Excitation sources-phosphors

Fig. 1. Common phosphor excitation sources, energies, and the type of soHd-state excitation caused by these sources. Following the initial excitation, high...

Luminescent Pigments. Luminescence is the abihty of matter to emit light after it absorbs energy (see Luminescent materials). Materials that have luminescent properties are known as phosphors, or luminescent pigments. If the light emission ceases shortly after the excitation source is removed (<10 s), the process is fluorescence. The process with longer decay times is referred to as phosphorescence. 

EXAMPLE 1.5 The sensitivity of luminescence. Consider a photoluminescence experiment in which the excitation source provides a power of 100 ptW at a wavelength of400 nm. The phosphor sample can absorb light at this wavelength and emit light with a quantum efficiency of r] = O.I. Assuming that kg = 10 fii.e., only one-thousandth of the emitted light reaches the detector) and a minimum detectable intensity of l(f photons per second, determine the minimum optical density that can be detected by luminescence. 

This UV radiation is converted into visible radiation by means of the fluorescence of the phosphor powder coating. The phosphor material uses the UV radiation as an excitation source and produces fluorescent emission in the visible region, with a broad spectrum to give off the white light that we can see. A good variety of combinations of phosphors are used (Shionoya and Yen, 1999). The principal field of application of fluorescent lamps is general lighting, for which they constitute efficient devices. 

Decay. The decay time requirements must be adhered to very precisely for cathode-ray tube phosphors. The measuring devices consist of fast excitation sources (flash lamps, lasers), photomultipliers with very low time constants, and an oscilloscope [5.440]. 

The Laboratory Data Control Fluoro Monitor (Fig.3.52) is a modular fluorescence detector similar in appearance to their UV detectors. With this detector, measurements may be made of the fluorescence of one stream or the differential fluorescence of two streams. The single-wavelength excitation source (a hot-cathode mercury lamp with a phosphor coating) emits a band of light with a maximum at 360 nm. The cell assembly is... 

Many minerals can be made to luminesce under various excitation sources, usually UV light, but in relatively few cases is the mechanism understood in detail. Best understood is luminescence due to transitions between localized states in the unfilled d-orbitals of transition metal ions and localized states in the unfilled f-orbitals of rare earth ions. Rare earth ions, important in the development of synthetic phosphor and laser materials, are uncommon among naturally occurring minerals. 

We have already briefly discussed luminescence decay times. Reiterating, the decay time of a phosphor has been defined as the time for the steady state luminescence intensity to decay to 1/e, or 0.368, of its original Intensity. It has been found that the intensity of photon emission builds up in the order of microseconds, i.e.- 10 sec. to a specific value, i.e.- the excitation process takes only a few microseconds. Since the intensity also decays in microseconds (if the excitation source is removed), there is an equilibrium value attained in the presence of the excitation source, which is a combination of both excitation time and decay time. This so-called steady state is called Iq, and is promulgated by the population of emitting... 

Although methods of preparing several phosphor compositions have been explored, we have not yet examined methods of measuring phosphor properties. The need to measmre the "brightness" or light output under a controlled excitation source should be apparent if one is to optimize any... 

Figure 3.1 schematically represents time-resolved experimental setup used in our experiments. The excitation sources were pulsed lasers, such as excimer XeCl (308 nm), nitrogen (337 run), three harmonics of Nd-YAG (266, 355 and 532 nm), and tunable dye and OPO, which deliver pulses of 10 ns duration. The spectra observed at the geometry of 90° are analyzed by intensified CCD matrix. Image intensifiers comprise three main components a photocathode, microchannel plate (MCP) and phosphor screen. The standard operation of these devices starts when the incident photons become converted into electrons at the photocathode. The electrons then accelerated towards the MCP where they are multiplied to an amount... 

A survey of the optical bandgap, excitonic recombination properties under low excitation and electron hole plasma recombination in AlxGai.xN has been given. Demand for UV applications, i.e. gas sensors or monitors, flare sensors, medical applications, chemical and biochemical applications and light sources for phosphors increases rapidly, which will surely lead to the further improvement of the quality of the AIN containing nitrides, and thus give us much more information about their luminescence properties. 

The first optical laser, the ruby laser, was built in 1960 by Theodore Maiman. Since that time lasers have had a profound impact on many areas of science and indeed on our everyday lives. The monochromaticity, coherence, high-intensity, and widely variable pulse-duration properties of lasers have led to dramatic improvements in optical measurements of all kinds and have proven especially valuable in spectroscopic studies in chemistry and physics. Because of their robustness and high power outputs, solid-state lasers are the workhorse devices in most of these applications, either as primary sources or, via nonlinear crystals or dye media, as frequency-shifted sources. In this experiment the 1064-mn near-infrared output from a solid-state Nd YAG laser will be frequency doubled to 532 nm to serve as a fast optical pump of a raby crystal. Ruby consists of a dilute solution of chromium 3 ions in a sapphire (AI2O3) lattice and is representative of many metal ion-doped solids that are useful as solid-state lasers, phosphors, and other luminescing materials. The radiative and nonradiative relaxation processes in such systems are important in determining their emission efficiencies, and these decay paths for the electronically excited Cr ion will be examined in this experiment. 

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