b. Spatial coherancy & source size Spetial coherancy is related to the size of the source. Source size governs spatial coherancy and maller source sizes give better coherancy. we define the distance dc, the effective source size for coherent illumination to be, D c << / 2  Where, = electron wavelength  = angle subtended by the source at the specimen Small beams are more spatially coherent than large beams. The more coherent the beam is, the better the quality of the high resolution phase contrast images, the sharper the diffraction patterns, better the diffraction contrasts in images of crystalline specimens. c. Stability The thermionic sources are generally very stable when compared to field emission sources.It is important that the electron current coming from the source should be stable, with stable high- voltage supply, otherwise, the screen intensity will vary, making it difficult to take correct exposed images & making microanalysis impossible in many cases. In summary, A smaller source size gives higher brightness & better spatial coherancy but less stability. 2.3 ELECTRONIC GUNS a.Thermionic gun Schematic diagram of a thermionic electron gun.A high voltage is placed between the filament and the anode,modified by a potential on the wehnelt which acts to focus the electrons into the crossover. The three major parts of a thermionic gun,from top to bottom: Cathode,the wehnelt cylinder and anode shown separately

Electrons in the thermionic gun are accelerated across a potential difference of the order 100KV between the cathode (at high negetive potential) and anode (at ground potential). Beam shaping & control of emission is a affected by properties of the shield (gun cop;wehnelt) and acceleration by the anode. In a biased gun the shield is maintained at a potential between 100 – 500 KV negative relative to the filament. This negative voltage serves to repel some emitted electrons back to the filament, and reduces total emission and brightness. The negetive potential of the shield with respect to the filament gives rise to a strong electrostatic field around the shield which acts as an electrostatic lens focusing the true source to form an image of the source below the anode. The gun cross over is used as the actual source of electrons for the electron microscope. b. Field emission gun The FEG is much simpler than the thermionic guns, Anode 1: provides the extraction voltage to pull electrons out of the tip. Anode 2: accelerates the electrons to 100KV or more. The electrons are accelerated through the applied potential by the second anode.The combined fields of the anodes act like a more refined electrostatic lens to produce a crossover. This lens controös the effective source size and position, but it isn‘t very flexible. Incorporating a magnetic lens into the gun gives a more controllable beam and larger brightness. Drawbacks of FEG: The source size is very small needs UHV conditions. UHV technology is expensive & requires much higher level of operator competence. Fig:Electron paths from a FES showing how a fine crossover is formed by two anodes acting as an electrostatic lens.sometimes an extra (gun)lens is added below the second anode

2.4 CONDENSER SYSTEM Operation of a two-lens condenser system for illuminating the specimen a.under focus and c. Over focus b. in-focus operation with condenser lens c2 d. Additional use of condenser lens c1 to demagnify the crossover,the demagnified image then being focused on the specimen with condenser lens C2 The function of the condenser lens system is to focus the electron beam emerging from the electron gun onto the specimen to permit optimal illuminating conditions for visualizing and recording the image A double condenser system adds considerable flexibility to the illuminating system by allowing a wider range of intensities with a given gun adjustment and making it possible to reduce the area of the object which is irradiated. A strong first lens (c1: short focal length) is used to produce a demagnified image (nearly 1  m diameter) of the electron source and a weaker second lens (c2: long focal length) projects this demagnified image onto the specimen plane producing a slight magnification so the final focused beam size is about 2-3  m. The focal length of c2 can be varied to spread the beam over a larger area of the specimen for example to record images at low magnification(<10000X). The fig. Shows the important modes of operation of a two lens condenser system. The beam divergence  0 i,e the divergence of radiation incident onto a specimen point is given by the diameter of the c 2 aperture. The image intensity is proportional to the current density J 0, which is directly related to  0 through brightness  (J 0 =    0 2 ). The current density and beam divergence are maximum when condenser 2 is focused. The diameter of the illuminating object region can be enlarged by the means of over focussing & under focussing of the condenser lens. The beam divergence is now determined by the second image of the crossover. Since this image is much smaller than the c2 aperture, the beam divergence is strongly decreased. At the same time the current density and thus the image intensity are reduced. Small beam divergences of < 1 mrad are required to obtain sharp diffraction spots. The minimum beam diameter which can be generated by the two condenser lenses is order of 1  m, but for analytical investigations for example X-ray microanalysis, it would be useful to have probes with a diameter << 0.1  m. this can be obtained by using condenser objective lenses.

2.5 OBJECTIVE LENS & INTERMEDIATE LENSES The optical enlarging system of an electron microscope consists of an objective lens followed by one or more projector lenses. The objective lens determines resolution & contrast in the image, and all subsequent lenses bring the final image to a convenient magnification for observation and recording. The objectve lens is more critical lens since it determines the resolving power of the instrument and performs the first stage of image forming. The optic of objective lens is shown in figure. A diffraction pattern is formed in the back focal plane of the lens. The first projector lens ( often called the intermediate or diffraction lens ) can usually be switched between two settings. (i) In the image mode it is focused on the image plane of the objective. The magnification of the final image on the microscope screen is then controlled by the strength of the remaining projector lenses. (ii) In the diffraction mode the intermediate lens is focused on the back focal plane of the objective and the diffraction pattern is projected on to the viewing screen. Requirements in the construction of the objective lens 1. The specimen must be situated close to the front focal plane of the objective to provide an initial magnification of 50 – 100 X. 2. Focal length should be as small as practical to insure minimum chromatic and spherical aberration. Since these decrease as the focal length decreases. The specimen has to be placed inside the lens field to obtain the necessary short focal length and this poses a problem of introducing a specimen in to the confined space of the lens. 3. There must be adequate clearance for insertion of specimen, aperture and anticontaminator. 4. There must be provision for inserting electrical or magnetic devices (stigmators) to correct for minute asymmetries in the lens field. Fig:The objective lens and first intermediate lens.the objective lens(OL)is focused on the specimen and forms an intermediate image as shown in (a).In imaging modes the intermediate lens(IL)magnifies this image further and passes it to the projector for display. In order to make the diffraction pattern visible,the intermediate lens is refocused on the back focal plane of the objective lens(BFP)and the diffraction pattern is passed to the projector system.