Monochromatic primaries

The construction of a TXRF system, including X-ray source, energy-dispersive detector and pulse-processing electronics, is similar to that of conventional XRF. The geometrical arrangement must also enable total reflection of a monochromatic primary beam. The totally reflected beam interferes with the incident primary beam. This interference causes the formation of standing waves above the surface of a homogeneous sample, as depicted in Fig. 4.1, or within a multiple-layered sample. Part of the primary beam fades away in an evanescent wave field in the bulk or substrate [4.28],... 

Laboratory X-ray sources emit highly divergent radiation. With conventional optics the major part of this radiation is discarded by a slit system and a monochromator. Both components can be replaced by a Gobel mirror [73,74], Figure 4.5 shows its construction and application. As a result a parallel and highly monochromatic primary beam is received. Replacement of conventional incident beam optics (cf. Fig. 2.2) by a Gobel mirror increases the primary beam intensity by a factor of 10-50. 

Suppose that we want to create the color sensation created by monochromatic light at wavelength k = 500 nm. The response of the three cones are given by S, < X) with i e r, g, b). In order to create the same color sensation using the monochromatic primaries Lj with j e 1,2, 3, we have to solve the following set of equations ... 

In other words, the response of the three receptors at the given wavelength k have to be equivalent to the response to a linear combination of the three monochromatic primary colors for some X, Y, and Z. If any one of the parameters X, Y. or Z should become negative, then we cannot create the particular color sensation using only these three primaries. 

Figure 4.2 Weights for monochromatic primaries located at 620 nm, 530 nm, and 400 nm for an observer whose response characteristics are described by the absorbance curves of the retinal curves as measured by Dartnall et al. (1983).

Figure 4.8 If three monochromatic primary colors are chosen for the display device then some colors cannot be created by the display device. The triangular-shaped subregion assumes three monochromatic primary colors at 400 nm, 520 nm, and 700 nm. Colors that are located inside the two gray regions lie outside the gamut of colors of the display device.

The adopted average is needed because it is very difficult to obtain monochromatic primary quantum values. e (x) is the spectral LSRPA on the surface of the catalytic wall. Substituting Equation (6) into Equation (4) gives... 

When fields of different spectral distributions can be adjusted in relative radiance to eliminate all color difference, the result is termed a metameric color match. In a color-matching experiment, a test field is presented next to a comparison field and the observer causes the two fields to match exactly by manipulating the radiances of so-called primaries provided to the comparison field. Such primaries are said to be added this can be accomplished by superposition with a half-silvered mirror, by superimposed images projected onto a screen, by veiy rapid temporal alternation of fields at a rate above the fusion frequency for vision, or by the use of pixels too small and closely packed to be discriminated (as in color television). If the primaries are suitably chosen (no one of them shonldbe matched by ai possible mixture of the other two), a human observer with normal color vision can uniqnely match atty test color by adjusting the radiances of three monochromatic primaries. To accomplish this, it sometimes proves necessary to shift one of the primaries so that it is added to the color being matched it is useful to treat this as a negative radiance of that primary in the test field. The choice of exactly three primaries is by no means arbitrary If only one or two primaries are used, matches are generally impossible, whereas if four or more primaries are allowed, matches are not uniquely determined. 

A strong point of EELS is that it detects losses in a very broad energy range, which comprises the entire infrared regime and extends even to electronic transitions at several electron volts. EELS spectrometers have to satisfy a number of stringent requirements. First, the primary electrons should be monochromatic. Second,... 

Figure 18.5 Schematic view of a diffraction experiment, (a) A narrow beam of x-rays (red) is taken out from the x-ray source through a collimating device. When the primary beam hits the crystal, most of it passes straight through, but some is diffracted by the crystal. These diffracted beams, which leave the crystal in many different directions, are recorded on a detector, either a piece of x-ray film or an area detector, (b) A diffraction pattern from a crystal of the enzyme RuBisCo using monochromatic radiation (compare with Figure 18.2b, the pattern using polychromatic radiation). The crystal was rotated one degree while this pattern was recorded.

Glocker and Frohnmayer determined the characteristic constant c for nine elements (Reference 2, Table 4) ranging in atomic numbers from 42 (molybdenum) to 90 (thorium). They proved that identical results could be obtained with the sample in the primary (polychromatic) or in the diffracted (monochromatic) beam. The method was applied with good results to the determination of barium in glass of antimony in a silicate of hafnium in the mineral alvite and of molybdenum, antimony, barium, and lanthanum in a solution of their salts—for example, 5.45% barium was found on 90-minute exposure by the x-ray method for a glass that yielded 5.8% on being analyzed chemically. 

Once we have done this, we now have our three primary colors in the form of standard lamps, and can proceed to determine Items 1,2 3, given above on page 421. To do this, we vary the wavelength of the monochromatic light, and determine relative amounts of red, green and blue light required to match the monochromatic color. This is done, as stated before, for about 5000 observers. 

In AFS, the analyte is introduced into an atomiser (flame, plasma, glow discharge, furnace) and excited by monochromatic radiation emitted by a primary source. The latter can be a continuous source (xenon lamp) or a line source (HCL, EDL, or tuned laser). Subsequently, the fluorescence radiation is measured. In the past, AFS has been used for elemental analysis. It has better sensitivity than many atomic absorption techniques, and offers a substantially longer linear range. However, despite these advantages, it has not gained the widespread usage of atomic absorption or emission techniques. The problem in AFS has been to obtain a... 

EELS spectrometers have to satisfy a number of stringent requirements. First, the primary electrons should be monochromatic, with as little spreading in energy as possible, preferably around 1 meV or better (1 meV = 8 cm ). Second, the energy of the scattered electrons should be measured with an accuracy of 1 meV or better. Third, the low energy electrons must be effectively shielded from magnetic fields. The resolution of EELS has steadily been improved from typically 50-100 cm-1 around 1975 to better than 10 cnf1 for the spectrometers that are available in 2000. The latter value comes close to the line width of a molecular vibration. 

The proportions of ground-state and (2P ) excited iodine atoms produced in a photolysis using monochromatic radiation can be approximately calculated from Fig. 1. To conserve momentum, essentially all the energy from the primary process in excess of that used in bond dissociation (HI -> H+I) =... 

If a single crystal is rotated in a monochromatic X-ray beam, a pattern of spots of reinforced X-rays can be recorded, traditionally on a photographic film placed behind the crystal perpendicular to the primary beam (giving the so-called Laue photographs). Nowadays, X-ray diffractometers use electronic photon counters as detectors. Since, as noted above, different atoms have different X-ray scattering powers, both the positions and... 

The primary act in a photochemical reaction is absorption of a quantum of radiation by the photoactive molecule. In a quantitative study, therefore, a radiation source of known intensity and frequency a suitable cel for the photolyte and an appropriate detector of light intensity are absolutely necessary for the determination of rates of reaction. To avoid experimental error due to geometry of the reaction cell, the best arrangement is to have a plane parallel beam of monochromatic radiation, incident upon a flat cuvette with proper stirring arrangement, as given in Figure 1.2. 

---