7.29 Optical Instruments
269
X-ray and gamma-ray images visible. Video transmissions were first performed by
kinescopic tubes, which projected a light image on a specially coated plate in a
vacuum which would become locally charged when light hit it. An electron beam
was swept in raster fashion across the plate to release and read the charge. The
collected electrons produced an electric signal which was in proportion to the charge
at the location the beam was directed. An amplified signal used to control the current
sent to the phosphor of a Cathode Ray Tube (CRT), with the electron beam of the
CRT swept across the phosphor mirroring the beam in the kinescopic tube, made up
an indirect image intensifier.
Direct image intensifiers cause incoming photons absorbed on a matrix of pixel
elements to emit electrons. These electrons are then accelerated in an electric field,
often along microchannels, so that when they collide with a phosphor, they emit
visible light with an intensity far higher than the original light. Cascading the
sequence of electron accelerations creates additional amplification of the light.
With intensifiers having light amplification, lower doses of X-rays for dental
and organ imaging is possible. In cameras and video systems, the initial conversion
from photons to electrons is made by Charge-Coupled Devices (CCDs) rather than
phosphors, and amplification is done electronically.
CCDs and electronic flat screens have largely replaced kinescopic tubes and
CRTs. This technology has dramatically lowered X-ray doses needed for diagnosis.
7.29.9 Fluorescent Tags
Biomolecular dynamics can be studied by taking advantage of ‘Förster Resonance
Energy Transfer’ (FRET). (Also called fluorescence resonance energy transfer.)
Donor chromophores send energy to nearby acceptor chromophores, which then
radiate. The sites for the energy transfer are called ‘fluorophores’, and may
be inserted into the molecules of interest by the experimentalist. Because the
interaction between chromophores is predominantly dipole-dipole, energy transfer
requires separation distances within a few nanometers. Small changes in distances
of this size become visible by the amount of light emission.
In single molecule FRET (smFRET), two fluorescent dye molecules are attached
to a biological molecule, such as each side of the DNA backbone. The interaction
between the two dyes can tell us about how far apart they are, say, during replication.
This distance information can be used to measure the shape changes of a single
molecule during its functional activity.
Applied in vivo, FRET has been used to detect the location and interactions of
genes and cellular structures, and to study metabolic pathways.
269
X-ray and gamma-ray images visible. Video transmissions were first performed by
kinescopic tubes, which projected a light image on a specially coated plate in a
vacuum which would become locally charged when light hit it. An electron beam
was swept in raster fashion across the plate to release and read the charge. The
collected electrons produced an electric signal which was in proportion to the charge
at the location the beam was directed. An amplified signal used to control the current
sent to the phosphor of a Cathode Ray Tube (CRT), with the electron beam of the
CRT swept across the phosphor mirroring the beam in the kinescopic tube, made up
an indirect image intensifier.
Direct image intensifiers cause incoming photons absorbed on a matrix of pixel
elements to emit electrons. These electrons are then accelerated in an electric field,
often along microchannels, so that when they collide with a phosphor, they emit
visible light with an intensity far higher than the original light. Cascading the
sequence of electron accelerations creates additional amplification of the light.
With intensifiers having light amplification, lower doses of X-rays for dental
and organ imaging is possible. In cameras and video systems, the initial conversion
from photons to electrons is made by Charge-Coupled Devices (CCDs) rather than
phosphors, and amplification is done electronically.
CCDs and electronic flat screens have largely replaced kinescopic tubes and
CRTs. This technology has dramatically lowered X-ray doses needed for diagnosis.
7.29.9 Fluorescent Tags
Biomolecular dynamics can be studied by taking advantage of ‘Förster Resonance
Energy Transfer’ (FRET). (Also called fluorescence resonance energy transfer.)
Donor chromophores send energy to nearby acceptor chromophores, which then
radiate. The sites for the energy transfer are called ‘fluorophores’, and may
be inserted into the molecules of interest by the experimentalist. Because the
interaction between chromophores is predominantly dipole-dipole, energy transfer
requires separation distances within a few nanometers. Small changes in distances
of this size become visible by the amount of light emission.
In single molecule FRET (smFRET), two fluorescent dye molecules are attached
to a biological molecule, such as each side of the DNA backbone. The interaction
between the two dyes can tell us about how far apart they are, say, during replication.
This distance information can be used to measure the shape changes of a single
molecule during its functional activity.
Applied in vivo, FRET has been used to detect the location and interactions of
genes and cellular structures, and to study metabolic pathways.