Thermal imaging

The first half of this chapter focuses on thermal imaging, describing the technique and the strenuous requirements of suitable laser-printable materials. We illustrate the latter with polyaniline composites specifically tailored for high-resolution printing over large areas with moderate throughput. Their use in active matrix backplane circuits for large, mechanically flexible sheets of electronic paper is then described. 

Printing by laser ablation involves transfer of a digital image from a donor film on to a flexible receiver, with the electronics device being built by sequential transfer of separate solid layers. The architecture of these films is illustrated in Fig. 10.2. The donor film comprises a multi-layer structure on a flexible polymeric sub- 

The pyroelectric elements used in the devices described so far are commonly square plates with sides about a millimetre long and thicknesses around 30 /mi. Because entire scenes are focused onto the plates in thermal imaging, they have to be larger, typically squares of side about 1 cm the thicknesses are the same as for the simpler devices. 

The charge on the surface of the plate must be removed before the electron-beam scan is repeated. This will occur automatically if the bias is high enough to ensure that sufficient electrons can reach the plate to neutralise the pyroelectric 

Lateral flow of heat in the pyroelectric detector plate tends to even out the temperature differences, blurring the charge pattern and hence the final image. This process will depend on the thickness h of the plate and the thermal diffusivity A = Xjc of the plate material. Yidicon performance improves as both h and A are reduced in value, and a suitable figure of merit is 

According to Eq. (7.36) PYDF is an attractive candidate for vidicon targets, and it has been used, although it does not approach the performance obtained using TGS and other members of that family. 

As an attractive alternative to the vidicon the array of elements can be connected directly to individual voltage detectors on a silicon integrated circuit (usually referred to as a ROIC read-out integrated circuit ) via solder bumps . The resulting signals can then be electronically processed to yield a picture in the standard way. Fig. 7.13 shows a section through part of such an imaging array. 

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A brief review of the figures of merit (1) for thermal imaging (4) and gas detection is given to show the various trades-off required to image the thermal environment and detect atmospheric contamination. 

Thermal imaging is sensitive to iafrared radiation that detects temperature changes over the surface of a part when heat has been appHed. Thermal diffusion ia a soHd is affected by variatioa ia composition or by the preseace of cracks, voids, delamiaatioas, etc the effects are detected by surface temperature changes. Defects cannot be detected if their depth below the surface is more than two to three times their diameter. Nondestmctive testing has been primarily used for composites and analysis of adhesive bonds or welds. Several studies are documented ia the Hterature (322—327). 

The most commercially important application that takes advantage of the pyroelectric effect ia polycrystalline ceramics is iafrared detection, especially for wavelengths ia excess of 2.5 p.m. AppHcations range from radiometry and surveillance to thermal imaging, and pyroelectric materials work under ambient conditions, unlike photon detectors, which require cooling. 

Isotherm A line in a flow system or on a graph connecting points of equal temperature, or a mathematical or graphical relationship between two variables at constant temperature. Or a display using lines on a drawing to show constant-temperature contour lines, as from thermal imaging with infrared techniques. 

CdS and CdSe are also useful phosphors. CdTe is a semiconductor used as a detector for X-rays and y-rays, ° and mercury cadmium tellurideriri has found widespread (particularly military) use as an ir detector for thermal imaging. 

Fig. 3. Thermal images of combustion plate (a) without and (b) with a gas distributor (Hydrogen and air were introduced to the combustion plate from the top).

A number of alternative sizing methods are available, and these are described in Table 8. The American Association of Pharmaceutical Scientists, Inhalation Focus Group conducted a comprehensive review of available methods, which was published in a series of articles identified in the last column of the table. All of the methods described either have been or are currently employed in the development of aerosol products. However, at this time only the inertial samplers, cascade impactors and impingers appear in compendial standards and in regulatory guidelines [44-46], Other methods such as thermal imaging are also under development and may give complementary size information to the current methods. 

The VIRTIS apparatus (Visible Infrared Thermal Imaging Spectrometer) on board can observe the atmosphere and the cloud layers at various depths (on both the day and the night side of the planet). VIRTIS has also provided data for the first temperature map of the hot Venusian surface. These data have led to the identification of hot spots and thus provided evidence for possible volcanic activity (www.esa.int/specials/venusexpress). 

Mikron Instruments Company Inc., Thermal Imaging Division, 1101 Elevation Street, Suite 3, Hancock Michigan 49939, USA. The address of the web site is www.mikroninst.com. 

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