Thermal detectors properties

IR spectrometers have the same components as UY/visible, except the materials need to be specially selected for their transmission properties in the IR (e.g., NaCl prisms for the monochromators). The radiation source is simply an inert substance heated to about 1500 °C (e.g., the Nernst glower, which uses a cylinder composed of rare earth oxides). Detection is usually by a thermal detector, such as a simple thermocouple, or some similar device. Two-beam system instruments often work on the null principle, in which the power of the reference beam is mechanically attenuated by the gradual insertion of a wedge-shaped absorber inserted into the beam, until it matches the power in the sample beam. In a simple ( flatbed ) system with a chart recorder, the movement of the mechanical attenuator is directly linked to the chart recorder. The output spectrum is essentially a record of the degree of... 

At the outset, one must understand certain principles of GC to assess if it is a proper analytical tool for the purpose. If so, how to achieve the best separation and identification of component mixtures in the sample with reasonable precision, accuracy, and speed And what kind of detector and column should be selected for the purpose It is, therefore, important to examine the type of compounds that are to be analyzed and certain physical and chemical properties of these compounds. Information regarding the structure and the functional groups, elemental composition, the polarity in the molecule, its molecular weight, boiling point, and thermal stability are very helpful for achieving the best analysis. After we know these properties, it is very simple to perform the GC analysis of component mixtures. To achieve this, just use an appropriate column and a proper detector. Properties of columns and detectors are highlighted below in the following sections. 

The hybrid circuit 10 comprises a buffer structure 16 which is comprised of a material which accommodates the difference in thermal expansion coefficients of the HgCdTe detector array 12 and the silicon read-out chip 14. The buffer layer is made of sapphire which also has good thermal conductivity properties. The buffer structure has laser drilled vias 18 which are formed in registration with unit cells of the detector array and the read-out circuit. Each of the vias is provided with indium bumps 20 at opposing ends thereof. The buffer structure is interposed between the detector array and the read-out chip. Cold weld indium bump technology is employed to couple the bumps 20 to the buffer structure. The buffer structure is further... 

IR radiation is emitted from the electrically modulated light source. The analytically relevant spectral range is transmitted through an interference filter, the sample chamber, and the membrane. This radiation is focused on a thermal detector (Dl), pyroelectrical or thermopile. The reflected radiation from the filter is used as a reference (D2). A comparison of the ATR-, the fiber-, and the transmission-method. Secs. 6.5.2.1, 6.5.4.2, and 6.5.4.4, shows that the ATR method is most versatile for all applications and that the transmission method allows the lowest limit of detection for gases (Hadziladzaru, 1994). The properties of the ATR method by employing wavelength selection with tunable interference filters has been studied by Lebioda (1994). 

The detector converts infrared radiation into an electrical signal. The two main classes of detectors are thermal and quantum detectors. The heating caused by impinging infrared radiation changes some physical properties of the thermal detector itself. In quantum detectors, the quantum nature of infrared radiation changes the detector s electrical properties. 

The performance of the radiation detectors depends on their intrinsic properties, temperature and external conditions of use. They can be compared by using a factor of merit D, known as the detectivity, equal to the inverse of the NEP for a detector with unit area used with an electrical band-width A/ of 1 Hz and expressed in cm Hz1/2 W-1. When a value of D is indicated for a thermal detector, it is considered to be independent of the radiation frequency and the time modulation frequency is assumed to be adapted to the intrinsic time constant t, of the detector. For a photoconductive detector, D peaks at a radiation frequency very close to the band gap for an intrinsic detector or to the ionization energy of the relevant centre for an extrinsic detector and decreases steadily at lower energies. 

Newton used a liquid in glass-thermometer to study heat radiation. Rumford and Leslie used a difierential gas thermometer. Herschel reverted to the liquid thermometer, but this was soon replaced by the thermopile (Melloni [3.4]). Some time later (Langley [3.5]) the first bolometers were used. More recently the use of the gas thermometer, in the shape of the Golay [3.6] and Luft cells has been reintroduced and is now widely used in spectrometers. Another type of thermal detector now widely used is that utilizing the pyroelectric effect. In addition to these, several other detection processes have been suggested, including thermal expansion and changed dielectric properties with temperature. 

Photovoltaic and photoconductive effects result from direct conversion of incident photons into conducting electrons within a material. The two effects differ in the method of sensing the photoexcited electrons electrically. Detectors based on these effects are called photon detectors, because they convert photons directly into conducting electrons no intermediate process is involved, such as the heating of the material by absorption of photons in a thermal detector which causes a change of a measurable electrical property. 

ASTM F 1769-97, Standard Test Method for Measurement qfDiffusivity, Solubility, and Permeability of Organic Vapor Barriers Using a Flame Ionization Detector (Philadelphia, 1997) ASTM C177-93, Steady-State Thermal Transmission Properties by Means of the Guarded Hot Plate (Philadelphia, 1993)... 

The second limitation is connected with spectral selectivity. Thermal detectors receive radiation in the whole electromagnetic spectmm. They are limited only by the optical properties of the material at the device surface. Their spectral characteristics may be modified e.g., by using a bandpass filter or by properly choosing the material at the device surface. 

All radiometric devices must convert infrared energy into electrical signals. The fundamental properties of infrared converters, commonly called detectors, are analyzed in Section 5.10. In Section 5.11 the operating principles, noise limitations, and several temperature to voltage conversion mechanisms of thermal detectors are treated. Properties and noise characteristics of quantum detectors are the subject of Section 5.12. In many cases radiometric instruments must be calibrated in intensity and wavenumber. For best results calibration techniques are part of the instrument design. Several calibration methods are treated and their merits discussed in Section 5.13. Finally, Section 5.14 deals with considerations encountered in the... 

The other class of detectors is based on the effect infrared photons exert directly on electrons in semiconducting materials such detectors are called photon or quantum detectors, a somewhat unfortunate name because thermal detectors absorb photons or quanta as well. The energy of an absorbed infrared photon may not be high enough to cause emission of an electron by the photoelectric effect, but it is sometimes sufficient to lift an electron from a valence band into a conduction band, thereby altering the macroscopic properties of the material. The change in the electrical resistance (in photoconductors) or in the electrical potential (in photovoltaic elements) may then be sensed electrically. 

For room temperature operation, a most attractive thermal detector is the pyroelectric element. It is a small capacitor with a dielectric material that possesses a temperature sensitive dipole moment. So far, the most successful dielectric is triglycine phosphate (TGS), particularly if doped with L-alanine. Its Curie point is at 49 °C and, consequently, it must be operated below that temperature. (Above the Curie point, these dielectrics lose their pyroelectric properties.) Other suitable materials include lithium tantalate and strontium barium niobate. The voltage across a capacitor of charge Q is... 

Unlike thermal detectors, which sense the power of the absorbed radiation, photon detectors respond to the number of photons arriving per unit time. Photon as well as thermal detectors are incoherent transducers, which means that the detection process is independent of the wave properties of the incident radiation field. Incoherent detectors produce an electrical signal proportional to the intensity of the radiation. In contrast, coherent detectors, such as the nonlinear elements in heterodyne receivers discussed in Section 5.9, register the amplitude and phase of the electric field associated with the absorbed radiation. Due to the simultaneous measurement of amplitude and phase, coherent detection is subject to a fundamental noise limit that has its origin in the quantum mechanical uncertainty principle. Incoherent detectors are free of this particular limit. However, as we shall see, they are subject to othernoise sources. 

Noise arises in semiconductor detectors from several mechanisms. Johnson noise is found in all resistive elements. It has already been discussed in coimection with thermal detectors [see Subsection 5.1 l.b and Eq. (5.11.20)]. If the load resistance in the circuit is larger than the detector resistance, the Johnson noise of the detector element dominates because load and detector act electrically in parallel as far as the noise properties are concerned. 

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