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Claims  |
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We claim:
1. Apparatus for monitoring conditions within a living subject comprising:
(a) a probe adapted for insertion into the body of a subject, said probe
including an elongated optically transmissive fiber having a proximal end
and a distal end, said probe also including a luminescent composition
mounted at said distal end of said fiber in optical communication
therewith, said luminescent composition having one or more luminescent
decay time parameters affected by said condition when said distal end of
said probe is disposed within the body of a subject;
(b) means for applying to said composition through said fiber excitation
light varying cyclically in amplitude, said excitation light incorporating
components varying in amplitude at a plurality of excitation modulation
frequencies simultaneously, whereby said luminescent composition will emit
response light varying cyclically in amplitude and including a plurality
of response components varying in amplitude at a plurality of response
modulation frequencies simultaneously and said response light will be
transmitted through said fiber to said proximal end thereof;
(c) means for detecting the cyclically varying amplitude of said response
light transmitted to said proximal end of said fiber and deriving a
frequency domain representation of said cyclically varying response light
amplitude including at least one characteristic of each of said plurality
of response components; and
(d) means for deriving values of one or more luminescence decay parameters
of said composition from said characteristics, whereby said derived
luminescence delay parameters will vary with said conditions.
2. Apparatus as claimed in claim 1 wherein said luminescence composition
has one or more rapidly decaying luminescence modes having decay time less
than about 100 nanoseconds and wherein said means for detecting said
cyclically varying response light amplitude and deriving a frequency
domain representation include means for deriving a frequency domain
representation of components in said response light representing said one
or more rapidly decaying luminescence modes.
3. Apparatus as claimed in claim 1 wherein said means for detecting said
cyclically varying response light amplitude and deriving a frequency
domain representation include means for directly sampling the amplitude of
said response light and providing a response series of digital sample
values each representing the amplitude of said response light at a
predetermined point in the repetitive response light cycle, and a means
for transforming said response series of digital sample values into said
frequency domain representation.
4. Apparatus as claimed in claim 3 wherein said means for transforming
includes means for applying a digital Fourier transform to said response
series of sample values.
5. Apparatus as claimed in claim 3 wherein said means for sampling the
amplitude of said light includes means for sampling said response light
over a plurality of cycles of said response light to provide raw sample
values from said plural cycles and means for averaging raw sample values
from a plurality of cycles to provide each value in said response series
of sample values.
6. Apparatus as claimed in claim 3 wherein said means for providing said
response series of sample values includes means for providing a plurality
of said response series of sample values and said means for transforming
includes means for applying a transformation independently to each said
response series of sample values to thereby provide a plurality of
frequency domain representations of said response light each including
said at least one characteristic of each of said plurality of response
components, said means for deriving a frequency domain representation
further comprising means for averaging values of each said characteristic
in a plurality of said independent frequency domain representations to
derive an averaged frequency domain representation, said means for
deriving luminescent decay parameters from said characteristics including
means for deriving said luminescent decay parameters from said
characteristics in said averaged frequency domain representation.
7. Apparatus as claimed in claim 3 wherein said means for sampling said
response light includes means for sampling said response light at a
frequency lower than the lowest one of said excitation modulation
frequencies.
8. Apparatus as claimed in claim 1 wherein said means for deriving a
frequency domain representation includes means for deriving a phase angle
for each said response component, and said means for deriving one or more
luminescence decay parameters includes means for deriving said parameters
at least in part from said phase angles.
9. Apparatus as claimed in claim 8 wherein said means for deriving one or
more luminescence decay parameters includes means for deriving one or more
decay exponents from said phase angles.
10. Apparatus as claimed in claim 1 wherein said means for deriving a
frequency domain representation includes means for deriving a modulation
value for each said response component, and said means for deriving one or
more luminescence decay parameters includes means for deriving said
parameters at least in part from said modulation values.
11. Apparatus as claimed in claim 1 wherein said fiber has a diameter less
than about 450 micrometers.
12. Apparatus as claimed in claim 11 wherein said probe includes a
plurality of fibers and said probe has an overall diameter less than about
450 micrometers.
13. Apparatus as claimed in claim 11 wherein said luminescent composition
is disposed in a mass abutting the distal end of said fiber and wherein
said mass has a maximum dimension in the direction transverse to the
direction of elongation of said fiber less than about 450 micrometers.
14. Apparatus as claimed in claim 1 wherein said luminescent composition
includes a luminescent material dispersed in a polymer.
15. Apparatus as claimed in claim 14 wherein said luminescent material is
sensitive to oxygen and wherein said polymer is selected from the group
consisting polyurethanes, polysiloxanes, polysiloxanepolycarbonate
copolymers and combinations thereof.
16. Apparatus as claimed in claim 14 wherein said luminescent material is a
compound selected from the group consisting of porphyrin; chlorin,
bacteriochlorin, phosphorinogen and derivatives and combinations thereof.
17. A method of monitoring conditions within a living subject comprising
the steps of:
(a) inserting a probe including an elongated optically transmissive fiber
and a luminescent composition mounted at a distal end of the fiber in
optical communication therewith, into the body of the subject so that the
distal end of the fiber is disposed within the body and one or more
luminescent decay time parameters of the composition are affected by the
condition to be monitored, and so that a proximal end of the fiber is
disposed outside of the body;
(b) directing excitation light through said fiber to said composition, said
excitation light varying cyclically in amplitude, and incorporating
components varying in amplitude at a plurality of excitation modulation
frequencies simultaneously, whereby said luminescent composition will emit
response light varying cyclically and including a plurality of response
components varying in amplitude at a plurality of response modulation
frequencies simultaneously and said response light will be transmitted
through said fiber to said proximal end thereof;
(c) detecting the cyclically varying amplitude of said response light
transmitted to said proximal end of said fiber;
(d) deriving a frequency domain representation of said cyclically varying
response light amplitude including at least one characteristic of each of
said plurality of response components; and
(e) deriving a value of one or more luminescence decay parameters of said
composition from said characteristics, whereby said derived luminescence
delay parameters will vary with said conditions.
18. A method as claimed in claim 17 further comprising the step of
translating said one or more derived luminescence decay parameters into a
value of the condition to be monitored.
19. A method as claimed in claim 17 wherein one or more luminescence decay
parameters include a decay time less than about 100 monoseconds.
20. A method as claimed in claim 17 wherein said step of detecting the
cyclically varying amplitude of said response light and deriving a
frequency domain representation includes the step of sampling the
amplitude of said response light and providing a response series of sample
values in digital form so that each said response light at a predetermined
point in the repetitive response light cycle, and transforming said
response series of sample values into said frequency domain
representation.
21. A method as claimed in claim 20 wherein said step of transforming
includes the step of applying a digital Fourier transform to said response
series of sample values.
22. A method as claimed in claim 20 wherein said step of sampling the
amplitude of said response light includes steps of sampling the response
light over a plurality of cycles of said response light to provide raw
sample values from said plural cycles and averaging raw sample values from
a plurality of cycles to provide each value in said response series of
sample values.
23. A method as claimed in claim 20 wherein said step of providing said
response series of sample values includes the step of sampling the
amplitude of said response light repeatedly to provide a plurality of said
response series of sample values and said transforming step includes the
step of applying a transformation independently to each said response
series of sample values to thereby provide a plurality of frequency domain
representations of said response light, each such representation including
said at least one characteristic of each of said plurality of response
components, said step of deriving a frequency domain representation
further including the step of averaging values of each said characteristic
in a plurality of said independent frequency domain representations to
derive an average frequency domain representation, said step of deriving
luminescent decay parameters from said characteristics including the step
of deriving said luminescent decay parameters from said characteristics in
said averaged frequency domain representation.
24. A method as claimed in claim 20 wherein said step of applying
excitation light includes the step of applying said excitation light as a
series of pulses at a fundamental frequency whereby said response light
will include components varying in amplitude at said fundamental frequency
and at harmonics thereof, and wherein said step of sampling the amplitude
of said response light includes the step of sampling the amplitude of said
response light at a sampling frequency lower than said fundamental
frequency.
25. A method as claimed in claim 20 wherein said step of deriving a
frequency domain representation includes the step of deriving a phase
angle for each said response component, and said step of deriving one or
more luminescence decay parameters includes the step of deriving said
parameters at least in part from said phase angles.
26. A method as claimed in claim 20 wherein said step of the deriving a
frequency domain representation includes the step of deriving a modulation
value for each said response component, and wherein said step of deriving
one or more luminescence decay parameters includes the step of deriving
said parameters at least in part from said modulation values.
27. A method as claimed in claim 17 wherein said condition is the PO.sub.2
of the blood in a mammal.
28. A method as claimed in claim 17 wherein said condition is the pH of the
blood in a mammal.
29. A method as claimed in claim 17 wherein said condition is the PCO.sub.2
of the blood in a mammal.
30. A method as claimed in claim 17 wherein said probe includes a plurality
of optically transmissive fibers each having a luminescent composition
mounted at the digital end thereof, each said luminescent composition
being sensitive to a different condition to be sensed, said step of
inserting said probe including the step of positioning all of said
luminescent compositions within said subject, said step of directing
excitation light, and response light including the steps of directing
excitation light along each of said fibers, said step of detecting the
cyclically varying amplitude of said response light including the step of
detecting the amplitude of response light transmitted to the proximal end
of each fiber, said step of deriving a frequency domain representation
including the step of deriving a frequency domain representation of the
response light amplitude from at least one of said fibers. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
It has long been recognized that the fluorescent and phosphorescent
properties of certain materials vary in accordance with properties of the
surroundings. For example, certain luminescent materials are subject to
"quenching" or extinction of their luminescent response by oxygen. Various
instruments have been proposed to exploit such phenomena in chemical
and/or physical measuring instruments. For example, U.S. Pat. No.
4,810,655 discloses an instrument for determining oxygen concentration by
applying excitation light to a fluorescent material and observing the time
dependence of fluorescence decay. As the oxygen concentration in the
environment surrounding the luminescent material changes, the pattern of
fluorescent decay with time also changes. The '655 instrument employs a
"light pipe" for transmitting the requisite excitation light to the
luminescent material and for transmitting the light back to a sensor.
European Patent Application 0,283,289 monitors the intensity of long lived
phosphorescent emissions from a phosphorescent material bonded to an end
of an optical fiber. The optical fiber is small enough that it can be
inserted through a small tube, such as an intravenous catheter or the
like, so that the phosphorescent material lies within a blood vessel and
acts as an in vivo PO.sub.2 sensor. Other fiber optic based PO.sub.2
sensors are disclosed in U.S. Pat. No. 4,476,870 and European Patent
Application 0,252,578. U.S. Pat. No. 4,576,173 discloses an instrument for
monitoring relatively long-lived "singlet oxygen emission" or
phosphorescence exhibited by certain bodily tissues such as tumors when
those tissues are treated with photosensitizing chemicals and exposed to
incident light. This instrument employs a chopped or pulsatile incident
light. In order to segregate the relatively long-lived "singlet oxygen
emissions" or phosphorescence from the relatively short lived fluorescence
of the sensitizing chemicals, the instrument employs a quadrature
detection system. A signal in phase with the chopped excitation light is
segregated from the quadrature component 90 degrees out of phase with the
chopped excitation light signal. The quadrature signal, out of phase with
the chopping signal, consists essentially of the desired long-lived
"singlet oxygen emission" signal. Although the reference mentions
"frequency domain signal processing", the signal processing involved is
nothing more than isolation of the quadrature signal from the in phase
signal. The amplitude of the isolated quadrature signal is monitored to
monitor the desired "singlet oxygen emission" intensity.
Although these and other fiber optic based luminescence probes and
instruments have been proposed for monitoring chemical and/or physical
conditions within the bodies of living subjects, the instruments available
heretofore have suffered from certain significant drawbacks. For ease of
insertion into the body, a fiber optic probe should be less than about 450
micrometers in diameter. It is especially important to keep the diameter
of each fiber optic probe to a minimum when a plurality of fiber optic
probes are to be passed into the body through a single opening, as through
a lumen of a single intravascular catheter or hypodermic needle. The
amount of luminescent material which can be accommodated in a probe of
such small diameter is limited. Moreover, the total energy which can be
applied to the luminescent material by excitation light transmitted along
the fiber optic is directly proportional to the cross-sectional area of
the fiber optic. Thus, only limited light energy can be applied to excite
the luminescent material in a fiber optic probe. All of these factors tend
to limit the amplitude of the response light emitted by the luminescent
material and transmitted back along the fiber to the proximal end. Even
highly sensitive photodetectors will provide only a weak signal. Further,
the signal is susceptible to interference from many sources, including
changes in optical and/or electronic components with time. The weak
response signal from the actual luminescent material at the probe may be
effectively hidden by the background noise. Stated another way, such
instruments have had poor signal to noise ratios. This problem has been
particularly severe in the case of instruments arranged to monitor the
decay rate of relatively short-lived luminescent phenomena such as
fluorescence or rapidly-decaying phosphorescence.
Thus, prior to the present invention, there have been significant, unmet
needs for still further improvement in luminescence based biomedical
monitoring apparatus and methods.
SUMMARY OF THE INVENTION
The present invention addresses these needs.
One aspect of the present invention provides apparatus for monitoring a
condition within a living subject. Apparatus according to this aspect of
the invention preferably includes a probe adapted for insertion into the
body of a subject, the probe including an elongated optically transmissive
fiber having a proximal end and a distal end. The probe also includes a
luminescent composition mounted at the distal end of the fiber in optical
communication therewith, such that the luminescent composition can receive
excitation light transmitted through the fiber and such that response
light produced by luminescent of the composition will be sent back to the
approximal end of the fiber. The luminescent composition has one or more
luminescent decay time parameters affected by the condition to be
monitored when the distal end of the probe is disposed within the body of
the subject. For example, the luminescent composition may include a
phosphorescent material which is quenched by oxygen such that its
phosphorescence decays more rapidly in the presence of oxygen.
The apparatus preferably also includes means for applying excitation light
to the luminescent composition through the fiber. Desirably, the
excitation light applying means is arranged to apply excitation light
varying cyclically in amplitude and incorporating components at a
plurality of excitation frequencies. Accordingly, the luminescent
composition will emit response light varying cyclically and including a
plurality of response components at a plurality cf response frequencies,
and this response light will be transmitted through the fiber to the
proximal end thereof. The apparatus preferably also includes means for
detecting the response light at the proximal end of the fiber and deriving
a frequency domain representation of the response light including at least
one characteristic of each of the plurality of response components. The
apparatus also includes means for deriving values of one or more
luminescence decay time parameters of the composition from the
characteristics included in frequency domain representation. As the
derived luminescence decay parameters represent the luminescence of the
material under the conditions prevailing within the subject, these
luminescence decay parameters will vary with the conditions within the
subject.
Conversion of the detected response light into a frequency domain
representation, as by Fourier or similar transforms, and determination of
the decay parameters from the transformed information provides several
advantages. Because the instrument monitors decay time parameters rather
than luminescence intensity, it is essentially insensitive to changes in
the luminescence intensity such as may be introduced by deterioration or
bleaching of the luminescent material, deviations in manufacture of the
probe or changes in the optical path of the instrument. Determination of
decay time parameters from the frequency domain information provides
markedly enhanced sensitivity to very brief decay times and very small
changes in decay time. Thus the preferred instruments according to this
aspect of the present invention can monitor the very brief decay times
associated with fluorescence and rapidly decaying phosphorescence, and can
detect the very small changes in those brief decay times occasioned by
changes in environmental conditions. Accordingly, those luminescent
materials which have only fluorescence or brief phosphorescence sensitive
to the condition to be monitored but which have other desirable properties
such as high sensitivity to the condition to be monitored can be employed
in the luminescent composition of the probe. Thus, the luminiscence being
monitored may have a decay time on the order of 100 nanoseconds, 10
nanoseconds or even less.
Moreover, the frequency domain transformation provides an inherent
averaging or noise suppression action. Thus the characteristics of the
frequency domain representation derived by the frequency domain
transformation and the luminescent decay time parameters determined from
the characteristics of the frequency domain representation have relatively
low sensitivity to noise or random variations in the response light
signal. Noise can also be suppressed by monitoring many cycles of the
cyclically varying response light and averaging the results in the time
domain. However, the noise suppression effect of the frequency domain
transformation permits derivation of decay time characteristics having a
given degree of accuracy with monitoring of a lesser number of response
light cycles than would be required with only time domain averaging under
equivalent conditions of signal to noise ratio in the response signal.
This in turn allows measurement of the conditions prevailing within the
subject in a relatively brief time, and hence permits monitoring of
rapidly charging conditions. Using the frequency domain transformation
approach, it is practical to monitor conditions within a living subject
using a probe of a very small diameter, without substantial loss of
accuracy. In preferred apparatus according to this aspect of the present
invention, the fiber optic is less than about 450 micrometers in diameter,
more preferably less than about 200 micrometers, and most preferably about
140 micrometers or less in diameter. The luminescent composition desirably
is provided as a mass at the distal end of the fiber optic, and the
diameter or dimension of the mass in the directions transverse to the
direction of elongation of the fiber desirably is also less than about 450
micrometers or preferably about 200 micrometers and most preferably about
140 micrometers or less. A plurality of such small-diameter fiber optics
may be employed in a composite probe including a plurality of luminescent
compositions while still maintaining the overall diameter of the probe
within reasonable limits.
Most preferably, the means for detecting the response light and deriving a
frequency domain representation are arranged to sample the amplitude of
the response light to thereby provide a response series of sample values
each representing the amplitude of the response light at a predetermined
point in the repetitive response light cycle. The means for deriving a
frequency domain representation most desirably also includes means for
transforming the response series of sample values into the frequency
domain representation, as by applying a Fourier transform to the response
series of sample values. Preferably, the sampling means includes means for
directly sampling the response light, i.e. for sampling the response light
or an electrical signal representing the response light, without any
intermediate frequency shifting or cross-correlation steps prior to
sampling. The apparatus according to the present invention may incorporate
elements disclosed in the commonly assigned, copending U.S. Patent
Application of Jose Ricardo Alcala, one of the coinventors herein,
entitled "Frequency Domain Fluorometry using Coherent Sampling" filed of
even date herewith. The disclosure of said copending application in its
entirety is hereby incorporated by reference herein. As set forth in
greater detail in the Alcala copending application, the sampling may be
conducted at a sampling frequency lower than the fundamental or lowest
frequency component in the response light signal, so that successive
samples will be taken from different cycles of the fundamental. This
permits sampling of very rapidly varying response light with sampling
devices operating at reasonable sample repetition rates. This technique is
referred to as wave skipping sampling. Most desirably, the response light
includes components of a fundamental frequency and at harmonics of that
frequency, and the wave skipping sampling is conducted at a sampling rate
which is coherent with the response light fundamental component, so that
after a given number of samples have been taken, the next sample falls on
the same portion of a cycle as the first sample.
Further aspects of the present invention provide methods of monitoring a
condition with a living subject. The methods according to this aspect of
the present invention preferably include the steps of inserting a probe
having any elongated optically transmissive fiber and a luminescent
material at its distal end into a subject so that the proximal end of the
fiber is accessible from outside the subject. The method further includes
the step of applying excitation light varying cyclically in amplitude as
discussed above to the luminescent material at the distal end of the fiber
by passing the excitation light through the fiber, and detecting the
response light emitted by the luminescent material by monitoring and
transmitted to the proximal end of the fiber. The method further includes
the steps of deriving a frequency domain representation of the response
light including at least one characteristic of each of a plurality of
response components at differing frequencies and deriving values of one or
more luminescence decay parameters of the luminescent composition from
these characteristics of the frequency domain representation. As discussed
above in connection with the apparatus, the frequency domain
transformation and derivation of the decay parameters from the
characteristics of the frequency domain representation provides
significant advantages such as speed and immunity from noise.
These and other objects, features and advantages of the invention will be
more readily apparent with reference to the detailed description of the
preferred embodiments set forth below, taken in conjunction with the
companying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic, practically block form diagram of apparatus in
accordance with a first embodiment of the invention.
FIG. 2 is a detailed view, on an enlarged scale, of a portion of the
apparatus shown in FIG. 1.
FIG. 3 is an idealized representation of certain waveforms occurring during
operation of the apparatus depicted in FIG. 1.
FIG. 4 is a further schematic, partially block diagrammatic view depicting
apparatus according to another embodiment of the invention.
FIG. 5 is a graph depicting results achieved in one monitoring method
according to an embodiment of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Apparatus in accordance with one embodiment of the invention incorporates a
central control computer 20 having a memory 22 and data input and output
devices 24. The apparatus further includes a programmable crystal clock 26
arranged to provide clock pulses at a predetermined frequency. A
programmable pulse generator 28 is arranged to provide electrical
rectangular wave pulses at an excitation frequency F.sub.e and at a
sampling frequency F.sub.s determined by counting clock signals from clock
26. The pulse generator 28 is arranged so that the frequencies F.sub.e and
F.sub.s of the rectangular wave pulses can be selected by computer 20, as
by adjusting the number of clock signals from clock 26 to be counted off
by pulse generator 28 during each cycle. Further, the pulse generator is
arranged to vary the breadth or duty cycle of the rectangular wave pulses
provided at frequencies F.sub.e and F.sub.s as directed by computer 20.
The output of pulse generator 28 at excitation frequency F.sub.e is linked
to the input connection of the light pulse source 30. The light pulse
source includes an electrically controllable light emitting structure.
This structure may be a device such as a light emitting diode or a
combination of elements such as a continuous wave laser coupled to an
acoustic-optic modulator or other device arranged to control passage of
light responsive to applied electrical signals. The light output of pulse
source 30 is connected through a controllable variable aperture 32 and a
bandpass optical filter 34 to the input of a conventional optical
switching and coupling apparatus 36. Filter 34 is arranged to permit
passage of light within a predetermined wavelength band but to block light
at wavelengths outside of such band. One output of switch and coupler 36
is connected to a probe 42.
Probe 42 includes an elongated optically transmissive fiber 43 having a
proximal end 44 and a distal end 45. Fiber 43 is a graded index quartz
optical fiber. It includes a core 46 and a cladding 47 surrounding the
core along the entire length of the fiber. Both the core and the cladding
are formed from transparent materials such quartz, but the cladding has a
slightly lower refractive index than the core. The fiber 43 is generally
circular in a cross section. Its outside diameter d.sub.o is less than
about 450 micrometers preferably less than about 200 micrometers and most
preferably less than about 140 micrometers. Its core diameter d.sub.c is
about 60-80% of the outside diameter, viz, about 100 micrometers where the
outside diameter is about 140 micrometers and preferably less than about
100 microns.
A mass 48 of a luminescent composition comprising crystals of a
phosphorescent material 49 embedded in an oxygen-permeable transparent
plastic resin 50 is mounted at the distal end 45 of the fiber. Desirably,
mass 48 is bonded to the fiber by adhesion of the plastic resin 50 to the
material of the fiber. The diameter of mass 48 (its dimension in the
direction transverse to the direction of elongation of fiber 43) is
substantially the same or only slightly larger than the outside diameter
d.sub.o of fiber 43. The oxygen permeable plastic resin may be a
polyurethane such as Pellethane sold by Dow Chemical or a silicone
polycarbonate resin such as that sold by General Electric Corporation. The
phosphorescent material has a substantial sensitivity to oxygen, and
desirably a substantial quenching of phosphorescence in the presence of
oxygen. Among the materials which may be employed are the metallo
derivatives of compounds selected from the group consisting of porphyrin;
chlorin; bacteriochlorin; porphyrinogen; and the alkyl or aryl substituted
derivatives of these compounds. All of the compounds have characteristic
multi-ring structures with plural nitrogen atoms juxtaposed with one
another adjacent the center of the structure. In the metallo derivatives,
a metal atom or ion is disposed adjacent the center of the structure and
is commonly considered as being bound to the nitrogen atoms of the
multi-ring structure. Among the metallo derivatives which may be employed
are those bearing metals selected from the group consisting of platinum
and palladium. Combinations of these metals may also be used. A
particularly preferred oxygen-sensitive luminescent material is platinum
tetraphenyl porphyrin, commonly referred to as "platinum porphyrin".
The luminescent composition in mass 48 is in light transmitting relation
with fiber 43. Thus, if light is passed along fiber 43 from its proximal
end 44 to its distal end 45, at least some of that light will enter mass
48. Conversely, if light is emitted by mass 48, as by phosphorescence of
the material 49, at least some of that emitted light will pass into the
core 46 of fiber 43 and will pass back along the fiber, towards the
proximal end 44.
The proximal end 44 of fiber 43 is connected to the optical switching and
coupling apparatus 36 via conventional fiber optic interconnecting
devices, schematically indicated at 51. The optical switch and coupler 36
has an output connected via a bandpass optical filter 53 to a detector 56.
Detector 56 is arranged to convert light into electrical signals such that
the amplitude of the electrical signals is directly related to the
amplitude of the incoming light supplied to the detector. Desirably, the
detector is a sensitive device having a very fast response time. Suitable
detectors include photomultiplier tubes such as those supplied under the
designation R928 by Hamamatsu Photonics K.K., Hamamatsu, Japan; and
include avalanche photodiodes and microchannel plates, also available from
the same supplier. The electrical output of detector 56 is connected to an
amplifier 58. The output of amplifier 58 is connected to an electrical
bandpass filter 60. Bandpass filter 60 desirably has a passband extending
from slightly below fundamental excitation frequency F.sub.e to an upper
frequency F.sub.u selected as discussed further below. F.sub.u typically
is about 5 times to about 100 times F.sub.e and preferably about 5 to
about 50 times F.sub.e. Thus, the passband of filter 60 typically is
arranged to encompass fundamental excitation frequency F.sub.e and a
predetermined set of harmonics of that frequency such as the fundamental
and the first five harmonics, the fundamental and the first 100 harmonics
or the like. The output of bandpass filter 60 is connected to the signal
input of a triggerable analog to digital or "A/D" converter 62. Converter
62 is arranged to capture the instantaneous amplitude of the electrical
signal passed through the filter 60 upon receipt of a triggering signal,
and to deliver the captured value in digital form. Output of converter 62
is connected through a direct memory access device 64 to the memory 22 of
the computer 20, so that digital values supplied by converter 64 can be
written into predetermined locations in memory 22 essentially
instantaneously without interrupting operation of the processor in
computer 20. The trigger input of converter 62 is connected to an output
of pulse generator 28 carrying pulses at sampling frequency F.sub.s, so
that each such pulse will trigger converter 62 to capture a further
sample.
The electrical output from bandpass filter 60 is also connected to a
feedback aperture control circuit 65. Control circuit 65 is linked to
computer 20 so that control circuit 65 can receive a target or set point
value for the amplitude of the electrical signal from filter 60. Aperture
control 65 is also linked to aperture 32, and aperture 32 is responsive to
control signals from circuit 65. Thus, the aperture control circuit is
arranged to adjust aperture 32 so as to maintain the peak amplitude of the
signal from filter 60 at the selected set point. Aperture control circuit
65 is also arranged such that it will only adjust the aperture 32 upon
appropriate command from computer 20 and, in the absence of such command,
aperture control 65 | | |
