Field lens

Required lenses. The field lens (fl = 95 mm), Fig. 1C and D, k, see Section II, b. 2., collecting all the light from the objective produces an image of the objective pupil near the reflection grating used for spectral dispersion or the mirror substituted for topographic operation. This ensures that all the available photons reach the effective area of the grating or mirror. 

Field lens Second lens Third lens... 

Illumination with an intermediate image. Here a field lens is used to produce an intermediate image on a diaphragm. To illuminate the collimator mirror fully, the appropriate zone can be selected with the aid of a lens placed immediately in front of the exit slit. A third lens is used to illuminate the entrance slit homogeneously (Fig. 16B). The magnification is then divided over all 3 lenses, thus chromatic aberrations... 

I is the diameter of the intermediate image, D the diameter of the field lens, % the entrance slit height and ai,a2 and a3 are the distances between the respective lenses. Furthermore ... 

For nondescanned detectors it is important to use a field lens in front of the detector. The general optical principle is shown in Fig. 5.92. In this figure it was assumed that the fluorescence light is separated from the excitation before it passes the tube lens the lens diameters are exaggerated. 

If a tube lens is in the fluorescence detection path, the beam configuration may be slightly different than that shown in Fig. 5.90. The microscope may also have additional lenses in the beam path to project an image on a camera, or to increase the light-collection area of direct detection. In any case, there is a simple way to find the image of the microscope lens behind the field lens Turn on the microscope lamp in the transmission beam path, so that the condenser lens fully illuminates the aperture of the microscope lens. The image of the microscope lens can then easily be found by holding a sheet of paper behind the field lens. 

Galileo also invented a compound microscope. In his 1623 book, the Assayer, Galileo discusses a telescope modified to see objects very close. Originally called an Occhialino (small eye glass), the word microscope was bestowed on this device by Galileo s fellow academician Johannes Faber. A telescope has a convex objective lens and concave eyepiece. In microscope, the eyepiece is convex and the objective lens is concave. Later on, a field lens was added at an intermediate position. 

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. 

If the telescope is to have a sizable field of view, as measured by the angle a, then the eyepiece could become prohibitively large. To reduce the diameter of the eyepiece, sl field lens is installed near the secondary focal plane of the objective. The field lens, which may be part of the eyepiece, directs the principal ray toward the axis and allows the diameter of the eyepiece to be reduced. Typically, the focal length of the field lens may be chosen so that the principal ray intersects the eyepiece at its periphery. This shifts the location of the exit pupil somewhat toward the eyepiece and slightly reduces the eye relief ht-tween the eye and the eyepiece. When the field lens is built into the eyepiece, the second lens in the eyepiece is called the eye lens. 

FIGURE 22 A terrestrial telescope, which includes a relay lens for inverting the image and a field lens for reducing the diameter of the relay lens. 

Unlike most field lenses, the field lens directly before the eyepiece does not project a pupil into the plane of the eye lens because, if it did so, the eye relief would be 0. In almost all other cases, however, the function of a field lens is to project a pupil into the plane of some other lens, and, indeed, the combination of the field lens and the eyepiece projects a pupil into the pupil of the eye. Any system of relay lenses similar to a terrestrial telescope, such as a periscope (which uses mirrors to look along a line of sight displaced from the line of sight of the eye), must necessarily use the principle of projecting a pupil into the apertures of successive lenses. The diameter of any lens is chosen to be equal to the diameter of the appropriate pupil, much as the diameter of a projection lens is matched to the image of the projection lamp. 

Blacker, A. et al. 1966. A new form of extended field lens for use in color television picture tube guns. IEEE Trans. Consumer Eke. (Aug.) 238 246. 

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