Digital Image Capture: • Direct Signal Capture (framegrabber) • Charge Coupled Device (CCD) • Transmitted Light Scanner • 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.
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: 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
ALWAYSsave 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.