Emitted Infrared

Geographers Estes and Golomb used the 8-14 jum spectral band to study the catastrophic oil slicks off Santa Barbara, California. The areas covered by oil are shown by radiance temperatures lower than those of the unpolluted water, ...these slicks dimmish air j water heat exchange and give a cold thermal picture. The precise nature of the emissivity characteristics of oil on water is a fascinating field in itself (Estes and Golomb, 1970, p. 677, italics mine.) 

We can agree, from just the facts themselves, on the paramount importance of emissivity. We will soon have the opportunity to see, still from the concrete facts, that, if emissivity plays the preponderant role during the day, its activity is very attenuated, relatively speaking, during the night. On the other hand, water pollution may easily be detected from the radiance temperatures thus, if for this reason alone, modern remote detection methods deserve to occupy a top position in our research techniques. 

Remembering both the importance of emissivity and of the time when radiations are recorded on board the satellites (around midnight), the basic principles of interpretation are fairly simple (Pouquet, 1968, NASA and 1969, Berlin). 

Of all the materials, it is water that reacts most like a blackbody, absorbing more, retaining longer, and reemitting more. Thus, at midnight, water is warmer than any other body subject to the natural laws of heat exchange with the atmosphere. 

In detail, this elementary principle of interpretation must take into account the materials conducting properties, structure, porosity, density, etc. Also, as we have already seen, the subsurface has neither liquid nor solid characteristics and is 

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Thermal Emission Laws. AH bodies emit infrared radiation by virtue of their temperature. The total amount of radiation is governed by Kirchhoff s law, which states that a body at thermal equiUbrium, ie, at the same temperature as its surroundings, must emit as much radiation as it absorbs at each wavelength. An absolutely blackbody, one that absorbs all radiation striking it, must therefore emit the most radiation possible for a body at a given temperature. The emission of this so-called blackbody is used as the standard against which all emission measurements are compared. The total radiant emittance, M., for a blackbody at temperature Tis given by the Stefan-Boltzmaim law,... 

When samples are heated, they emit infrared radiation with a characteristic spectrum. The IR emission of ceramics, coals, and other complicated solids and thin films can be studied. Also, if conditions make it difficult to use an infrared source... 

In an FTIR spectrometer, a source (usually a resistively heated ceramic rod) emits infrared radiation that is focused onto an interferometer whose main components consist of a beamsplitter, fixed mirror, movable mirror, and detector. The beamsplitter divides the beam into two beams. One beam is reflected off the beamsplitter toward the fixed mirror and is then reflected back through the beamsplitter to the detector. The other beam is transmitted through the beamsplitter toward the movable mirror and is then reflected off of the beamsplitter and to the detector [1],... 

It is scenario 3 that is most consistent with the data depicted in Fig. 1-2. Given that the physical climate system is strongly influenced by gases in the atmosphere that absorb and emit infrared radiation (e.g., H2O, CO2, CH4, etc.), and since the amounts of these species in the air depend to some extent (for some, a great extent) on the functioning of the biosphere, it is logical to view the climate of the Earth as a coupled physical, chemical, and biological entity. 

C02-IR detection systems for example contain a light source emitting infrared light ideally at wavelengths around 4.2 pm (often a simple light bulb), an absorption path (the so called cuvette), a spectral filter and a detector (thermopile). The filter is normally integrated into the detector housing. 

Radiation in the infrared region of the spectrum is obtained from heated ceramic devices such as the Nemst glower or Globar. The Globar is made of silicon carbide and is heated to approximately 800-1500°C to emit black-body radiation in the infrared region of the spectrum. Coils of nichrome wire also emit infrared radiation when electrically heated. 

Fig. 12.21 shows the combustion products of AP-HTPB and RDX-HTPB composite propellants. Large amounts of H2O, HCl, and CO2 are formed when an AP-HTPB propellant composed of a.p(0.85) is burnt. The molecules of H2O, HCl, and CO2 each emit infrared radiation. On the other hand, no COj or C(g) is formed when an RDX-HTPB propellant composed of ri3x(0.85) is burnt. Instead, large amounts of CO, H2, and Nj molecules are formed as its major combustion products. However, no infrared radiation is emitted from H2 or N2 molecules. Though CO molecules are formed at ri3x(0.85), the infrared radiation emitted from these is less than that from H2O or CO2 molecules. 

The greenhouse effect in Earth s atmosphere. Visible light from the sun is absorbed by the ground, which then emits infrared radiation. Carbon dioxide, water vapor, and other greenhouse gases in the atmosphere absorb and reemit heat that would otherwise be radiated from Earth into space. 

In emission spectrometry, the sample is the infrared source. Materials emit infrared radiation by virtue of their temperature. KirchhofF s law states that the amounts of infrared radiation emitted and absorbed by a body in thermal equilibrium must be equal at each wavelength. A blackbody, which is a body having infinite absorptivity, must therefore produce a smooth emission spectrum that has the maximum possible emission intensity of any body at the same temperature. The emissivity, 8, of a sample is the ratio of its emission to that of a blackbody at the same temperature. Infrared-opaque bodies have the same emissivity at all wavelengths so they emit smooth, blackbody-like spectra. On the other hand, any sample dilute or thin enough for transmission spectrometry produces a structured emission spectrum that is analogous to its transmission spectrum because the emissivity is proportional to the absorptivity at each wavelength. The emissivity is calculated from the sample emission spectrum, E, by the relation... 

Thermal emission is based on measuring the fundamental absorption bands of glucose at 10pm, using the body s naturally emitted infrared radiation as the source. The detection equipment is similar to that used for infrared absorption spectroscopy. Malchoff et al.82 reported the evaluation of a prototype that measures the infrared emission from the tympanic membrane. 

Pyroelectric materials respond to changes in the intensity of incident radiation and not to a temporally uniform intensity. Thus humans or animals moving across the field of view of a detector will produce a response as a result of the movement of their warm bodies which emit infrared radiation (a 10 /rm). To obtain a response from stationary objects requires the radiation from them to be periodically interrupted. This is usually achieved by a sector disc rotating in front of the detector and acting as a radiation chopper. 

A Globar is a SiC infrared light source A rod of diameter 5 to 10 mm and length 20 to 50 mm, when heated to between 1000°C and 1700°C, emits infrared radiation with wavelengths between 4 and 15 /tm (overall wave-number range 6000 to 100 cm-1). 

So far, we have been concerned mainly with emission of radiation from electronically excited states. Emission may also arise from vibrational transitions in various reaction systems. The species HO2 has long been postulated as an important chain carrier in combustion reactions, although emission from electronically excited HO2 has yet to be demonstrated unequivocally. However, Tagirov has observed radiation in flames at a frequency of 1305 cm which he ascribes to transitions from vibrationally excited HO2. Investigations of vibrational quenching processes are of great interest, and if the vibrationally excited species emit infrared radiation, then emission spectrometry may be the most satisfactory way of following the reaction. Davidson et describe a shock-tube study of the relaxation of... 

Since the spectrometer will itself be emitting infrared radiation to the detector (at 77 K), determination of the emissivity requires the following measurements ... 

Notice that drops of electrons to the lowest level emit ultraviolet frequencies, and shorter drops of electrons to the third level emit infrared frequencies. 

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