Accessory aperture

Consistent sampling conditions are a necessity for careful quantitative work. In transmission spectrometry it is critical to fill the entire accessory aperture with the sample. If the actual sample area is smaller than the beam that passes through the aperture, stray light or excess radiation will reach the detector (Figure 9.2). The radiation that does not pass through the sample provides a spurious signal to the detector, and this excess radiation establishes a minimum transmittance. That is, no matter how strongly a sample absorbs, no band can have a transmittance below the minimum... 

Fig. 1. Typical locations for CAM components, showing the photometer, 1 filter wheel, 2 monochromator, 3 shutter and aperture unit, 4 beam splitter, 5 accessories for polarized light such as a rotary analyzer and a compensator, 6 beam splitter for epi-excitation fluorescence, 7 objective lens, 8 stage, 9 substage condenser, 10 condenser aperture, 11 polarizer, 12 field aperture for photometry, 13 shutter, 14 primary illuminator, 15 arc lamp, 16 shutter, 17 monochromator, 18 filter wheel, 19 and ocular, 20.

FTIR Microspectroscopy.3 A microscope accessory coupled to a liquid-nitrogen-cooled mercury-cadmium-telluride (MCT) detector can be used to obtain an IR spectrum. This is possible in both the transmission and reflectance modes. Several beads are spread on an IR-transparent window (NaCl, KBr, diamond) and possibly flattened via a hand-press or a compression cell. The IR beam is focused on a single bead using the view mode of the microscope. The blank area surrounding the bead is isolated using an adjustable aperture, and a spectrum is recorded using 32 scans (<1 min). A nearby blank area of the same size on the IR transparent window is recorded as the background. 

Figure 6.2 General layout of the FTIR imaging spectrometer. Optical elements F and G illuminate the sampling accessory with light from the spectrometer optical elements B and E focus the light from the sampling accessory onto the FPA detector aperture C is used to control the overall light level reaching the detector, and filter B is used to block light from outside the desired spectral region.

Type of decks angle Aperture" Screen deck type Accessories... 

As seen in Fig. 4.20, for a given degree of scattering, an increase in the illumination of zone S will enhance the illumination of the entrance slit (aperture) and, as a consequence, the SNR of the measured spectrum. This effect was exploited by Tolstoy [126], who suggested using a KBr condenser of relative aperture 0.5 (Fig. 4.20b) to focus the incident beam onto the sample. This allowed the area of the source image on the sample to be reduced by a factor of >2. In place of the lens, any commercially available microfocus accessory may be employed. 

Another mechanical solution to the problem of Fresnel specular reflectance is cutting off the specular component at the exit aperture of the accessory, as shown in Fig. 4.25 [154]. The waveguide that filters the KM component from the scattering radiation in the cup-on-the-saucer DRIFTS accessory (Fig. 4.19) may also be regarded as a mechanical device. 

Large-scale diamond compression (high-pressure) cells have been available for some time. However, during recent years, a microversion of the cell, also known as the diamond anvil cell, has been produced. This small diamond anvil cell is ideal for the study of physically hard (but compliant) samples, certain intractable samples, and samples that are optically too thick for normal transmission measurements. Note that this small form of the accessory is intended for samples 1 mm or less in size. The aperture through the diamonds is small, and therefore the use of a beam condenser accessory is recommended for optimal throughput. Alternatively, the cell may be used in combination with an IR microscope. 

Fig. 6.1 Schematic of a laser confocal scanning microscope. The laser illumination spot is scanned across the sample. Reflected light is de-scanned and passed through the beam splitter and the confocal aperture to the detector. The detector of transmitted light may be present as an accessory, but it gives a normal, not a confocal, optical microscope image.

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