Tungsten filament source

FIGURE 8.1 A graph showing approximately how the intensity of light from a tungsten filament source varies with wavelength. 

Figure 25-1 9 The Spectronic 20 spectrophotometer. A photograph of the instrument is shown in (a), while the optical diagram is seen in (b). Radiation from the tungsten filament source passes through an entrance slit into the monochromator. A reflection grating diffracts the radiation, and the selected wavelength band passes through the exit slit into the sample chamber. A solid-state detector converts the light intensity into a related electrical signal that is amplified and displayed on a digital readout. (Courtesy of Thermo Electron Corp., Madison, WI.)...

FIGURE 13-19 (aj Tho Spectronic 20 spectrophotomeler and (bj its optical diagram. Radiation from the tungsten filament source passes through an entrance slit inlo the monochromator,... 

Figure 21-9 Block diagram of a tungsten filament source.

Deuterium or tungsten filament sources with interference filters were also used to provide a simple means of detecting absorbing species a.s they eluted from a column. Some instruments were dual wavelength devices, or they were equipped with filter wheels containing several filters that could be rapidly switched to detect various species as they eluted. Today, filler instruments... 

FIGURE 5.23 Spiral-type tungsten filament. Source Varian/Eimac.)... 

Electron Gun and Optics. The electron source is usually a tungsten filament source, although field emission guns ate also employed for high-resolution work. The electrons are accelerated to an energy between 1 and... 

The tungsten filament source has a limit to the beam current that can be obtained in a small spot. Replacing it with a lanthanum hexaboride (LaB6) gun provides a beam about 30 times brighter. This results in an improved signal to noise ratio and allows better resolution at high scan speeds. The increased current is also useful for analytical microscopy. A disadvantage is that this gun requires a better vacuum which must be provided by the addition of an ion pump to the gun chamber. 

Vacuum Tubes. In the manufacture of vacuum tubes for use in polarized ion sources, vaporized cesium is used as a getter for residual gaseous impurities in the tube and as a coating to reduce the work function of the tungsten filaments or cathodes of the tube. The cesium vapor is generated by firing, at about 850°C within the sealed and evacuated tube, a cesium chromate pellet and zirconium (12) (see Vacuum technology). 

Clearly, standardized light sources are desirable for color matching, particularly in view of the phenomenon of illuminant metamerism described below. Over the years CIE has defined several standard illuminants, some of which can be closely approximated by practical sources. In 1931 there was Source A, defined as a tungsten filament incandescent lamp at a color temperature of 2854 K. Sources B and C used filtering of A to simulate noon sunlight and north sky daylight, respectively. Subsequently a series of D illuminants was estabUshed to better represent natural daylight. Of these the most important is Illuminant E). ... 

Real or gray bodies deviate from these ideal blackbody values by the A-dependent emissivity, but the color sequence remains essentially the same. This mechanism explains the color of incandescent light sources such as flames in a candle, tungsten filament light bulb, flash bulb, carbon arc, limelight, lightning in part, and the incandescent part of pyrotechnics (qv). 

There are three major types of electron sources thermionic tungsten, LaBg, and hot and cold field emission. In the first case, a tungsten filament is heated to allow... 

The usual source of electrons is a tungsten filament electron gun held at a negative potential (typically 10-30 kV), and magnetic lenses focus the beam into a fine probe incident on the surface of the specimen. A probe diameter of 0.2-1 pm is typical, with a current of 1-100 nA. 

Both fixed and variable wavelength uv/visible detectors are available. The variable types use a deuterium and/or a tungsten filament lamp as the radiation source and can operate between about 190-700 nm. They will have a number of absorbance ranges (ranges are given... 

Arkel refining a sample of the impure metal, for example zirconium, is heated to a temperature around 550 K in contact with low pressure iodine gas in a sealed system which has a heated tungsten filament in the centre. The filament temperature is normally about 1700 K. At the source the iodides of zirconium and some of the impurities are formed and these diffuse across the intervening space, where the total pressure is maintained at 10 3 atmos, and are decomposed on the filament. The iodine then returns to form fresh iodide at the source, and the transport continues. 

The most common conventional gas source is an electron impact (El) source. This consists of a metal chamber with a volume of a few cm3, through which the sample flows in the form of a gas. Electrons produced by thermionic emission from a heated tungsten filament are passed through this gas, and accelerated by a relatively low voltage ( 100eV), causing ionization within the sample gas. A plate inside the chamber carries a low positive potential (the repeller ) which ejects the positive ions into a region which contains a series of plates (called lenses) and slits, which serve to focus, collimate, and accelerate the ion beam into the next part of the system... 

---