“透镜系统”的版本间的差异

2017年4月28日 (五) 10:30的版本

1. einzel lens

An electrostatic lens that has the same potential at the entrance and exit of the lens. Normally, the "einzel lens" consists of three electrodes. Two electrodes at both ends are set at the ground potential, and a positive or negative potential is given to the central electrode. The lens with a positive potential at the central electrode is called the acceleration-type einzel lens, whereas the lens with a negative potential is called the deceleration-type einzel lens. Both are convex lenses. The electrons passing through the einzel lens undergo lens action without acceleration and deceleration at the exit of the lens. This lens is used for ion-beam instruments, for example, an ion mass analyzer. Related term immersion lens

2．anastigmat

A lens that corrects all of five Seidel aberrations. Almost all the commercial cameras, which have been recently developed, are fitted with an anastigmat lens. Related term Five Seidel aberrations

3．anaplat

A lens that corrects spherical and coma aberrations. A telescope and an optical microscope are equipped with an achromatic (chromatic aberration corrected) "anaplat." The anaplat is accomplished in the following way. The surface at the object side of the lens is finished in a spherical shape to eliminate the spherical aberration. The surface at the image side of the lens is fabricated to suppress the occurrence of the coma aberration (surface satisfying the sine condition). Related term spherical aberration (off-axial) coma aberration

4. underfocus

"Underfocus" means that the excitation of the objective lens in a TEM is slightly decreased from that at the in-focus (focused on a specimen). At this excitation, the image produced on the selector aperture is the specimen image when the objective lens is focused on a position above the specimen. To enhance contrast in a bright (or dark) field image (to make the outline of the image clear), the image is often taken at a slight underfocus, instead of at the in-focus. Related term overfocus 5. phase plate Keywords "phase plate" A plate that causes changes in phases of electron waves. To obtain the Zernike phase contrast in a TEM, the phase plate is made of a carbon film with a small absorption. The thickness of the phase plate is adjusted so that scattered waves undergo a phase shift of π/2 with respect to a transmitted wave. The plate is inserted into the scattered waves. Then, transmitted and scattered waves cause interference. As a result, the phase change of an object is converted into amplitude (intensity) change. Since the phase plate improves the wave interference at a small spatial-frequency range (corresponding to a long-distance range in a real space), it is effective in obtaining high contrast for biological specimens. The spatial resolution obtained by the phase plate is approximately 0.5 nm. Related term Zernike phase contrast 6. immersion lens Keywords "immersion lens" An electrostatic lens that has different potentials at the entrance and exit of the lens. The term of "immersion lens" originates from the immersion lens of light optical lenses, where the refraction index is different at the entrance and exit of the lens. The extraction electrode of the electron gun is an acceleration-type immersion lens. The Wien filter is normally a deceleration-type immersion lens. It is noted that lens action arises when electrons are accelerated or decelerated. Related term einzel lens 7. image wobbler Keywords "image wobbler" A function to perform focusing of the illumination beam by the use of deflection coils below the condenser lens. The electron beam is repeatedly tilted at slightly positive and negative directions with respect to the optical axis. The excitation of the objective lens or the specimen position is adjusted so that the image does not move while the "image wobbler" is operated. The image wobbler is useful for rough focusing at magnifications up to ～50,000×.