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Fiber-Optically Interfaced Cameras
Princeton Instruments, Inc., offers specialized imaging cameras with fiber optic interfaces. This type of interface provides much higher throughput than even the best lenses, with no vignetting. Cameras can utilize fiber optic tapers for a magnification ratio as high as 5:1.
Fiber Optic Advantages
Fused fiber optics provide very high throughput optical coupling from any flat, light emitting surface to the surface of a CCD array. The efficiency of this form of optical coupling is typically 50-60%. The efficiency of lens coupling, by comparison, is typically 1-10%. Fiber optic coupling is also not restricted to 1:1 imaging. Fused fiber optics are available formed into tapers, with magnification or demagnification ratios up to 5:1. Fiber optic coupling (1:1) has the added advantage that it does not incur the vignetting, distortion, or long range crosstalk of a lens system.

Special Applications

Applications of Fiber Optic Technology
Because of their high optical throughput, fiber optically interfaced cameras are often the design of choice for imaging phosphor screens. Applications that use this type of technology include X-ray imaging, streak cameras, MCP detectors, image intensifiers, and electron microscope scintillators. Fiber-optically interfaced cameras are also used for imaging flat biological samples, including the leaves of luminescently tagged transgenic plants, fluorescent or chemiluminescent electrophoresis gels or blots, and tissue sections. With an appropriate scintillator, they can also be used to image autoradiographic samples. By using a fiber optic interface specifically designed to fit between the bond wires on an integrated circuit or probes of a probe station, this type of camera can also be used for high sensitivity imaging of luminescent semiconductors in operation.

This photograph shows a CCD coupled to a fiber optic taper. These tapers allow magnification ratios (N) as high as 5:1, with throughput of (50/N2)% or more. coupling between intensifier and CCD, the only cooled, slow scan intensified camera on the market to use such an arrangement. The performance of these CCD cameras is covered in great detail in the CCD Cameras section of this catalog, which includes the CCD Selection Chart. Performance characteristics such as dark charge, readout noise, and readout rate appear on these pages. Thermoelectric temperatures shown on these pages are generally not applicable to these cameras, however, for a couple of reasons. First of all, the fiber optics can Princeton Instruments conduct heat to the CCD, limiting the attainable range of temperatures. Secondly, cameras which have open fiber optics are not operated in vacuum. This also increases thermal conductivity. Contact the factory for information on cooling specifications for a specific fiber optic system.

Standard or Intensified Models Available
Princeton Instruments offers a wide variety of CCD cameras with fiber optic coupling. These are based on our standard thermoelectrically cooled CCD cameras. For gated operation or maximum sensitivity, we also provide intensified cameras with fiber optic input windows. In addition to the fiber optic input, these cameras use fiber optic 84

X-Ray Cameras
For X-ray detection, Princeton Instruments provides cameras with either 1:1 or tapered fibers. The fibers can be directly coated with Tl:CsI, a structured crystalline scintillator with high efficiency and resoluTel: 609-587-9797


tion. Alternatively, the front of the fiber optics can be left uncoated, for customer provided or interchangeable phosphor screens. Cameras are available with the fiber optics exposed, or with the entire fiber optic window and scintillator inside a vacuum chamber with a Beryllium window. The former allows easy access to the fiber optic input side, appropriate for a system using several different scintillators. The vacuum arrangement provides better cooling and eliminates any possibility of condensation when operating at cryogenic temperatures.

Electron Microscopy
For electron microscope imaging (TEM), Princeton Instruments provides cameras with either 1:1 or tapered fiber optics. A variety of electron scintillators can be provided on the fiber optic input surface, including phosphors, YAG:Ce single crystal scintillators, or scintillating fiber optic windows. These scintillators can also be provided on a separate removable fiber optic window, optically coupled to the fiber optic window of the CCD.

Many, but not all, CCDs are available with fiber optic windows, as shown on the array at right. Tapers or other fiber optics are mounted directly to the window of the CCD. For other applications, the source of the light will need to be turned off or shuttered off during CCD readout. In electronic systems such as streak cameras or electron microscopes, this can be done electronically using a signal from the PI camera controller. In X-ray systems, the X-rays can either be turned off or shuttered between the X-ray source and the scintillator. in fiber spacing, and fiber damage. Because the bundles are hexagonally shaped, this can lead to a hexagonal pattern in the flat field image. This pattern is called "chicken wire". It can be very effectively removed from digital images by applying a flat field correction to the data, an automatic feature of Princeton Instruments WinView software package.

Special Applications

Other Applications
For streak cameras, MCP detectors, image intensifiers, and other systems with fiber optic output, Princeton Instruments provides cameras with mechanical interfacing for direct fiber window to fiber window contact. For optimum coupling, we recommend use of an optical index matching material at this interface. For imaging luminescent biological samples, no scintillator is required. The fiber optic window is simply pressed against the sample. For optically thin samples, a mirror on the other side of the sample can increase signal, at some loss in resolution. Princeton Instruments can provide fiber optically interfaced CCD cameras based on many different CCD arrays. Contact your Princeton Instruments representative for a list of the currently supported arrays.

Exposed Fiber Optics and Condensation
All Princeton Instruments fiber optically interfaced cameras use thermoelectric cooling to enhance the performance of the CCD. Because the fiber optics are in direct contact with the CCD, they will be cooled to a lesser degree. On some systems the front of the fiber window may be exposed to the ambient air. If the air proves to be sufficiently humid, condensation can result. Since many scintillator materials are hygroscopic, condensation of this type will cause permanent damage. Thus cameras with exposed fiber optics should generally be operated in a dry atmosphere, or with gentle heating applied to the air near the exposed front window. Cameras with the front surface in a vacuum eliminate this problem entirely.

Distortions and Cosmetic Defects of Fiber Optic Tapers
Fiber optic tapers are produced by stretching near molten fused fiber optic bundles. The stretching that occurs in this process can introduce both global distortion and local shearing (spatial discontinuities). Individual fibers can also break, resulting in point defects in the image. Secondly, the fibers at the outside of the bundle will typically lose some light transmission ability, leading to a nonuniformity which appears similar to the vignetting of a lens system. While these problems can in principle be substantially mitigated in software, it is best to begin with the highest quality fiber optic material, to minimize these problems. Princeton Instruments works with every major supplier of fused fiber optic tapers and provides only the highest quality tapers available. Taper quality depends on diameter, since smaller formats can be made with lower distortion, and a higher magnification ratio. Contact your Princeton Instruments representative for up-to-date information on fiber optic specifications. 85

Shuttering
Because there is no place in these cameras between the light source and the CCD for a mechanical shutter, smearing can occur if the subject is still emitting light when the CCD is read out. For long exposure times and fast CCD readouts, this smearing may be negligible. Smearing can be further reduced by use of a frame transfer camera, as described in a separate section of this catalog. Fax: 609-587-1970

Uniformity of Fiber Optic Windows
Fiber optic windows are produced by assembling and fusing increasingly large bundles of fibers. At the interface between these bundles, there can be discontinuities Princeton Instruments