Viewing Screen/Fluorescent Screen
- Practical Electron Microscopy and Database -
- An Online Book -
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Television (TV) camera for EMs is used to observe and/or record electron microscope (EM) images for computer input, in situ observation, and multi-user observation. The TV observation is performed by a TV camera in combination with a dedicated fluorescent screen. In this case, the EM image is converted to a light image by the fluorescent screen.
In TEM systems, the response of a fluorescent screen with standard thickness decreases with increase of accelerating voltage of incident electrons. For high voltage beam, most of the incident electrons may pass right through the screen and the energy lost per unit path length decreases with increase of beam voltage. Therefore, a thicker screen must be used for high voltage beam. For instance, a thickness increase of about 4 times is needed from 120 keV to 800 keV. On the other hand, the beam laterally spreads more significantly for high voltage beam, blurring the fluorescent image. Attempt had been made to find an acceptable compromise thickness of phosphor without too much loss of resolution. In some cases, for fine focusing, a thinner layer of fluorescent screen (e.g. made of ZnS) should be employed to avoid the backscatter of electrons that occur from the solid substrate.
Note that the magnification and camera length projected on the fluorescent screen are slightly smaller than the ones on the film because the fluorescent screen is located at a higher position than the film.
The fluorescent screen plate in TEM is made of aluminum coated with phosphor powder, while the viewing window is made of lead glass to shield the hard X-rays generated in the TEM. For the same reason, the lead glass is thicker for higher accelerating voltages so that it is more difficult to observe the fine contrast through the screen in a higher voltage TEM. Therefore, we usually use a television (TV) camera that is installed below the camera chamber instead of viewing through the lead glass screen.
As shown in Figure 3247 (a), in most modern TEMs, the electron gun, top lenses, and specimen chamber are maintained at ultra-high vacuum by an ion pump, while the viewing screens and photographic chamber are maintained at a lower vacuum, which is referred to as high vacuum, by either a diffusion pump or a turbomolecular pump. This vacuum level is backed by a mechanical (rotary) pump. However, some TEMs have lower vacuum in the specimen chamber as shown in Figure 3247 (b).
Figure 3247. Vacuum in TEMs: (a) Modern TEMs, and (b) Some TEMs.
Because of the limit of depth of focus and because no lens in between the recording media and TEM viewing screen can be used to compensate for the out-of-focus, the recording film or CCD camera is arranged to locate just several centimeters below the screen. However, the specimen image on the recording media is still out of focus if the image has been brought to exact focus on the TEM screen.
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