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Materials Science on CD-ROM User Guide

Image Fundamentals

Version 2.1

Peter Goodhew, University of Liverpool
Abigail Callanan, MATTER

October 1997

General Comments

This module is one of several devoted to microscopy. It particularly treats those features of imaging systems which are common to all microscopies. No attempt is made to cover particular microscopes or their image contrast mechanisms, diffraction or analysis. These topics are covered in companion modules.

Assumed Pre-knowledge

This is an introductory module which assumes little pre-knowledge. It introduces some of the features which are common to all images, particularly the concept of the pixel.

It would be helpful (but not essential) before embarking on this module, to be familiar with some kind of microscope (light or electron) and to understand the function of a lens. It would be better to have some familiarity with the action of a thin lens, the ray diagrams associated with a convex lens and the concept of the focal length of a lens.

Related MATTER modules which refer to these topics include:

• Introduction to Electrons in Crystals
• Image Fundamentals
• Beam-Specimen Interactions
• Using the Scanning Electron Microscope
• Crystallography

Module Structure

The module contains seven sections;

• Introduction
• Pixels
• Magnification
• Image Recording
• Contrast
• Resolution
• Image Aberrations

Introduction

On this simple page the idea of an object and its (magnified) image is introduced using a familiar object.

Pixels

In this simulation the relationship between the sizes of the object and image pixels can be explored. Using a single object the pixel size at the object can be selected from a list of four values, while the image pixel size can be selected from three. This gives twelve magnifications, and therefore image sizes. The magnifications range from 40 x (where only part of the image can be shown but the pixels are very clearly visible) to 0.02 x (at which the image size is just a dot on the screen). The concept that magnification is simply the ratio of the image and object pixel sizes is introduced, and should be contrasted with the thin lens optics approach used elsewhere, particularly in the module Introduction to Electron Microscopes.

Magnification

The idea that magnification is controlled by pixel sizes is reinforced and this section also emphasises the importance of study at low magnification and introduces the idea of empty magnification. The image on this page is first viewed at high magnification, with the image pixels clearly visible. Only when lower magnifications are selected does the pixel structure disappear and the nature of the object becomes clear.

Image Recording

The important concept animated here is the distinction between serial and parallel collection of images. The term raster is also introduced and defined via its glossary entry.

Contrast

The ideas of signal, noise and natural (intrinsic) contrast are introduced here. The Rose criterion, in the form Smax - S > 5Ö n, is stated and the user can experiment with the appearance of an image at a variety of natural contrast levels.

Resolution

The Rayleigh criterion for resolving power is illustrated, using an animated pair of Airy discs. This needs to be backed up by reference to diffraction through a pinhole or slit, for instance via the glossary entry defining the Airy rings.

Image aberrations

In this section the main image aberrations are described.

Animated ray diagrams, combined with aberrated images, show the origin and effect of spherical and chromatic aberrations.

Astigmatism is treated in terms of two plane sections through a lens and the notional corrections which need to be applied. Finally pincushion and barrel distortion are shown, applied to the same original image.

Bibliography

For further study the following texts are recommended: