Abstract:
An optical probe, which is particularly suited to for use in measurements on tissue material of a patient. In one embodiment, the probe comprises upper and lower housing elements incorporating a light energy source and corresponding detector. The tissue material of the patient is disposed between the upper and lower housing elements such that the light energy emitted by the source passes through the tissue material to the detector. A plurality of light shields are attached to one or both of the housing elements to reduce the amount of ambient and reflected light reaching the detector. Additionally, various portions of the upper and lower housing elements and shields utilize light absorbent coloration and/or coatings which further mitigate the effects of undesired ambient and reflected light, thereby reducing noise generated within the instrument and increasing its accuracy. In one embodiment, the light shields are made removable from the optical probe, thereby facilitating replacement. A circuit for monitoring the condition of the probe, and indicating when replacement of the probe is desirable, is also disclosed.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to low-noise optical probes which may be used to sense optical energy passed through or reflected from a medium to determine the characteristics of the medium.  
           [0003]    2. Description of the Related Art  
           [0004]    The physical characteristics of a given medium may often be determined by transmitting electromagnetic or acoustic energy through, or reflected energy from, portions of the medium. For example, in the context of medical diagnosis, light or sound energy may be directed onto a portion of a patient&#39;s body, and the fraction of that energy transmitted through (or reflected by) the patient&#39;s body measured to determine information about the various physical attributes of the patient. This type of non-invasive measurement is both more comfortable for and less deleterious to the patient than invasive techniques, and can generally be performed more quickly.  
           [0005]    Non-invasive physiological monitoring of bodily function is often required. For example, during surgery, blood oxygen saturation (oximetry) is often continuously 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 an accessible part of the body such as a finger or an earlobe. A typical transmissive non-invasive monitoring device includes a light source such as a light-emitting diode (LED) placed on one side of the body part, while a photodetector is placed on an opposite side of the body part. Light energy generated by the LED is transmitted through the tissue, blood, and other portions of the body part, and detected by the photodetector on the other side. Alternatively, in a reflective device, the detector is placed on the same side of the body part as the light source, and the amount of light energy reflected by the body part measured.  
           [0006]    The transmission of optical energy passing through the body is strongly dependent on the thickness of the material through which the light passes (the optical path length). Many portions of a patient&#39;s body are typically soft and compressible. For example, a finger comprises a number of components including skin, muscle, tissue, bone, and blood. Although the bone is relatively incompressible, the tissue, muscle, and skin are easily compressible or deformed with pressure applied to the finger, as often occurs when the finger is bent. Thus, if optical energy is made incident on a patient&#39;s finger, and the patient moves in a manner which distorts or compresses the finger, the optical properties, including optical path length, may change. Since a patient generally moves in an erratic fashion, the compression of the finger is erratic and unpredictable. This causes the change in optical path length to be erratic, making the absorption of incident light energy erratic, and resulting in a measured signal which can be difficult to interpret. Similarly, movement of the patient during a reflective measurement can dramatically affect the quality of the signal obtained therefrom.  
           [0007]    In addition to the typical problem of patient movement, the presence of unwanted ambient and/or reflected light energy interferes with the measurement of the intensity of the light transmitted through or reflected by the body part. Optical transmission/reflection systems as described above utilize a light energy detector which measures, inter alia, the intensity of light transmitted to or reflected from the body part being analyzed. Since ambient light incident on the detector affects the intensity measurement, noise or error is introduced into the measured signal by such ambient light. Similarly, light generated by the light source within the measuring device (typically, an LED) which is not transmitted through or reflected by the body part under examination will also result in signal error if such light is received by the detector. These “secondary” reflections arise when light emitted by the light source is reflected by structures within the optical probe onto the detector. Accordingly, to increase the accuracy of the measurement process, both ambient light and “secondary” reflections from the light source should be mitigated.  
           [0008]    [0008]FIG. 1 a  illustrates an ideal signal waveform obtained from an optical probe system. FIG. 1 b  illustrates an actual spectra obtained from a typical optical probes not corrected for the effects of patient motion or ambient/reflected light. Note the significant increase in noise (and resulting loss of signal clarity) in FIG. 1 b  due to these effects.  
           [0009]    Prior optical probes have successfully addressed the issue of ease of use and patient motion during measurement. See, for example, U.S. Pat. No. 5,638,818 entitled “Low Noise Optical Probe,” assigned to the Applicant herein, which discloses a system utilizing a chamber which isolates that portion of the patient&#39;s tissue under examination from compression or movement by the patient. The device is attached to the finger of a patient, thereby readily and accurately positioning the tissue of the patient&#39;s finger over the chamber.  
           [0010]    However, attempts at limiting the effects of ambient and “secondary” reflected light have been less successful, not due to their ineffectiveness, but rather due to their obtrusiveness and relative complexity of use. A need exists, especially in the health care context, for a simple, fast, unobtrusive, and largely error-free means of non-invasive measurement of a patient&#39;s physical parameters. Especially critical is the attribute that such means be easily adapted to a variety of different patient types and characteristics with little or no adjustment, as is the device disclosed in the aforementioned patent. Prior art methods of mitigating ambient and reflected light interference have involved coverings or shrouds which substantially envelop the optical probe and tissue, thereby requiring substantial sizing and adjustment of the covering for each different patient being measured. Another disadvantage of such methods is that the placement of the patient&#39;s appendage (such as a finger) in relation to the light source and detector can not be reliably verified by the person administering the measurement unless the probe is first placed on the appendage, and the covering installed thereafter, or alternatively, unless the patient is queried. This necessitates additional time and effort on the part of the patient and the person making the measurement.  
           [0011]    Another factor relating to the efficacy of an optical probe is force distribution on the body part or tissue material being measured. Specifically, if force is distributed on the tissue material being measured unevenly or disproportionately, varying degrees of compression of the tissue may result, thereby producing a broader range of optical path lengths in the region of the light source and detector. Furthermore, if the force that the probe exerts on the tissue material is highly localized, the ability of the patient to move the tissue material with respect to the source/detector is enhanced, thereby leading to potentially increased noise levels within the signal generated by the probe.  
           [0012]    Yet another consideration relating to non-invasive optical probe measurement involves cost. In recent times, the demand has increased significantly for both disposable and reusable optical probes which are suitably constructed to provide accurate, low-noise measurements. The aforementioned prior art methods of attenuating ambient and reflected light employing coverings or shrouds carry with them a significant cost, especially if the probe (or components thereof) must be replaced on a frequent basis. Therefore, in many applications, it would be useful to have a low-cost reusable optical probe capable of attenuating ambient and reflected light, with only the degradable components being easily and cost-effectively replaced as required, without necessitating the replacement of the entire probe. Similarly, it would be useful to have a disposable probe capable of attenuating ambient and reflected light, which could be routinely replaced in its entirety a cost-effective manner.  
           [0013]    Finally, existing optical probes do not include an easy to use and reliable means for determining when to replace the probe. At present, the probe operator or health care provider must keep a record or log of the date of installation of a given probe, and replace it at a given periodicity or simply replace the probe when it seems worn out. This approach is problematic, however, not only from the standpoint of additional time and effort consumed in maintaining the record, but more significantly from the perspective that the measurement of installed time is not necessarily representative of the wear on the probe. For example, two probes installed on the same date may experience significantly different levels of wear, depending on the level of use. Alternatively, the operator could keep a log of usage, but this is too burdensome and time consuming.  
           [0014]    Based on the foregoing, a need exists for an improved low-noise optical probe which (i) is simple in design and easy to use under a variety of different operating conditions; (ii) is capable of attenuating ambient and reflected light without necessitating probe adjustment or fitting to each different patient; (iii) is capable of alerting the operator when replacement is required; and (iv) is cost effective. Such an improved probe would also ideally shield against noise caused by electromagnetic interference (EMI).  
         SUMMARY OF THE INVENTION  
         [0015]    The present invention satisfies the foregoing needs by providing an improved optical probe for use in non-invasive energy absorption or reflection measurements, as well as a method of using the same.  
           [0016]    In a first aspect of the invention, an improved shielded optical probe assembly is disclosed which incorporates a light energy source and light energy detector embedded within a multi-part housing adapted to receive and clamp onto tissue material from the patient. When the probe is operating, light energy is directed from the light energy source through a first aperture formed within a first element of the housing and onto the tissue material of the patient, which is received within the probe. A portion of this light is transmitted through (or reflected from) the tissue material onto the detector via a second aperture. In this fashion, a light generated by the light source and transmitted through or reflected from the tissue material at a localized point is received by the detector. A light shield is fitted to the housing so as to partially surround the tissue material when it is received within the housing, thereby attenuating ambient light incident on the optical probe. In one embodiment, the light shield is made removable in order to facilitate its replacement after degradation and wear. Additionally, portions of the shield and housing are colored and/or coated such that light incident on these portions is absorbed or attenuated. The foregoing light attenuation features act to reduce the effects of noise induced within the detector (and associated processing circuitry) due to light energy not transmitted directly through or reflected from the tissue material from the light source. The probe is also optionally fitted with a diffraction grating and Faraday shield to mitigate the effects of unwanted optical modes and electromagnetic interference on probe accuracy.  
           [0017]    In a second aspect of the invention, the foregoing optical probe includes a mechanism for equalizing the force applied to the tissue of the patient when the probe is clamped thereon. In one embodiment, a series of elongated apertures each receive hinge pins which are biased apart by springs wound around the axis of the pins. When the housing elements of the probe are grasped and compressed together by the user, the hinge pins are forced against one edge of the elongated apertures, thereby providing a fulcrum for opening the probe. After the probe is opened, and the patient&#39;s finger inserted, the compressing force is removed, thereby allowing the housing elements to clamp onto the finger. As the compression force is removed, the spring bias allows the previously compressed ends of the housing elements apart, and urging the pins to the opposite edge of the elongated apertures, and “leveling” the housing elements into a more parallel orientation. This parallel orientation distributes force on the patient&#39;s finger more evenly.  
           [0018]    In a third aspect of the invention, a monitoring device is disclosed which is integrated with the optical probe circuitry in order to assist the operator in determining when to replace the probe. In one embodiment, the monitoring device is a counter which counts the number of electrical pulses generated by the detector circuitry, and correlates this number to the time of actual probe operation and percent of useful lifetime. A light emitting diode visible on the exterior of the probe is used to alert the operator to the need for probe replacement.  
           [0019]    In another aspect of the invention, a method of measuring the amount of light transmitted or reflected by the tissue material of a patient using the aforementioned optical probe is disclosed. In one embodiment of the method, the tissue material is inserted into the shielded probe housing, and light generated by the light source of the probe is transmitted via the first aperture into the tissue material. Light energy transmitted (or reflected) by the tissue material is then detected by the detector via the second aperture, and a signal relating to the intensity of the detector generated. Ambient light incident on the probe, and light generated by the light source and scattered off components other than the tissue material, are attenuated or absorbed by the shield and absorptive coating(s) during detection and signal generation in order to reduce any noise component associated therewith. The operating time of the probe is also counted in order to monitor probe remaining lifetime.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1 a  illustrates an ideal optical transmittance signal that would be measured by a typical prior art optical probe when utilized for blood oximetry.  
         [0021]    [0021]FIG. 1 b  illustrates a non-ideal optical transmittance signal measured by a typical prior art optical probe when utilized for blood oximetry.  
         [0022]    [0022]FIG. 2 is an exploded perspective view of a first embodiment of the optical probe of the present invention, configured to measure optical transmission.  
         [0023]    [0023]FIG. 2 a  is a perspective view of the optical probe of FIG. 2 when assembled.  
         [0024]    [0024]FIG. 2 b  is a perspective view of an upper support surface element of the optical probe of the present invention with a nonreflecting portion depicted with shading.  
         [0025]    [0025]FIG. 2 c  is a perspective view of the lower support surface element shown in FIG. 2 with a nonreflecting portion depicted with shading.  
         [0026]    [0026]FIG. 3 is a cross-sectional view of the optical probe of FIG. 2 when assembled, taken along line  3 - 3  thereof.  
         [0027]    [0027]FIG. 4 is a cross-sectional view of the optical probe of FIG. 2 when assembled, taken along line  4 - 4  thereof.  
         [0028]    [0028]FIG. 5 is a perspective view of the detector shield of the present invention, shown during assembly.  
         [0029]    [0029]FIG. 6 a perspective view of a second embodiment of the optical probe of the present invention configured to measure optical transmission.  
         [0030]    [0030]FIG. 6 a  is a detail plan view of the removable shield elements and channels of the optical probe of FIG. 6.  
         [0031]    [0031]FIG. 7 is a cross-sectional view of a third embodiment of the optical probe of the present invention configured to measure optical reflectance.  
         [0032]    [0032]FIG. 8 is a block diagram illustrating one embodiment of a monitoring device circuit according to the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0033]    The invention is described in detail below with reference to the figures, wherein like elements are referenced with like numerals throughout.  
         [0034]    It is noted that the term “tissue material” as used herein includes, without limitation, the skin, tissue, blood, cartilage, ligaments, tendons, muscle, or bone of a given portion of a patient&#39;s body, such as the distal end of a finger, or any portion thereof.  
         [0035]    The term “light energy” as used herein refers to any type of electromagnetic radiation or energy, whether comprised of a narrow, discrete frequency or multiple frequencies. Examples of light energy include visible light, infrared radiation, and ultraviolet radiation. While described as an “optical” probe, the invention disclosed herein may also feasibly be used in conjunction with other forms of energy or radiation, whether optically visible or not.  
         [0036]    As shown in FIGS. 2 and 3, a first embodiment of the improved optical probe of the present invention is described. As shown in FIGS. 2 and 3, the present embodiment of the probe  100  generally comprises a two-piece housing  102 , a light energy source  103 , and a light energy detector  105 , and an electrical supply and signal cable  107 . The housing  102  consists of a first (upper) housing element  104  and a second (lower) housing element  106 , which are rotatably attached to one another via a pivot element  108 . The light source  103  is disposed within the upper housing element  104 , while the detector is disposed within the lower housing element  106 . The housing  102  of the present embodiment is adapted to receive the distal end of a finger  112  as shown in FIG. 3, with the “upper” housing element  104  engaging the upper surface  113  of the finger  112 , and the “lower” housing element  106  engaging the lower surface  118  of the finger  112 . It will be recognized, however, that the probe  100  may be used in any orientation, such as with the first housing element  104  being located below the second housing element  106 . Furthermore, the light source  103  may alternatively be placed in the lower housing element  106 , and the detector in the upper housing element  104  if desired, subject to modification of other probe components as described further below. It is also noted that while the following discussion describes a series of exemplary embodiments based on measuring the optical characteristics of a finger  112 , the present invention may be adapted for use with any number of other body parts, such as earlobes or loose skin, with equal success. Hence, the specific embodiments described herein are merely illustrative of the broader invention.  
         [0037]    The first and second housing elements  104 ,  106  of the probe  100  of FIGS. 2 and 3 are generally rectangular in form, with the pivot element  108  being disposed near a common end  109  of each of the elongate housing elements  104 ,  106 . The housing elements  104 ,  106  are in the present embodiment formed from an opaque plastic using an injection molding process of the type well known in the polymer sciences, although other materials and formation techniques may be used. The first housing element  104  includes a monitoring light emitting diode (LED)  426  visible to the operator, as is described in greater detail below with respect to FIG. 8. The first and second housing elements  104 ,  106  further each include support surface elements  114 ,  116 , and one or more pairs of vertical risers  110   a - 110   d  with pin apertures  111   a,    111   b  , the latter which are used to form the basis of the pivot element  108 . The two housing elements  104 ,  106  are biased around the rotational axis  123  of the pivot element  108  by a biasing element, in this case a hinge spring  133  as described further below.  
         [0038]    As shown in FIG. 2, the pivot element  108  of the present embodiment comprises a hinge, and includes the aforementioned vertical risers  110   a - 110   d,  two hinge pins  125   a,    125   b,  and biasing spring  133  located along the central axis  123  of the hinge pins  125   a,    125   b.  The hinge pins  125   a,    125   b  each include an outward retaining element  127 , and are of a “split pin” design such that a ridge  141  located on the distal end  144  of each pin  125  engages a corresponding edge  143  of the respective interior vertical riser  110   c,    110   d  of the upper housing element  104  when each pin  125  is fully received within the probe  100 , as shown in FIG. 4. This arrangement, specifically the ridges  141  of the pins  125  engaging the edges  143  of their respective vertical risers  110  under an outward biasing force generated by the split in the pin, permits the pins  125  to be readily “snapped” into the apertures  111  within the vertical risers  110 , thereby forming a hinge with pivot or rotational axis for the upper and lower housing elements  104 ,  106 . The biasing spring  133  fits around the pins  125   a,    125   b  as shown in FIG. 4, the two free ends  145   a,  and the connecting section  1456  being received with respective holders  146   a,    146   b  formed within the interior surfaces of the upper and lower housing elements  104 ,  106 , respectively. Using this arrangement, the biasing spring  133  is preloaded (i.e., partially wound) so as to bias the upper housing element  104  against the lower housing element  106 . A pair of finger recesses  150   a,    150   b  are formed within the outward portion of each of the housing elements  104 ,  106 , at a location between the common end  109  of each housing element and the pivot axis  123 , thereby permitting the user to grasp the probe  100  between his or her fingers using the recesses  150   a,    150   b  and separate the probe housing elements  104 ,  106  by applying force counter to the spring biasing force. In this fashion, the user simply grasps the probe  100 , opens it by applying a light force with the grasping fingers, and inserts the distal end of the patient&#39;s finger  112  into the opened end  154  of the probe.  
         [0039]    As depicted in FIG. 2, the pin apertures  111  of the lower housing element  106  are somewhat elongated in the vertical direction (i.e., in a direction normal to the plane of the housing element  106 ). This feature has the practical effect of making the upper and lower housing elements  104 ,  106  conform more readily to the shape of the patient&#39;s finger  112  when the latter is received within the probe  100 . Specifically, the elongated pin apertures allow the portion of the patient&#39;s finger  112  inserted into the open end  154  of the probe (FIG. 3) to act as a fulcrum for a “separating” force generated by the biasing springs  133  such that the common ends  109  of the upper and lower housing elements  104 ,  106  are forced apart by, inter alia, the bias spring separating force. This separating force is generated by the offset  160  of the bias spring ends  145   a,  and connecting section  145   b  from the axis  123  of the spring, as shown in FIG. 3. When the user grasps the recesses  150  of the housing elements  104 ,  106  and squeezes, the pins  125  are forced to the fully compressed position within the elongated pin apertures  111 ; that is, the pins are forced against the bottom edge of the elongated apertures  111  in order to allow the probe  100  to be opened. However, once the finger  112  is inserted into the probe, the disproportionate compression of the finger  112  (due to the interaction of the angled housing elements  104 ,  106  and the substantially cylindrical finger  112 ) and the aforementioned bias spring separating force, act to force the common end  109  of the probe housing elements  104 ,  106  apart, thereby making upper and lower housing elements  104 ,  106  more parallel to each other as shown in FIG. 3. This “dislocation” of the upper element  104  with respect to the lower element  106  allows more of the surface area of the upper and lower support surface elements  114 ,  116  (described below) to contact the finger  112 , and for more even pressure distribution thereon.  
         [0040]    As previously discussed, the housing elements  104 ,  106  are adapted to receive first (upper) and second (lower) support surface elements  114 ,  116 , respectively, which provide support and alignment for the tissue material, such as the finger  112  shown in FIG. 3, when the probe  100  is clamped thereon. When assembled as in FIGS. 2 a  and  3 , the housing elements  104 ,  106  and support surface elements  114 ,  116  form interior cavities  115   a,    115   b  within the upper and lower housing elements  104 ,  106 , respectively, which contain, inter alia, the light source  103  and photodetector  105  as described in greater detail below. The upper support surface element  114  is fashioned from a substantially pliable polymer such as silicone rubber, so as to permit some deformation of the element  114  when in contact with the fairly rigid upper portion  113  of the patient&#39;s finger  112 . In one embodiment, the upper element  114  is constructed as a membrane of polymer. The lower surface element  116  is fashioned from a substantially solid and rigid (i.e., higher durometer) polymer. This harder, solid polymer is used for the lower surface element  116  since the lower portion of the finger  112  is generally more fleshy and deformable, thereby allowing the skin and tissue material thereof to deform and contour to the shape of the inner region  122  of the lower surface element.  
         [0041]    The upper and lower surface elements  114 ,  116  also include first and second apertures  117 ,  119 , respectively, which communicate with the patient&#39;s tissue material when the finger  112  is inserted in the probe  100 . The apertures allow for light energy to be transmitted between the light source  103  and tissue material, and similarly between the tissue material and detector  105 . The first aperture  117  is also axially located with the second aperture  119  in the vertical dimension, such that when the probe  100  is in the closed configuration with the patient&#39;s finger  112  disposed between the upper and lower surface support elements  114 ,  116 , light emitted by the light source  103  through the first aperture  117  is transmitted through the finger  112  and the second aperture  119  and received by the detector  105 . Hence, the light source  103 , first aperture  117 , second aperture  119 , and detector  105  are substantially axial in this configuration.  
         [0042]    The lower support element  116  is further provided with a positioning element  196  disposed near the pivot element  108  and common end  109  of the probe  100 , as shown in FIGS. 2 and 3. The positioning element  196  is oriented vertically with respect to the lower support element  116  so as to stop the distal end of the patient&#39;s finger from being inserted into the probe past a certain point, thereby facilitating proper alignment of the finger  112  within the probe  100 , especially with respect to the source and detector apertures  117 ,  119 . While the present embodiment uses a semi-circular tab as the positioning element  196 , it will be recognized that other configurations and locations of the element  196  may be used. For example, the tab could be bifurcated with a portion being located on the upper support surface element  114 , and a portion on the lower support surface element  116 . Alternatively, the positioning element could be in the form of a tapered collar which receives, aligns, and restrains only the distal portion of the patient&#39;s finger. Many such alternative embodiments of the positioning element are possible, and considered to be within the scope of the present invention.  
         [0043]    As further described below, the lower surface element  116  optionally includes a chamber  126  coincident with the second aperture  119  to assist in mitigating the effects of patient movement during light transmission or reflection. The structure of such chambers is described in detail in U.S. Pat. No. 5,638,818, entitled “Low Noise Optical Probe”, which is incorporated herein by reference. In general, the chamber  126  acts to isolate a portion of the tissue material directly over the chamber  126  and aperture  119 , thereby reducing compression of that tissue during movement. This tends to stabilize the signal generated by the detector  105 , since the optical transmission path through the tissue material is effectively stabilized and constant. The chamber  126  is deep enough that the detector  105  and the bottom of the chamber  126  do not come into contact with the easily compressible portion of the tissue material, even when the tissue is material is caused to move. The movement of venous blood due to compression is also minimized in the field of view of the detector  105 .  
         [0044]    In the embodiment of FIGS.  2 - 4 , the light source  103  is comprised of one or more devices such as semi-conductive light emitting diodes (LEDs), although it will be appreciated that other light generating devices may be used. The light source  103  may be chosen to emit light at a single known discrete wavelength, at multiple discrete wavelengths, or across a portion of the spectrum (such as that emitted by a “white light” LED), depending on the needs of the particular application. In the present embodiment, the light source  103  consists of two diodes emitting light energy in the infrared and red regions of the electromagnetic spectrum, and a parallel resistor (or resistors) used for security. The construction and operation of such light source drive circuitry is described in U.S. Pat. No. 5,758,644 incorporated herein by reference.  
         [0045]    As shown in FIG. 2, the light source  103  is affixed in a recess  170  formed in the interior portion of the upper support surface element  114 , and aligned with the aperture  117  formed within the upper support surface element  114 . An adhesive such as a UV-cured silicone-based gel is used to affix the LED  103  to the recess  170 , although other adhesives or attachment schemes may be employed. The light energy detector  105 , in the present case a semi-conductive photodetector, is received within a corresponding recess  172  within the lower support surface element  116 . As with the LED  103 , the photodetector  105  may be fixed within its recess  172  according to a number of different methods, including but not limited to adhesive, a press fit, or clear epoxy resin which transmits light over a range of wavelengths of interest.  
         [0046]    As illustrated in FIG. 2, the upper support surface element  114  further includes an optical energy shield  130  which, in the present embodiment, is comprised of a plurality of shield tabs  132   a,    132   b,    132   c  which protrude from the upper support surface element  114 . The shield  130  and tabs  132   a,    132   b,    132   c  are sized and shaped so as to conform substantially to the outer circumference of the patient&#39;s finger  112 , providing at least a partial seal against ambient light incident on the probe exterior and otherwise exposed portions of the finger  112 . Since the shield  130  is also formed from the same pliable polymer as the first support surface element  114 , both the shield and upper support surface element are capable of automatically adapting their shape substantially to that of the patient&#39;s finger  112  without further adjustment. Specifically, as the probe  100  is closed around the finger  112 , the central region of the pliable upper support surface element  114  engages the more rigid upper portion of the patient&#39;s finger, thereby compressing the element  114  in this region. This tends to draw the proximal portions  190   a,    190   b  of the shield tabs  132   a,    132   b,    132   c  toward the finger  112 , thereby forming a better seal. In this fashion, patients having fingers of different circumferences can be accommodated with the same probe shield  130 .  
         [0047]    As shown in FIGS. 2 and 3, the present embodiment of the optical probe  100  includes optional optically transparent covers  198   a,    198   b  which are fitted at least partly within the apertures  117 ,  119  formed in the upper and lower support surface elements  114 ,  116 , respectively. The covers  198   a,    198   b  are fabricated from a rigid or semi-rigid transparent polymer (such as polycarbonate), although other materials may be substituted. The two covers  198   a,    198   b  act as protection for the source  103  and detector  105  disposed thereunder, respectively. Specifically, the covers  198  are bonded to the interior surface of the upper and lower support surface elements  114 ,  116  such that the outer surfaces  199   a,    199   b  of the covers  198  are essentially flush with the outer surfaces of their respective support surface elements  114 ,  116 . The outer surfaces  199   a,    199   b  (FIG. 3) of the covers  198   a,    198   b  may also be curved or contoured if desired. It is further noted that while the present embodiment utilizes covers  198  which are optically transparent, the physical and optical properties of the covers may be adjusted to produce the desired characteristics. For example, one or both of the covers  198   a,    198   b  may include a scattering medium as described further below.  
         [0048]    In an alternative embodiment, the windows for the emitter and detector are filled with a UV-cured silicone that is transparent to the wavelengths of the emitter, and the transparent covers described above are not used.  
         [0049]    In addition to the aforementioned features, the upper and lower support surface elements  114 ,  116  and light shield  130  are advantageously formed from or coated with a light absorbing material which further mitigates the effects of ambient light, as well as stray (i.e., “secondary”) reflected light within the probe generated by the light source  103 . Specifically, light generated by the light source  103  can take several paths in reaching the detector, only one of which is the desired path via the aforementioned first and second apertures  117 ,  119  and through the interposed tissue material. Preferably, in order to obtain more accurate measurement of transmitted light intensity, these other paths are eliminated or attenuated. Hence, in one embodiment, the light absorbing material is disposed on the entire upper and lower surface elements  114 ,  116  and shield  130  so as to substantially absorb any secondary reflections prior to being received by the detector  105 . In the embodiment of FIGS.  2 - 4 , the upper surface element  114  (and integral shield  130 ) and lower surface element  116  may be formed from a black, opaque material which both inhibits the transmission of light energy through its thickness and absorbs at least a portion of the incident light incident onto its surface. In this fashion, the absorbing material mitigates the effects of both reflections transmitted from the light source  103 , and ambient light incident on the surface elements  114 ,  116 . It will also be appreciated that while the present embodiment employs surface elements  114 ,  116  which are formed from an opaque material, absorptive coatings or coverings may be used as well. For example, all or a portion of the upper and lower surface elements  114 ,  116  and shield  130  could be coated with a light-absorbing paint or other absorbing substance as an alternative to or in addition to the use of the aforementioned opaque material. In one embodiment, the elements  114  and  116  are white or reflective in the vicinity immediately surrounding the apertures  117 ,  119 .  
         [0050]    For example, FIGS. 2 b  and  2   c  depict one embodiment of the upper and lower elements  114 ,  116 , showing nonreflective surfaces  140   a  and  140   b  with shading. In this embodiment, the area directly around the apertures for the LED and photodetector remain white or reflective to promote light transmission in the area of the photodetector.  
         [0051]    In addition to the light shield and absorptive materials described above, the present embodiment utilizes an electromagnetic noise shield  185  as illustrated in FIG. 5. The noise shield  185  operates on Faraday principles to block or attenuate electromagnetic interference (EMI). The noise shield  185  has a grating  187  which permits light to pass while still blocking electromagnetic energy. In a preferred embodiment, this is a very fine screen with a large open percentage to permit as much light as possible yet still block EMI. In the illustrated embodiment, the detector shield  185  is an etched copper shield made of copper foil approximately 4 mils thick, which is wrapped around the detector  105  in a box-like fashion so as to substantially enclose the detector, significantly insulating it from a external EMI. The screen or grating  187  is etched through the shield to allow light from the light source  103  to transmit through the shield  185  to the detector  105 . The noise shield  185  is also electrically grounded to the detector ground, thereby precluding the buildup of electrical charge.  
         [0052]    In the optical probe of FIGS.  2 - 4 , the apertures  117 ,  119  optionally may be filled wholly, or in part, by a scattering medium  180 . The scattering of the light energy within a scattering medium has been found to increase the signal-to-noise ratio of the signal generated by the detector. Ideally, the scattering medium  180  scatters but does not significantly absorb light energy at the wavelengths of significance for the operation of the probe. In other words, the material is substantially transparent to optical absorption, but none-the-less effectively scatters light energy. In general, the scattering medium  180  may comprise one of a number of fixotropic substances (i.e., substances having two or more mixed materials which are conducive to scattering). The construction and use of scattering media are further described in the aforementioned U.S. Pat. No. 5,638,818.  
         [0053]    [0053]FIGS. 6 and 6 a  illustrate a second embodiment of the optical probe of the present invention. As illustrated, this second embodiment of the probe  200  comprises, inter alia, first and second housing elements  204 ,  206 , upper and lower support surface elements  214 ,  216 , and pivot assembly  208  as previously described. The embodiment of FIG. 6 further includes a plurality of discrete, removable shield tabs  220  which are mounted to the optical probe  200  using a sliding key and channel arrangement. Specifically, each of the tab elements  220  are formed such that the edge  222  which engages the optical probe  200  is shaped or keyed so as to fit within the channels  224  located on the outer portions of and running longitudinally along the upper housing element  104  of the optical probe, as shown in FIG. 6 a.  The present embodiment employs channels  224  and edges  222  which are substantially square in cross-section, although it will be recognized that other cross-sectional shapes including rectangles, polygons, and triangles and circles may be used with equal success. The use of a square (or other shape) for the edges  222  and their corresponding channels  224  prevents significant lateral movement or rotation of the edges  222  in the channels  224  when installed. As shown in FIG. 6 a,  the channels  224  may be further formed such that a pair of support ridges  225   a,    225   b  act to support the tabs  220  when the latter are inserted into the channels  224 , thereby adding additional rigidity to the shield tabs  220 . The shield tabs  220  may also be made interchangeable from side to side if desired.  
         [0054]    The shield tabs  220  of the embodiment of FIGS. 6 and 6 a  are fabricated from a somewhat rigid yet flexible polymer such as silicone rubber of durometer  45 A. This construction permits their removal from and insertion into the channels  224  of the housing element  204 , while allowing some degree of flexibility so as to adapt to the shape and size of each individual patient&#39;s finger when the probe is fitted on the patient.  
         [0055]    It will be appreciated that while the shield tabs  220  of the present embodiment are mounted using channels disposed along the outer portions of the upper housing element  204 , other locations and mounting configurations may be used. For example, the channels  224  could alternatively be located in the lower housing element  206 , with the shield tabs  220  extending upward when inserted rather than downward as in FIG. 5. Similarly, the channels  224  could be located within the upper or lower support surface elements  214 ,  216 . Furthermore, retaining mechanisms other than the aforementioned channels  224  may be used, such as pins and corresponding holes, adhesives, or other devices well known in the mechanical arts. In yet another alternative embodiment, the shield tabs  220  could be part of a single component which is received within the channels  224  or other retaining mechanism on the optical probe. A great variety of optical shield configurations according to the present invention are possible, all of which achieve the goals of minimizing noise within the probe  200  while allowing removability of the shield.  
         [0056]    As illustrated in FIG. 7, another embodiment of the optical probe of the present invention is described. In this embodiment, the probe  300  includes a light energy source  303  and detector  305 , upper and lower housing elements  304 ,  306 , upper and lower support surface elements  314 ,  316 , and pivot element  308 , as in the prior embodiment. However, the embodiment of FIG. 7 utilizes the principle of optical reflectance rather than optical transmission; hence, the light source  303  and detector  305  are both co-located within the upper housing element  304 , in direct proximity to one another. Source and detector apertures  317 ,  319  are formed within the upper support surface element  314 , their axes being canted at a predetermined angle  320  relative to the normal direction  321  and along a common plane  323 . This arrangement permits light to impinge on the surface of the patient&#39;s finger  112 , be reflected therefrom, and received by the detector  305  via the detector aperture  319 .  
         [0057]    It will be recognized that while the source  303  and detector  305  and their respective apertures  317 ,  319  are located generally in the upper housing element  304  and upper support member  314 , they may equally as well be located in the lower housing element  306  and support member  316  if desired. Many of such alternative orientations are possible.  
         [0058]    Additionally, although the embodiment of FIG. 7 has the axis of the apertures  317 ,  319  canted at a predetermined angle, such angle is not necessary for the operation of the optical probe. For example, the apertures  317 ,  319  could alternatively be closely co-located in a vertical orientation; light scattered from various portions of the tissue material of the patient&#39;s finger  112  beneath the source aperture  317  would be reflected into the detector aperture  319  and received by the detector  305 .  
         [0059]    [0059]FIG. 8 illustrates one embodiment of the probe monitoring circuit of the present invention provided with a sensor and a monitor. In this embodiment, the monitoring circuit is comprised generally of a counter  406 , non-volatile storage device  420 , and monitoring LED  426 . Circuitry useful for driving the light source and processing the detector signals is known in the art. The monitoring circuit counts the number of LED activations which is obtained from the modulated drive signals. For example, in one embodiment, the modulated drive pulses to at least one LEDs are also communicated to the counter  406 . Specifically, the light source LEDs  103   a,    103   b  are pulsed (modulated) to alternatively emit red and infrared light energy, respectively, as controlled by the LED driver  424  and the microprocessor  418 . The drive signals to at least one of the LEDs also clocks the counter  406 . In one preferred embodiment, the counter only increments once for a predefined number of activations. In other words, a divide by X circuit forms a portion of the counter. This is because the number of cycles is very high, and a divide circuit reduces the capacity requirements of the counter. The construction and operation of electronic counters are well known in the electrical arts, and accordingly will not be described further. Advantageously, the counter is maintained in a non-volatile RAM (NVRAM) which maintains a running count. When a predetermined number is reached, an LED  426  is activated. This number is determined by, for example, empirical data relating to the longevity of the probe to a given number of detector pulses. The monitoring LED  426  is mounted in the probe housing  102  or remotely from the probe if desired (such as on the probe control module, not shown), and is readily visible to the operator.  
         [0060]    It will be recognized that the aforementioned probe monitoring function can be accomplished using a number of different circuit configurations well known in the art. For example, in one alternative embodiment, circuitry is employed which measures the actual time the detector  105  generates an output signal. In another embodiment, the number of pulses applied to each of the LEDs  103   a,    103   b  in the light source  103  is counted. Many other such alternative embodiments are possible.  
         [0061]    It will be further recognized that while the circuit of FIG. 8 utilizes an LED  426  for indicating probe age, other indicating devices such as incandescent bulbs, liquid crystal displays (LCDs), or even audio generators may be used.  
         [0062]    The pulsed emissions from the LEDs are also detected by the detector  105 . The detected signals are amplified by the amplifier  402 . The amplified signals are then filtered using a band-pass or other filter  404 , thereby passing only that portion of the detected signals corresponding to the desired band of light intensity.  
         [0063]    The probe of the present invention may be employed in any circumstance where a measurement of transmitted or reflected energy is to be made, including but not limited to measurements taken on a finger, an earlobe, a lip, or a forehead. Thus, there are numerous other embodiments which will be obvious to one skilled in the art, including but not limited to changes in the shape of the probe and its components, changes in the materials out of which the probe is made, and changes in the shape, dimensions, and location and orientation of the apertures and shield. Furthermore, the probe of the present invention may be employed in measurements of energy or radiation other than light energy as defined herein. Depending upon the type of energy which is most advantageously utilized in a measurement, the type of transmitter or receiver of energy may be changed. The invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.