Other Light Sources

Before discussing light sources generally, it may be useful to consider some basic characteristics of light. 

The smallest unit (packet) of electromagnetic energy (a photon) is related to frequency by the formula, E = hv, in which E is the energy and h is Planck s constant. Alternatively, the relation can be written, E = hc/A,. Frequency (v) is a number with units of cycles per second (cps, the number of times a wavefront passes a given point in unit time, sec ) and is given the name Hertz (Hz), Planck s constant is a fundamental number, measured in J sec or erg-sec. 

The units in which light is measured are summarized briefly here. 

The standard unit of light measurement (the light flux) is the lumen, which is the amount of energy (power, watts) emitted or received (Joules per second). At 555 nm, 1 lumen = 0.00147 watts = 0.00147 J.s-1. This is sometimes called a lightwatt . 

Since the energy for one photon at this wavelength is 3.6 x 10- J, then the number of photons represented by 1 lumen is approximately 4 X 10i per second, radiated or received. Thus, the luminous flux (lumens, Im) gives the power radiating from an object or the power received by an object. 

Lasers have been shown to serve as an efficient light source for some reactions. 

In particular, the 308 nm emission of the XeCl exdmer laser is a convenient source that, unlike arc lamps, is monochromatic and does not emit heat. 

A commercial 3 kW laser of this type (60 cm long, vertically mounted) has been used to build a falling film reactor capable of converting 10 g or more in 10-20 h [7]. At least at present, however, these light sources are rather expensive and require considerable care for their maintenance consequently, they cannot be considered for adoption by an organic photochemistry laboratory requiring a versatile tool for preparative applications. 

Having available a certain number of LED sets might also be an alternative to having the above-mentioned sets of phosphor-coated lamps. Whilst the former set-up would be more expensive than the latter, it would be much more versatile, because in the case of LEDs a much wider choice of X is available (unfortunately, this is not as 

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A schematic representation of a PR apparatus is shown in Figure 2. In PR a pump beam (laser or other light source) chopped at frequency 2 creates photo-injected electron-hole pairs that modulate the built-in electric field of the semiconductor. The photon energy of the pump beam must be larger than the lowest energy gap of the material. A typical pump beam for measurements at or below room temperature is a 5-mW He-Ne laser. (At elevated temperatures a more powerful pump must be employed.)... 

The light produced by a laser has a much more narrow wavelength than the light of an LED or other light sources (Fig. 15.3). In addition, laser light is coherent, i.e., the photons travel in parallel paths from the source. Lasers made of thin-films are similar to bulk lasers (He-Ne, ruby) except that they are more compact and efficient. Due to the short lifetime of the photons, high-frequency modulations are possible. 

Energy Source Photo trophic Uses sunlight or other light source... 

Other light sources, however, emit a distinct number of discontinuous colors. Examples include streetlights, neon signs, sparklers, and fireworks. The spectral patterns... 

In photochemical research and its applications, lasers and arc lamps are very important. Lasers will be considered in section 7.2. A few other light sources of interest will be mentioned here. 

The above protocol provides a basic idea of a typical LAPS set-up. The number of measurement spots and the size of the sensor chip can vary for different applications. Electronic and software need to be adapted to the specific requirements and can be developed from the above description. Other light sources (e.g., laser) and different ways to modulate them might be investigated. The position of the light sources as well as the position of the amplifier stage is of great importance for the signal stability and should be as close as possible to the sensor chip. Furthermore, the conductivity of the electrolyte solution has to be... 

Flower RW, Hochheimer BE A clinical technique and apparams for simultaneous angiography of the separate retinal and choroidal circulation. Invest Ophthalmol 1973 12 248-261. Geeraets WJ, Berry ER. Ocular spectral characteristics as related to hazards from lasers and other light sources. AmJ Ophthalmol 1968 66 15-20. 

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