Ellipsoidal mirror

In the diffuse reflectance mode, samples can be measured as loose powders, with the advantages that not only is the tedious preparation of wafers unnecessary but also diffusion limitations associated with tightly pressed samples are avoided. Diffuse reflectance is also the indicated technique for strongly scattering or absorbing particles. The often-used acronyms DRIFT or DRIFTS stand for diffuse reflectance infrared Fourier transform spectroscopy. The diffusely scattered radiation is collected by an ellipsoidal mirror and focussed on the detector. The infrared absorption spectrum is described the Kubelka-Munk function ... 

An ellipsoidal mirror has the same focusing properties but collects over two axes rather than one... 

Figure 12. Optical arrangement by Fuller and Griffiths (25) for diffuse reflectance spectroscopy P, paraboloidal mirror E, ellipsoidal mirror S, sample D, detector...

Figure 7. Uncalibrated VCD absorbance curves (upper) and ordinary absorbance (lower) for a-pinene. The signs of the enantiomers and the racemic mixture of a-pinene are indicated. The spectra on the left were take with the lens focusing optics and those on the right with the ellipsoidal mirror focusing optics.

We used an imaging system as shown in Fig. 9.6 to observe the focal spot of the harmonic beams. The harmonics were focused onto a Ce YAG scintillator by the ellipsoidal mirror. Figure 9.15 shows the 1/e2 beam spot size... 

Fig. 9.14. A schematic of an ellipsoidal mirror used for focusing the harmonic beams...

Fig. 9.15. Focused beam spot size of the harmonic waves as a function of distance from the ellipsoidal mirror. The open circles and filled squares correspond to the horizontal and vertical directions, respectively. The best-fit curve based on (9.1) is also shown by the solid line...

Figure 2-5 Collection optics with an off-asis ellipsoidal mirror for the UV region. FL, focusing lens S, sample and ES, entrance slits.

Other conic sections exist paraboloidal mirrors, ellipsoidal mirrors, and hyperboloidal mirrors. In paraboloidal mirrors, all rays (from infinity or not) converge at the same focus. In ellipsoidal mirrors, all rays emanating at focus... 

For DRIFT studies, a wood wafer, paper sheet, or milled wood sample dispersed in KBr (or KC1) is placed in a cup at the focal point of the concave, ellipsoidal mirror so that the incident light is focused on the sample. The scattered light coming from the sample is collected from the concave mirror and directed by a suitable mirror system to the detector of the FTIR instrument. The pressure used for smoothing the sample has to be adjusted so that reproducible results can be obtained (Yeboah et al. 1984). The contribution of specular reflectance can be diminished by reducing the particle size and by increasing the sample dilution. For powder samples, as indicated above, the diluent is KBr or KCI. Good results are normally obtained with alkali halide powders that contain 1-2% of sample. In certain cases, the sample concentration may be increased up to 10%. 

Figure 3.2 Schematic view of a commercial LIF detector for CE (from Beckman Instruments. Fullerton, CA). A fiber-optic cable transmits laser light from the laser to the detector and illuminates a section of the capillary. Fluorescence is collected by the ellipsoidal mirror and focused back onto the photomultiplier tube. To reduce unwanted laser light, a centered hole in the mirror allows most of the beam to pass. A beam block is used to attenuate scattered laser light. (Reprinted with permission from Schwartz and Gunman, Beckman Instruments Primer Series, Vol. VH, 1995. Copyright Beckman Instruments, Inc.)...

With ellipsoid mirrors, all rays passing through the first focus are reflected to pass the second focus. 

Beam condensers, by using a pair of ellipsoid mirrors, produce very small images of the Jacquinot stop or the entrance slit at the sample position. The size of these images may be even further reduced by making use of a Weierstrass sphere. Weierstrass (1856) showed that a spherical lens has two aplanatic points . If a sphere of a glass with a refractive index n is introduced into an optical system which has a focus at a distance of r n from its center, then the beam is focused inside the sphere at a distance of r/n from the center (Fig. 3.5-9). In this case the angle O in Eq. 3.4-5 may approach 90°. Thus, a sample with a very small area can fully fit the optical conductance of the spectrometer (Fig. 3.4-2d). Microscopes usually have an optical conductance which is considerably lower than that of spectrometers. In this case, the sample and the objective are the elements limiting the optical conductance (Schrader, 1990 Sec. 3.5.3.3). 

The excitation source that emits UV light over a wide range of wavelengths (usually a deuterium lamp) is situated at the focal point of an ellipsoidal mirror shown at the top left hand comer of the diagram. 

Between the spherical mirror and the ellipsoidal mirror, in the center of the diagram, is a beam splitter that reflects a portion of the incident... 

Fig. 1 Accessories for diffuse reflectance spectroscopy (A) Integrating sphere with hemispherical radiation collection (B) Accessory based on ellipsoidal mirrors, used within the sample compartment of the spectrometer (C) Rotational ellipsoidal mirror device with dedicated detector and (D) Bifurcated fiber optic-based accessory (also shown is the random mixture of fibers for illumination and detection compared with devices based on reflection optics the acceptance cone for radiation delivery and collection is limited and depends on the refractive indices of the core and cladding material).

Figure 6,5 Schematic diagram of the ellipsoidal mirror optical counter (Hu-sar. 1974),...

Figure 6.6 The relative response curves of the ellipsoidal mirror optical counter. The lines wen calculated from Mie theory, and the points were measured experimentally. (Courtesy S. L. HeisEe and R. B. Husar.)...

Fig. 4.1. Schematic of an experimental set-up for absorption measurements at low temperature incorporating a Perkin-Elmer Model 99G monochromator. Si, S2 and S3 are IR sources selectable with plane mirrors Mi and M2. FM focusing spherical mirrors. Ei and E2 entrance and exit slits. CM off-axis paraboloid collimating mirror. G plane reflection grating. Beam 1 from Si is converted by CM into a parallel beam dispersed by G. One wavelength is diffracted in a direction where it can be intercepted by first mirror M as beam 2 and focused on the internal chopper Ch. Modulated beam 2 is redirected toward G as beam 3 and re-dispersed a second time as beam 4. Beam 4 intercepted by IM is focused on E2 and re-focused on the sample by FM. The divergent monochromatic beam is finally focused on thermocouple D by ellipsoidal mirror EFM. Fi, F2 and Pol are locations for transmission filters and a polarizer. Beam 1 can be blocked by shutter Sh (after [37]). With permission from the Institute of Physics...

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