- A measure of the relative aperture of a lens; its light
gathering capacity. It is equal to the ratio of the focal length of the lens
divided by the diameter of its limiting opening (aperture): f-number = focal
length/iris diameter. Note that the number becomes smaller as the aperture
grows larger and that it must be squared to directly measure the area which is
the light gathering capacity. Also see numerical aperture (NA).
false Becke line
- A second bright line which moves in the direction
opposite to the Becke line. It is usually observed with thick particles or when
the refractive index difference between particles and mountant is large. It
appears as a concentration of reflected light at the high index side of the
far point of eye
- For the normal eye, the far point is at infinity. The
rays of light from an infinitely distant point source are parallel and can be
focused with the accommodation muscles of the eye entirely relaxed. See
accommodation; near point of the eye.
- The fast ray or fast component for a crystal or fiber
corresponds to the lower refractive index.
- The area of the object that is seen when the image is observed.
It may range in diameter from several millimeters to less than 0.1 mm. The
magnification and the size of the diaphram opening in the eyepiece governs the
diameter of the field of view.
- The thickness of the object space within which objects
focused by a lens will all appear in good simultaneous focus. Penetration is a
- In a photomicrographic system particularly, an iris diaphram
that is imaged in the field of view with Kohler illumination. This limits the
extent of the illuminated field and eliminates much extraneous light. See
- The iris diaphram that is located in front of
the collecting lens of the light source. With Koehler illumination, the
condenser focuses the image of the field diaphram onto the image plane.
- Either one of two kinds of point-projection
microscopes, both invented by E. W. Muller:
(1) The older device (1936) is a specialized 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,
(2) A later device (field-ion-mission microscope, 1950) emits absorbed
helium ions from an anode.
- The lower lens in an ocular; the lens nearest the object
field of view
- The extent of the visible image field that can be seen.
- The set of planes in a microscope adjusted for Koehler
illumination that are conjugate with the focused specimen. They include the
plane of the specimen, the field diaphram, the intermediate image plane, and
the image on the retina, photographic emulsion, or the faceplate of the video
- An electrically heated wire used as a source of radiation,
such as electrons, or as a source of heat such as for the vaporization of a
filar micrometer or filar eyepiece
- an eyepiece equipped with a fiducial line in its focal plane that is movable by means of a calibrated micrometer screw in order to make accurate measurements of length.
filar micrometer ocular
- A micrometer ocular with crossed
lines which can be simultaneously focused in and moved across the field by
means of a micrometer screw. The amount of displacement can be read in 0.01
steps on the micrometer drum head. This makes a very delicate measuring device,
since calibration of the micrometer scale by means of a stage micrometer gives
values for each interval on the drum head which are much less than the
resolving power of the highest-aperture objectives.
- Unwanted light in an optical instrument that arises by
reflection at lens surfaces (including the observer's eyeglasses) and lens
barrel, etc., and sometimes from lens aberration. Flare reduces image contrast
and may form undesirable focused images and hot spots. Flare in a microscope is
reduced by immersion of the condenser and objective lenses, anti-reflection
- Microscope objective lenses considerably better
corrected than achromats but not quite as well corrected as the apochromats. By
using fluorite crystals (which have lower dispersion) in place of some of the
glass elements, a fluorite objective corrects for spherical aberrations in
three wavelengths at considerably lower cost than the apochromats.
- Dyes which have little coloring effect under ordinary
lighting conditions but which fluoresce when irradiated with ultraviolet
- The absorption of radiation to produce radiation of longer
wavelengths, i.e., lower energy.
- A microscope illuminated by ultraviolet or blue
light so that the object may re-radiate light of longer wavelengths. To protect
the eyes, a W-absorbing filter should be provided if not built into the
fluorescence recovery after photobleaching (FRAP)
- A new
technique in light microscopy using a pulse from a focused laser microbeam to
deplete the fluorescence in a local region in a living cell. The subsequent
recovery of fluorescence in the irradiated region is measured to establish the
mobility of the molecules that carry the fluorescent tag.
- Microscope objective lenses considerably
better corrected than achromats but not quite as well corrected as the
apochromats. By using fluorite crystals (which have lower dispersion) in place
of some of the glass elements, a fluorite objective corrects for spherical
aberrations in three wavelengths at considerably lower cost than the
- Sometimes called luminous flux; the visible portion of the
radiant energy emitted by a light source. It is measured in lumens per solid
angle. In electrical engineering, it is analogous to the lines of force in a
magnetic field, spoken of as magnetic flux.
- The distance from the optical center of the lens to the focal point
- The focal length of an objective and its working distance are directly proportional.
- The point on the principal axis of a lens at which
impinging parallel rays converge to a common point or focus. The point is
virtual for a negative lens. See virtual image.
- A point at which rays, originating from a point in the object, converge.
- The point or plane in which light rays or an electron beam
form a minimum-sized spot that has the proper intensity distribution. Also the
act of bringing light or electron beams to a fine spot.
- the point at which a lens focuses an axial object
point. Synonymous with
focal point see focal point
focusing device (electrons)
- A device which effectively increases the
angular aperture of the electron beam illuminating the object, rendering the
focus more critical.
focusing glass (light microscopy)
- A hand magnifier, used at the focal
plane of a camera, usually with the ground glass removed. Its purpose is to
examine the image for critical focus. It is, as a rule, a low-power magnifier
so mounted that it is in focus when supported on a piece of clear glass at the
focal plane of the cam- era, for studying the image. It operates equally well
on the aerial image.
focusing magnifier (electron microscopy)
- A low-power light microscope,
telescope, or simple lens used to observe the electron image formed on a
- A class of diffraction fringes formed when the source
of illumination and viewing screen are at a finite distance from a diffracting
edge. In the electron microscope these fringes are best seen when the object is
slightly out of focus.
- A lens built up, progressively, in zones or steps, each
zone with its own individual radius. Considerable spherical correction is
attained, and the weight of the lens is greatly reduced. Fresnel lenses were
originally designed for lighthouses but they are now attainable for small
spotlights, automobile headlights, and similar uses.
- Process by which radiant flux is reflected from an
optically plane boundary between two transparent dielectric materials.
- The front element of a compound lens system; the first lens
element which the entering light encounters. See back lens.