ABERRATIONS AND THEIR

CORRECTIONS IN OPTICAL AND

ELECTRON MICROSCOPES

Muhammed Labeeb

CONTENTS

▪ ABERRATIONS

▪ SPHERICAL ABERRATION AND ITS CORRECTION

▪ COMA AND ITS CORRECTION

▪ ASTIGMATISM AND ITS CORRECTION

▪ DISTORTION AND ITS CORRECTION

▪ CURVATURE OF FIELD AND ITS CORRECTION

▪ CHROMATIC ABERRATION AND ITS CORRECTION

▪ REFERENCES

ABERRATIONS

▪ An aberration is a departure of the performance of a system from the predicted path

▪ Aberration leads to blurring of the image produced by an image-forming optical system

▪ Aberrations are classified into two types

▪ Monochromatic : Monochromatic aberrations are caused by the geometry of the lens or mirror and appear even when using monochromatic light

▪ Chromatic : Chromatic aberrations are caused by dispersion, the variation of a lens's refractive index with wavelength. They do not appear when monochromatic light is used

TYPES OF OPTICAL ABERRATIONS

Monochromatic Aberrations

SPHERICAL ABERRATION

COMA

ASTIGMATISM

DISTORTION

CURVATURE OF FIELD

Chromatic AberrationsLateral, or transverse

Axial, or longitudinal

SPHERICAL ABERRATION

▪ Spherical aberration occurs when light from a point object on the optical axis is more strongly refracted at the periphery OR near the edge of the lens

Light rays passing through the outer portion of the lens are more strongly refracted than those passing through the central portion and are focused at a different point along the optical axis

SPHERICAL ABERRATION

On top is a depiction of a perfect lens without spherical aberration: all incoming rays are focused in the focal point.

The bottom example depicts a real lens with spherical surfaces, which produces spherical aberration: The different rays do not meet after the lens in one focal point.

CORRECTIONS

▪ In optical microscopes:

▪ Spherical aberration can be minimized by using an aperture or diaphragm to restrict the light path to the central part of the objective only

▪ Lens design such as using Aspheric lens can minimize spherical aberration, but not generally employed

▪ In electron microscopes

▪ In Transmission Electron Microscopes (TEM), due to the use of Electron lens, spherical aberration exist.

▪ By using Apertures which are annular metallic plates, the effect is minimized

▪ Intense research is carrying out in US to design and develop Transmission Electron Aberration-Corrected Microscope (TEAM), with minimal aberrations

An aspheric biconvex lens

COMA OR COMATIC ABERRATION

▪ A lens aberration occurring in the part of the image field that is some distance from the principal axis (off-axis) of the system

▪ It results from different magnification in the various lens zones

▪ Most severe when the microscope is out of proper alignment

Extra-axial object points appear as short, comet-like images (to have a tail (coma) like a comet)

CORRECTIONS

▪ Since it is due to off-axis points, proper alignment of optical axis is to be ensured

▪ The degree of coma aberration is greater for lenses with wider apertures, and can be corrected (partially) by reducing aperture size

▪ By designing lens of different shapes and can be eliminated along with spherical aberration

▪ Lenses in which both spherical aberration and coma are minimized at a single wavelength are called bestform or aplanatic lenses

ASTIGMATISM

▪ Astigmatism is one where rays that propagate in two perpendicular planes have different focal points

▪ As a result, image appears stretched in one direction at one plane of focus and stretched in opposite direction at another plane of focus, thus image lack sharpness

ASTIGMATISM

CORRECTIONS

▪ Astigmatism may arise due to imperfect lens surfaces or misalignments. So using lens with perfect surface and proper alignment reduces astigmatism

▪ ‘Stigmators’ which apply correcting fields are used in electron microscopes for correcting astigmatism

DISTORTION

▪ Distortion takes place when the features located at different distances from the optical axis exhibit different magnification

TYPES OF DISTORTION

▪ Barrel distortion

▪ When magnification decreases with distance from the optical axis and give rise to effect of an image which has been mapped around a barrel

TYPES OF DISTORTION

▪ Pincushion distortion

▪ When magnification increases with distance from the optical axis and corners of squares form elongated points, as in a cushion

CORRECTION

▪ Distortion is difficult to detect when observing specimen without any pattern as well as due to its minor scale

▪ Because the image surface of optimum focus is curved, compensating eyepieces with equal but opposite curvature are used to produce a flat image

▪ In TEM, complex set of quadrapole, hexapole and octapole magnetic lenses are employed to reduce distortion even at high magnification and resolution as high as 50 picometers

CURVATURE OF FIELD

▪ Curvature of Field is an optical aberration in which a flat object normal to the optical axis cannot be brought into focus on a flat image plane

▪ When light is focused through a curved lens, the image plane produced by that lens will be curved

CURVATURE OF FIELD

▪ When the image is projected on a flat surface, either the centre or peripheral part appears to be out of focus or Objects in the center and edges of the field are never in focus simultaneously

CORRECTION

▪ In optical microscopes lenses made of glasses of special formulations are used

▪ Use plano-convex lens (plane in one side spherical on the other)

▪ Using aperture to reduce light passing through edges of lens can minimize the effect with the cost of reduced brightness

CHROMATIC ABERRATION

▪ Chromatic aberration is aberration in which there is a failure of a lens to focus all colors to the same focal point

▪ It occurs because lenses have a different refractive index for different wavelengths of light (the dispersion of the lens)

▪ The refractive index decreases with increasing wavelength – so violet/blue, having smallest wavelength will be refracted more and red with highest wavelength will be refracted less.

▪ Hence violet/blue are focused near to lens and red away from lens

▪ There are two types of chromatic aberration: axial (longitudinal), and transverse (lateral)

CHROMATIC ABERRATION

TYPES

▪ Axial or longitudinal aberration occurs when different wavelengths of light are focused at different distances from the lens

TYPES

▪ Transverse aberration occurs when different wavelengths are focused at different positions in the focal plane

CORRECTION

▪ Use monochromatic lighting

▪ By using achromatic lenses, which are corrected to bring two wavelengths (typically red and blue) into focus in the same plane

Achromatic lenses are composed of two individual lenses made from glasses with different amounts of dispersion.

Typically, one element is a negative (concave) element made out of flint glass having relatively high dispersion, and the other is a positive (convex) element made of crown glass having lower dispersion

CORRECTION

▪ In electron microscopes, due to variation of energies of beam, electrons have different wavelengths.

▪ Electron of lower energy are bent more compared to high energy electrons by objective lens field, thus giving rise to chromatic aberration

▪ To minimize the effect, variation in high voltage source should be minimized.

▪ More the semi-angle of electron beam, more will be the aberration. Hence semi-angle of electron beam should also be reduced

▪ Avoid mechanical vibrations

REFERENCES

▪ ASM Metals Hand Book, 9th edn, Vol 9, Metallography and Microstructures, ASM, Metals Park, (1983)

▪ P.C. Angelo, Materials characterization, Elsevier, 2014

▪ http://en.wikipedia.org/wiki/Optical_aberration

▪ http://en.wikipedia.org/wiki/Transmission_electron_microscope

▪ http://en.wikipedia.org/wiki/Transmission_Electron_Aberration-Corrected_Microscope