Monochromatic radiation incoherent

Lasers with short pulses are not used in Raman spectrometers, mainly because the detectors in Raman spectrometers are tuned to high sensitivity. Such detectors are very easy to saturate and this is a case where short and intense laser pulses are employed for excitation of Raman scattering. It must be noted, that gas lasers are not perfect sources of monochromatic radiation. Together with intense coherent radiation such lasers produce weak incoherent radiation, caused by a different transition between electronic energy levels of the gas. The intensity of this incoherent and noncollimated radiation can be suppressed by increasing the distance between the laser and the sample, by placing a spatial filter (consisting of two lenses and a pinhole) or a narrow-band filter (usually an interference filter) into the laser beam. 

Molybdenum target. X-ray, 304,311 Momentum, angular, in NMR, 499 Monochromatic radiation Beer law, 158 diode lasers, 173 incoherent, 169 Monochromator, 180-189 dispersion, 185 echelle spectrograph, 261 effect of slit width on resolution. 188... 

Figure 2.4 shows some types of refractive microlenses that can be fabricated utilizing the standard microfabrication procedures in materials convenient for the MWIR and LWIR ranges. Most of them are loosely based on the solutions for microlenses used in fiber optics to improve coupling between laser sources and fibers [98]. These immersion lenses were thus intended for the operation with coherent and monochromatic radiation, while most of the microlenses in the field of IR detector technology are intended for incoherent, mono- or polychromatic Lambertian sources and, of course, they operate in different atmospheric windows. 

Strictly monochromatic radiation propagating in a unique direction (e.g., from a point source) is never realized. A monochromatic wave implies a periodic process of infinite duration. Such waves do not exist, although the signal from a stable, singlemode laser provides a fair approximation. Ordinary incoherent radiation emitted and reflected from real atmospheres and surfaces consists of individual wave packets of finite length and duration a few meters and 10 seconds are typical values. Similarly, point sources are replaced by extended sources in practice. Radiation from such sources tends to be incoherent and covers a range of frequencies and directions. Thus, it is more convenient to work with a distribution of plane waves and their associated Poynting vectors. 

Lasers are another source of excitation radiation used in fluorescence detection systems. The high-directional output of a laser maximizes the fraction of total output that can be easily focused down to a spot size compatible with the dimensions of CE detection cells. The output of a laser is also typically monochromatic, or a discrete set of spectrally narrow lines. This type of output makes it relatively easy to filter out low-level incoherent plasma radiation and undesired emission lines without greatly diminishing the overall output power. In addition, many lasers provide flexibility in terms of pulse width and repetition rate, which allows one to optimize excitation with respect to analyte photostability. 

If the phase of the wave can be calculated from its phase at nearby points and times, the radiation is coherent, monochromatic and parallel light, for example, is coherent. The phase and amplitude of completely incoherent light vary randomly in space and time. When two coherent waves of amplitudes a and b come together they interfere with each other. The result is a wave of intensity (a + bf if they are exactly in phase (constructive interference) and intensity (a - bf if they are completely out of phase (destructive interference). In general the waves must be added by vector sum rules. Incoherent waves interfere momentarily, but over any period of observation the phase effects average out. The resulting average intensity is the sum of the intensities of the two waves, a + b ). 

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