Electron microscopy
 
Photographic Plate/Photo-film for Image Recording in EMs
- Practical Electron Microscopy and Database -
- An Online Book -
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This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.
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TEM images had mostly been recorded with photo-films traditionally before the digital age. There have been various sheet films in the market. In image recording, the film is exposed to electrons directly in the camera chamber. The sensitivity of the sheet films for electrons is similar to that for light. The characteristics of the films can be described by characteristic curve, optical density, contrast, latitude and emulsion absorption.

The term of optical density (also called photographic density) refers to the degree of the blackening on the film, which is directly related to the intensity of radiation. When it is illuminated with light, the optical density D of the film is given by,
         D = log10(1/T) ----------------------- [3327a]
         T = It/I0 ----------------------- [3327b]
where,
         T -- The transmittance of the film and refers to the electron stopping effect or opacity.
         I0 -- The intensity of the illuminating light.
         It -- The intensity of transmitted light.
Figure 3327a shows the relationship between optical density and transmittance.

Relationship between optical density and transmittance.

Figure 3327a. Relationship between optical density (D) and transmittance (T).

The relation between optical density D and exposure E is plotted as a curve, called characteristic curve, or H-D curve (after Hurter and Driffield, who first created this type of curves in 1890) The curve contains three parts, called toe, straight line and shoulder as shown in Figure 3327b. The toe is the low exposure region and the shoulder is the high exposure region, while the straight line is used for practical exposure. The characteristic curve depends on both the accelerating voltage and the conditions of film development. The film exposure is usually set to satisfy this useful range of D = 0.5 - 1.0.

characteristic curve of photo films (for 200 keV accelerating voltage)

Figure 3327b. Schematic illustration of characteristic curve (for 200 keV accelerating voltage).

The resolution of photo-films is in the level of ~10 µm, depending on the type of the films used.

In TEM, the photographic plate can cause problems if there is something wrong on it, for instance:
         i) A photographic plate can be stuck in the film mechanism in TEM so that no GIF images are observed.
         ii) The level of GIF camera chamber vacuum can be evaluated by comparing the current camera chamber vacuum reading to the good ones recorded historically. If the vacuum is poor, one step in the examination procedure to locate any leaks is to check if the location of photographic film in the camera chamber causes any leaks.

In addition, the photographic method also has other drawbacks:
         i) It is very time-consuming.
         ii) Factors such as the non-linear response of the photographic film and optical artifacts induce additional complications of data interpretation.

Therefore, CCD cameras have been developed to overcome such problems.

 

 

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