Diffraction grating. A transparent or
reflective substrate containing an array of parallel lines having the form of
alternating grooves and ridges with spacings close to the wavelength of light.
Light that is reflected by or transmitted through such a grating becomes
strongly diffracted. Depending on the geometry of illumination and wavelength, a
grating can generate color spectra and patterns of diffraction spots.
reflective substrate containing an array of parallel lines having the form of
alternating grooves and ridges with spacings close to the wavelength of light.
Light that is reflected by or transmitted through such a grating becomes
strongly diffracted. Depending on the geometry of illumination and wavelength, a
grating can generate color spectra and patterns of diffraction spots.
Diffraction plane. One of the aperture
planes of the light microscope containing the focused diffraction image of the
object. Under conditions of Koehler illumination, the diffraction plane is
located in or near the back focal plane of the objective lens.
planes of the light microscope containing the focused diffraction image of the
object. Under conditions of Koehler illumination, the diffraction plane is
located in or near the back focal plane of the objective lens.
Distortion. An aberration of lenses,
where the magnification factor describing an image varies continuously between
the central and peripheral portions of the image. Depending on whether the
magnification is greater at the center or at the periphery, the distortion can
be of the barrel or the pincushion type, respectively.
where the magnification factor describing an image varies continuously between
the central and peripheral portions of the image. Depending on whether the
magnification is greater at the center or at the periphery, the distortion can
be of the barrel or the pincushion type, respectively.
Double refraction. In polarization
optics, the splitting of light into distinct O and E rays in a birefringent
material. When a birefringent crystal of calcite is placed on a page of printed words, the effects of double
refraction are clearly observed as an overlapping, double image of the text.
optics, the splitting of light into distinct O and E rays in a birefringent
material. When a birefringent crystal of calcite is placed on a page of printed words, the effects of double
refraction are clearly observed as an overlapping, double image of the text.
Emission filter. In fluorescence
microscopy, the final element in a fluorescence filter cube, which transmits
fluorescence emission wavelengths while blocking residual excitation
wavelengths. Commonly called a barrier filter. Emission filters are colored
glass or interference filters and have the transmission properties of a
bandpass or long-pass filter.
microscopy, the final element in a fluorescence filter cube, which transmits
fluorescence emission wavelengths while blocking residual excitation
wavelengths. Commonly called a barrier filter. Emission filters are colored
glass or interference filters and have the transmission properties of a
bandpass or long-pass filter.
Emission spectrum. In fluorescence, the
spectrum of wavelengths emitted by an atom or molecule after excitation by a
light or other radiation source. Typically, the emission spectrum of a dye
covers a spectrum of wavelengths longer than the corresponding excitation
spectrum.
spectrum of wavelengths emitted by an atom or molecule after excitation by a
light or other radiation source. Typically, the emission spectrum of a dye
covers a spectrum of wavelengths longer than the corresponding excitation
spectrum.
Epi-illumination. A common method of
illumination in fluorescence microscopy, where the illuminator is placed on the
same side of the specimen as the objective lens, and the objective performs a
dual role as both a condenser and an objective. A dichroic mirror is placed in
the light path to reflect excitatory light from the lamp toward the specimen
and transmit emitted fluorescent wavelengths to the eye or camera.
illumination in fluorescence microscopy, where the illuminator is placed on the
same side of the specimen as the objective lens, and the objective performs a
dual role as both a condenser and an objective. A dichroic mirror is placed in
the light path to reflect excitatory light from the lamp toward the specimen
and transmit emitted fluorescent wavelengths to the eye or camera.
Excitation filter. In fluorescence
microscopy, the first element in a fluorescence filter cube and the filter that
produces the exciting band of wavelengths from a broadband light source such as
a mercury or xenon arc lamp. Commonly the excitation filter is a high-quality
bandpass interference filter.
microscopy, the first element in a fluorescence filter cube and the filter that
produces the exciting band of wavelengths from a broadband light source such as
a mercury or xenon arc lamp. Commonly the excitation filter is a high-quality
bandpass interference filter.
Excitation spectrum. In fluorescence,
the spectrum of wavelengths capable of exciting an atom or a molecule to
exhibit fluorescence. Typically the excitation spectrum covers a range of
wavelengths shorter than the corresponding
fluorescence emission spectrum.
the spectrum of wavelengths capable of exciting an atom or a molecule to
exhibit fluorescence. Typically the excitation spectrum covers a range of
wavelengths shorter than the corresponding
fluorescence emission spectrum.
Eyepiece or ocular. The second
magnifying lens of the microscope used to focus a real magnified image on the
retina of the real intermediate image produced by the objective. The added
magnification provided by the eyepiece increases the angular magnification of
the virtual image perceived by the eye. The typical range of eyepiece
magnifications is 5–25.
magnifying lens of the microscope used to focus a real magnified image on the
retina of the real intermediate image produced by the objective. The added
magnification provided by the eyepiece increases the angular magnification of
the virtual image perceived by the eye. The typical range of eyepiece
magnifications is 5–25.
Eyepiece telescope. See Bertrand lens.
Field diaphragm. A variable diaphragm
located in or near the aperture plane of the light source that is used to
reduce the amount of stray light in the object image. Since the edge of the
diaphragm is conjugate with the object plane under conditions of Koehler
illumination, the field diaphragm is used as an aid in centering and focusing
the condenser lens.
located in or near the aperture plane of the light source that is used to
reduce the amount of stray light in the object image. Since the edge of the
diaphragm is conjugate with the object plane under conditions of Koehler
illumination, the field diaphragm is used as an aid in centering and focusing
the condenser lens.
Field planes. That set of conjugate focal
planes representing the field diaphragm, the object, the real intermediate
image, and the retina.
planes representing the field diaphragm, the object, the real intermediate
image, and the retina.
Flat-field correction. In image
processing, the procedure used to obtain a photometrically accurate image from
a raw image. A so-called dark frame containing bias and thermal counts is
subtracted from the raw image and from a “flat” or “background” image. The
dark-subtracted raw image is then divided by the dark-subtracted flatfield
image to produce the corrected image. With operation, all optical faults are
removed. The photometric relation of pixel values to photoelectron count is
also lost during division, although the relative amplitudes of pixel values
within an image are retained. See also Dark frame and Flat-field frame.
processing, the procedure used to obtain a photometrically accurate image from
a raw image. A so-called dark frame containing bias and thermal counts is
subtracted from the raw image and from a “flat” or “background” image. The
dark-subtracted raw image is then divided by the dark-subtracted flatfield
image to produce the corrected image. With operation, all optical faults are
removed. The photometric relation of pixel values to photoelectron count is
also lost during division, although the relative amplitudes of pixel values
within an image are retained. See also Dark frame and Flat-field frame.
Fluorescence. The process by which a
suitable molecule, transiently excited by absorption of external radiation
(including light) of the proper energy, releases the energy as a
longer-wavelength photon. This process usually takes less than a nanosecond.
suitable molecule, transiently excited by absorption of external radiation
(including light) of the proper energy, releases the energy as a
longer-wavelength photon. This process usually takes less than a nanosecond.
Fluorescence microscopy. A mode of
light microscopy whereby the wavelengths of fluo-rescence emission from an
excited fluorescent specimen are used to form an image.
light microscopy whereby the wavelengths of fluo-rescence emission from an
excited fluorescent specimen are used to form an image.
Fluorite or semiapochromat lens.
Objective lenses made of fluorite or Ca2F, a highly transparent material of low
color dispersion. The excellent color correction afforded by simple fluorite
elements accounts for their alternative designation as semiapochromats. The maximum numerical aperture is usually
limited at 1.3.
Objective lenses made of fluorite or Ca2F, a highly transparent material of low
color dispersion. The excellent color correction afforded by simple fluorite
elements accounts for their alternative designation as semiapochromats. The maximum numerical aperture is usually
limited at 1.3.
Fluorochrome. A dye or molecule capable
of exhibiting fluorescence.
of exhibiting fluorescence.
Fluorophore. The specific region or
structural domain of a molecule capable of exhibiting fluorescence. Examples
include the fluorescein moiety in a fluoresceinconjugated protein and the
tetrapyrrole ring in chlorophyll.
structural domain of a molecule capable of exhibiting fluorescence. Examples
include the fluorescein moiety in a fluoresceinconjugated protein and the
tetrapyrrole ring in chlorophyll.
Focal length. The distance along the
optic axis between the principal plane of a lens and its focal plane. For a
simple converging (positive) lens illuminated by an infinitely distant point
source of light, the image of the point lies precisely one focal length away
from the principal plane.
optic axis between the principal plane of a lens and its focal plane. For a
simple converging (positive) lens illuminated by an infinitely distant point
source of light, the image of the point lies precisely one focal length away
from the principal plane.
Focal ratio or f-number. The ratio of
the focal length of a lens to the diameter of its aperture.
the focal length of a lens to the diameter of its aperture.
Fovea. A 0.2–0.3 mm diameter spot in
the center of the macula on the retina that lies on the optic axis of the eye
and contains a high concentration of cone cell photoreceptors for color vision
and visual acuity in bright light conditions.
the center of the macula on the retina that lies on the optic axis of the eye
and contains a high concentration of cone cell photoreceptors for color vision
and visual acuity in bright light conditions.
Frame averaging or Kalman averaging. In
electronic imaging, the method of averaging a number of raw image frames to
reduce noise and improve the signal-to-noise ratio. The signal-to-noise ratio
varies as the square root of the number of frames averaged.
electronic imaging, the method of averaging a number of raw image frames to
reduce noise and improve the signal-to-noise ratio. The signal-to-noise ratio
varies as the square root of the number of frames averaged.
Halo. In phase contrast microscopy,
characteristic contrast patterns of light or dark gradients flanking the edges
of objects in a phase contrast image. Halos are caused by the phase contrast
optical design that requires that the image of the condenser annulus and
objective phase plate annulus have slightly different dimensions in the back
focal plane of the objective.
characteristic contrast patterns of light or dark gradients flanking the edges
of objects in a phase contrast image. Halos are caused by the phase contrast
optical design that requires that the image of the condenser annulus and
objective phase plate annulus have slightly different dimensions in the back
focal plane of the objective.
Huygens’ principle. A geometrical
method used to show the successive locations occupied by an advancing
wavefront. An initial source or wavefront is treated as a point source or a
collection of point sources of light, each of which emits a spherical wave
known as a Huygens’wavelet. The surface of an imaginary envelope encompassing
an entire group of wavelet profiles describes the location of the wavefront at
a later time, t. Huygens’ principle is commonly used to describe the
distribution of light energy in multiple interacting wavefronts as occurs
during diffraction and interference.
method used to show the successive locations occupied by an advancing
wavefront. An initial source or wavefront is treated as a point source or a
collection of point sources of light, each of which emits a spherical wave
known as a Huygens’wavelet. The surface of an imaginary envelope encompassing
an entire group of wavelet profiles describes the location of the wavefront at
a later time, t. Huygens’ principle is commonly used to describe the
distribution of light energy in multiple interacting wavefronts as occurs
during diffraction and interference.