Optical systems radiation sources

A. J. LaRocca, in Infrared and Electro-Optical Systems Handbook Sources of Radiation, Vol. 1 (G. J. Zissis, ed.), pp. 51-138, SPIE Optical Engineering Press, Bellingham, Washington (1993). 

Current instruments use complex and refined optical systems in order to radiate the cuvette with monochromatic light selected from the radiant spectrum of the light source. A radically different approach may be practical when tunable lasers become available at reasonable prices. 

In recent years, the evolution of the technological components required for IR sensor systems has been denoted by a significant miniaturisation of light sources, optics and detectors. Essentially, an IR sensor consists of (i) a polychromatic or monochromatic radiation source, (ii) a sensor head and (iii) a spectral analyser with a detector. As sensors where all optical elements can be included in the sensor head are the exception rather than the rule, also various optics, waveguides and filters may form essential parts of IR-optical chemical sensors. Another important building block, in particular when aiming at sensors capable of detecting trace levels, are modifications of the sensor element itself. 

When a radiation source is placed inside a closed cavity, its radiation energy is distributed among all of the modes following Equations (2.1) and (2.2), once the system has reached equilibrium. As we have seen in Example 2.1, in spite of the large number of modes in such a closed cavity, the mean number of photons per mode corresponding to the optical region is very small. Specifically, it is very small compared to unity. This is the ultimate reason why, in thermal radiation fields, the spontaneous emission per mode by far exceeds the stimulated emission. (Remember that the stimulated emission process requires the presence of photons to induce the transition, opposite to the case of the spontaneous emission process.)... 

Here we consider some of the principle features of x-ray optics for beam lines on synchrotron radiation sources, with particular reference to the special requirements of small specimens. The most important factors involved are the size and position of the virtual source, the distance between the virtual source and the focussing elements relative to that between the focussing elements and the focus, and the presence and performance of the focussing systems. These points are considered briefly below. 

Research based on time-resolved XAS in an optical pump-x-ray probe scheme has first been implemented at synchrotron radiation sources. Mills et al. [2] used a 20 Hz repetition rate Nd YAG laser to photolyse carbonmonomyoglobin (MbCO) and monitor the photolysis product with time-resolved XAS around the K-edge of the iron atom. Other studies were carried out on different types of photolyzed systems in liquids, by Thiel et al. [3], Clozza et al. [4], Chance et al. [5,6] and Chen et al. [7,8,9]. All these studies were limited to the nanosecond or longer time domain. We recently reported on time-resolved XANES studies of a Ruthenium complex in water solution reaching the picosecond time scale [10]. This work allows us to evaluate the feasibility of future time-resolved XAS experiments, which we present below together with our new results. 

In January 1992. E. Desurvire (Columbia University Center for Telecommunications Research) reported that optical fibers made from silica glass and traces of erbium can amplify light signals when they are energized by infrared radiation. Desurvire developed an efficient radiation source (referred (o as a laser diode chip) that, when integrated into a fiber optic communication system, can increase transmission capacity by a factor of 10(1. 

Blackbody radiation sources are accurate radiant energy standards of known flux and spectral distribulion. They are used for calibrating other infrared sources, detectors, and optical systems. The radiating properties of a blackbody source are described by Planck s law. Energy distribution... 

Fig. 3.8 Generic layout of a system suitable for studying very fast and ultrafast processes. Appropriate radiation sources may be a flash lamp, a laser or an electron accelerator, while optical, conductivity, or ESR detection systems may be employed.

Fluorescence is spontaneous radiation that arises because of the stimulation of an atomic or molecular system to energies higher than equilibrium. This is illustrated in Figure 1 for a simple two-level atom. The atom is excited by absorption of a photon of energy hv. If the fluorescence is observed at 90° to a collimated excitation source, then a very small focal volume may be defined resulting in fine spatial resolution. The fluorescence power an optical system will collect is... 

Stray-light errors are more likely to be observed near the wavelength limits of an instrument, where the radiation intensity of the source and the efficiency of the optical system are reduced, especially below 220 nm and at the crossover point between the ultraviolet and the visible lamps (about 320 to 400 nm). Errors may become serious where the solvent absorbs strongly or where a strongly-absorbing sample is measured by difference spectrophotometry. 

The radiant flux

thermal radiation source through a spectrometer is calculated by multiplying the spectral radiance by the spectral optical conductance, the square of the bandwidth of the spectrometer, and the transmission factor of the entire system (Eq, 3.1-9). Fig. 3.3-1 shows the Planck function according to Eq. 3.3-3. The absorption properties of non-black body radiators can be described by the Bouguer-Lambert-Beer law ... 

Nagano (30) markets the Light-Tron Xenon LTX-01 type test chamber, which is pictured in Figure 27 (A), along with a diagram of its optical system (B). The optical system is unique in that it utilizes an isolated, air-cooled, high-powered, short-arc, and filtered xenon lamp as its source. It also utilizes a turntable to compensate for variations in radiation across the sample surface. 

A typical IR spectrometer consists of the following components radiation source, sampling area, monochromator (in a dispersive instrument), an interference filter or interferometer (in a non-dispersive instrument), a detector, and a recorder or data-handling system. The instrumentation requirements for the mid-infrared, the far-infrared, and the near-infrared regions are different. Most commercial dispersive infrared spectrometers are designed to operate in the mid-infrared region (4000-400 cm ). An FTIR spectrometer with proper radiation sources and detectors can cover the entire IR region. In this section, the types of radiation sources, optical systems, and detectors used in the IR spectrometer are discussed. 

A typical optical system is shown in Fig. 26. A lens of short focal length (7-10 cm) projects a nearly parallel beam of radiation from the source A through a filter F, to remove unwanted radiation. The stop Sj prevents unfiltered radiation from reaching the RV. It is sometimes useful to converge the beam slightly with a second lens (focal length 40 cm) such that the beam reaches its smallest diameter in the centre of the RV. The latter may be divided into two compartments, one of which contains a compound used for actinometry, or alternatively the beam is focused by the lens L3 onto a photocell or thermopile P. The intensity of the beam is suitably reduced by the density filter F2. To provide maximum possible intensity... 

Atomic spectrometric methods of analysis essentially make use of equipment for spectral dispersion so as to isolate the signals of the elements to be determined and to make the full selectivity of the methodology available. In optical atomic spectrometry, this involves the use of dispersive as well as of non-dispersive spectrometers. The radiation from the spectrochemical radiation sources or the radiation which has passed through the atom reservoir is then imaged into an optical spectrometer. In the case of atomic spectrometry, when using a plasma as an ion source, mass spectrometric equipment is required so as to separate the ions of the different analytes according to their mass to charge ratio. In both cases suitable data acquisition and data treatment systems need to be provided with the instruments as well. 

In optical atomic spectrometry the radiation emitted by the radiation source or the radiation which comes from the primary source and has passed through the atom reservoir has to be lead into a spectrometer. In order to make optimum use of the source, the radiation should be lead as complete as possible into the spectrometer. The amount of radiation passing through an optical system is expressed by its optical conductance. Its geometrical value is given by ... 

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