Blackbody emission spectrum

Figure 22. Schematic overlay of the most intense IR absorptions for neutral acetone and acetone-dg with a 300-K blackbody emission spectrum. The intensity axis refers to the blackbody radiation only.

The blackbody emission spectrum at right changes with temperature, as shown in the graph. Near 300 K, maximum emission occurs at infrared wavelengths. The outer region of the sun behaves like a blackbody with a temperature near 5 800 K, emitting mainly visible light. 

Figure 17.6. (a) Emission spectra of a 3-mm-thick polycarbonate sheet made by the TIRES technique and by uniformly heating the sample. A blackbody emission spectrum is shown below for comparison, (fo) Emittance spectra of polycarbonate derived from the upper panel spectra compared to an absorption spectrum of polycarbonate recorded by photoacoustic spectrometry. (Reproduced from [7], by permission of the American Chemical Society copyright 1990.)... 

One of the problems which must be solved for quantitative measurements by emission is the need for a blackbody source at the temperature of measurement. And a variety of blackbody references have been used including a V-shaped cavity of graphite 164), a metal plate covered with a flat black paint1S6 160) and a cone of black paper l53). However, none of these methods of producing a blackbody reference spectrum are adequate. In most cases the efficiency of the reference has not been established. The most recent recommendation 1S0) is an aluminium cup painted with an Epley-Parsons solar black lacquer which has an emittance of greater than 98% over the infrared spectral range. 

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... 

Fig. 12.23 (a) Photograph showing light emission of carbon black upon 785-nm laser excitation in an argon atmosphere. The corresponding emission spectrum was recorded using external UV-VIS-NIR spectrometer. The lines represent the calculated blackbody emission curves of 50-nm carbon black particles at different temperatures... 

The spectral composition of the emitted light was recorded using an external UV-NIR spectrometer. The temperature is determined by comparing the corrected experimental data [59, 116] with the calculated emission spectrum [59, 117, 118]. The blackbody spectrum of the nanocrystalline graphite sample was calculated for three different temperatures. Based on the maxima of the emission curve, the local sample temperature was 2,600°C (Fig. 12.23b). 

The Stefan-Boltzmann law in Eq. 12-3 gives the total blackbody emissive power f. i, which is the sum of the radiation emitted over all wavelengths. Sometimes we need to know the spectral blackbody emissive power, which is the amount of radiation energy emitted by a blackbody at a thermodynamic temperature T per unit time, per unit surface area, and per unit wavelength about the wavelength X. For example, we are more interested in the amount of radiation an incandescent lighthulb emits in the visible wavelength spectrum than we are in the total amount emitted. 

Most of what is known about atomic (and molecular) structure and mechanics has been deduced from spectroscopy. Fig. 1.7 shows two different types of spectra. A continuous spectrum can be produced by an incandescent solid or gas at high pressure. Blackbody radiation, for example, gives a coiilinuum. An emission spectrum can be produced by a gas at low pressure excited In heat or by collisions with electrons. An absorption spectrum results when light Irom a continuous source passes through a cooler gas. consisting of a scries ol daik lines characteristic of... 

A blackbody is one that absorbs and emits all frequencies. Experimentally, we accomplish this by the following process. As you heat a body (say, a stove burner), you will observe that the perceived color of the object will make a transition from red to yellow-orange. It could turn white, and even blue, as the, temperature increases. The color appears to change because the maximum frequency of the observed light moves to higher frequencies (lower wavelength) as the blackbody is heated. We can record the output, the emission spectrum, of the blackbody and the result is shown in Fig. 2.5 for two different temperatures. 

Transient infrared spectroscopy (TIRS) is a mid-infrared technique [82] that has been developed to obtain spectra of moving solids and viscous liquids. TIRS spectra are obtained from the generation of a thin, short-lived temperature differential that is introduced by means of either a hot or cold jet of gas. When a hot jet is used, an emission spectrum is obtained from the thin, heated surface layer. This technique is known as transient infrared emission spectroscopy (TIRES). When a cold jet is used, the blackbody-like thermal emission from the bulk of the sample is selectively absorbed as it passes through the thin, cooled surface layer. The result is a transmission spectrum convoluted with the observed thermal spectroscopy. This method is known as transient infrared transmission spectroscopy (TIRTS). TIRS is ideally suited for online analysis because it is a single-ended technique that requires no sample preparation. This technique has been applied to the lignin analysis of wood chips [83]. 

For many practical applications, the blackbody emission is needed within finite, relatively narrow wavelength intervals, rather than the entire spectrum. To calculate the blackbody energy between wavelengths A,] and X2, we write... 

Another potential application for LEDs is in illumination. The requirements for devices that serve as illumination sources are somewhat different than the monochromatic OLEDs described above. OLEDs targeted for RGB displays have to give electroluminescent spectra with a relatively narrow line shape centered on the peak wavelength. On the other hand, an illumination source is meant to approximate the blackbody solar spectrum and needs to have a broad line shape with roughly equal intensity across the entire visible spectrum. Therefore, in order to attain complete coverage across the visible spectrum, an OLED used for illumination purposes typically employs multiple emitters are that are either co-deposited into a single emissive layer or distributed into different layers or regions of the device. A number of the different device architectures have been reported to achieve efficient white EL and are discussed below. 

Knowing the wavenumber Vnja, which corresponds to the maximum of the distribution of blackbody emission at temperature T, is important for estimating the wavenumber region over which an emission spectrum can be observed. is given by the following formula which can be derived using dp(v, T)/dv = 0. 

Figure 3 The infrared emission spectrum measured using an FT-IR spectrometer from an 18 torr CH4/O2 flame and the blackbody radiation (1173 K) emitted over the same region. The infrared emission spectrum has not been corrected for the responsivity of the detector.

Blackbody radiator Object that absorbs or emits radiation with 100% efficiency at all wavelengths and whose emission spectrum follows the laws derived by Max Planck and others. 

Fig. 6.5.4 Voyager 1 IRIS emission spectrum of the Pele region of lo and three blackbody spectra corresponding to different temperatures and filling different fractions of the field of view. The sum of the three blackbody spectra matches the measured spectrum well (Pearl Sinton, 1982). The feature near 750 cm is an artifact.

Table IB tabulates the average transmissivity for these same polymers and radiation sources. A comparison of average reflectivity and transmissivity values between the gas-fired radiant panel and a 1143°K blackbody radiator indicates a general difference of less than five percent in either values for a given polymer. A comparison indicates that the radiant panel emission spectrum is nearly the same as a blackbody source and that for most studies it can be considered to be a blackbody.

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