United States Patent 19
`Diab et al.
`
`[54] LOW NOISE OPTICAL PROBE
`
`[75]
`
`Inventors: Mohamed Kheir Diab; Esmaiel
`Kiani-Azarbayjany, both of Laguna
`Niguel; James M. Lepper, Jr., Trabuco
`Canyon,all of Calif.
`
`[73] Assignee: Masimo Corporation, Mission Viejo,
`Calif.
`
`[21] Appl. No.: 333,132
`
`[22] Filed:
`
`Nov. 1, 1994
`
`Related U.S. Application Data
`
`[63] Continuation-in-part of Ser. No. 672,890, Mar. 21, 1991,
`abandoned.
`
`FSD] Wnt. CUO csessssssessssssssssssscscesssssssssssseesenseceeeees A61B 5/00
`[52] US. C1. ccccsccscscsessesees 128/653.1; 128/633; 128/632;
`128/665; 128/666; 356/41
`[58] Field of Searchy o.......ccsscsecssesesseess 128/632, 633-4,
`128/664-7, 653.1; 356/41
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`9/1963 Smith .
`3,103,214
`8/1969 Harte.
`3,463,142
`3,704,706 12/1972 Herczfeld et al. .
`4,129,124 12/1978 Thalmann...css 128/666
`4,321,930
`3/1982 Jobsis et al. .
`4,334,544
`6/1982 Hill etal. .
`4,380,240
`4/1983 Jobsis etal. .
`4,528,986
`7/1985 Arundel etal. .
`4,621,643
`11/1986 New, Jr. et al. .
`4,824,242
`4/1989 Frick et al...sesssessscscesseseee 128/666
`4,825,872
`5/1989 Tan et al. .
`-
`4,825,879
`5/1989 Tan et al. .
`4,865,038
`9/1989 Rich etal. .
`4,867,165
`9/1989 Nolleret al.
`4,880,304
`11/1989 Jaeb et al. .
`4,907,594
`3/1990 Muz.
`4,913,150
`4/1990 Cheungetal. .
`4,927,264
`5/1990 Shiga et al. .
`4,928,691
`5/1990 Nicolson et al. .
`
`.
`
`
`
`|
`
`|
`
`US005638818A
`
`[111 Patent Number:
`
`[45] Date of Patent:
`
`5,638,818
`Jun. 17, 1997
`
`7/1990 Goodman etal. .
`4,938,218
`7/1991 Weinstein .
`5,031,608
`5,058,588 10/1991 Kaestle ............ssuscescssssscesereees 128/633
`5,080,098
`1/1992 Willett et al. .
`5,086,229
`2/1992 Rosenthal etal. .
`5,099,842
`3/1992 Mannheimeretal. .......cscseee 128/633
`5,109,848
`5/1992 Thomas etal. .
`5,125,403
`6/1992 Culp.
`5,224,478
`7/1993 Sakai et al. .
`FOREIGN PATENT DOCUMENTS
`
`3/1983 European Pat. Off..
`74428
`404562 12/1990 European Pat. Off.
`.
`9201894
`5/1992 WIPO.
`
`Primary Examiner—Robert L. Nasser
`Attorney, Agent, or Firm—Knobbe, Martens, Olson & Bear
`
`[57]
`
`ABSTRACT
`
`An optical probe for measurements, which is particularly
`suited to reduce noise in measurements taken on an easily
`compressible material, such as a finger, a toe, a forehead, an
`eatlobe, or a lip. The probe includes a base having an
`aperture which leads to a chamber. The base is placed
`adjacent a portion of the material, the chamber being placed
`directly adjacent any easily compressible portion of the
`material. A photodetectoris located within the chamber and
`does not contact the material. A light emitting diode (LED)
`is affixed to the material, opposite the photodetector and
`above the chamber. The material which is supported by the
`aperture and therefore rests above or has intruded into the
`chamberis inhibited from compression since nothing comes
`in contact with this portion of the material, even when the
`material moves. Thus,
`light from the LED is directed
`through a stabilized portion of the material, i.e., the optical
`path length through which light travels is stabilized, even
`during motion of the material. This reduces noise in the
`signal measured by the photodetector. A scattering medium
`is interposed between the LED and the material, between the
`material and the photodetector, or between the LED and the
`material as well as between the material and the photode-
`tector. The scattering medium is used to improvethe signal-
`to-noise ratio of the received optical signal.
`
`30 Claims, 13 Drawing Sheets
`
`1
`
`APPLE 1006
`
`1
`
`APPLE 1006
`
`

`

`USS. Patent
`
`Jun.17, 1997
`
`Sheet 1 of 13
`
`5,638,818
`
`FIG. I
`
`[jh
`
`2
`
`

`

`USS. Patent
`
`Jun.17, 1997
`
`Sheet 2 of 13
`
`5,638,818
`
`124
`
`100~
`
`150.
`
`3
`
`

`

`U.S. Patent
`
`Jun. 17, 1997
`
`Sheet 3 of 13
`
`5,638,818
`
`| 124
`100— 130 r
`
`FIG.5
`
`4
`
`

`

`US. Patent
`
`Jun. 17, 1997
`
`Sheet 4 of 13
`
`5,638,818
`
`124
`Lo
`
`100
`
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`
`FIG. 7
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`

`U.S. Patent
`
`Jun. 17, 1997
`
`Sheet5 of 13
`
`5,638,818
`
`24
`
`wo 1 FIG. 10
`||
`
`217
`
`6
`
`

`

`U.S. Patent
`
`Jun. 17, 1997
`
`Sheet 6 of 13
`
`5,638,818
`
`500
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`

`

`USS. Patent
`
`Jun. 17, 1997
`
`Sheet 7 of 13
`
`5,638,818
`
`424
`
`FIG.16
`
`
`
`MUSCLE
`TISSUE
`
`
`ARTERIAL BLOOD
`VENOUS BLOOD
`
`
`408
`
` “”™
`
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`8
`
`

`

`Sheet 8 of 13
`baad
`
`5,638,818
`400
`
`U.S. Patent
`
`Jun. 17, 1997
`
`450
`
`
`9
`
`

`

`Sheet 9 of 13
`
`5,638,818
`
`US. Patent
`
`Jun.17, 1997
`
`
`
`10
`
`

`

`U.S. Patent
`
`Jun. 17, 1997
`
`Sheet 10 of 13
`
`5,638,818
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`

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`USS. Patent
`
`Jun.17, 1997
`
`Sheet 11 of 13
`
`5,638,818
`
`
`
`12
`
`

`

`US. Patent
`
`Jun. 17, 1997
`
`Sheet 12 of 13
`
`5,638,818
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`

`US. Patent
`
`Jun. 17, 1997
`
`Sheet 13 of 13
`
`5,638,818
`
`
`
`14
`
`

`

`5,638,818
`
`1
`LOW NOISE OPTICAL PROBE
`
`This is a continuation-in-part of U.S. patent application
`Ser. No. 07/672,890, filed Mar. 21, 1991, now abandoned.
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`The present invention relates to the sensing of energy.
`Morespecifically, the present invention relates to the reduc-
`tion of noise in signals via an improved sensing mechanism.
`2. Description of the Related Art
`Energy is often transmitted through or reflected from a
`medium to determine characteristics of the medium. For
`example, in the medical field, instead of extracting material
`from a patient’s body for testing, light or sound energy may
`be caused to be incident on the patient’s body and trans-
`mitted (or reflected) energy may be measured to determine
`information about the material through which thelight has
`passed. This type of non-invasive measurement is more
`comfortable for the patient and can be performed more
`quickly.
`Non-invasive physiological monitoring of bodily function
`is often required. For example, during surgery, blood pres-
`sure and the body’s available supply of oxygen, or the blood
`oxygen saturation, are often monitored. Measurements such
`as these are often performed with non-invasive techniques
`where assessments are made by measuring the ratio of
`incident to transmitted (or reflected). light through a portion
`of the body, for example a digit such as a finger, or an
`earlobe, or a forehead.
`
`Transmission of optical energy as it passes through the
`body is strongly dependent on the thickness of the material
`through which the light passes, or the optical path length.
`Many portions of a patient’s body are typically soft and
`compressible. For example, a finger comprises skin, muscle,
`tissue, bone, blood, etc. Although the bone is relatively
`incompressible, the tissue, muscle, etc. are easily compress-
`ible with pressure applied to the finger, as often occurs when
`the finger moves. Thus,if optical energy is made incident on
`a finger and the patient moves in a manner which distorts or
`compressesthe finger, the optical path length changes. Since
`a patient generally moves in an erratic fashion, the com-
`pression of the finger is erratic. This causes the change in
`optical path length to be erratic, making the absorbtion
`erratic, resulting in a difficult to interpret measured signal.
`Many types of non-invasive monitoring devices have
`been developed to try to produce a clear and discernabie
`signal as energy is transmitted through a medium, such as a
`finger or other part of the body. In typical optical probes a
`light emitting diode (LED) is placed on one side of the
`medium while a photodetector is placed on an opposite side
`of the medium. Manyprior art optical probes are designed
`for use only when a patient is relatively motionless since, as
`discussed above, motion induced noise can grossly corrupt
`the measured signal. Typically, probes are designed to
`maximize contact between the LED and the medium and the
`photodetector and the medium to promote strong optical
`coupling between the LED,
`the medium, and the
`photodetector,
`thereby generating a strong output signal
`intensity. In this way, a strong, clear signal can be transmit-
`ted through the medium when the patient
`is generally
`motionless.
`
`For example, U.S. Pat. No. 4,880,304 to Jaeb, ef al.
`discloses an optical probe for a pulse oximeter, or blood
`oxygen saturation monitor, comprising a housing with a flat
`
`2
`lower face containing a central protrusion in which a plu-
`tality of light emitting diodes (LEDs) and an optical detector
`are mounted. When the probe is placed on the patient’s
`tissue, the protrusion causes the LEDs and the detector to
`press againstthe tissue to provide improved optical coupling
`of the sensor to the skin. In another embodiment(FIGS. 4a
`and 4b in the Jaeb patent), the LEDs and the detector are
`arranged within a central chamber, generally horizontal with
`respectto the tissue on which the probe is placed. A set of
`mirrors or prisms causeslight to be directed from the LEDs
`onto the tissue through a polymer sealant within the
`chamber, the sealant providing a contact with the tissue for
`good optical coupling with the tissue.
`US. Pat. No. 4,825,879 to Tan,et al. discloses an optical
`probe wherein a T-shaped wrap, having a vertical stem and
`a horizontal cross bar,is utilized to secure a light source and
`an optical sensor in optical contact with a finger. Thelight
`source is located in a window on one side of the vertical stem
`while the sensoris located in a window on the other side of
`the vertical stem. Thefinger is aligned with the stem and the
`stem is bent such that the light source and the sensor lie on
`opposite sides of the finger. Then, the cross bar is wrapped
`around the finger to secure the wrap, thereby ensuring that
`the light source and the sensor remain in contact with the
`finger to produce good optical coupling.
`US. Pat. No. 4,380,240 to Jobsis, et al. discloses an
`optical probe wherein a light source and a light detector are
`incorporated into channels within a slightly deformable
`mounting structure which is adhered to a strap. Annular
`adhesive tapes are placed over the source and the detector.
`The light source and detector are firmly engaged with a
`bodily surface by the adhesive tapes and pressure induced by
`closing the strap arounda portion of the body. An alternative
`embodiment provides a pressurized seal and a pumping
`mechanism to cause the body to be sucked into contact with
`the light source and detector.
`US. Pat. No. 4,865,038 to Rich,et al. discloses an optical
`probe having an extremely thin cross section such thatit is
`flexible. A die LED and a die photodetector are located on
`a flexible printed circuit board and encapsulated by an epoxy
`bead. A spacer, having circular apertures positioned in
`alignment with the LED and photodetector, is placed over
`the exposed circuit board. A transparent top coveris placed
`over the spacer and is sealed with a bottom cover placed
`under the circuit board, thereby sealing the probe from
`contaminants. A spine may be added to strengthen the
`device. The flexibility of the device allows it to be pinched
`onto the body causing the epoxy beads over the LED and the
`photodetector to protrude through the apertures in the spacer
`andpress against the top cover such that good optical contact
`is made with the body.
`USS. Pat. No. 4,907,594 to Muz discloses an optical probe
`wherein a dual wall rubberized sheath is fit over a finger. A
`pump is located at thetip of the finger such that a pressurized
`chamber may be formed between the two walls, thereby
`causing an LED and a photodetector located in the inner wall
`to be in contact with the finger.
`Each of the above described optical probes is designed to
`cause a strong measured signal at the photodetector by
`optimizing contact between the LED,the patient, and the
`probe. However, this optimization forces compressible por-
`tions of the patient’s body to be in contact with surfaces
`which compress these portions of the patient’s body when
`the patient moves. This can cause extreme changes in the
`thickness of material through which optical energy passes,
`ie., changes in the optical path length and changes due to
`
`40
`
`55
`
`60
`
`65
`
`15
`
`15
`
`

`

`5,638,818
`
`3
`scattering as a result of venous blood movement during
`motion. Changes in the optical path length can produce
`enough distortion in the measured signal to make it difficult
`or impossible to determine desired information. Thus, a need.
`exists for a probe which inhibits motion induced noise, or
`motion artifacts, during measurement of a signal while still
`generating a transmitted or reflected signal of sufficient
`intensity to be measured by a detector.
`
`SUMMARYOF THE INVENTION
`
`The present invention involves a probe for use in both
`invasive and non-invasive energy absorption (or reflection)
`measurements. A base is formed in a shape generally cor-
`responding to the material on which measurements are to be
`made, for example, a section of a patient’s body such as a
`finger, an earlobe, a forehead, a toe, an organ, or a portion
`of tissue. The base has a forward end, a rear end, a top and
`a bottom. An aperture is formedin the top of the base. The
`aperture is the entrance to a chamber. A detector, such as a
`photodetector, is mounted within the chamber,typically in
`the bottom of the chamber. The material on which measure-
`ments are to be made is placed on the base such that any
`compressible portion of the material
`is located directly
`adjacent the chamber. Thus, the compressible portion of the
`material is caused to rest above or enter into the chamber.
`The chamber is deep enough that any material which
`intrudes into the chamber does not contact anything which
`might cause compression.
`A light source, such as an LED,is affixed to the material,
`opposite the photodetector. The LED emits light energy
`which propagates through and is absorbed by the material
`along the optical path length, or thickness of material
`through which light propagates. An attenuated light energy
`signal emerges from the material, into the chamber. As light
`propagates through the material, it is scattered by the mate-
`rial and is thus transmitted into the chamber over a broad
`range of angles. The photodetector produces an electrical
`signal indicative of the intensity of the signal transmitted by
`the material. The electrical signal is input to a processor
`which analyzes the signal to determine information about
`the medium through which light energy has been transmit-
`ted.
`
`The probe of the present invention has an aperture and
`chamber which enable an easily compressible portion of the
`material that light energy passes through to rest in the
`chamber and not be compressed. This results in less distur-
`bance of the optical path between the light source and the
`detector. Since the LED is generally aligned with the cham-
`ber and the photodetector, the light energy signal propagates
`through the portion of the material which rests above or is
`accommodated within the chamber. The chamber allows the
`compressible portion of the material to remain substantially
`uncompressed, even during motion, since nothing within the
`chamber physically contacts the material through which
`light energy passes to cause compression. Thus, the thick-
`ness of the material, or the optical path length, is stabilized,
`and the movement of venous blood during motion is
`minimized, thereby improving the signal-to-noise ratio of
`the measuredsignal. Thus,the probe of the present invention
`producesa strong, clear signal wherein noise due to motion,
`or motion artifacts, is substantially reduced.
`In one preferred embodimentof the present invention, the
`chamberis filled with a scattering medium. The scattering
`medium is advantageously formed of a conformable plastic
`or a highly compressible material so that the material on
`which measurements are to be made is not compressed upon
`
`4
`contact with the scattering medium. Thescattering medium
`helps to minimize the effects of local artifacts and pertur-
`bations within the material. Thus, an increased optical
`signal-to-noise ratio is observed. The scattering medium
`also improves the optical coupling with the material.
`In another preferred embodiment, the scattering medium
`is interposed between thelight source and the material, and
`in yet another preferred embodiment, the scattering medium
`is interposed between the light source and the material as
`well as between the material and the photodetector. Each of
`these embodiments results in an improved optical signal-to-
`noiseratio.
`.
`
`In yet another preferred embodiment, an immersion lens
`is utilized in combination with the light source and/or the
`photodetector. In a particularly preferred embodiment, the
`immersion lens is formed by placing an epoxy bump form-
`ing a partial sphere over semiconductor diodes used as the
`light source and/or the photodetector to improve the signal-
`to-noise ratio.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 illustrates a schematic medium comprising N
`different constituents.
`
`FIG. 2a illustrates an ideal plethysmographic signal that
`would be measured by the optical probe of the present
`invention when utilized for pulse oximetry.
`FIG. 2b illustrates a realistic signal measured by the
`optical probe of the present invention whenutilized for pulse
`oximetry.
`FIG. 3 is a perspective view of a probe of the present
`invention having a single segment chamber.
`FIG.4 is a cross-sectional view of an optical probe of the
`present invention illustrating a single segment chamber
`having a detector within it.
`FIG. 5 is a cross-sectional view of a probe of the present
`invention having a detector resting on a shell of base
`material.
`
`FIG.6 is a cross-sectional view of a probe of the present
`invention incorporating a light collecting lens.
`FIG.7 is a cross-sectional view of a probe of the present
`invention illustrating a single segment chamber having an
`LED within it.
`
`FIG.8 is a cross-sectional view of a probe of the present
`invention incorporating a collimating lens assembly.
`FIG. 9 is a cross-section view of a probe of the present
`invention wherein the LED and the detector are not aligned
`along the central axis of the chamber.
`FIG. 10 is a perspective view of another embodiment of
`a probe of the present invention having a two segment
`chamber.
`FIG.11 is a cross-sectional view of another embodiment
`of the probe of FIG. 10 incorporating a two segment
`chamber having a detector within it.
`FIG.12 is a cross-sectional view of another embodiment
`of the probe of FIG. 10 incorporating a light collecting lens
`in a two segment chamber.
`FIG. 13 is a perspective view of probe of the present
`invention having a three segment chamber.
`FIG.14is a cross-sectional view of the probe of FIG. 13
`incorporating a three segment chamber having a detector
`within it.
`FIG.15is a cross-sectional view of another embodiment
`of the probe of FIG. 13 incorporating a light collimating
`lens.
`
`50
`
`16
`
`16
`
`

`

`5,638,818
`
`@)
`
`6 N
`
`=~ S egeiny
`I~ Ie =
`
`wheree,is the absorption coefficient of the iconstituent; x,
`is the thickness of the i® constituent through which light
`energy passes, or the optical path length ofthe i”; and c, is
`the concentration of the i” constituent in thickness x,.
`Since energy absorption is strongly dependent on the
`thicknesses of the constituents A, through A, which make
`up the medium 20 through which the energy passes, when
`the thickness of the medium 20 changes, due to motion for
`example, the thicknesses of the individual constituents A,
`through A, change. This causes the absorption characteris-
`tics of the medium 20 to change.
`Often a medium 20 is under random orerratic motion. For
`example,if the medium 20 is an easily compressible portion
`of a patient’s body, such as a digit, and the patient moves,
`the medium 20 compresseserratically causing the individual
`thicknesses X, through X,, of the constituents A, through Ay
`to vary erratically. This erratic variation may cause large
`excursions in the measured signal and can makeit extremely
`difficult to discern a desired signal, as would be present
`without motion induced noise, or motion artifacts.
`For example, FIG. 2a illustrates an ideal desired signal
`waveform, labelled Y, measured in one application of the
`present invention, namely pulse oximetry. FIG. 2b illustrates
`a morerealistic measured waveform S, also measured in a
`pulse oximetry application, comprising the ideal desired
`signal waveform Y plus motion inducednoise, n, ie. S=¥+
`n. It is easily seen how motion artifacts obscure the desired
`signal portion Y.
`FIG. 3 is a perspective view of one embodiment of an
`optical probe 100 of the present invention which greatly
`diminishes the effects of motion artifacts on the measured
`signal. FIG. 4 shows a cross-sectional view of the optical
`probe 100 of the present invention taken along line 44 in
`FIG. 3. For clarity in the perspective view of FIG. 3, a
`material 128 on which measurements are to be taken is not
`shown placed adjacent the probe 100. However, the material
`128 on which measurementsare to be made is shown in FIG.
`4, Asillustrated in FIGS. 3 and 4, a base 110, having a top
`112, a bottom 114, a forward end 116, and a rear end 118,is
`made of a material which is preferably rigid and opaque.It
`will be understood, however, that the probe 100 may be
`made of materials which may berigid,resilient, opaque, or
`transparent, for example.
`An aperture 120 is formed in the top 112 of the base 110.
`Typically, the aperture 120 is located at a point between
`one-quarter and one-half of the length of the base 100. The
`aperture 120 may be of any shape, including but not limited
`to circular, square, or triangular. The aperture 120 forms the
`opening to a chamber 122 which mayalso be of any shape.
`In one embodiment, a lateral cross-section (not shown) of
`the chamber 122 is the same shapeas the aperture. A central
`axis 124 of the chamber 122 is defined by a line aligned
`perpendicular to the aperture 120 and extending generally
`through a central portion of the aperture 120.
`In the embodimentof FIG.4, a light source 130, typically
`a light emitting diode (LED),is affixed adjacent the material
`128, aligned along the central axis 124 of the chamber 122
`opposite the chamber 122. Typically, an adhesive such as
`medical tape is usedto affix the LED 130 to the material 128.
`A detector 126, such as a photodetector, is placed within the
`chamber 122. A central portion of the photodetector 126 is
`generally aligned with the central axis 124 of the chamber
`122, typically at the bottom 114 of the chamber 122. The
`photodetector 126 may be fixed within the chamber 122
`
`20
`
`25
`
`35
`
`40
`
`45
`
`55
`
`65
`
`17
`
`5
`FIG. 16 is a perspective view of a probe of the present
`invention specifically designed to be used with a digit.
`FIG. 17 illustrates a schematic finger comprising
`fingernail, skin, bone, tissue, muscle, blood,etc.
`FIG. 18 is a cross-section view of the probe of FIG. 16.
`FIG. 19 is a longitudinal cross-sectional view of the probe
`of FIG. 16.
`FIG. 20 is a cross-sectional view of another embodiment
`of the probe of FIG. 16 incorporatinga light collecting lens.
`FIG.21 is a cross-sectional view of a probe of the present
`invention designed to be utilized for reflectance measure-
`ments.
`
`FIG. 22 is a cross-sectional view of a probe which is
`advantageously used for non-invasive measurements when a
`material is compressible on more than one side. The probe
`has two bases, each with a chamber to house a detector or
`an energy source and thereby reduce motion artifacts.
`FIG. 23 is a cross-sectional view of a probe having a
`generally cone-shaped chamber with a reflective surface
`which advantageously causes energy to be concentrated, or
`“funneled,” onto the surface of a detector within the
`chamber, improving the measured signal.
`FIG. 24 is a schematic of one system which may advan-
`tageously employ a probe of the present invention.
`FIG. 25 is a cross-sectional view of a probe wherein the
`apertureis filled with a compressible scattering medium.
`FIG. 26 is a cross-sectional view of a probe wherein the
`LED is spaced from the material to be measured by a
`transmission assembly having a scattering medium inter-
`posed between the LED and the material.
`FIG. 27 is a cross-sectional view of a probe wherein a
`scattering medium is interposed between the LED and the
`material as well as between the material and the photode-
`tector.
`
`FIG. 28 is a cross-sectional view of a preferred embodi-
`ment of a probe in accordance with the present invention
`having an immersion lens for the photodetector and for the
`LEDandhaving scattering medium interposed between the
`LED and the test material as well as between the test
`material and the photodetector.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`Examination of a material is often advantageous, espe-
`cially when it is difficult or expensive to procure and test a
`sample of the material. For example, in physiological
`measurements, it is often desirable to monitor a patient
`without unnecessary drawing of blood or tissue from the
`patient. The known properties of energy absorption as
`energy propagates through a material may be used to deter-
`mine information about the material
`through which the
`energy has passed. Energy is made incident on a material,
`and a measurement is made of energy cither transmitted by
`or reflected from the material.
`
`The amplitude of the measured signal is highly dependent
`on the thickness of the material through which the energy
`passes, or the optical path length, as well as other properties
`such as the erratic movement of venous blood during
`motion. A schematic medium 20 comprising N different
`constituents A, through A, is shown in FIG. 1. Energy
`transmitted through the medium 20 is approximately attenu-
`ated according to the equation:
`
`17
`
`

`

`5,638,818
`
`7
`according to a numberof different methods, including but
`not limited to adhesive, a press fit, or clear epoxy resin
`which transmits light over a range of wavelengthsof inter-
`est. Typically, no matter how the photodetector 126 is held
`within the chamber 122,
`the bottom surface 114 of the
`chamber 122 is made opaque either via the press fit or via
`paint or tape, for example.
`It is often the case that materials 128 on which absorption
`measurements are performed are, at least in part, easily
`compressible. Easily compressible portions of the material
`128is placed directly adjacent(i.e., above) the chamber 122.
`The area surrounding the aperture 120 supports the material
`covering the chamber 122. The chamber 122 is wide enough
`that any compressible portion of the material 128 located
`above the aperture 120 may intrude into the chamber 122.
`Thus, the material 122 may rest above or penetrate slightly
`into the chamber 122 and is thereby shielded from pertur-
`bations which compress the material 128, such as pressure
`caused when the material 128 is touched.
`In the present embodiment,the depth of the chamber 122
`may range from 0.5 mm to 10 mm in depth, with 2~4 mm
`preferred, and 3-4 mm morepreferred. Similarly, the diam-
`eter of the aperture 120 may, in the present embodiment.
`range from 3 mm to 20 mm, as required by the specific
`application. For instance, the aperture would be smaller for
`neonates than for adults. These sizes have been found to be
`effective in reducing perturbations and compression of the
`material 128, when the material is human skin.
`The chamber 122 is deep enough that the photodetector
`126 and the bottom 114of the chamber 122 do not comeinto
`contact with the easily compressible portion of the material
`128, even when the material 128 is caused to move. Thus,
`along the central axis 124 of the chamber 122 nothing comes
`into physical contact with the easily compressible portion of
`the material 128 and causes it to compress. With little or no
`compression of the material 128 in this region, the thickness
`of the material 128, or the optical path length oflight energy
`propagating through the material 128, is substantially sta-
`bilized in the field of view of the photodetector. The move-
`ment of venous blood due to compression is also minimized
`in the field of view of the photodetector.
`The LED 130 emits light at a known wavelength. The
`light propagates through the material 128 and an attenuated
`signal is transmitted into the chamber 122 to be received by
`the photodetector 126. As light from the LED 130 propa-
`gates through the material 128,it is scattered by the material
`128 and is thus transmitted into the chamber 122 over a
`broad range of angles in a very complex manner. Thus, some
`of the light is caused to be incident on the opaque walls 123
`of the chamber 122 and is absorbed. Although the signal
`travels through a greater optical distance to reach the pho-
`todetector 126 at the bottom 114 of the chamber 122 than if
`the photodetector 126 were immediately adjacent the mate-
`rial 128, thus eliminating direct coupling between the pho-
`todetector 126 and the material 128, the resulting degrada-
`tion to signal
`intensity is compensated for by the
`stabilization of the optical path length and the resultant
`reduction of noise in the measured signal. The photodetector
`126 produces an electrical signal indicative of the intensity
`of light energy incident on the photodetector 126. The
`electrical signal is input to a processor which analyzes the
`signal to determine characteristics of the media 128 through
`which the light energy has passed.
`The opaque quality of the base 110 absorbs ambientlight
`which can interfere with the signal measured at the photo-
`detector 126. This further improves signal quality. Further,
`the opaque bottom 114 of the chamber 122 protects the
`
`8
`photodetector 126 from ambient light which can obscure the
`desired signal measured at the photodetector 126. Thus, an
`accurate measurement of the intensity of the attenuated
`signal may be made at the photodetector 126.
`An alternative embodiment of the chamber 122 is shown
`in frontal cross-section in FIG. 5. A shell 131 of base 110
`material covers the bottom 114 of the chamber 122. The
`photodetector 126 is mounted on the shell 131, within the
`chamber 122, generally aligned with the LED 130. The
`photodetector 126 is electrically connected to a processor
`through a small hole (not shown) in the shell 131. The shell
`131 shields the photodetector 126 from ambientlight which
`can seriously degrade the signal-to-noise ratio of the signal
`measuredat the photodetector 126. It will be understood that
`the bottom 114 of the chamber 122 may be formed. with or
`without the shell in any embodiment of the probe of the
`present invention.
`FIG. 6 shows a frontal cross sectional view of another
`embodiment of the probe 100 of the present invention
`wherein a light collecting lens 132 is placed within the
`chamber 122, between the material 128 which rests above or
`enters into the chamber 122 and the photodetector 126. The
`lens 132 has one generally planar surface 132a aligned
`parallel to the aperture 120 in the top 112 of the base 110,
`located deep enough within the chamber 122 that any
`material 128 which intrudes into the chamber 122 does not
`contact the planar surface 132a of the lens 132. Another
`surface 1326 of the lens 132 is generally convex havingits
`apex directed toward the photodetector 126 in the bottom
`114 of the chamber 122. The lens 132 may be held in the
`chamber 122 by a number of means, including but not
`limited to optical adhesive, a lens retaining ring, or a press
`fit. The chamber 122 functions in the same manner as
`described above to stabilize the optical path length and
`reduce motion artifacts. The light collecting lens 132 gathers
`muchof the light which wasscattered as it was transmitted
`through the material 128 and causes it to be incident on the
`photodetector 126. This produces a stronger measured sig-
`nal,
`FIG. 7 shows another embodimentof the probe 100 of the
`present invention wherein the positions of the photodetector
`126 and the LED 130 are interchanged. The LED 130 is
`placed within the chamber 122,typically at the bottom 114
`of the chamber 122, generally aligned with the central axis
`124 of the chamber 122. The LED 130 maybe fixed within
`the chamber 122 according to a number of different
`methods, including but not limited to a pressfit, adhesive, or
`clear epoxy resin which transmits light over a range of
`wavelengths of interest, such as around the wavelength
`which the LED emits. Again, a material 128 is placed on the
`base 110 having a compressible portion of the material 128
`located directly above the chamber 122. The photodetector
`126is attached to the material 128, opposite the LED 130,
`such that the LED 130, the photodetector 126, and the
`chamber 122 are aligned along the central axis 124 of the
`chamber 122. The photodetector 126 is typically attached by
`an opaque material. For example, the photodetector 126 may
`be attached to the material 128 with opaque tape, thereby
`limiting signal degradation caused by ambient light. The
`ph