OPTICAL TERMINOLOGY
Figure 9-5 Astigmatism and Change in Spot Shape in Different Focus Positions
(a) (b) (c)
(a) | (b) | (c) |
(4) Field curvature
An image plane of an object on a plane perpendicular to an optical axis does not always become a plane perpendicular to the optical axis, but it generally becomes a curved plane. This symptom is called “field curvature.”
When field curvature is present, the image is more displaced as it becomes closer to the periphery of the visual field. Therefore, when the center of an image is brought into focus, blur occurs in the peripheral areas of the image. To bring the entire image, including the periphery, into clear focus, it is necessary to adequately compensate for this type of aberration.
(5) Distortion
When there is no similar relation between a planar shape on an object and a shape on the image plane, this is called “distortion.” When distortion is present, a square image appears in a shape of a barrel or
Figure 9-6 Distortion
(a) Barrel shape (a)
typetype
The microscope optical system contains some distortion. When distortion is present, it can bring erroneous results of shape measurements. When a microscope is used for precision measurements, pay close attention to this aberration, for example, by providing it with an aberration compensation function.
(6) Chromatic aberration
Glasses used for optical systems have different refractive indexes depending on the wavelength. This causes differences in focal length between wavelengths and generates displacement of image forming position. This phenomenon is called “chromatic aberration,” which is sometimes subdivided into axial displacement on the optical axis, called “axial chromatic aberration” (or lateral chromatic aberration) and displacement on the image plane, called “chromatic
aberration of magnitude.”
Many special glass materials are used, e.g., for apochromats (MPlanApo in Olympus), to eliminate chromatic aberration in a wide range from violet light
9.3 Wavefront Aberration
Since a long time ago, aberrations have been used in “geometric optics,” which considers light as “light rays.” Microscope optical systems are often used for observation of very small specimens at a wavelength level, and sometimes adopt “wave optics,” which regards light as “waves” and handles the phase information, taking account of the influence of diffraction.
In such a case, “wavefront aberration” is used for evaluation. As shown below, when requirements for ideal imaging are satisfied in a microscope optical system, the spherical wavefront (spherical waves) coming from a single point on an object (specimen) is converted to plane waves through an ideal objective lens. The plane waves are converted to spherical waves through an ideal tube lens, and condensed to a single point. The wavefront of these waves is called the “ideal wavefront.”
Figure 9-7 Ideal Microscope Optical System
Specimen Ideal |
| Ideal | Image plane | |||||
|
| objective |
| tube lens |
|
| ||
|
| lens |
|
|
|
|
| |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Spherical | Plane | Spherical |
wave | wave | wave |
Based on the figure indicated for (1) spherical aberration, the behavior of the wavefront in an optical system that has an aberration is described below.
Figure | |
| Actual |
| wavefront |
Specimen | Ideal |
| wavefront |
Objective lens with |
|
spherical aberration |
|
A difference (a degree of disagreement) between the ideal wavefront and the actual wavefront shown above is called “wavefront aberration.”
46
