Lens imaging system, refractive

Figure 11. Image formation by a refractive lens system showing the object O in the mask or object plane imaged to I in the wafer or image plane.

Figure 16. A refractive lens imaging system using partially coherent light, condenser lens and objective lens.

Beam condensers, by using a pair of ellipsoid mirrors, produce very small images of the Jacquinot stop or the entrance slit at the sample position. The size of these images may be even further reduced by making use of a Weierstrass sphere. Weierstrass (1856) showed that a spherical lens has two aplanatic points . If a sphere of a glass with a refractive index n is introduced into an optical system which has a focus at a distance of r n from its center, then the beam is focused inside the sphere at a distance of r/n from the center (Fig. 3.5-9). In this case the angle O in Eq. 3.4-5 may approach 90°. Thus, a sample with a very small area can fully fit the optical conductance of the spectrometer (Fig. 3.4-2d). Microscopes usually have an optical conductance which is considerably lower than that of spectrometers. In this case, the sample and the objective are the elements limiting the optical conductance (Schrader, 1990 Sec. 3.5.3.3). 

A ray passing through the front focal point will be refracted in a direction parallel to the axis. Sketch the light paths from object to image in a single lens system in following situations. 

Fig. 12.28 The light path in a typical microscope. Light is focused by the condenser lens (typically a system of two lenses), scattered by the sample, and refocused by an objective lens. The ability of the objective lens to resolve two objects into distinct images depends on the numerical aperture, which is related to the refractive index of the lens material and the angle a, as discussed in the text.

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