Intensity, threshold

Fig. 7. Dependence of threshold intensity required for lasing on DCM-dendrimer concentration...

Cavitation threshold, Intensity of cavitation, rate of chemical reaction Transient threshold Size of the nuclei (cavitation threshold)... 

As expected the threshold for aerated water is lower than that for gas free water (see earlier) and the threshold intensity increases ivith increase in frequency. In fact ten... 

Fig.2.n. Variation in threshold intensity with frequency (a) aerated water (b) air free water. 

This early investigation of Lindstrom and Lamm was also particularly important in that it proved there had to be a threshold intensity of ultrasound before polymerisation took place. 

Figure 17.6 shows the results for a typical composite image of the entire flame in which the image intensity, or gray scale value, is plotted on the vertical axis against radial and axial distance coordinates. To find the flame area, a threshold intensity is first picked and a binary image is created with all pixels with intensities above the threshold set to 1 (white) and the rest set to 0 (black). The number of white pixels is then proportional to the surface area of the flame at that intensity contour. This process is then repeated to build up a plot of flame area versus a normalized threshold number (intensity value/256) such as that shown in Fig. 17.7a. 

False positives may be reduced by raising the threshold intensity for alarming on a target explosive however, this comes at the expense of a lower detection probability. 

Although paraxial approximation becomes imsuitable for higher-WA optics and for non-Gaussian beams, the above insights should remain qualitatively valid in these cases as well. Since only the above-the-threshold intensity part of the spatio-temporal envelope of the beam is important for photomodification, usually this part can be reasonably well approximated by a Gaussian. 

On the other hand, with increasing exposure (more precisely, its intensity), significant irreversible changes in transmissivity are observed (Fig. 7.8). We see that the characteristic at / < 7th and I > Ith, where 7th is the threshold intensity, are quite different. In general, the versus E curve (E denotes exposure magnitude) may be divided into three parts ... 

Figure 7.11 shows the evolution of Raman spectra by increasing the light intensity of illumination. It is important to point out the existence of a certain threshold intensity of the incident laser beam—below 7th, the initial shape of the spectrum was recovered after turning off the illumination above it, the spectrum further transforms... 

The threshold intensity (relative to 100.0) for reporting reflections has been decreased to 0.01. This means that more weak reflections tend to be listed for the high-symmetry structures. If there are too many reflections to list on one page, the intensity threshold is increased so that only the 165 most intense reflections out to 50° 26 are listed. 

The photographs from which our original data set was obtained were used to do this, taking as a guide the intensities of weak reflections superimposed upon a background similar in intensity to that where the unobserved reflection would be expected. Having established values for our own data set, it was assumed that threshold intensities of unobserved reflections in other sets would be in a similar ratio to the intensities of nearby observed reflections. 

Cavitation begins at much smaller intensities when low sound frequencies are applied. Fig. 5 describes how the threshold intensity increases with increasing frequency. Drawing a vertical line at approximately 20 kHz, as one moves up this vertical line, wave intensity increases [W/cm2]. The first thing one encounters as the intensity is increased is the curve for aerated water, or water saturated with air. The intensity at this point is sufficient to produce cavitation as desorbed air contributes to bubble nucleation. As one continues to increase intensity, one will encounter the curve for degassed cavitation. This intensity is the absolute maximum intensity allowed (at standard conditions) for sound traveling in water at this frequency. Most of sonochemistry are performed at intensity levels between these two values. 

Figure 2.1 shows the ionization mechanisms for atoms in high intensity laser fields. Non-resonant multiphoton ionization (NRMPI) is expected at an irradiation intensity of around 1013 W cm 2. Optical field ionization (OFI), which comprises tunneling ionization (TI) and barrier suppression ionization (BSI), occurs at an intensity above 1014Wcm 2. The original Coulomb potential is distorted enough for the electron to either tunnel out through or escape over the barrier. The threshold intensity of BSI for atoms can be estimated by (2.1) [14] ... 

The viscosity of a iiquid has been reported to affect the onset of acoustic cavitation. Viscosity is a quaiitative measure of molecular interaction in a liquid. The higher the viscosity is, the higher are the attractive forces between the molecules and hence, the higher is the threshold intensity of US where cavitation starts. Based on experimental evidence, Briggs et al. [58] developed a quantitative relation between liquid viscosity q and the experimental value of the cohesive pressure Pco. which is defined as the difference between the hydrostatic pressure Po — which coincides with ambient pressure when no... 

Photopolymerization is a process wherein a liquid monomer is converted into a solid polymer after exposure to light of appropriate wavelength and threshold intensity. Recently, by employing incident intensity light below the threshold, it was shown how photopolymerization can be induced around the hot spots (that augment the intensity by the factor of the introduction) leading to solid hills in the vicinity of the hot spots [37]. 

Additionally, a hybrid scheme applying a high electric AC field simultaneously with the intense pulse laser is investigated to reduce the damage threshold intensity. This may reduce the damage threshold fluences considerably for relatively longer pulses, whereas it becomes ineffective for laser pulses shorter than 1.0 ps. The threshold fluences are found to be independent of AC frequencies of the applied electric fields except for very high frequencies. 

Below a certain threshold intensity, no in-phase grating could be formed even if the out-of-phase grating was completely saturated and the writing continued. This threshold intensity at 488 nm was about 24 W/cm for PD03 and 22 W/cm for HPAA-N02. These systems also showed interesting optical erasure characteristics, which are discussed in Section 14.4.10. 

It is important to have reliable laser diagnostics, preferably on a shot-to-shot basis this is possible for a 10 Hz system. To characterise the temporal profile of the pulses, single-shot spectra and autocorrelation data can establish whether the laser pulses are Fourier-transform limited. For lasers with a sech profile, a product AvAtxO.32 should be achieved. It is also important to monitor the focal spot, its Airy disc and average intensity. Alternatively, a reasonable measure of the focused intensity can be obtained using Xe gas and the known threshold intensities for producing the various stages of ionization [12]. 

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