Beam-limiting apertures

In many cases, the profile a spectroscopist sees is just the instrumental profile, but not the profile emitted by the source. In the simplest case (geometric optics, matched slits), this is a triangular slit function, but diffraction effects by beam limiting apertures, lens (or mirror) aberrations, poor alignment of the spectroscopic apparatus, etc., do often significantly modify the triangular function, especially if high resolution is employed. 

The final beam-limiting aperture was originally in the gap of the final lens, but only the design in Fig. 3.17A has sufficient room. The apertures shown are virtual apertures at an optically equivalent position, and their actual diameter must be divided by the demagnification of the final lens to give the effective aperture diameter D. 

Figure 2.4 shows the elements of a high resolution diffractometer. The beam conditioner controls the divergence and wavelength spread of the beam by a combination of diffracting elements and angular-limiting apertures. The latter may also control the spatial width of the beam. This falls upon a specimen,... 

Figure 8.22 Schematic of an atomic beam diffractometer. Pressure-limiting apertures separate the zones with different vacuum quality.

The purpose of the diffractometer goniostat is to bring a selected reflected beam into the detector aperture (see figure A1.6) or a number of reflected beams onto an area detector of limited aperture (i.e. an aperture which does not intercept all the available diffraction spots at one setting of the area detector. 

The least powerful class of laser is class I. In this class, the power and energy of the unit are such that the TLV fordirect viewing ofthe laser beam, if the entire beampasses through the limiting aperture of the eye, cannot be exceeded for the classification duration (the... 

A beam of a TEMqo laser (Vol. 1, Sect. 5.3) with a Gaussian intensity profile is focused to a diffraction-limited spot with the diameter d 2Xf D by an adapted lens system with the focal length / and the limiting aperture D. For example, with... 

Historically, XPS instruments have relied upon the energy analyzer to define the analysis area because technology was not available to finely focus the X-ray beam, limiting the spatial resolution of XPS. A commonly used approach to improve the spatial resolution is shown in Figure 14.4, where an aperture is placed in the analyzer s input lens to restrict the analysis area. Only the photoelectrons from a given small area of the sample pass through the aperture and into the analyzer. The practical lower limit for small-area XPS analysis with this approach is 25 pm. 

When a parallel light beam passes a limiting aperture with diameter a, a Fraunhofer diffraction pattern is produced in the plane of the focusing lens L2 (Fig. 4.9). The intensity distribution 7(0) as a function of the angle 0 with the optical axis of the system is given by the well-known formula [4.3]... 

Returning to the flow cell, the starting point for PDA design. Fig. 2 shows how to squeeze the most light through what has been characterized as the optical bottleneck of the system. The light source is focused on one end of the cell, which makes the other end the limiting aperture of the optical system. Every point on the flow cell entrance cross section will then be connected by a ray to every point on the exit cross section. Note that the beam envelope expands beyond the cell at either end. 

In Fig. 5.34, the ratio yio/yoo of the diffraction losses for the TEMio and the TEMoo modes in a symmetric resonator with gi = gi = g is plotted for different values of g as a function of the Fresnel number Np. From this diagram one can obtain, for any given resonator, the diameter 2a of an aperture that suppresses the TEMio mode but still has sufficiently small losses for the fundamental TEMoo mode with beam radius w. In gas lasers, the diameter 2a of the discharge tube generally forms the limiting aperture. One has to choose the resonator parameters in such a way that a 3w/2 because this assures that the fundamental mode nearly fills the whole active medium, but still suffers less than 1 % diffraction losses (Sect. 5.2.6). 

In any image-forming system such as the optics of an FT-IR spectrometer, the optical throughput (see Section 2.7) of the beam is fixed by two limiting apertures, the entrance pupil and the field stop. The aperture stop limits the diameter of the beam at each optical component, while the field stop determines the size and shape of the image. In an FT-IR spectrometer, the aperture stop is usually decided by the diameter of the beamsplitter or one of the larger mirrors in the optical path. However, in one instrument, a variable aperture, called the beamplitter stop or... 

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