Image Devices

							Digital Image Capture:

1. Direct Signal Capture (framegrabber)
2. Charge Coupled Device (CCD)
3. Transmitted Light Scanner
4. Image Plates (TEM only)
                                       A framegrabber is a piece
                                       of hardware designed to
                                       capture an image from a
                                       camera. It is an interface
                                       board which digitizes the
                                       analog signal sent by a
                                       camera. The camera
                                       outputs data as an analog
video signal and the framegrabber captures the video data and
sends it to the computer's memory. The framegrabber is also
responsible for subsampling the data, which involves changing
the incoming datastream into a form which is better for viewing or
processing. Typical operations a framegrabber can perform
include the use of look-up tables to convert the image to a
standard image format, gain-control and region-of interest
selection.
In the case of an SEM or STEM the analog output
of the detector (SE, BS, PMT, etc.) is sent directly
to the framegrabber without the use of a camera.
The spatial resolution being defined by the number
of scanned points in the raster pattern.
For the owners of older SEMs there are
products that can take the analog output and
create a digital image on any PC
For microscopes that do not create an image via a
raster pattern a different approach is needed. The
most common solution is to use a CCD camera
which operates on the principal of capturing the
entire image at once, similar to the way film does.
In fact the design of most CCDs is more
similar to that of the human retina which is
an array of light sensing neurons.
At the heart of every digital camera is a Charge
Coupled Device (CCD) typically about a square
centimeter in size.
 Willard Boyle          George E. Smith

 Awarded the Noble Prize in Physics for
the invention of the CCD. October 6, 2009
The CCD is
comprised of
many individual
signal capture
units, each of
which
corresponds to a
single pixel in the
final digital
image.
Light in the form of incoming
photons falls onto the surface
of the CCD chip. This
generates free electrons in the
silicon of the CCD in
proportion to the number of
photons striking it. These
electrons collect in little
packets created by the
geometry of the silicon and
surrounding electrical circuitry
laid out in a two dimensional
grid on the chip. Typical CCD
chips have from one to five
million such packets of
charge.
At the heart of the CCD is these metal oxide
semiconductors (MOS) which allow the
charge of electrons to build up in wells in the
silicon base.
In a TEM the CCD
camera can be
mounted in any
number of positions.
Either above the
viewing screen (35mm
port), on-axis below
the screen or off-axis.
The long depth of
focus in a TEM makes
this possible.
The scintillator converts the electron image into a photon
image. Fiber optics transfer this image to the CCD where the
photons generate electrical charge. During the readout, the
charge is shifted line by line to the serial register from where
it is transferred pixel by pixel to the output node and exits to
the analog-to-digital converter. The main features of slow
scan CCD cameras are high sensitivity, low noise, a high
dynamic range and excellent linearity.
In better TEM cameras the fiber optic coupling
(FO) is shaped to exactly fit the shape of the
CCD thus making for a more efficient transfer
of photons to the CCD.
The CCD operates on the principle of charge coupling. The
packets of charged electrons can be moved one row at a time by
varying the voltage of adjacent rows thereby creating a potential
well which couples two rows and causes the charge to move over.
Buckets on conveyor belts depict how each bucket contains a
different amount of light (shown as rainwater) and how these buckets
are shifted in an orderly fashion first to a collecting row, and then to a
final measuring device at the front. In this way the quantity of water
(or electrons representing light) in each bucket (or packet) are
counted. In a typical CCD this happens very fast: about 30 times per
second for every one of the million or so "buckets" on the CCD.
To increase the efficiency of reading the output of
the CCD array there are several different designs.
One type transfers the entire frame into an empty
storage array, while others alternate empty rows
with collecting rows.
                             One can also
                             increase the light
                             capturing
                             capability of a
                             CCD array by a
                             process known as
                             “binning” in
                             which the output
two or more pixels are combined. This
improves the signal gathering capability (and
reduces noise) but at a sacrifice of spatial
resolution.
CCDs are nearly ideal detectors

    High Quantum Efficiency (QE)
The QE of a detector is the ratio of the number of photons
detected to the number of photons incident. In the visible region
of the spectrum (400 - 700 nm) our eye has a QE of less than 1
%. Photographic film is slightly better with a QE of 5 - 20 %
(typically at the low end of this range.) CCDs, on the other hand,
usually have QEs of 50 - 90 %. (Quantum efficiency is, of
course, a highly wavelength dependent property.) Because of
their high QE, CCDs can achieve the same signal-to-noise as
film with exposure times approximately a factor of 10 shorter.
CCDs are nearly ideal detectors

    Large dynamic range
The dynamic range of a detector refers to its ability to
simultaneously detect objects of both low and high brightness.
Photographic film has a dynamic range of only 100, whereas
CCDs are sensitive to objects differing by a factor of 10,000 in
brightness.
    High linearity
In a perfectly linear detector the digital signal per photon is
constant independent of the number of photons detected.
Photographic film is highly non-linear because too few photons
result in no detector response, while too many cause saturation.
(CCDs will also saturate, but because of their high dynamic range
they are linear over a much larger region.) Linearity is required if
images are to be combined.
  CCDs are nearly ideal detectors

Uniform response
   One obvious disadvantage of using photographic film is that the
   detector (the film) is different every time. Because a CCD
   detector is permanent, its response is easily characterized. Pixels
   to pixel differences can be calibrated and removed using a "flat
   field" frame.
Low noise
   By cooling CCDs to liquid nitrogen temperatures (77 K) it is
   possible to eliminate most of the thermal noise. Also, because
   CCDs are linear (and digital), many exposures may be combined
   to reduce Poisson noise.
CCDs can be used to collect an image in one
of three ways, either one pixel at a time, one
row at a time, or as an entire area at once.
An original document is
placed on the surface of
the scanner and
illuminated by means of
a fluorescent tube. A
reflector system projects
the light reflected from
the document onto the
CCD chip which
scans each line dot by
dot.
Imaging plates are yet another technology
whereby images from an electron microscope
can be digitally recorded.
The Imaging Plate is a flexible electron
detector, where an active layer of tiny
crystals locally store high energetic
radiation. The storage crystals are
made from doted barium fluoro-
bromide embedded in some blue
colored resin. The electron irradiation
excites the crystals in their
luminescence center to a semi-stable
state. The image information, formed
by this excitation is stable for many
hours and decays within days.
By an illumination with red
laser light, the crystals are
excited again and stimulated
to release the stored
information as blue
luminescence signal. The
amount of blue light released
depends on the first
excitation with electrons and
is a direct measure of the
electron dose. As this is a
physical process it is fully
reversible without
degradation, so the Imaging
Plate can be reused many
times.
The Imaging Plate
reader micron reads
with a pixel size of
15µm up to 50µm, and
can use the full area of
80x90mm resulting in
images with up to
6000x5000 Pixel.
Compared with CCD
cameras that have
pixel sizes in the same
range, the detected
area of the Imaging
Plate is about the
tenfold of the CCD.
Up to 20 plates at a time can be read by the
plate reader and reused. The reading process
can take several hours.
Despite the advantages Image Plate technology
has not been very successful due to the cost
($100K+) and inconvenience (similar to film).
Digital Image Files:

There are currently a large number of formats
in use to store the data in a digital image.

Likewise there are many different software
programs available that will read the
information in a digital image file and
reconstruct it as a displayed image on a
computer screen.
Aspect Ratio:
While many pixels
represent a square
area they sometimes
do not and in order
to faithfully
reconstruct the
image it is important to know the aspect ratio
of the pixel (1:1, 1:1.3, etc)
If aspect ratio used to collect the image is
different from the one used to reconstruct
the image a distorted image will result.
Digital Image Files:
Digital Image Files:

Uncompressed file formats (e.g. TIFF, BMP,
GIF) store the digital data as a complete
matrix, thus the reconstruction of the image
from one of these formats is a faithful
rendering of the original image.
Digital Image Files:




Various data compression methods can be used
to reduce the number of bits needed to
reconstruct the image.
Run-length coding can
be especially useful if
there are large
stretches in either the
horizontal or vertical
columns in which the
values of the pixels
remain unchanged and
can result in a large
reduction in image
size.
Digital Image Files:              JPEG (Joint
                                  Photographic
                                  Experts Group) is
                                  an image format
                                  that uses various
                                  forms of
                                  compression to
                                  reduce the size of
 the file. It is known as a “lossy” format because
 one loses some spatial resolution depending on
 the level of compression.
The file size savings can be large for B&W
images and very large for color images.




.TIF 150 Kbytes           .JPG 34 KBytes
ALWAYS save the
original in the
uncompressed
format. Once the
data is lost through
compression it can
never be recovered.
Mass image storage
of images is no
longer a major
problem.