Off-axis illumination

The SIM technique uses the intensity infomiation in two dimensions (2D), which can enhance the sensor detection sensitivity. The intensity distribution in 2D is the function of the optical excitation and the boundary conditions of the optical fiber, whereby changing the boundary conditions results in intensity modulation in 2D [36]. It is known that in SIM technique applications, higher-order modes are excited by off-axis illumination of the optical fiber [16, 38]. Those modes have more interactions with the core/cladding interface therefore, they are more sensitive to changes in the refractive index of the cladding material. 

A number of optical techniques to enhance resolution have been proposed and have played an important role in extending the resolution and life of optical lithography, although they cannot be applied to any pattern sizes or shapes but are pattern-dependent. These techniques include annular and off-axis illumination, phase shift masks, multiple exposures, etc. [505]. 

This is the intensity of the exposure radiation in the plane of the wafer. The extended source method, or Hopkins method,is often used to predict the aerial image of a partially coherent, diffraction-limited, low-numerical-aperture-aherrated projection system based on scalar diffraction theory. For very high NA, vector calculations involving the complete solution of Maxwell s equation are used. The illumination may be of a single wavelength or it may be broadband. The illumination source may be a conventional disk shape or other more complicated shapes as in off-axis illumination. ... 

Some of these problems can be overcome using a top barrier coating to isolate the resist layer from the immersion medium [21]. Current state-of-the-art immersion photolithography systems utilise a wavelength of 193 nm [22]. Using optical techniques such as off-axis illumination and water immersion projection lenses, these systems can define feature sizes as fine as 38 nm. 

Figure 5. Schematic arrangement for hologram formation with an electron biprism. A plane wave illuminates the specimen placed off-axis. After the object lens a wire is placed between two earthed plates. The wire is the electron optical analog of a Fresnel biprism and causes the unperturbed and perturbed waves forming the electron hologram to interfere. The object phase-shift causes a displacement in the hologram fringes, and is thus observable.

Figure 9.5. A differential radiometer using back-to-back off-axis paraboloidal dishes. The feeds are designed to mainly illuminate the center of the dishes with minimal spillover past the edges.

More challenging is the question of how to get the light into the sample with good efficiency. Different methods have been compared in Ref. 72. In the early times of CIDNP, the NMR probe was frequently modified as to allow the insertion of a quartz rod from below, which was either orientated in line with the NMR tube or off-axis. In the first case, the light entered through the bottom of the tube in the second, a prism on top of the quartz rod effected side-on illumination of the region... 

FIGURE 26.4 Spectrofluorometer. Fluorescence methods can be extremely sensitive to the low background interference. Since the detector is off-axis from the incident light and a second monochromator blocks light of wavelengths illuminating the sample, virtually no signal reaches the detector other than the desired fluorescence. 

Projection of a reflective LV is more difficult than the corresponding transmissive one. The two demonstration units which are described in following section both employ a reflex system in which the cell is illuminated through the lens as in Fig. 8. The lens is used in a telecentric manner and the aperture must be larger than the diameter of the cell. As the size of the cell is increased it becomes increasingly difficult to find a lens (for laboratory use) which will do the job. A custom-designed telecentric lens will also increase in cost rapidly as the cell size is increased. An alternative which does not have this problem is an off-axis reflective Type I system in which the aperture of the lens may be much smaller than the cell. The problem with this system is that in order to have a short projection throw, which is necessary for a desk-top rear-projection screen, the lens must have a wide field of view (60 or more). This is not easy to achieve with a high resolution flat-field lens. 

An elegant approach which overcomes these difficulties was suggested in the group of Sackmann et al.. A parallel polarized laser beam was focused to an off axis spot in the back focal plane of a microscope objective of high numerical aperture. The sample which is located in the front plane is then illuminated by a parallel polarized beam of light incident on the SEimple under an oblique angle. The exact angle of incidence is controlled by the... 

We now discuss the use of fluorescence to study rotational motions of molecules on a finer scale. Suppose we have a sample of randomly oriented molecules that we illuminate with linearly polarized light. Let the polarizarimi be parallel to the laboratory s z-axis. The light will selectively excite molecules that have their transition dipole (/< a) oriented parallel to this same axis. However, molecules with off-axis orientations also will be excited with a probability that depends oti cos 0, where 0 is the angle from the z-axis (Eqs. 4.8a—4.8c). 

Equation (1) describes the selective reflection from a cholesteric material for normal incidence only. As the illuminating radiation becomes off-axis, the optics of the system become highly complicated. However, it has been shown that the angular dependence of the selectively reflected wavelengths varies approximately as... 

For quantitative determination of the shear stress, the two cameras and lighting source are set at optimum off-axis (A), observation (a), and illumination fi) angles, respectively. The calibration curve between the relative hue of both the cameras (left and right), that is,... 

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