Focal spot

The sensitivity curves are plots of maximum achieved sensitivity as a function of thickness of the object for a given focal spot size and source to detector distance. The best attainable sensitivity in image intensifier systems is a function of tube voltage, current, scattered radiation and the screen gamma. As a first step, stainless steel plates with thicknesses ranging from 5 mm-30 mm in steps of 5 mm were chosen. These plates had a length of 950 mm and width of 280 mm. The plate is positioned very close and at the center to the LI. tube. The extraneous... 

As the application of the tubes is mostly in radiometric measurement, the focal spot dimensions are not too critical. A 1mm x 1mm square optical focal spot on a 21° anode angle is normal. 

For the radiographic testing of the samples a microfocus X-ray tube, manufactured by Feinfokus, Garbsen was used. The X-ray tube has a focal spot, depending on the power, with a diameter from 5 pm to 50 pm. 

This focal spot diameter is very much smaller than the spots of conventional X-ray tubes. The goodness of a X-ray image is influenced by contrast and sharpness. Caused by the almost punctual point of origin of the X-rays, a low geometrical unsharpness according to equation 1 is reached. 

Similarly, the focusing capability of an array is the strongest focused beam which can be steered. The simplest way to evaluate it is to test a theoretical focusing time delay law, in the near-field and in the natural direction of propagation of the array. The beam pattern characteristics depth, lateral size and length of the focal spot must be found consistent with modelling and no lobe must appear above a predetermined level. 

Because the laser beam is focused on the sample surface the laser power is dissipated in a very smaU area which may cause sample heating if the sample is absorbing and may cause break-down if the sample is susceptible to photodecomposition. This problem sometimes may be avoided simply by using the minimum laser power needed to observe the spectrum. If that fails, the sample can be mounted on a motor shaft and spun so that the power is dissipated over a larger area. Spinners must be adjusted carefully to avoid defocusing the laser or shifting the focal spot off the optic axis of the monochromator system. 

The resolution of an acoustic lens is determined by diffraction limitations, and is 7 = 0.51 /N.A [95], where is the wavelength of sound in liquid, and N.A is the numerical aperture of the acoustic lens. For smaller (high-frequency) lenses, N.A can be about 1, and this would give a resolution of 0.5 Kyj. Thus a well designed lens can obtain a diameter of the focal spot approaching an acoustic wavelength (about 0.4 /Ltm at 2.0 GHz in water). In this case, the acoustic microscope can achieve a resolution comparable to that of the optical microscope. 

Fig. 1—Profile measurement technique of Champper 2000+. A surface measurement is made with a linearly polarized laser beam that passes to translation stage which contains a penta-prism. The beam then passes through a Nomarski prism which shears the beam into two orthogonally polarized beam components. They recombine at the Nomarski prism. The polarization state of the recombined beam includes the phase information from the two reflected beams. The beam then passes to the nonpolarizing beam splitter which directs the beam to a polarizing beam splitter. This polarizing beam splitter splits the two reflected components to detectors A and B, respectively. The surface height difference at the two focal spots is directly related to the phase difference between the two reflected beams, and is proportional to the voltage difference between the two detectors. Each measurement point yields the local surface slope [7].

The analytical area of interest is positioned in the focal spot of the He Ne laser beam, the transient recorder is armed to record, and the Nd YAG laser is fired. This laser pulse of between 5 and 15 ns duration produces a packet of ions that is accelerated from the sample surface and injected into the TOF-MS. All the... 

The maximum power of a conventional X-ray tube is 2.4 kW for broad focus (approx.. 2x 12 mm focal spot size). Modern rotating anodes consume 18 kW and deliver fine focus (approx.. 0.1 x 1 mm focal spot size). Most important for high intensity is not the power consumption, but the product of focal spot power density and focal spot size or, more accurately, the flux on the sample measured in photons/s (cf. Sect. 7.6). 

The position of the focal spot between the two PMs can be varied using an external motorized stage. After reflection on each PM, the beam is collimated by a second identical off-axis parabola and sent as a parallel beam in the experimental chamber. The entire setup is maintained under clean vacuum down to at least 10 6 mbar. 

Fig. 10.4. Temporal profiles of the laser in the DPM configuration (circles), using a SPM (triangles) and without correction (line). On the right, the focal spots obtained without using the DPM (top) and with the DPM (bottom)...

The natural way to increase the efficiency of such a frequency conversion process is to use a focused fundamental beam (or, alternatively, a waveguide structure). An established theory of SHG using focused cw beams " predicts, for negligible birefringence waUc-off, an optimal focusing condition which is expressed by the ratio L/b 2.83, where b is the confocal parameter (b = k wQ, where Wqi and ky are the focal spot radius and the wave vector of the fundamental wave respectively). However, this theory applies only to the long-pulse or cw case, where GVM is negligible... 

In another study of SHG in the femtosecond regime, an experiment to determine the optimal focal position within a bulk KNbOj crystal has been carried out. It was shown that, at low powers below the saturation regime, the optimal position of the focal spot is indeed in the center of the crystal as predicted by our present model. 

Shaping of the cathode and hooded anode to minimize focal spot size is a speciahzed art. Apparent focal spots of 1 mm (as viewed from the detectors) are typical for baggage systems. Tube currents are commonly in the range of... 

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