Chromatical aberration

Once the primary electron beam is created, it must be demagnified with condenser lenses and then focused onto the sample with objective lenses. These electron lenses are electromagnetic in nature and use electric and magnetic fields to steer the electrons. Such lenses are subject to severe spherical and chromatic aberrations. Therefore, a point primary beam source is blurred into a primary beam disk to an extent dependent on the energy and energy spread of the primary electrons. In addition, these lenses are also subject to astigmatism. AH three of these effects ultimately limit the primary beam spot size and hence, the lateral resolution achievable with sem. 

A low dispersion is desirable in optical glasses used for lenses in cameras, telescopes, etc, because dispersion causes chromatic aberration, a condition which reduces the sharpness of an image. However, it is possible to correct for chromatic aberration by using a combination of glasses having different Abbu numbers (9). 

TEM signal the energy losses upon scattering accentuate chromatic aberration effects. 

Through a curious set of circumstances, Newton failed to solve the problem of chromatic aberration, and so, he abandoned the attempt to construct a refracting telescope which should be achromatic, and instead designed a reflecting telescope, probably on the model of a small one that he had constructed in 1668. The form he used is known by his name today. 

The focusing of radiation within the instrument was formerly done by means of lenses, but these suffer from chromatic aberration and particularly in respect of the relationship between the visible and ultraviolet parts of the spectrum. Focusing is now usually carried out by means of suitably curved mirrors having a reflecting surface coated with aluminium which is protected by a silica film. 

Even if the first attempts at making aspherical lenses had been successful, chromatism (Fig. 6, which was not properly understood, would have prevented such lenses to deliver acceptable images anyway. Solutions to the problem of chromatic aberrations had been delayed by Newton himself, who incorrectly... 

Apochromat Microscope objective that has better color correction than the much more common achromat objectives corrected chromatically for two wavelengths of light (red and blue). Apochromatic objectives are corrected chromatically for three colors (red, green and blue) and spherically for two colors, which practically eliminates chromatic aberration. 

Chromatic aberration is the phenomenon of different wavelengths of light being focused to different positions within your sample. 

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