectrum of sampling regions down to about 10 pm resolution to be taken. Consequently, FTIR microscopy is ideal for compositional mapping and analysis of heterogeneous samples whose domain sizes are in the tens of micrometre range. 

Large-scale diamond compression (high-pressure) cells have been available for some time. However, during recent years, a microversion of the cell, also known as the diamond anvil cell, has been produced. This small diamond anvil cell is ideal for the study of physically hard (but compliant) samples, certain intractable samples, and samples that are optically too thick for normal transmission measurements. Note that this small form of the accessory is intended for samples 1 mm or less in size. The aperture through the diamonds is small, and therefore the use of a beam condenser accessory is recommended for optimal throughput. Alternatively, the cell may be used in combination with an IR microscope. 

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