显微镜学课件Chapter 2 OM-d MJS3 no QAs 英文资料,工程,材料学必备

Camera Lamp House Condenser Eyepieces Aperture
Clusters of Objectives Specimen stage Focus control
Stand
Fig. 2.1 An optical microscope (Nikon Optiphot tion
The resolution of an optical microscope is defined as the shortest distance between two points on a specimen that can still be distinguished by the observer or camera system as separate entities. The resolving power of a microscope is the most important feature of the optical microscopy. As seen from Eq. (1.1) of Chapter 1, the primary factor in determining resolution is the objective numerical aperture and wavelength of light. Correct alignment of the microscope optical system is also of paramount importance to ensure maximum resolution. The numerical aperture of a microscope objective is a measure of its ability to gather light and resolve fine specimen detail at a fixed object distance. All modern microscope objectives have the numerical aperture value inscribed on the lens barrel (see Fig. 2.3), which allows determination of the smallest specimen detail resolvable by the objective and an approximate indication of the depth of field (See section 2.5). The numerical aperture is given by the simple expression: NA = n • sin (2.1)
• • •
M - Metallographic; D - Dark field; Plan – field curvature corrections. This kind of objective compensates for curvature of field and is excellent for metallurgy observation. 20 – Magnification of the objective; 0.40 – Numerical aperture (NA) which indicates the light acceptance angle. NA is basically a value that describes the quality of a lens (see below for full definition). The highest NA encountered in practice is about 1.4. If there is no indication that the lens is an immersion type, it is called a “dry” lens (refraction index n = 1). An immersion type lens requires the presence of an oil between the lens and the object that is imaged. - Tube length between the eyepiece and the objective. The older objectives may mark a finite length (160mm as standard) which can work only in a fixed tube length. Adding optical accessories into the light path of a fixed tube length microscope increases the effective tube length to a value greater than 160 mm and subsequently introduces spherical aberration into the optical system. For this reason, a lens is placed within the body tube between the objective and the eyepieces (oculars) to produce the parallel optical path between the objective and the tube lens with only a minimal effect on focus and aberration corrections. The magnified image is focused at infinity so that you do not have to focus your eyes up close to see it, simply imagine you are looking into the distance, say at a landscape. Such an objective is indicated as . 0 – No cover glass used in the metallographic microscope (reflected light). Other number represent the thickness of cover glass which is used in transmitted light microscope.
where n represents the refractive index of the medium between the objective front lens and the specimen (for air, n=1), and is the one-half angular aperture of the objective. The wavelength of light is an important factor in the resolution of a microscope. Shorter wavelengths yield higher resolution and visa versa. The greatest resolving power in optical microscopy is realized with near-ultraviolet light, the shortest effective imaging wavelength. Near-ultraviolet light is followed by blue, then green, and finally red light in the ability to resolve specimen detail. Under most circumstances, microscopists use white light generated by a tungsten-halogen bulb to illuminate the specimen. As our eyes are most sensitive to green light, the green region of the visible light spectrum is used for illumination by use of filters.
Chapter 2 Optical Microscopy (OM)
Contents
2.1 Introduction to OM 2.2 Objective lens 2.3 Resolution 2.4 Magnification 2.5 Depth of field 2.6 Confocal laser scanning microscopy 2.7 Sample preparation
2.7.1 Cutting 2.7.2 Mounting 2.7.3 Grinding 2.7.4 Polishing 2.7.5 Etching
2.8 Case studies 2.9 Online resources 2.10 Questions
2.1 Introduction to optical microscopy
Flat-field correction Tube Length
Magnification Numerical Aperture
Focal length
Cover glass thickness range
Fig. 2.3 One objective lens (Olympus 1-LM541 20)
Optical microscopes have been used to magnify the image for one and half centuries. They are still widely used at present because of the low cost and relative ease of application. A variety of models have been built to satisfy different requirements. Every model of optical microscope is slightly different from another but basically contains the following major parts: stand; specimen stage; objective; light source; focus controls; eyepieces; condenser; field aperture and camera. Based on the modes of illumination, optical microscopes can be divided in to two types: transmitted or reflected. If the specimens are transparent, for example sections of organic tissues, optical microscopes with transmitted illumination are used. Since most solid materials are opaque, the vast majority of work is performed in reflected optical microscopes. Fig. 2.1 shows such a reflected microscope. The principal formation of image is shown in Fig. 2.2. The objective lens is the most important part of a microscope as it is the major factor in determining the resolution and choosing the magnification of optical microscope. Thus it is worth to have a close look at it in the next session.