| field diaphragm | <microscopy> In a photomicrographic system particularly, an iris diaphragm that is imaged in the field of view with Kohler illumination. This limits the extent of the illuminated field and eliminates much extraneous light. The iris diaphragm that is located in front of the collecting lens of the light source. With Kohler illumination, the condenser focuses the image of the field diaphragm onto the image plane. (05 Aug 1998) |
|---|---|
| field-emission microscope | <instrument, microscopy> Either one of two kinds of point-projection microscopes, both invented by E. W. Muller: (1) The older device (1936) is a specialised cathode-ray tube, employing field-emission of electrons from a negatively charged tip of a very sharp needle in a vacuum, by point-projection of the image onto a positively charged, fluorescent screen. (2) A later device (field-ion-mission microscope, 1950) emits absorbed helium ions from an anode. (05 Aug 1998) |
| field emission tube | An X-ray tube that uses a cold cathode, relying on the tube voltage to pull electrons from it to the anode. (05 Mar 2000) |
| field fever | A leptospirosis caused by leptospira. Synonym: canefield fever. (05 Mar 2000) |
| field gradient | In magnetic resonance imaging, a magnetic field that varies with location, superimposed on the uniform field of the magnet, to alter the resonant frequency of nuclei and allow recovery of their spatial position. Synonym: field gradient. (05 Mar 2000) |
| field identification | <zoology> The determination of the taxonomic identity of an individual specimen, under field conditions, often with the aid of keys etc. See: Identification. (09 Jan 1998) |
| field ion microscope | <instrument> Type of microscopy in which the specimen is illuminated with ions, often gallium ions, that are focussed electrostatically. The ions remove components of the specimen, lower atomic masses first. These are imaged and provide information on elemental distribution with a resolution of perhaps 30 nm. (18 Nov 1997) |
| field lens | <physics> The lower lens in an ocular, the lens nearest the object field. (05 Aug 1998) |
| field lines | <radiobiology> Lines in space along which a field is either changing or not changing (depends on the kind of field) but which help to create diagrams which characterise the behaviour and effects of the field. For instance, electric field lines run in the direction that the electric field will push charged particles, the strength of the field is proportional to the density of the field lines. On the other hand, the magnetic force pushes particles in a direction perpendicular to both the particle's velocity and the direction of the magnetic field line. (09 Oct 1997) |
| field of consciousness | The content of awareness at any given moment. (05 Mar 2000) |
| field of fixation | In ophthalmology, the angular distance around which the line of fixation can be turned. (05 Mar 2000) |
| field of view | <microscopy> The extent of the visible image field that can be seen. (05 Aug 1998) |
| field planes | <microscopy> The set of planes in a microscope adjusted for Kohler illumination that are conjugate with the focused specimen. They include the plane of the specimen, the field diaphragm, the intermediate image plane, and the image on the retina, photographic emulsion, or the faceplate of the video pickup device. (05 Aug 1998) |
| field-reversed configuration | <radiobiology> A compact torus produced in a theta pinch and having (in principle) no toroidal field. The potential advantages for a fusion reactor include a simple (linear) machine geometry, an average plasma pressure close to the confining field pressure, and physical separation of formation and burn chambers. The are predicted to be violently unstable to tilting, but this is rarely observed. (09 Oct 1997) |
| field shaping coils | <radiobiology> Type of poloidal field coils (in a tokamak) which create magnetic fields which shape and control the plasma. Used to constrain horizontal and vertical displacements of the plasma, as well as (in some configurations) produce non-circular plasma cross-sections (poloidal cross-section) and/or create one or more divertor separatrices. (09 Oct 1997) |