Radiant efficiency

The blue phosphor of long standing is ZnS Ag, which has a radiant efficiency close to the theoretical limit. It is a donor-acceptor type phosphor, with silver ions acting as the acceptor in the ZnS with either aluminium or chlorine as the donors on zinc or sulfate sites. 

Eximer light sources may require water cooling for optimal operation. Their life times have not yet been determined, but excimer lamps (see Table 1) have been operational for more than 3000 hours with radiant efficiencies of approximately 6%. 

An infinite plate with a temperature Kstrl and radiant efficiency j will transmit, independent of the distance, to a similar plate of frozen product with a temperature Kstr2 and a radiant efficiency e2 an amount of radiation energy. The surface heat flux q is ... 

In the case of photoluininescence we distinguish the quantum efficiency (q), the radiant efficiency (17) and the luminous efficiency (L). The quantum efficiency q is defined as the ratio of the number of emitted quanta to the number of absorbed quanta. In the absenee of competing radiationless transitions its value is I (or 100%). In the fundamental literature this is the important efficiency. The other two have a more technical importance. 

The radiant efficiency rf is defined as the ratio of the emitted luminescent power and the power absorbed by the material from the exciting radiation. The luminous efficiency L is the ratio of the luminous flux emitted by the material and the absorbed... 

For cathode-ray (CR) excitation q is irrelevant. The radiant efficiency is defined as the ratio of the emitted power to the power of the electron beam falling on the luminescent material. This means that r/cR refers to the total power incident on the material, whereas tjuv refers to the power absorbed. The luminous efficiency for CR excitation is defined in a similar way as for photoexcitation. The radiant efficiency for X-ray excitation is defined as for CR-ray excitation. 

In Table 4.5 we have gathered a few data on the several efficiencies of luminescent materials known to be efficient. These data were measured at room temperature [12]. The factors which restrict the photoluminescence efficiency, viz. the radiationless processes, were discussed above. For high-energy excitation, like cathode-ray or X-ray excitation, the situation is more complicated. Data like those presented in Table 4.3 for ricR and nx suggest that the maximum values of the radiant efficiency for host lattice excitation are restricted to values in between lO and 20%. This in fact is the case. This observation has already drawn attention decades ago, and early explanations were offered in the sixties. The most detailed one is that by Robbins [13]. The next paragraph presents a summary of this work. 

Assuming that all excitation energy is absorbed (i.e. r = 0), that all electron-hole pair energy arrives at the luminescent center (i.e. S = I), and that q = 100%, we arrive at the maximum radiant efficiency... 

ITie host lattices which yield the highest radiant efficiency (Sect 4.3) for cathode-ray excitation are undoubtedly ZnS and its derivatives. For the blue-emitting ZnS Ag values higher than 20% have been reported. We note that this fits our discussion in Sect. 4.4, where the maximum efficiency for host lattice excitation was found to occur for small values of the band gap Eg and the vibrational frequency i lo (Eqs (4.5)-(4.7)). For ZnS Eg = 3.8 eV and I l.o = 350 cm . These values satisfy our requirements. This can be illustrated by Y2O3 with Eg = 5.6 eV and i>lo 550 cm", yielding a maximum efficiency of only 8%. Although the latter value is for red emission, this value is much lower than for ZnS. 

We will discuss the possible materials for the blue-, the green-, and the red-emining phosphor in turn. For the blue the phosphor ZnS Ag has been continuously in use. As mentioned above, its radiant efficiency is very high and close to the theoretical limit. In Fig. 7.5 its emission spectrum is given. 

In the fields of projection television phosphors the situation is less. satisfactory. It is clear that luminescent materials have difficulties in meeting the high requirements. At the moment the largest need is for a blue-emitting phosphor with an acceptable saturation. The bad situation is well illustrated by the blue phosphor used, viz. ZnS Ag. Its high radiant efficiency in direct-view television tubes ( 20%) decreases to less than 5% under the conditions of the projection-television tube. Nevertheless it has not been possible, up till now, to find an acceptable alternative. 

These materials have a very high light yield (except for undoped Csl). For Nal Tl, for example, the radiant efficiency calculated from the light yield is about 5 of the maximum possible efficiency (sec Table 9.4). The low light yield of Csl is, certainly for a part, due to thermal quenching [26]. For certain applications the decay time of these scintillators (< I /us) is acceptable. Unfortunately the afterglow is considerable and the stability poor. It depends on the application whether these scintillators can be applied or not (see Table 9.3). Nal Tl is probably the most extensively used scintillator. 

Electrically heated radiators In these radiators, the IR radiation is obtained by passing an electric current through a resistance, which raises its temperature (Hallstrom et al., 1988, p. 217). The most common are metal sheath radiant rods, quartz tube, and quartz lamp. A typical cross section of a tube emitter is sketched in Figure 19.6a. One of the most important characteristics of such emitters is the radiant efficiency, which may be defined as... 

FIGURE 19.7 Radiant efficiency and relationship between voltage and temperature of various radiators. (Courtesy of Fostoria Industries, Inc., Fostoria, OH.)... 

Gas-fired radiators These radiators consist of a perforated plate (metal or refractory), which is heated by gas flames in one of the surfaces so the plate raises its temperature and anits radiant energy. The porosity of the plate determines the temperature of the other surface so as to ensure a safe process. Figure 19.6b shows a sketch of this type of radiator (van t Land, 1991, p. 250). The temperature of such a radiator is generally between 1500°C and 1700°C with wavelengths from 2.7 to 2.3 pm (van t Land, 1991, p. 249). The radiant efficiency of such radiators is typically about 60%. 

Radiant Efficiency Ratio of optical power output to the electrical power input of the device. 

In order to be appHcable as incendiaries, pyrotechnic compositions must fulfil a number of thermo-chemical requirements such as high enthalpy of reaction, low activation energy, suitable rate of combustion, high radiant efficiency and large radiating combustion plume and/or highly conductive slag. In addition, these... 

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