Lamp emission characteristics

Essentially the same spectrometer as is used in atomic absorption spectroscopy can also be used to record atomic emission data, simply by omitting the hollow cathode lamp as the source of the radiation. The excited atoms in the flame will then radiate, rather than absorb, and the intensity of the emission is measured via the monochromator and the photomultiplier detector. At the temperature achieved in the flame, however, very few of the atoms are in the excited state ( 10% for Cs, 0.1% for Ca), so the sample atoms are not normally sufficiently excited to give adequate emission intensity, except for the alkali metals (which are often equally well determined by emission as by absorption). Nevertheless, it can be useful in cases where elements are required for which no lamp is available, although some elements exhibit virtually no emission characteristics at these temperatures. 

For recording of the emission spectrum, the emitted radiation is focussed on the slit of a monochromator and intensities measured attach wavelength. Since sensitivities of photocells or photomultipliers are wavelength dependent, a standardization of the detector-monochromator combination is necessary for obtaining true emission spectrum This can be done by using a standard lamp of known colour temperature whose emission characteristics is obtained from Planck s radiation law. The correction term is applied to the instrumental readings at each wavelength. Very often substances whose emission spectra have been accurately determined in the units of relative quanta per unit wavenumber intervals are... 

Many technical applications require high radiant power, which cannot be furnished by an operationally reasonable number of lamps having otherwise optimal emission characteristics. Examples of this situation are mostly found in applications oTthe 254-nm fine, where a number of low-pressure mercury lamps may be replaced by one medium-pressure mercury arc. This substitution represents a compromise where spectral selectivity and energy wasting (VIS and IR radiation) is traded against a compact production unit which is less expensive (number of reactors, quartz, safety requirements) and easier to operate (number of reactors, space, and overview). 

The choice of the light source—form, emitted wavelengths, radiant power— depends inter alia on whether Ti02 is unsupported or supported, and on the type and shape of the supporting material. For example, a Ti02-coated flexible material can be wrapped around a cylindrical lamp placed inside the reactor. A plate covered by Ti02 can be installed perpendicularly to the beam of a lamp located outside the reactor. Obviously, the choice of the lamp, especially of the emission characteristics, also depends on the objective. For instance, in view of solar photocatalytic applications, lamps mimicking solar irradiation at the Earth s level or a solar box can be used in the laboratory. 

The submitters used a Helios Italquartz 500-W lamp which has emission characteristics similar to those of the Hanovia 450-W medium pressure mercury lamp used by the checkers. 

The emission characteristics of a particular lamp are dependent on tube current, mercury pressure, phosphor composition, phosphor coating procedure, tube composition, and the frequency of the ballast used. Since these lamps are produced for a very competitive commercial market, much of the production information is considered proprietary and not published. Changes can be made, sometimes without the knowledge of customers. 

Tungsten-base materials are used for cathodes in power grit tubes, radio valves. X-ray tubes, as electrode material for gas-discharge lamps, electric arc welding, electron guns, electron microscopes, and plasma generators. The electron emission characteristic is also an important property for their use as electrical contact materials. Finally, tungsten is a... 

Table I. Emission Characteristics and Spectral Irradiance Values for FS40 Lamp and Trifluralin Absorption Coefficients ...

Let us suppose that we have prepared a series of phosphor compositions wherein the activator concentration was the variable. We would need to measure these samples to determine which concentration was near to the optimum. Let us further suppose that we wish to use our phosphor in a LPMV lamp application. Although we could manufacture a test-lamp, usually this is not very feasible. What we do is to prepare a thick layer on a holder and compare it to a "standard" phosphor having similar emission characteristics. The layer is called a "plaque" and the apparatus is called a "plaque-tester", as shown in the following ... 

Figure 2. Emission characteristics for the ultraviolet lamps used in this work (RPR-3500A from the Southern New Bigland Ultraviolet Oo, Hamden, CT)...

The spectral response of the photocell and monochromator varies with wavelength, and must be properly calibrated if measurements at different wavelengths are to be compared. This is conveniently done by determining the emission characteristics of a tungsten lamp over the spectral range at various temperatures and estimating the intensity of... 

Detailed information on the emission characteristics of lamps and the transmission properties of glasses and filter solutions can be found in A. J. Gordon and R. A. Ford, The Chemists Companion Wiley-Interscience, New York, 1972, pp. 348-368, or in S. L. Murov, Handbook of Photochemistry, Marcel Dekker, New York, 1973. 

The light sources commonly used for laboratory-accelerated weathering tests include filtered xenon arcs, two types of filtered carbon arcs, fluorescent UV lamps, and metal halide lamps. The spectral power distributions are shown in Figures 8.3 through 8.6 in comparison with solar radiation. The spectral emission characteristics of the sources differ significantly in both the UV and visible regions. 

Fig. 9. Spectrum of a bright line emission source (e.g., deuterium lamp). The characteristic broadening is an illustration of the bandpass of a spectrophotometer.

Correlation between devices of the same type, either in the same laboratory or at different locations, can be very good if they are operated under the same test conditions. The results between devices with different sources of radiation (xenon lamp or carbon arc) will vary according to the spectral emission characteristics of the sources and spectral absorption characteristics... 

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