Surface mirrors

A majority of traditional NIR measurements are made on solid materials and these involve reflectance measurements, notably via diffuse reflectance. Likewise, in the mid-IR not all spectral measurements involve the transmission of radiation. Such measurements include internal reflectance (also known as attenuated total reflectance, ATR), external reflectance (front surface, mirror -style or specular reflectance), bulk diffuse reflectance (less common in the mid-IR compared to NIR), and photoacoustic determinations. Photoacoustic detection has been applied to trace-level gas measurements and commercial instruments are available based on this mode of detection. It is important to note that the photoacoustic spectrum is a direct measurement of infrared absorption. While most infrared spectra are either directly or indirectly correlated... 

Figure 5.23 — Flow-through ionophore-based sensor for the determination of lithium in serum. (A) Mechanism involved in the sensor response (symbol meanings as in Fig. 5.20). (B) Diffuse reflectance flow-cell (a) upper stainless steel cell body (A) silicon rubber packing (c) quartz glass window (d) Teflon spacer (0.05 mm thickness) (e) hydrophobic surface mirror (/) lower stainless steel cell body. For details, see text. (Reproduced from [90] with permission of the American Chemical Society).

The streak camera viewed the chge upward thru a periscope in which the line of sight was reflected to a horizontal direction by a front surface mirror. Measurements of the peak pressures by the aquarium technique were found to be the C-J or detonation pressures of the thermohydrodynamic theory... 

Double-beam atomic absorption instrumentation a = rotating half-silvered mirror b = front surface mirror. 

The growth of the importance of infrared spectrography and spectrophotometry in determining the structure of compounds and the composition of substances has led to the development of many infrared spectroscopes and other instruments. Most infrared spectroscopes and spectrophotometers employ front-surface mirrors instead of lenses. This eliminates the necessity for energy to pass through glass, quartz, or similar material. Furthermore,... 

Figure 7. Front surface mirror arrangement arrows indicate light path. (A) sample cup assembly, (B) 45° front surface mirror, (C) camera.

The plastic sample cup was polished and machined to the same dimensions as the standard metal cup. To contain the plastic cup, the standard body assembly was used except that one 11/i6-inch hole was made in the bottom of the assembly to permit the sample to be photographed. Two 3/4-inch steel rods were screwed to the base of the tester. They had a 1-inch thick recessed plate to support the sample cup assembly and to provide space for the front surface mirror which was used to reflect the image to the camera (Figure 7). 

Figure 3.5-6 Arrangement of a, a rectangular cell and b, a spherical cell at the focus of the entrance lens of a Raman spectrometer. The effective solid angle is by a factor of 2.5 larger for a spherical cell compared to the rectangular cell. FR focal range, SM surface mirror.

At the same time Hass [29] invented the silicon oxide protected aluminum surface mirror. Protection against mechanical damage became important in mirror fabrication with the exception of Al mirrors for astronomical applications. And finally, for use in the near ultra violet range, pure aluminum films have to be protected primarily against degrading oxidation on exposure to air by covering them with layers of MgF, or LiF, as was found out by Hass et al. [30] in the time between 1955 and 1961. 

REAR SURFACE MIRRORS, SURFACE MIRRORS AND BEAM SPLITTER MIRRORS... 

The conventional mirror, a glass plate which is silver-plated on the rear surface by a chemical process, is a rear surface mirror. Radiation first passes through the glass substrate and is then reflected on its rear surface. 

In the surface mirror, modern technology has enabled an improved method of construction the reflecting metal film is coated under high vacuum on the side on which radiation is incident. Various metals are applied for the different types of surface mirrors. Their reflectance is shown in Fig.26 of Chapter 8. The most frequently used metal for surface mirrors is aluminium. Its reflectance as a function of wavelength and its dependence on preparation conditions is shown in Fig.l 1. 

Surface mirrors are used on the side on which the mirror is coated, the substrate providing only the surface for the mirror, and radiation does not pass through it. By these means, the above mentioned disadvantages of the rear surface mirror are eliminated in front surface mirrors by the method of construction. 

Commercially available are various types of aluminium front surface mirrors to suit different requirements. For the visible spectral range, there are standard mirrors such as Alflex A . If improved reflection is required, a multiple film mirror Alflex B can be used. Both types of mirrors are provided with a hard and resistant dielectric protection coating. Such mirrors were first made by Hass et al. [73, 74]. The aluminium film on the surface mirror Alflex is even protected by an interference film system, which also enhances the reflectance for the visible range. In the visible and infrared, the spectral curve of the reflectance is approximately the same for Alflex A as that of an unprotected aluminium surface. With a mirror type Alflex B. the increase in reflection in the visible, with a maximum at 550 nm, can be clearly seen in Fig. 12. If required, this maximum can also be shifted to other wavelengths in the visible spectrum. 

A surface mirror for the UV is an aluminium mirror which is deposited very fast in a very good vacuum to prevent detrimental oxidation, and is then coated immediately with a protective magnesium fluoride film [75-83] sometimes LiF is also used. This film combination gives excellent reflectance in the ultraviolet region of200 nm up to 400 nm, as can be seen in Fig.12. 

All coated surface mirrors have good adherence and can be carefully cleaned with cloth and organic solvents. 

Aluminium surface mirrors are produced as plane mirrors for ... 

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