Optical spectrometers

O Keefe A and Deacon DAG 1988 Cavity ring-down optical spectrometer for absorption-measurements using pulsed laser sources Rev. Sol. Instrum. 59 2544-51... 

The two essential elements of an electron spectrometer are the electrodes that accelerate electrons and focus them into a beam and the dispersive elements that sort electrons according to their energies. These serve the fimctions of lenses and prisms in an optical spectrometer. The same parameters are used to describe these elements in an electron spectrometer as in an optical spectrometer the teclmology is referred to as electron optics. 

In photoluminescence one measures physical and chemical properties of materials by using photons to induce excited electronic states in the material system and analyzing the optical emission as these states relax. Typically, light is directed onto the sample for excitation, and the emitted luminescence is collected by a lens and passed through an optical spectrometer onto a photodetector. The spectral distribution and time dependence of the emission are related to electronic transition probabilities within the sample, and can be used to provide qualitative and, sometimes, quantitative information about chemical composition, structure (bonding, disorder, interfaces, quantum wells), impurities, kinetic processes, and energy transfer. 

The intensity of absorbed radiation. Sunlight or room lights may alter the rate of a reaction. Usually this effect is to be avoided unless the object is to study photochemical effects. The light level in an optical spectrometer that uses monochromatic light is not likely to cause problems, but if white light strikes the sample, as in a diode-array spectrophotometer, this is a possibility. 

An industrial microscope with a long-working distance 20 X objective is used for the collection of the chromatic interference patterns. They are produced by the recombination of the light beams reflected at both the glass/chromium layer and lubricant/steel ball interfaces. The contact is illuminated through the objective using an episcopic microscope illuminator with a fiber optic light source. The secondary beam splitter inserted between the microscope illuminator and an eyepiece tube enables the simultaneous use of a color video camera and a fiber optic spectrometer. 

The devices described are mostly used in portable instruments. For a deeper understanding, extended studies are recommended. After a general review of the most important parts of optical spectrometers, we proceed now to colorimetric particularities. 

The spectrum excited by an electrical discharge through gas at low pressme can be studied with two simple types of spectrum tube seen in Figure 75. These tubes also act as weak sources of certain radiations, such as monochromatic light for an optical spectrometer. The... 

R.A. Crocombe, Miniature optical spectrometers (Part IV), Spectroscopy Magazine at www.spectroscopyonline. com (2008), accessed 13 November 2009. 

Traditional optical spectrometers for both mid-IR and NIR were based on a scanning monochromator. This design features a single source and detector, and a mechanically scanned dispersion element in combination... 

Reeves, J.B. Ill and Van Kessel, J.S. (2000) Determination of ammonium-N, moisture, total C and total N in dairy manures using a near infrared fibre-optic spectrometer. Journal of Near Infrared Spectroscopy 8, 151-160. 

O Keefe, A., and D. A. G. Deacon, Cavity Ring-Down Optical Spectrometer for Absorption Measurements Using Pulsed Laser Sources, Rev. Sci. Instrum, 59, 2544-2551 (1988). 

Before we can confront the problem of undoing the damage inflicted by spreading phenomena, we need to develop background material on the mathematics of convolution (the function of this chapter) and on the nature of spreading in a typical instrument, the optical spectrometer (see Chapter 2). In this chapter we introduce the fundamental concepts of convolution and review the properties of Fourier transforms, with emphasis on elements that should help the reader to develop an understanding of deconvolution basics. We go on to state the problem of deconvolution and its difficulties. 

Conversely, we may observe an exceedingly narrow spectral line, so that o(x ) is approximated by <5(x ). Now the data i(x) represent the response function. This principle can, in fact, be used to determine the response function of a spectrometer. The laser, for example, is a tempting source of monochromatic radiation for measuring the response function of an optical spectrometer. Coherence effects, however, complicate the issue. We present further detail in Section II of Chapter 2. 

Typically, t(co) is small for co large. A spectrometer suppresses high frequencies. If the data i(x) have appreciable noise content at those frequencies, it is certain that the restored object will show the noise in a more-pronounced way. It is clearly not possible to restore frequencies beyond the band limit Q by this method when such a limit exists. (Optical spectrometers having sine or sine-squared response-function components do indeed band-limit the data.) Furthermore, where the frequencies are strongly suppressed, the signal-to-noise ratio is poor, and T(cu) will amplify mainly the noise, thus producing a noisy and unusable object estimate. 

Sometimes the spectrometer completely obliterates the information at all Fourier frequencies co beyond some finite cutoff Q. This is specifically true of dispersive optical spectrometers, where the aperture determines 1. The cutoff Q may be extended to high Fourier frequencies by multipassing the dispersive element or employing the high orders from a diffraction grating. 

The material YCaA104 is of interest for solid state lasers. The orientational dependence of the HFEPR spectra at 250 GHz was measured32 for an iron-doped crystal using a goniometer in a quasi-optical spectrometer at 253 K The spectra of Fe3+, S — 5/2, revealed the existence of two magnetically inequivalent sites of roughly equal concentration. Only the site with its z-axis along the c-axis of the crystal was studied in detail and was found to have a 0-factor close to 1.99 and a ZFS of about 29 GHz. 

The HFEPR of Ni2+ in the host Ni2CdCl6.2H20 was studied in a single crystal. The orientation dependence of the spectra at 250 GHz was measured32 using a goniometer in a quasi-optical spectrometer at 253 K. Two inequivalent sites were detected with the magnetic axes of both sites parallel to the c-axis. The 0-factor of both sites was close to 2.24 but the ZFS were -6.781 and -30.97 GHz. 

Double beam optical spectrometers (scanning type)... 

Figure 29.9 Comparison of an optical spectrometer with a simple EPR spectrometer.

Optical fibers are now used extensively to bring the analyzing light to and from the sample for optical spectrometers. What was not known about optical fibers in early implementations was the problems they can bring to spectroscopy. In early applications of fiber optics, they were found to have limited lifetime, cause signal drift and attenuation, and cause process contamination.4 Fiber optics were developed for the on/off application... 

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