Critical underfocus in low‐magnification electron microscopy: Rationale and practice
Identifieur interne : 000967 ( Main/Exploration ); précédent : 000966; suivant : 000968Critical underfocus in low‐magnification electron microscopy: Rationale and practice
Auteurs : Timothy Fahrenbach [États-Unis] ; John H. Luft [États-Unis]Source :
- Journal of Electron Microscopy Technique [ 0741-0581 ] ; 1988-04.
English descriptors
Abstract
Focus in transmission electron microscopy, especially at low‐to‐moderate powers, is usually achieved by an empirical, though largely haphazard, process of defocusing the objective below what is known to be perfect focus as defined by the absence of Fresnel diffraction fringes at the edges of holes. In this article we provide a precise, empirical method of focus, whose rationale resides in the less‐than‐perfect image transfer from the image space of the objective to the negative, i.e., the modulation transfer function. In practice, “perfect focus,” as defined above, is established with a beam deflector (“wobbler”), to which underfocus is then applied routinely by reference to a table. Such “critical underfocus” values have to be calibrated, as described here, for each microscope and depend on all factors that influence contrast, that is, accelerating voltage, condenser defocus and coherence of illumination, focal length of the objective, aperture sizes, combination of intermediate and projection lenses, and the properties of the photographic emulsion.
Url:
DOI: 10.1002/jemt.1060080411
Affiliations:
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Luft</name><affiliation wicri:level="2"><country xml:lang="fr">États-Unis</country><placeName><region type="state">Washington (État)</region></placeName><wicri:cityArea>Department of Biological Structure, University of Washington School of Medicine, Seattle</wicri:cityArea></affiliation></author></analytic><monogr></monogr><series><title level="j">Journal of Electron Microscopy Technique</title><title level="j" type="abbrev">J. Elec. Microsc. Tech.</title><idno type="ISSN">0741-0581</idno><idno type="eISSN">1553-0817</idno><imprint><publisher>Wiley Subscription Services, Inc., A Wiley Company</publisher><pubPlace>Hoboken</pubPlace><date type="published" when="1988-04">1988-04</date><biblScope unit="volume">8</biblScope><biblScope unit="issue">4</biblScope><biblScope unit="page" from="433">433</biblScope><biblScope unit="page" to="439">439</biblScope></imprint><idno type="ISSN">0741-0581</idno></series><idno type="istex">8F1082240E68FEC4DF54BEFDF5AA2A352E288C64</idno><idno type="DOI">10.1002/jemt.1060080411</idno><idno type="ArticleID">JEMT1060080411</idno></biblStruct></sourceDesc><seriesStmt><idno type="ISSN">0741-0581</idno></seriesStmt></fileDesc><profileDesc><textClass><keywords scheme="KwdEn" xml:lang="en"><term>Amplitude contrast</term><term>Emulsion</term><term>Modulation transfer function</term><term>Phase contrast</term></keywords></textClass><langUsage><language ident="en">en</language></langUsage></profileDesc></teiHeader><front><div type="abstract" xml:lang="en">Focus in transmission electron microscopy, especially at low‐to‐moderate powers, is usually achieved by an empirical, though largely haphazard, process of defocusing the objective below what is known to be perfect focus as defined by the absence of Fresnel diffraction fringes at the edges of holes. In this article we provide a precise, empirical method of focus, whose rationale resides in the less‐than‐perfect image transfer from the image space of the objective to the negative, i.e., the modulation transfer function. In practice, “perfect focus,” as defined above, is established with a beam deflector (“wobbler”), to which underfocus is then applied routinely by reference to a table. Such “critical underfocus” values have to be calibrated, as described here, for each microscope and depend on all factors that influence contrast, that is, accelerating voltage, condenser defocus and coherence of illumination, focal length of the objective, aperture sizes, combination of intermediate and projection lenses, and the properties of the photographic emulsion.</div></front></TEI><affiliations><list><country><li>États-Unis</li></country><region><li>Oregon</li><li>Washington (État)</li></region></list><tree><country name="États-Unis"><region name="Oregon"><name sortKey="Fahrenbach, Timothy" sort="Fahrenbach, Timothy" uniqKey="Fahrenbach T" first="Timothy" last="Fahrenbach">Timothy Fahrenbach</name></region><name sortKey="Luft, John H" sort="Luft, John H" uniqKey="Luft J" first="John H." last="Luft">John H. Luft</name></country></tree></affiliations></record>Pour manipuler ce document sous Unix (Dilib)
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