Incandescent lamps lamp selection

Most incandescent lamps today exhibit a coiled-coil geometry of the filament (Fig. 6.7). Coiled-coils are made by winding the filaments on mandrels of molybdenum and subsequent winding of the coil on a larger mandrel to form a secondary coil [6.2]. The coil is then heated to about 1700°C to relieve the stresses formed during coiling and to stabilize the geometry [6.7]. The mandrels are then selectively dissolved in nitric acid or mixtures of nitric and sulfuric acid. 

The selection of the source of light appropriate to the circumstances depends on several factors. It is important to consider efficiency, ease of installation, costs of installation and running, maintenance, lamp life characteristics, size, robustness and heat and colom output. The efficiency of any lamp (often termed efficacy) can be expressed in terms of light output per unit of electricity used (lumens per watt). Generally speaking, incandescent lamps are less efficient than discharge sources. 

We will not discuss the last method since it is merely a mechanical procedure which uses centripetal force on a spinning disc to disperse the slurry material. All of the other techniques require a substantial vacuum in order to form the thin film and to prevent oxidation and contamination of the film during its formation on a selected substrate. The history of vacuum pumps covers many years. Both incandescent and gas-discharge lamps would not have been developed and made a part of our life if the vacuum pump had not been invented and made practical. Nor would we have the color television if vacuum pumps had not been invented. AU three of these devices use thin films in one form or another. The past 12 years have accelerated the use of thin films for AMLCD s, OLED s, PDF s, LED s and LCD s, to the degree that the vacuum pump is Indispensable. 

There are several types of photomultipliers, which differ in the nature of the entrance window, either crystal or fluoride, and in the nature of the sensitive layer on the photocathode. Some are only sensitive in the far-UV while others are more sensitive in the visible. The type of photomultiplier to be used is selected according to the wavelength of the line to be detected. A fatigue lamp (a small incandescent light source) is often used with photomultipliers to keep the temperature of the tube and its associated electronics constant. The fatigue lamp is switched on when the emission source is off and vice versa. 

In principle, incandescent sources, such as quartz-halogen lamps and different types of arc lamps filled with deuterium or xenon may be used for HR-CS AAS. But the most critical parameter for the selection of a radiation source is its spectral radiance in the important far-UV spectral region down to 190nm. In order to compete with the detection limits of LS AAS the spectral radiance per picometer bandwidth increment of a CS should be at least one order of magnirnde higher than the narrow emission lines of hollow cathode lamps, since the geometrical conductance of a HR-CS AAS spectrometer is lower by the same order of magnitude. 

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