MACO™ Electron Microscope Film
Exposure, speed and contrast considerations
General comments:
Although electron micrographs are made on conventional photographic material
and like with conventional photographic images, a positive print is made
from the negative image, some important differences remain between the
production of negatives using light and that using electrons.
In conventional photography, the latent image on the film is produced by
photons. For a silver halide crystal to become developable, it must usually
be hit by 5 to10 photons.
For electron micrography, the situation is somewhat different. A single
electron suffices to render up to 10 silver halide crystals developable.
This difference is due to the different energies carried by a photon and an
electron. While the typical photon in the visible light range carries an
energy of about 2-3 electron volts (eV), an electron in an electron
microscope carries about 50 to 100 KV, i. e., 25,000 to 35,000 time this
energy. The film, accordingly will behave differently. While image noise,
which is the pendant to grain in conventional photography, is essentially
determined by the size of the silver halide crystals in conventional
photography, it is determined by statistical fluctuations of electron beam
intensity in electron micrography. The actual image signal, i.e. the useful information,
increases as a linear function of exposure. Image noise, on the other hand, increases
as the square root of exposure. This gives rise to some conclusions with regard to the
best possible image quality, or best possible signal-to-noise ratio:
If the required image density and contrast are achieved by prolonging development,
the signal-to-noise ratio is not improved, noise being amplified by the same factor
as the useful signal. However, statistical fluctuations, the cause of the noise
signal, become less significant as the number of electrons increases. Increasing
the number of electrons, or increasing the sampling rate, therefore, results in a
decrease of image noise and an improvement of detail resolution. A further factor
of influence on the choice of exposure is the stability of the sample. Where the
(in-)stability of the sample forbids extended exposure, reducing the device magnification
(magnification of the microscope proper) and achieving the required magnification of the
final print by optically enlarging the negative can be one way of improving image quality.
Reducing the device magnification means that for the same exposure of the sample, more electrons
are available for a unit of negative area. Consider, for example, a final magnification of 80,
achieved, on the one hand, by using a device magnification of 80, and, on the other hand, by
using a device magnification of 20 and optical enlarging of the negative by a factor of 4.
Exposure may be as long as it takes in the case of a stable sample, and in that case, the first
method would lead to optimum results. In the case of an unstable sample, allowing only limited
exposure to electrons, however, a lower device magnification followed by optical enlarging,
would yield better results.
This shows that films used for electron micrography must comply with different requirements than
conventional photographic material. In particular, it must be possible to achieve similar values of
density and contrast by pursuing different approaches (either weak exposure followed by energetic
development or strong exposure followed by reduced development) determined according to the requirements
of the sample.
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Thursday February 09, 2012
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