Patent Publication Number: US-10768107-B2

Title: Interface detector for blood processing system

Description:
RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 15/414,717, filed Jan. 25, 2017, which is a divisional of U.S. patent application Ser. No. 14/422,188, filed Feb. 18, 2015, which is a U.S. national stage application of PCT Patent Application No. PCT/US13/31494, filed Mar. 14, 2013, which claims the benefit of and priority of U.S. Provisional Patent Application Ser. No. 61/696,343, filed Sep. 4, 2012, the contents of each of the above applications being incorporated by reference herein. 
    
    
     DESCRIPTION 
     Technical Field 
     The disclosure relates to blood treatment systems and methods. More particularly, the disclosure relates to systems and methods for optically detecting or monitoring characteristics of fluid (e.g., the location of an interface between separated blood components) within a centrifugal blood processing device. 
     Background 
     Various blood processing systems now make it possible to collect particular blood constituents, instead of whole blood, from a blood source. Typically, in such systems, whole blood is drawn from a blood source, the particular blood component or constituent is separated, removed, and collected, and the remaining blood constituents are returned to the blood source. Removing only particular constituents is advantageous when the blood source is a human donor, because potentially less time is needed for the donor&#39;s body to return to pre-donation levels, and donations can be made at more frequent intervals than when whole blood is collected. This increases the overall supply of blood constituents, such as plasma and platelets, made available for transfer and/or therapeutic treatment. 
     Whole blood is typically separated into its constituents through centrifugation. This requires that the whole blood be passed through a centrifuge after it is withdrawn from, and before it is returned to, the blood source. To reduce contamination and possible infection (if the blood source is a human donor or patient), the blood is preferably processed within a sealed, sterile fluid flow system during the centrifugation process. Typical blood processing systems include a disposable, sealed, and sterile flow circuit, including a centrifuge chamber portion, that is mounted in cooperation on a durable, reusable assembly containing the hardware (centrifuge, drive system, pumps, valve actuators, programmable controller, and the like) that rotates a centrifuge chamber and controls the flow through the fluid circuit. 
     The centrifuge rotates the centrifuge chamber of the disposable flow circuit during processing. As the centrifuge chamber is rotated by the centrifuge, the heavier (greater specific gravity) components of the whole blood in the centrifuge chamber, such as red blood cells, move radially outwardly away from the center of rotation toward the outer or “high-G” wall of the centrifuge chamber. The lighter (lower specific gravity) components, such as plasma, migrate toward the inner or “low-G” wall of the centrifuge chamber. The boundary that forms between the denser red blood cells and the lighter plasma in the centrifuge chamber is commonly referred to as the interface. Various ones of these components can be selectively removed from the whole blood by providing appropriately located channeling structures and outlet ports in the flow circuit. For example, in one blood separation procedure, plasma is separated from cellular blood components and collected, with the cellular blood components and a replacement fluid being returned to the blood source. Alternatively, red blood cells may be harvested from the centrifuge chamber and the rest of the blood constituents returned to the donor. Other processes are also possible including, without limitation, platelet collection, red blood cell exchanges, plasma exchanges, etc. In these procedures, the efficiency of the process is often dependent upon accurate identification and control of the position of the interface during centrifugation. 
     It is known to employ an optical sensor system to monitor the flow of blood and/or blood components through the flow circuit in the centrifuge and determine various characteristics of the flow. For example, U.S. Pat. No. 6,899,666 to Brown relates to an optical sensor system for viewing into the centrifuge chamber for detecting and controlling the location of an interface between separated blood components in a centrifuge. While this system functions satisfactorily, there remains an opportunity to provide optical monitoring systems with improved interface detection and greater robustness. 
     SUMMARY 
     There are several aspects of the present subject matter which may be embodied separately or together in the devices and systems described and claimed below. These aspects may be employed alone or in combination with other aspects of the subject matter described herein, and the description of these aspects together is not intended to preclude the use of these aspects separately or the claiming of such aspects separately or in different combinations as set forth in the claims appended hereto. 
     In one aspect, a blood processing system includes a centrifuge assembly having a light-transmissive portion, a light reflector, and a fluid processing region at least partially positioned between the light-transmissive portion and the light reflector. The blood processing system also includes an optical sensor system configured to emit a scanning light beam along a path toward the light-transmissive portion of the centrifuge assembly. The light-transmissive portion of the centrifuge is configured to transmit at least a portion of the scanning light beam to the fluid processing region and the light reflector. The light reflector is configured to reflect at least a portion of the scanning light beam toward the optical sensor system along a path substantially coaxial to the path of the scanning light beam from the optical sensor system toward the light-transmissive portion of the centrifuge assembly. 
     In another aspect, a method is provided for monitoring fluid within a blood processing system having a centrifuge assembly. The method includes separating blood in a centrifuge assembly into at least two blood components and directing a scanning light beam along a path toward and into the centrifuge assembly so as to intersect the blood or at least one of the blood components. At least a portion of the scanning light beam is reflected after intersecting the blood or blood component, with the reflected light being directed along a path out of the centrifuge assembly that is substantially coaxial to the path of the scanning light beam toward and into the centrifuge assembly. At least a portion of the reflected light is received and analyzed. 
     In yet another aspect, an optical sensor system is provided for use in combination with a blood processing system. The optical sensor system includes a light source, a light detector, and an optical fiber providing a light path between the light source and the light detector. 
     In another aspect, a blood processing system includes a centrifuge assembly having a light-transmissive portion, a light reflector, and a fluid processing region at least partially positioned between the light-transmissive portion and the light reflector. The blood processing system also includes an optical sensor system having a light source configured to emit a source light beam, a light detector, and an optical fiber providing a light path to the light detector. The light-transmissive portion of the centrifuge assembly is configured to transmit at least a portion of the source light beam to the fluid processing region and the light reflector. The light reflector is configured to reflect at least a portion of the source light beam toward the optical sensor assembly. The optical fiber is configured to conduct at least a portion of the reflected source light beam toward the light detector. 
     In yet another aspect, a method is provided for monitoring fluid within a blood processing system having a centrifuge assembly. The method includes separating blood in a centrifuge assembly into at least two blood components and generating a source light beam. At least a portion of the source light beam is directed into the centrifuge assembly so as to intersect the blood or at least one of the blood components. At least a portion of the source light beam is reflected after intersecting the blood or blood component and is then directed toward a light detector through an optical fiber. 
     In another aspect, an optical sensor system for use in combination with a blood processing system includes a white light source. 
     In yet another aspect, a blood processing system includes a centrifuge assembly having a light-transmissive portion and a fluid processing region positioned at least partially adjacent to the light-transmissive portion. The blood processing system also includes an optical sensor system having a light source that emits a white light directed toward the light-transmissive portion of the centrifuge assembly. 
     In another aspect, a method is provided for monitoring fluid within a blood processing system having a centrifuge assembly. The method includes separating blood in a centrifuge assembly into at least two blood components and generating a source light beam comprising a white light. At least a portion of the source light beam is directed toward and into the centrifuge assembly so as to intersect the blood or at least one of the blood components. At least a portion of the source light beam is reflected after intersecting the blood or blood component and at least one characteristic of the reflected source light beam is detected. 
     In yet another aspect, a blood processing system includes a centrifuge assembly having a light-transmissive portion, a light reflector, and a fluid processing region at least partially positioned between the light-transmissive portion and the light reflector. The blood processing system also includes an optical sensor system having a light source configured to emit a source light beam and a plurality of light detectors. The light-transmissive portion of the centrifuge assembly is configured to transmit at least a portion of the source light beam to the fluid processing region and the light reflector. The light reflector is configured to reflect at least a portion of the source light beam toward the optical sensor system. The plurality of light detectors are configured to detect at least one characteristic of the reflected source light beam at different locations. 
     In another aspect, a method is provided for monitoring fluid within a blood processing system having a centrifuge assembly. The method includes separating blood in a centrifuge assembly into at least two blood components and generating a source light beam. The source light beam is directed toward and into the centrifuge assembly so as to intersect the blood or at least one of the blood components. At least a portion of the source light beam is reflected after intersecting the blood or blood component and at least one characteristic of the reflected source light beam is detected at a plurality of different locations. 
     In yet another aspect, a blood processing system includes a centrifuge assembly having a rotational axis. The blood processing system also includes an optical sensor system having a light source that emits a source light beam directed along a path parallel to a radius passing through the rotational axis of the centrifuge assembly. The path of the source light beam is oriented so as to not pass through the rotational axis of the centrifuge assembly. 
     In another aspect, a method is provided for monitoring fluid within a blood processing system having a centrifuge assembly with a rotational axis. The method includes separating blood in a centrifuge assembly into at least two blood components and generating a source light beam. At least a portion of the source light beam is directed along a path parallel to a radius passing through the rotational axis of the centrifuge assembly, but oriented so as to not pass through the rotational axis of the centrifuge assembly, and into the centrifuge assembly so as to intersect the blood or at least one of the blood components. At least a portion of the source light beam is reflected after intersecting the blood or blood component and then at least one characteristic of the reflected source light beam is detected. 
     In yet another aspect, a blood processing system includes a centrifuge assembly having a rotational axis. The centrifuge assembly has a light-transmissive portion, a fluid processing region positioned radially inwardly of the light-transmissive portion, and a yoke including a first support arm configured to rotate the light-transmissive portion and the fluid processing region about the rotational axis. The blood processing system also includes an optical sensor system configured to direct a light toward the light-transmissive portion of the centrifuge assembly. The yoke is positioned between the light-transmissive portion and the optical sensor system and is configured to allow passage of at least a portion of the light through the first support arm as the light is directed toward the light-transmissive portion. 
     In another aspect, a blood processing system includes a centrifuge assembly having a rotational axis. The centrifuge assembly has a light-transmissive portion, a fluid processing region positioned radially inwardly of the light-transmissive portion, and a yoke. The yoke includes a first support arm configured to rotate the light-transmissive portion and the fluid processing region about the rotational axis. An optical fiber bundle extends between first and second ends and is associated with the support arm of the yoke. The blood processing system also includes an optical sensor system configured to direct a light toward the first end of the optical fiber bundle. The second end of the optical fiber bundle directs the light toward the light-transmissive portion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side elevation view, with portions broken away and in section, of one example of a blood separation system employing aspects of the present invention, with a centrifuge bowl and spool of the system being shown in their operating position; 
         FIG. 2  is a side elevation view, with portions broken away and in section, of the system shown in  FIG. 1 , with the bowl and spool shown in an upright position for receiving a blood separation chamber; 
         FIG. 3  is a top perspective view of the spool of the centrifuge shown in  FIG. 2  in its upright position and carrying the blood separation chamber; 
         FIG. 4  is a plan view of the blood separation chamber shown in  FIG. 3 , out of association with the spool; 
         FIG. 5  is an enlarged perspective view of an interface ramp carried by the centrifuge in association with the blood separation chamber, showing the centrifugally separated red blood cell layer, plasma layer, and interface within the chamber when in a desired location on the ramp; 
         FIG. 6  is an enlarged perspective view of the interface ramp shown in  FIG. 5 , showing the red blood cell layer and interface at an undesired high location on the ramp; 
         FIG. 7  is an enlarged perspective view of the interface ramp shown in  FIG. 5 , showing the red blood cell layer and interface at an undesired low location on the ramp; 
         FIG. 8  is a front perspective view of the bowl of the centrifuge of  FIG. 1  and an optical sensor system or assembly, inverted from the usual operating position for clarity, which may form a part of an interface controller to view the interface ramp during rotation of the bowl; 
         FIG. 9  is a rear perspective view of the bowl and optical sensor system or assembly of  FIG. 8 ; 
         FIG. 10  is a cross-sectional view of the optical sensor system or assembly of  FIG. 8 ; 
         FIG. 11  is a perspective view of selected internal components of the optical sensor system or assembly of  FIG. 8 , with a housing or case of the optical sensor assembly omitted for illustrative purposes; 
         FIG. 11A  is a perspective view of an alternative embodiment of the selected internal components of the optical sensor system of  FIG. 11 ; 
         FIG. 11B  is a perspective view of another alternative embodiment of the selected internal components of the optical sensor system of  FIG. 11 ; 
         FIG. 12  is a top plan view of the bowl and optical sensor system or assembly of  FIG. 8 , with a housing or case of the optical sensor system omitted for illustrative purposes; 
         FIGS. 13 and 14  illustrate a light beam from the optical sensor system of  FIG. 8  passing through the interface ramp of the centrifuge bowl and a centrifuge container or other fluid passage containing blood or blood components; 
         FIG. 15  is a schematic view of the interconnectivity of selected electronic components of the optical sensor system of  FIG. 8 ; 
         FIG. 16  is a schematic view of the interface controller, incorporating the optical sensor system of  FIG. 8 ; 
         FIG. 17  is a perspective view of an alternative centrifuge yoke for use with optical sensor systems according to the present disclosure; 
         FIGS. 18 and 19  are cross-sectional views of the centrifuge and yoke of  FIG. 17 , showing sight lines into the centrifuge in different centrifuge positions; 
         FIG. 20  is a partial cross-sectional view of another embodiment of an alternative centrifuge yoke for use with optical sensor systems according to the present disclosure; 
         FIG. 21  is a cross-sectional detail view of a lower end of an optical fiber bundle associated with the yoke of  FIG. 20 ; 
         FIG. 22  is an end view of the lower end of the optical fiber bundle associated with the yoke of  FIG. 20 ; 
         FIG. 23  is an end view of the upper end of the optical fiber bundle associated with the yoke of  FIG. 20 ; and 
         FIG. 23A  is an end view of an alternative embodiment of the upper end of the optical fiber bundle associated with the yoke of  FIG. 20 . 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
     The embodiments disclosed herein are for the purpose of providing a description of the present subject matter, and it is understood that the subject matter may be embodied in various other forms and combinations not shown in detail. Therefore, specific embodiments and features disclosed herein are not to be interpreted as limiting the subject matter as defined in the accompanying claims. 
       FIGS. 1 and 2  show a centrifugal blood processing system  10  with an interface controller  12  ( FIG. 16 ) having improved interface detection capabilities. The illustrated system  10  shares many centrifuge design aspects with a system currently marketed as the AMICUS® separator by Fenwal, Inc. of Lake Zurich, Ill., which is an affiliate of Fresenius Kabi AG of Bad Homburg, Germany, as described in greater detail in U.S. Pat. No. 5,868,696, which is hereby incorporated herein by reference. The system  10  can be used for processing various fluids, but is particularly well suited for processing whole blood, blood components, or other suspensions of biological cellular materials. 
     While interface control and optical detection principles will be described herein with reference to one particular system  10  and centrifuge assembly  14 , it should be understood that these principles may be employed with other fluid processing systems (e.g., other centrifugal blood separation systems and centrifuges) without departing from the scope of the present disclosure. 
     A. The Centrifuge Assembly 
     The system  10  includes a centrifuge assembly  14  used to centrifugally separate blood components. The system  10  may be programmed to separate blood into a variety of components (e.g., platelet concentrate, platelet-rich plasma, and red cells). It may be used for platelet collection, therapeutic plasma exchange, red cell exchange, red cell or plasma collection, or other blood processing applications. For illustrative purposes only, a platelet collection procedure and a therapeutic plasma exchange procedure will be described herein. However, the principles described and claimed herein may be employed with other blood separation procedures without departing from the scope of the present disclosure. 
     The illustrated centrifuge assembly  14  shares certain design aspects with the one shown in U.S. Pat. No. 5,316,667 to Brown et al., which is incorporated herein by reference. The illustrated centrifuge assembly, which is shown for purposes of illustration and not limitation, comprises a bowl  16  and a spool  18 . In one embodiment, the bowl  16  and spool  18  are pivoted on a yoke  20  between an operating position ( FIG. 1 ) and a loading/unloading position ( FIG. 2 ). Other methods of accessing the bowl  16  and the spool  18  may also be employed without departing from the scope of the present disclosure. The present subject matter may be used with centrifuges that do not employ such a spool and bowl, such as molded centrifuge chambers, centrifuge bowls with pre-formed processing chamber slots, or other designs. 
     When in the loading/unloading position, the spool  18  can be opened by movement at least partially out of the bowl  16 , as  FIG. 2  shows. In this position, the operator wraps a flexible blood separation chamber  22  (see  FIG. 3 ) about the spool  18 . Closure of the spool  18  and bowl  16  encloses the chamber  22  between the inner surface of the bowl  16  and the outer surface of the spool  18  (which collectively define the fluid processing region in which the chamber  22  is received) for processing. When closed, the spool  18  and bowl  16  are pivoted into the operating position of  FIG. 1  for rotation about a rotational axis. 
     B. The Blood Separation Chamber 
     The blood separation chamber  22  can be variously constructed.  FIG. 4  shows a representative embodiment. 
     The chamber  22  shown in  FIG. 4  allows for either single- or multi-stage processing. When used for multi-stage processing of whole blood, a first stage  24  separates whole blood into first and second components. Depending on the nature of the separation procedure, one of the components may be transferred into a second stage  26  for further processing. 
     As  FIGS. 3 and 4  best show, there are three ports  28 ,  30 , and  32  associated with the first stage  24 . Depending on the particular blood processing procedure, the ports may have different functionality but, in an exemplary procedure, the port identified at  32  may be used for conveying blood (which may include anticoagulant) from a blood source or donor into the first stage  24 . During such a procedure, the other two ports  28  and  30  may serve as outlet ports for separated blood components exiting the first stage  24 . For example, the first outlet port  30  may convey a low density blood component from the first stage  24 , while the second outlet port  28  may convey a high density blood component from the first stage  24 . 
     In a method of carrying out single-stage processing, one of the separated components is returned to the donor, while the other is removed from the first stage  24  and stored. For example, when carrying out a therapeutic plasma exchange procedure, whole blood in the first stage  24  is separated into cellular components (i.e., a high density red blood cell component) and substantially cell-free plasma (i.e., a low density component). The plasma is removed from the first stage  24  via the first outlet port  30  for collection and storage, while the cellular components are removed from the first stage  24  via the second outlet port  28  and returned to the donor or patient. Alternatively, rather than collecting and storing the plasma, it may instead be discarded after separation or treated by a secondary device and returned to the donor or patient. 
     If multi-stage processing is required, for example in a platelet collection procedure, one of the components (platelet-rich plasma) will be transferred from the first stage  24  to the second stage  26  via a port  34  associated with the second stage  26 . The component transferred to the second stage  26  is further fractionated into sub-components such as plasma and platelet concentrate, with one of the sub-components (plasma in one embodiment) being removed from the second stage  26  via an outlet port  36  and the other sub-component (platelet concentrate in one embodiment) remaining in the second stage  26 . In the illustrated embodiment, the ports  28 ,  30 ,  32 ,  34 , and  36  are arranged side-by-side along the top transverse edge of the chamber  22 . 
     While the same ports  28 ,  30 , and  32  of the first stage  24  are used as in the above-described therapeutic plasma exchange procedure, the ports  28  and  32  may have different functionality in a multi-stage separation procedure. In the method of multi-stage operation for platelet collection, blood enters the first stage  24  via the port  28  and is separated into red blood cells (i.e., the high density blood component) and platelet-rich plasma (i.e., the low density blood component). The red blood cells are returned to the donor (via the port  32 ), while the platelet-rich plasma is conveyed out of the first stage  24  (via the first outlet port  30 ) and into the second stage  26  (via the inlet port  34 ). In the second stage  26 , the platelet-rich plasma is separated into platelet-poor plasma and platelet concentrate. The platelet-poor plasma is removed from the second stage  26  (via the outlet port  36 ), leaving platelet concentrate in the second stage  26  for eventual resuspension and transfer to one or more storage containers. 
     As best shown in  FIG. 3 , a tubing umbilicus  38  is attached to the ports  28 ,  30 ,  32 ,  34 , and  36 . The umbilicus  38  interconnects the rotating first and second stages  24  and  26  with each other and with pumps and other stationary components located outside the rotating components of the centrifuge assembly  14  (see  FIGS. 1 and 2 ). As  FIG. 1  shows, a non-rotating (zero omega) holder  40  holds the upper portion of the umbilicus  38  in a non-rotating position above the spool  18  and bowl  16 . A holder  42  on the yoke  20  rotates the mid-portion of the umbilicus  38  at a first (one omega) speed about the suspended spool  18  and bowl  16 . Another holder  44  ( FIGS. 2 and 3 ) mounts the lower end of the umbilicus  38  to the centrifuge assembly  14 . The inherent strength of the umbilicus  38  causes the centrifuge assembly  14  to rotate at a second speed twice the one omega speed (the two omega speed). This known relative rotation of the umbilicus  38  keeps it from accumulating twisting, in this way avoiding the need for rotating seals. In an alternative embodiment, rather than the holder  42  rotating the umbilicus  38  to turn the centrifuge assembly  14 , a gear system may be employed to rotate the umbilicus  38  and/or the centrifuge assembly  14  separately. It should be noted that the present subject matter can also be employed in direct-drive centrifuge assemblies (i.e., systems that rely on a gear train to rotate the centrifuge) and centrifuge assemblies using rotating seals, and is not limited to use in a seal-less centrifuge system. 
     As  FIG. 4  shows, a first interior seal  46  is located between the low density outlet port  30  and the high density outlet port  28 . A second interior seal  48  is located between the high density outlet port  28  and the blood inlet port  32 . The interior seals  46  and  48  form a fluid passage  50  (an inlet for whole blood in an exemplary platelet collection procedure or an outlet for high density blood components in an exemplary therapeutic plasma exchange procedure) and a low density collection region  52  in the first stage  24 . The second seal  48  also forms a fluid passage  54  (an outlet for high density blood components in an exemplary platelet collection procedure or a blood inlet in an exemplary therapeutic plasma exchange procedure) in the first stage  24 . 
     In a platelet collection procedure, the fluid passage  50  channels blood into the first stage  24 , where it separates into an optically dense layer  56  ( FIG. 5 ), which forms as larger and/or heavier blood particles move under the influence of centrifugal force toward the high-G (outer) wall  62 . The optically dense layer  56  will include red blood cells (and, hence, may be referred to herein as the “RBC layer”) but, depending on the speed at which the assembly  14  is rotated, other cellular components (e.g., larger white blood cells) may also be present in the RBC layer  56 . 
     Rather than flowing blood into the first stage  24  by the fluid passage  50  (as in a platelet collection procedure), blood enters the first stage  24  by the fluid passage  54  in a therapeutic plasma exchange procedure, but is still separated into an RBC layer  56 . In comparison to a platelet collection procedure, the centrifuge assembly  14  rotates at a higher speed during a therapeutic plasma exchange procedure, creating a stronger separation field in the first stage  24 . As a result of the stronger separation field, additional cellular components, namely white blood cells and platelets, will be present in a greater quantity in the RBC layer  56 . 
     In both cases, the movement of the component(s) of the RBC layer  56  displaces less dense blood components radially toward the low-G (inner) wall  64 , forming a second, less optically dense layer  58 . In an exemplary platelet collection procedure, the less optically dense layer  58  includes platelet-rich plasma (and, hence, will be referred to herein as the “plasma layer”). In an exemplary therapeutic plasma exchange procedure, the less optically dense layer  58  includes substantially cell-free plasma. However, depending on the speed at which the centrifuge assembly  14  is rotated and the length of time that the blood is resident in the centrifuge assembly, other components (e.g., smaller white blood cells) may also be present in the plasma layer  58 . 
     The transition between the RBC layer  56  and the plasma layer  56  is generally referred to as the interface  60  ( FIG. 5 ). Platelets and white blood cells (which have a density greater than plasma and usually less than red blood cells) typically occupy this transition region, although that also varies with centrifuge speed and residence time, as is well known in the technical field. 
     The location of the interface  60  within the chamber  22  can dynamically shift during blood processing, as  FIGS. 6 and 7  show. If the location of the interface  60  is too high (that is, if it is too close to the low-G wall  64  and the removal port  30 , as  FIG. 6  shows), cellular components can spill over and into the low density collection region  52 , potentially adversely affecting the quality of the low density components (typically plasma). On the other hand, if the location of the interface  60  is too low (that is, if it resides too far away from the low-G wall  64 , as  FIG. 7  shows), the collection efficiency of the system  10  may be impaired. 
     In the illustrated embodiment, as  FIG. 5  shows, a ramp  66  extends from the high-G wall  62  of the bowl  16  at an angle “A” across the low density collection region  52 . The angle “A,” measured with respect to the axis of the first outlet port  30  is about 25° in one embodiment.  FIG. 5  shows the orientation of the ramp  66  when viewed from the low-G wall  64  of the spool  18 .  FIG. 4  shows, in phantom lines, the orientation of the ramp  66  when viewed from the high-G wall  62  of the bowl  16 . 
     Further details of the angled relationship of the ramp  66  and the first outlet port  30  can be found in U.S. Pat. No. 5,632,893 to Brown et al., which is incorporated herein by reference. The ramp  66  shown in  FIGS. 5-7  may be considered a simplified or representational version of an actual ramp that would be used in practice. For example,  FIGS. 8-9 and 13-14  illustrate a particular ramp configuration that may be particularly advantageous for imaging and interface-detection purposes, as will be described in greater detail below. However, the ramp  66  may be variously configured without departing from the scope of the present disclosure. 
     The ramp  66  forms a tapered wedge that restricts the flow of fluid toward the first outlet port  30 . The top edge of the ramp  66  extends to form a constricted passage  68  along the low-G wall  64 . The plasma layer  58  must flow through the constricted passage  68  to reach the first outlet port  30 . 
     As  FIG. 5  shows, the ramp  66  makes the interface  60  between the RBC layer  56  and the plasma layer  58  more discernible for detection, displaying the RBC layer  56 , plasma layer  58 , and interface  60  for viewing through a light-transmissive portion of the high-G wall  62  of the chamber  22 , as will be described in greater detail below. 
     Further details of the separation chamber  22  and its operation may be found in U.S. Pat. No. 5,316,667, which is incorporated by reference. 
     C. The Interface Controller 
     In one embodiment, the interface controller  12  ( FIG. 16 ) includes an optical sensor system or assembly  70  (see  FIGS. 8-12 ) positioned at a location outside of the centrifuge assembly  14 . The optical sensor system  70  is oriented to detect the location of the interface  60  the RBC layer  56  and the plasma layer  58  on the ramp  66 . If the interface  60  detected by the optical sensor system  70  is at an improper location (e.g., in the locations of  FIG. 6 or 7 ), the interface controller  12  is functional to correct the location of the interface  60 , as will be described in greater detail herein. 
     Referring to  FIGS. 8-12 , the optical sensor system  70  is secured to a fixture or wall  74  of the system  10 . The wall  74  includes an opening  76  ( FIG. 9 ) through which light from the optical sensor system  70  may be directed toward and into the centrifuge assembly  14  via a light-transmissive portion thereof. In the illustrated embodiment, the ramp  66  is translucent and comprises the light-transmissive portion of the centrifuge bowl  16 , such that light from the optical sensor system  70  passes through the ramp  66  ( FIGS. 13 and 14 ) to intersect the separated blood components thereon to determine the location of the interface  60 , as will be described in greater detail herein. 
     The optical sensor system  70  includes a variety of components, some of which are contained within a housing or case  78 . Among the components mounted within the housing  78  is at least one light source  80  ( FIGS. 10-12 ), which emits a source beam  82  of light. The optical sensor system  70  may include one or more components (e.g., the achromatic prism pairs  84  and aperture stop  86  of  FIGS. 10-12 ) configured to condition and/or focus the source beam  82  that exits the light source  80 . For example, if provided, an achromatic prism pair  84  provides a color correction function by directing two color wavelengths (e.g., blue and red) along a desired path or angle, while an aperture stop  86  controls and limits the amount of light from the light source  80  allowed to pass further through the optical sensor system  70 . It should be understood that, depending on the nature of the light source  80 , selected components (e.g., the achromatic prism pairs  84 ) may be omitted from the optical sensor system  70 . Similarly, additional components may also be incorporated into the optical sensor system  70  without departing from the scope of the present disclosure. 
     In the illustrated embodiment, the light source  80  comprises a light-emitting diode which emits a source light beam  82  or a plurality of light-emitting diodes that combine to emit a source light beam  82 . The light source  80  may emit a single- or multiple-wavelength source light beam  82 , but in a preferred embodiment, comprises a white light source that is configured to emit a multi-wavelength, white source light beam  82 . If provided as a white light source, the light source  80  may comprise one or more true white lights (e.g., incandescent or filament lights or light-emitting diodes) or a plurality of differently colored light sources (e.g., red, green, and blue light-emitting diodes arranged on a common die) that combine to simulate or approximate a white light. In one embodiment, the light source  80  is of the type which emits a white source light beam  82  exhibiting a relatively high spectral power distribution in the red and blue wavelength spectra, such as a warm white LUXEON® light-emitting diode of Philips Lumileds Lighting Company of San Jose, Calif. 
     In other embodiments, other types of light sources and source beams may be employed without departing from the scope of the present disclosure. For example, in another embodiment, the light source comprises one or more non-white, narrow spectrum light sources. The nature of the narrow spectrum light sources (e.g., whether they are provided as light-emitting diode or in some other form) and the source light beam emitted by the narrow spectrum light sources (e.g., the color of the light, if it is within the visible spectrum) may vary and is not limited to a particular type of light source or a particular wavelength of light. In one exemplary embodiment, a narrow spectrum light source comprises a light-emitting diode configured to emit a red source light beam, in which case the light source may be provided as a deep red LUXEON® light-emitting diode of Philips Lumileds Lighting Company of San Jose, Calif. Other narrow spectrum red light sources may also be employed, as well as other narrow spectrum light sources configured to emit a beam having any other suitable wavelength. If the light source is configured to emit a relatively wide bandwidth source beam, it may be preferred to also provide one or more filters configured to narrow the bandwidth of either the source beam emitted by the light source and/or the bandwidth of a light beam returning to the optical sensor system  70  after having interacted with the fluid processing region. 
     The optical sensor system  70  also includes a plurality of light detectors  88 ,  88   a  ( FIG. 15 ). The light detectors  88 ,  88   a  may be variously configured without departing from the scope of the present disclosure, but in one embodiment they comprise silicon PIN photodiodes, which may be particularly well-suited for use with a white or red light source. In the illustrated embodiment, the light detectors  88 ,  88   a  are positioned outside of the housing  78 , and may be mounted in a separate housing. This may be advantageous for the purpose of spacing the light detectors  88 ,  88   a  and other sensitive components (e.g., analog electronics and amplifier components) away from the drive systems that rotate the centrifuge assembly  14 . Additionally, such a configuration allows for a compact optical module design, which is relatively immune to electrical noise and vibration while allowing for electrically immune light transmission from the optical module to a separate electronics module that can be modified and upgraded for different functionality without affecting the optical module design. For example, if not otherwise provided, a separate electronics module could be modified and upgraded to include spectral splitting and analysis without modifying the optical module. 
     In embodiments having the light detectors  88 ,  88   a  mounted outside of the housing  78 , they may be in communication with the interior of the housing  78  via optical fibers  90 - 90   c  ( FIGS. 8-10 and 15 ). In the illustrated embodiment, there are four optical fibers  90 - 90   c  (one referred to herein as a reference fiber  90  and the others referred to herein as scanning fibers  90   a - 90   c ) extending between the housing  78  and four light detectors  88 ,  88   a  connected thereto by FC/PC connectors or the like to define a portion of the light path between the light source and the light detectors ( FIG. 15  is simplified to show only one detector  88   a , but in the illustrated embodiment there are a plurality, preferably three or six, of such detectors  88   a ). However, in other embodiments, there may be a different number of optical fibers and light detectors, at least one and preferably a plurality, such as three or more. For example, a beam splitter may be positioned at a downstream end of an optical fiber to split a beam exiting the optical fiber into two beams, with each beam going to a different light detector. 
     In the illustrated embodiment, the upstream or inlet (light-receiving) ends of the optical fibers  90 - 90   c  are oriented at an angle to the initial direction  92  of the source light beam  82 , as shown in  FIGS. 10-12 . In one embodiment, the upstream or inlet ends of the optical fibers  90 - 90   c  are positioned to receive light along a direction perpendicular to the initial direction  92  of the source light beam  82 . The optical fibers  90 - 90   c  are configured to receive at least a portion of the light emitted by the light source  80 , so selected components of the optical sensor system  70  may be configured to direct light from the light source  80  toward one or more of the optical fibers  90 - 90   c . For example, in the illustrated embodiment, the optical sensor system  70  includes a beam splitter  94  that is configured to split the source beam  82  from the light source  80  into two beams  96  (reference beam or first split beam) and  98  (scanning beam or second split beam) ( FIGS. 11 and 12 ). In one embodiment, the beam splitter  94  comprises a beam splitter cube which splits the source beam  82 , with a first split beam  96  being a reference beam that is reflected at an angle (e.g., 90° in the illustrated embodiment) toward the optical fiber  90  and a second split beam  98  being a scanning beam that is transmitted through the beam splitter cube  94  and toward the centrifuge assembly  14 . 
     The optical sensor system  70  may include one or more components (e.g., an achromatic prism pair  84  for color correction, as shown in  FIGS. 10-12 ) configured to condition and/or focus the reference beam  96  before it reaches the associated optical fiber  90 , but the light received by the optical fiber  90  is essentially a direct view of the source beam  82  (albeit at a fraction of its original intensity) and gives an indication of the power level of the light source  80 . For this reason, the optical fiber  90  that receives the reference beam  96  may be referred to as the reference fiber. As such, the reference fiber  90  may be associated with a light detector  88  that forms a feedback loop with a driver  100  of the light source  80  ( FIG. 15 ). It may be advantageous for the light source  80  to emit a source beam  82  having a substantially uniform or constant brightness, and any fluctuations in the brightness of the source beam  82  are directly reflected in the brightness of the reference beam  96  and, hence, the strength of the signal transmitted from the light detector  88  to the driver  100 . The driver  100  or a controller may make adjustments to the power delivered to the light source  80  to maintain the brightness of the source beam  82  at a substantially uniform level. In other embodiments, the brightness of the source beam  82  may be measured and used as an input to measure fluid characteristics (e.g., lipemia or hemolysis), apart from or in addition to the brightness being controlled. It is also within the scope of the present disclosure for the brightness to be measured to determine light output degradation over time, either apart from or in addition to the brightness being controlled. The light detector  88  may be directly associated with the driver  100  or, as shown in  FIG. 15 , include one or more intermediate devices (e.g., an interface processing module  126 ) that may measure or condition or otherwise interact with the signal from the light detector  88  prior to reaching the driver  100  or otherwise use the signal for other purposes. 
     In an alternative embodiment, the reference fiber  90  is eliminated and the light detector  88  that is positioned downstream of the reference fiber  90  in the above-described embodiment is instead positioned within the housing  78 . For example,  FIG. 11A  shows an embodiment in which the light detector  88  is placed in substantially the same location and in the same orientation as the reference fiber  90  in  FIGS. 10 and 11 . In such an embodiment, the light detector  88  directly receives light, rather than having light transmitted thereto by the reference fiber  90 . By such a configuration, other components of the optical sensor system  70  (e.g., lens  84 ) may also be eliminated or modified. In other embodiments, one or more of the other optical fibers  90   a - 90   c  may be eliminated and replaced with a light detector that is positioned in the same or a similar position and orientation in the housing  78 . 
     In another alternative embodiment which omits the reference fiber  90 , the light detector  88  is placed in a different location within the housing  78  ( FIG. 11B ), rather than being positioned at the location of the reference fiber  90  in  FIGS. 10 and 11 . In the embodiment of  FIG. 11B , the light detector  88  is positioned adjacent to the light source  80 , which may include being positioned on the same printed circuit board as the light source  80  (if the light source  80  is mounted on a printed circuit board), but may include any other suitable location. By such a configuration, other components of the optical sensor system  70  (in addition to the reference fiber  90 ) may be eliminated or modified, as appropriate. 
     Depending on the exact location of the light detector  88 , its orientation may vary, provided that it is oriented so as to be in at least partial light-receiving relationship with respect to the light source  80 . In one embodiment, the light detector  88  is oriented at an angle with respect to the general path of the source beam  82 . In the illustrated embodiment, a substantially side-looking light detector  88  is provided, with the light detector  88  being oriented generally perpendicular to the path of the source beam  82 . In other embodiments, the light detector  88  may be positioned elsewhere within the housing  78  and oriented differently, but it has been found that a side-looking light detector  88  positioned adjacent to the light source  80  is particularly advantageous in terms of monitoring and controlling the level of light emitted by the light source  80 . 
     Regardless of the exact location of the optical fibers and/or light detectors, the scanning beam  98  is transmitted through the beam splitter  94  (or other suitable light-directing member) and toward the centrifuge assembly  14 . The scanning beam  98  may pass through a lens or protective window  102  prior to reaching the centrifuge assembly  14 . The window  102  may serve a number of purposes, which may include focusing the scanning beam  98  at the proper location of the centrifuge assembly  14  and/or protecting the components of the optical sensor system  70  contained within the housing  78  from debris present within the system  10 . As will be described in greater detail herein, the scanning beam  98  passes through the interface ramp  66  and the fluids positioned thereon (including the interface  60 ) before being reflected back to the optical sensor system  70 . The reflected second split beam or reflected scanning beam  104  passes through the window  102  and encounters the beam splitter  94 , which directs at least a portion of the reflected scanning beam  104  at an angle to the path  92  of the scanning beam  98  ( FIG. 12 ). The path  92  of the scanning beam  98  coincides with the direction in which the reflected scanning beam  104  returns to the optical sensor system  70  (as well as the initial direction of the source light beam  82 ). In the illustrated embodiment, the beam splitter  94  directs at least a portion of the reflected second scanning  104  at a 90° angle to the path  92  of the scanning beam  98 . Hence, it will be seen that the reflected scanning beam  104  is directed in the opposite direction of the reference beam  96  by the beam splitter  94 . 
     In one embodiment, one or more optical fibers  90   a - 90   c  may be positioned to directly receive the reflected scanning beam  104  from the beam splitter  94  (i.e., being positioned along or adjacent to the same axis as the reference fiber  90 , but oriented on the opposite side of the beam splitter  94  and facing the opposite direction). In another embodiment, such as the one illustrated in  FIGS. 8-11 , the optical fiber(s)  90   a - 90   c  configured to receive the reflected scanning beam  104  (which may be referred to as scanning fibers) are positioned generally adjacent to the reference fiber  90 . More particularly, the illustrated scanning fibers  90   a - 90   c  are positioned below and in the same plane as the reference fiber  90 , on the same side of the beam splitter  94  and facing in the same direction. Such a configuration may be advantageous for a number of reasons, such as space considerations and accessibility of the fibers for maintenance, replacement, and/or upgrade purposes. An optical barrier or other shielding surface may be interposed between the reference fiber  90  and the scanning fibers  90   a - 90   c  to prevent the reference beam  96  from illuminating the scanning fibers  90   a - 90   c  or the reflected scanning beam  104  illuminating the reference fiber  90 . 
     In the illustrated embodiment, to facilitate the fiber positioning described above, a beam directing member  106  (e.g., a pair of mirrors) is employed between the beam splitter  94  and the scanning fibers  90   a - 90   c  to direct the reflected scanning beam  104  to the scanning fibers  90   a - 90   c . The optical sensor system  70  may include one or more components (e.g., the achromatic prism pairs  84 , direct vision prism  108 , and aperture stop  86  of  FIGS. 10-12 ) configured to condition and/or focus the reflected scanning beam  104  prior to encountering the beam directing member  106 . A direct vision prism  108  may be particularly advantageous for undoing any dispersion of a reflected beam having passed through the ramp  66  (which may be prismatic, as described below), thereby color-correcting the reflected beam. 
     As for the relative position of the optical sensor system  70  with respect to the centrifuge assembly  14 ,  FIG. 12  shows that the path  92  of the scanning beam  98  may be parallel to, but offset from, a radial line perpendicular to and passing through the rotational axis  110  of the centrifuge assembly  14 . Hence, it can be seen that the beam emitted by the optical sensor system  70  to analyze the blood and/or blood components in the centrifuge assembly  14  neither passes through nor is parallel to the rotational axis  110  of the centrifuge assembly  14 . This may be advantageous depending on the configuration of the ramp  66 . For example,  FIGS. 13 and 14  are cross-sectional views of the ramp  66 , which shows it with an angled inner face  112  and an angled outer face  114 , which effectively make the ramp  66  a prism. As used in reference to the faces of the ramp  66 , the term “angled” refers to the fact that the inner and outer faces of the ramp  66  are non-tangential to the substantially circular perimeter of the bowl  16 . 
     In the illustrated embodiment, the inner ramp face  112  is angled at approximately 29° (from a horizontal line, in the orientation of  FIGS. 13 and 14 ) and the outer ramp face  114  is angled at approximately 26.4° (from a horizontal line, in the orientation of  FIGS. 13 and 14 ), resulting in an approximately 55.4° prism. To minimize ghosting (show in  FIG. 14  as rays  116 ) and maintain focus of light through the ramp  66  in view of the material (which may be a polycarbonate material in one embodiment) and configuration of the ramp  66  (i.e., as a prism), it has been found that causing the scanning beam  98  to encounter the outer ramp face  114  at an angle is advantageous. By offsetting the path  92  of the scanning beam  92  from the rotational axis  110  of the centrifuge assembly  14 , the ramp  66  will be at an angle to the path  92  when the scanning beam  98  encounters the outer ramp face  114 . In one embodiment, the ramp  66  is approximately 10° from center (see  FIG. 12 ) when it comes into the field of vision of the optical sensor system  70 . In other embodiments, it may be advantageous for the ramp  66  to be at a different angle or even centered when it comes into the field of vision of the optical sensor system  70 . 
     As for the individual faces of the ramp  66 , the inner ramp face  112  is angled to display the location of the interface  60 , as described in greater detail above with respect to  FIGS. 5-7 . Accordingly, it is the inner ramp face  112  that the scanning beam  98  is focused upon to detect the location of the interface  60 . The outer ramp face  114  is angled to contribute to focusing the scanning beam  98  on the inner ramp face  112  at all times that the ramp  66  is within the field of vision of the optical sensor system  70 . Depending on the configuration of the optical sensor system  70 , multiple samples or readings (of the order of one hundred, in some embodiments) can be taken each time the ramp  66  rotates through the field of view of the optical sensor system  70 .  FIGS. 13 and 14  illustrate two exemplary positions of the ramp  66  during a single pass of the ramp  66  through the field of vision of the optical sensor system  70 , with  FIG. 13  showing a portion of the right side of the inner ramp face  112  being scanned or viewed and  FIG. 14  showing a portion of the left-center side of the inner ramp face  112  being scanned or viewed. It will be seen that, in both positions, the scanning beam  98  is focused on the inner ramp face  112 , where the interface  60  is displayed. 
     As shown in  FIGS. 13 and 14  and noted above, at least a portion of the source beam  82  (which takes the form of the scanning beam  98  in the illustrated embodiment) is directed toward the rotating bowl  16  to pass through the light-transmissive portion thereof, the ramp  66 , and the blood or blood component displayed thereon. In the illustrated embodiment, the bowl  16  is transparent to the light emitted by the light source  80  only in the region at which the interface ramp  66  is incorporated into the bowl  16  ( FIGS. 8 and 12 ). In the illustrated embodiment, the region comprises a window or opening cut out in the bowl  16 , which receives at least a portion of the ramp  66 . The remainder of the bowl  16  that passes through the path of the optical sensor system  70  comprises an opaque or light absorbing material. In the illustrated embodiment, the optical sensor system  70  remains stationary during operation of the blood processing system  10 , as the spool  18  and bowl  16  rotate at a two omega speed. Thus, the optical sensor system  70  may be provided as a continuous or an always-on system (i.e., shining light on the centrifuge assembly  14  even when the ramp  66  is out of the field of vision of the optical sensor system  70 ) or as an intermittent or gated system that only emits a source beam when the ramp  66  is within the field of vision. 
     The light from the source  80  passes through the ramp  66 , to be focused on the inner ramp face  112  and the fluid displayed thereon (e.g., the separated blood components and interface  60 ). At least a portion of the light (i.e., the portion not absorbed or reflected by the fluids) continues through the blood separation chamber  22  and hits the spool  18 . The spool  18  may carry a light-reflective material or light reflector  118  ( FIGS. 13 and 14 ) behind the interface ramp  66  to return the light passing through the ramp  66 , the fluid on the ramp  66 , and the blood separation chamber  22 . In the illustrated embodiment, the light reflector  118  comprises a retroreflector configured to reflect the light along the same path by which it strikes the retroreflector, as shown in  FIGS. 13 and 14 . It may be advantageous for the path  92  of the scanning beam  98  to coincide with the direction in which the reflected scanning beam  104  returns to the optical sensor system  70 . For example, by employing coaxial scanning and reflected scanning beams  98  and  104 , it is ensured that both beams  98  and  104  pass through the same optical components at substantially the same angles from the point where the source beam  82  enters the beam splitter  94  to the point where the reflected scanning beam  104  exits the beam splitter  94  to be focused on the scanning fibers  90   a - 90   c . As used herein, the term “optical components” refers to the surfaces and objects through which a light beam passes. In the case of the scanning and reflected scanning beams  98  and  104 , the optical components include the walls of the blood separation chamber  22 , the fluids contained within the blood separation chamber  22 , the ramp  66 , the beam splitter  94 , and the window  102 . While it is preferred to employ a retroreflector to provide substantially coaxial scanning and reflected scanning beams  98  and  104 , it is also within the scope of the present disclosure to employ a light reflector  118  comprising a mirror or the like, which reflects a light at the same angle at which the light is incident to the mirror. Light reflectors that reflect the scanning beam  98  at some other angle may also be employed without departing from the scope of the present disclosure. 
     The light reflected by the light reflector  118  passes again through the ramp  66 , but in the other direction toward the optical sensor system  70  as a reflected beam or reflected scanning beam  104 . The reflected beam  104  returned to the optical sensor system  70  is ultimately directed to one or more of the light detectors  88   a  for analysis. The reflected beam  104  may be directed to the light detector(s) in any suitable way without departing from the scope of the present disclosure, but in the illustrated embodiment, it is directed to a plurality of light detectors  88   a  via the operation of the beam splitter  94 , the beam deflecting mirror  106 , and the scanning fibers  90   a - 90   c  associated with the light detectors  88   a , as described above in greater detail. 
     The reflected beam  104  is larger than the individual scanning fibers  90   a - 90   c , so each scanning fiber will only receive a portion of the reflected beam  104 . Accordingly, by arranging the scanning fibers in different configurations, different locations and portions of the reflected beam  104  may be captured and analyzed. For example, in the illustrated embodiment, three scanning fibers  90   a - 90   c  are arranged in a generally vertical line below the reference fiber  90  ( FIG. 8 ), thereby taking readings of upper, lower, and central portions or locations of the reflected beam  104 . While it is within the scope of the present disclosure for a single reference fiber or light detector to be used to analyze the reflected beam  104 , it may be advantageous to employ a plurality of fibers and detectors to develop a more complete picture of the interface location. Additionally, the effect of noise on the signals ultimately received by the light detectors may be reduced by considering a plurality of readings from different locations, and accuracy improved. 
     As noted above, the ramp  66  may be oriented at an approximately 25° angle with respect to the rotational axis  110  of the centrifuge assembly  14 , which results in the interface  60  appearing on the inner ramp face  112  as a line angled at an approximately 25° angle with respect to the rotational axis  110 . If the scanning fibers  90   a - 90   c  are arranged in a vertical line (as shown in  FIG. 8 ), they will register the presence of the interface  60  at different times. For example, in one embodiment, the upper end of the angled interface  60  may move into the field of vision of the optical sensor system  70  before the lower end does. In this case, at some point during a particular scanning session, the upper portion of the scanning beam  98  will pass through the interface  60  on the ramp  66  while the central and lower portions of the scanning beam  98  will pass through some other fluid on the ramp  66  (e.g., the RBC layer  56  or the plasma layer  58 ). At this point, the reflected beam  104  is returned to and received by the scanning fibers  90   a - 90   c , with only the lowermost scanning fiber  90   c  being positioned to receive that portion of the reflected beam  104  that has passed through the interface  60  (on account of the illustrated beam directing member  106  inverting the image of the reflected beam  104 ). As the centrifuge assembly  14  continues to rotate through the field of vision of the optical sensor system  70 , the lower portions of the scanning beam  98  will eventually pass through the interface  60 , to be registered by the central and uppermost scanning fibers at later points in time. Accordingly, the “interface” signals transmitted to the light detectors  88   a  associated with the scanning fibers  90   a - 90   c  will occur at different times to reflect the fact that the interface  60  appears as an angled line on the ramp  66 . 
     In an alternative embodiment, rather than positioning the scanning fibers  90   a - 90   c  in a vertical line, they may be oriented at an angle, such as at an approximately 25° to coincide with the angle at which the ramp  66  is oriented with respect to the rotational axis  110  of the centrifuge assembly  14 . As described above, the interface  60  appears on the ramp  66  as a line oriented at approximately the same angle as that of the ramp  66  with respect to the rotational axis  110  of the centrifuge assembly  14 . Thus, by orienting the scanning fibers  90   a - 90   c  along a line at the same approximate angle as the ramp  66 , they will be also be oriented at approximately the same angle as the interface  60  on the ramp  66 . With the scanning fibers  90   a - 90   c  arranged at the same angle as the interface  60 , the “interface” signals transmitted to the light detectors  88   a  associated with the scanning fibers  90   a - 90   c  will occur substantially simultaneously. 
     By considering the previous two examples of optical fiber orientations, it will be seen that the location of the scanning fibers  90   a - 90   c  effectively determines the locations on the ramp  66  that are being monitored by the optical sensor system  70 . Thus, while the two different scanning fiber arrangements will detect the same location of the interface  60  on the ramp  66 , they consider different regions of the ramp  66  in doing so. In one embodiment, to give the optical sensor system  70  additional flexibility, the scanning fibers  90   a - 90   c  may be mounted together on an adjustable module. In the illustrated embodiment, the scanning fibers  90   a - 90   c  are mounted together on an adjustable module  120  having a tubular collar  122  ( FIG. 8 ) extending outside of the housing  78 , which may be grasped and rotated to simultaneously adjust the arrangement of all of the scanning fibers  90   a - 90   c . In other embodiments, the scanning fibers may be arranged for individual, rather than simultaneous adjustment, such as by providing an adjustable module or a surface of the housing with a plurality of sockets into which the various scanning fibers may be selectively inserted or removed to create different (e.g., non-linear) one- or two-dimensional scanning profiles. The optical sensor system  70  may be configured to have a horizontal resolution (i.e., a resolution in the plane of the centrifuge assembly  14 ) of approximately 100 μm or better, resulting in an accurate determination of the location of the interface  60 . 
     As for the light detectors  88 ,  88   a  and their contribution to determining and adjusting the location of the interface  60  on the ramp  66 ,  FIG. 15  shows a plurality of representative light detectors  88 ,  88   a . The lower detector  88  is associated with the reference fiber  90 , as described above to form a feedback loop with the light source driver  100  to control the brightness of the light source  80 . The upper light detector  88   a  of  FIG. 15  is associated with one of the scanning fibers  90   a - 90   c .  FIG. 15  only shows one such detector  88   a , but there may be one or more such detectors  88   a  for each scanning fiber  90   a - 90   c  provided in the optical sensor system  70 . Each of these light detectors  88   a  receives the portion of the reflected beam  104  transmitted thereto by the associated scanning fiber  90   a - 90   c . Each light detector  88   a  converts the light into a signal that may pass through one or more amplifiers  124  (e.g., a transimpedance amplifier, a gain amplifier, and/or a buffer amplifier), if provided. The individual signals represent a characteristic of the fluid (e.g., the location of its interface) or the nature of the fluid on the ramp  66  at the location monitored by the associated scanning fiber  90   a - 90   c . For example, in one embodiment, as the ramp  66  comes into alignment with the optical sensor system  70 , the detector(s)  88   a  will first sense light reflected through the plasma layer  58  on the ramp  66 . Eventually, the RBC layer  56  adjacent the interface  60  on the ramp  66  will enter the optical path of the optical sensor system  70 . The RBC layer  56  absorbs at least a portion of the light and thereby reduces the previously sensed intensity of the reflected light. The intensity of the reflected light transmitted to the detector(s)  88   a  is indicative of the amount of light that is not absorbed by the RBC layer  56  adjacent to the interface  60 . 
     The signal(s) from the optical sensor system  70  are transmitted to an interface processing module  126  ( FIG. 16 ), which can determine the location of the interface  60  on the ramp  66  relative to the constricted passage  68 . A more detailed discussion of the algorithms by which an exemplary interface controller receives and processes signals to determine the location of the interface on the ramp may be found in U.S. Pat. No. 6,312,607 to Brown et al., which is incorporated herein by reference. 
     When the location of the interface  60  on the ramp  66  has been determined, the interface processing module  126  outputs that information to an interface command element or module  128  ( FIG. 16 ). The interface command module  128  may include a comparator, which compares the interface location output with a desired interface location to generate an error signal. The error signal may take a number of forms but, in one embodiment, may be expressed in terms of a targeted red blood cell percentage value (i.e., the percentage of the ramp  66  which should be occupied by the RBC layer  56 ). 
     When the control value is expressed in terms of a targeted red blood cell percentage value, a positive error signal indicates that the RBC layer  56  on the ramp  66  is too large (as  FIG. 6  shows). The interface command module  128  generates a signal to adjust an operational parameter accordingly, such as by reducing the rate at which plasma is removed through a tube  130  associated with the first outlet port  30  under action of a pump  132  ( FIG. 16 ). The interface  60  moves away from the constricted passage  68  toward the desired control position (as  FIG. 5  shows), where the error signal is zero. 
     A negative error signal indicates that the RBC layer  56  on the ramp  66  is too small (as  FIG. 7  shows). The interface command module  128  generates a signal to adjust an operational parameter accordingly, such as by increasing the rate at which plasma is removed through the first outlet port  30  and associated tube  130 . The interface  60  moves toward the constricted passage  68  to the desired control position ( FIG. 5 ), where the error signal is again zero. 
     Besides determining the location of an interface, the optical sensor system  70  may determine other information about the fluid in the blood separation chamber  22 . For example, the optical sensor system  70  may be configured to detect and read notations (e.g., bar codes) present on the centrifuge assembly  14  and/or the blood separation chamber  22 . Alternatively, rather than intensity-based information, the optical sensor system  70  may be configured to gather spectrally-based information, thereby acting as a spectrometer. For example, when employing a white light source, different wavelengths of the light passing through the ramp  66  and fluid thereon will be absorbed by the different types of fluid that may appear on the ramp  66 . The light that is reflected to a scanning fiber  90   a - 90   c  may be passed through a spectral beam splitter and then to a pair of light detectors  88   a , with each detector receiving the unique wavelengths passed thereto and generating signals based on that data. The signals may be passed to a controller or processing module that considers the individual signals (e.g., considering red vs. blue light absorption) and/or compares them to historical signals (e.g., considering the difference in blue light absorption over time) to generate information about the fluid in the blood separation chamber  22  (e.g., lipid concentration, the presence of cellular blood components in separated plasma, platelet concentration, and hemolysis) and/or to cause adjustments in the operation of the system  10 . 
     Furthermore, the optical sensor system  70  may include additional or alternative components without departing from the scope of the present disclosure. For example,  FIG. 15  shows one or more power or status indicators  132  (which can be a visual indicator that the optical sensor system  70  is functional) and one or more voltage regulators  134  associated with the indicators  132 , the driver  100 , and various amplifiers  124 . The system may also include various connectors  136  between the various components (e.g., BNC connectors, 3-pin connectors to a power source, etc.), as well as to other components that are not illustrated. In other embodiments, a non-white, non-LED light source and/or non-photodiode light detectors (e.g., a camera sensor or an area sensor array or a linear sensor array) may be employed and/or other illustrated components may be replaced with non-illustrated components suited to perform a similar or comparable function. 
     D. Alternative Centrifuge Yokes 
     As described above, centrifuge assemblies according to the present disclosure may be provided as umbilicus-driven (as illustrated in  FIGS. 1 and 2 ) or as direct-driven. If the centrifuge assembly is umbilicus-driven, additional steps may be taken to reduce the risk of the view of the ramp  66  by the optical sensor system  70  being blocked or obscured by the yoke  20  or umbilicus  38  during use. 
     According to one approach, a centrifuge assembly  14   a  having a modified yoke  20   a  is provided, as shown in  FIGS. 17-19 . The yoke  20   a  includes first and second support arms  200  and  202 , which are shown as being generally diametrically opposed, with the centrifuge bowl  16  positioned therebetween. The yoke  20   a  is configured and operates generally according to the above description of the yoke  20  of  FIGS. 1 and 2 , with the exception that one of the support arms (illustrated as second support arm  202 ) defines an opening or aperture or window  204  therethrough. As will be described in greater detail, the yoke window  204  is configured to provide a sight line through the support arm  202  to allow the optical sensor system  70  to view and monitor the ramp  66 . Accordingly, the yoke window  204  is preferably significantly larger than the ramp  66  to maximize the visibility of the ramp  66  through the support arm  202 , with a height H (the vertical dimension in the orientation of  FIG. 17 ) that is greater than the height of the ramp  66  (shown in  FIG. 17  as a pair of broken lines  206  to represent the multiple possible positions of the ramp  66  as the centrifuge bowl  16  is rotated) and a width or angular extent W ( FIG. 19 ) that is greater than the width or angular extent of the ramp  66 . Preferably, the yoke window  204  is positioned with the ramp  66  centered along the height H of the yoke window  204  (i.e., with the vertical center of the ramp  66  being at the same elevation as the vertical center of the yoke window  204  in the orientation of  FIG. 17 ), but it is also within the scope of the present disclosure for the ramp  66  to be closer to the top or bottom of the yoke window  204 . 
     Increasing the width or angular extent W of the yoke window  204  increases the visibility of the ramp  66  by the optical sensor system  70 . As best shown in  FIGS. 18 and 19 , the yoke window  204  preferably has a width or angular extent W equal to or greater than that of the opposing support arm  200  at the same elevation, with the other support arm  200  being diametrically opposed to the yoke window  204 . By such a configuration, there is never one visual obstruction or obstacle (e.g., one of the support arms  200 ,  202 ) positioned 180° from another visual obstruction or obstacle (e.g., the other support arm). By way of example,  FIG. 18  shows opposing first and second sight lines  208  and  210  into the centrifuge bowl  16  from a position outside of the centrifuge assembly  14   a  (e.g., from the position of the optical sensor system  70 ). When the first sight line  208  is blocked by the first support arm  200  ( FIG. 18 ), there is visibility into the centrifuge bowl  16  180° away along the second sight line  208  via the yoke window  204 . When the second sight line  210  is blocked by the second support arm  202  ( FIG. 19 ), there is visibility into the centrifuge bowl  16  180° away along the first sight line  200  to the side of the first support arm  200 . 
     The illustrated configuration may be preferred because of the fact that the yoke  20   a  rotates at one half the speed of the centrifuge bowl  16 , as described above in greater detail. In such a rotational relationship, a 180° rotation of the yoke  20   a  will result in a 360° rotation of the centrifuge bowl  16 . Thus, the ramp  66  will be at the same position (e.g., in position to be viewed by the optical sensor system  70 ) upon each 180° rotation of the yoke  20   a . Accordingly, if the yoke is provided with visual obstructions or obstacles positioned 180° apart, then it may be that the view of the ramp  66  will be obstructed during consecutive 360° rotations of the centrifuge bowl  16 . In contrast, if the yoke is provided so as to eliminate any obstructions positioned 180° apart (as in the embodiment of  FIGS. 17-19 ), then even if the view of the ramp  66  is obstructed at one time, the view of the ramp  66  by the optical sensor system  70  will be clear during the next 360° rotation of the centrifuge bowl  16 . 
     In connection with the yoke  20   a  of  FIGS. 17-19  (or provided separately), the optical sensor system  70  may include a component that can distinguish between an obstructed or partially obstructed view and an unobstructed view. This functionality may be incorporated one of the existing components (e.g., the interface processing module  126 ) or instead be provided by a separate component. In one embodiment, this is accomplished by bracketing the time it takes to scan the ramp  66  twice and comparing the pulse-widths of the two scans obtained during that time period. A partially obstructed scan will have a shorter pulse-width than an unobstructed scan, while a fully obstructed scan will have no pulse-width. By bracketing the time it takes to scan the ramp  66  twice, a fully obstructed scan with no pulse-width may be considered, whereas such a scan may be ignored or missed if the distinguishing device only detects and measures non-zero pulse-widths. When one of the scans has a greater pulse-width than the other, the scan having the larger pulse-width may be selected for further processing and use in the control system. If the pulse-widths of the scans are the same or approximately the same, either one or both of the scans may be selected for further processing and use in the control system. It should be understood that this bracketing method is only one way of distinguishing between obstructed and unobstructed views of the ramp  66 , and other methods of distinguishing between obstructed and unobstructed views of the ramp  66  may be employed without departing from the scope of the present disclosure. 
     While  FIGS. 17-19  illustrate a two-armed yoke  20   a , with one of the support arms  202  having a window  204  therethrough for improved visibility into the centrifuge bowl  16  from an externally located optical sensor system, it is also within the scope of the present disclosure to omit one of the support arms. For example,  FIG. 20  shows only one support arm  302  of a yoke  20   b . If the yoke  20   b  includes only one support arm  302 , then the above-described concern of a visual obstruction located 180° away from the support arm  302  is effectively eliminated. 
     According to another aspect of the present disclosure that is illustrated in  FIG. 20 , a yoke  20   b  having an optical fiber bundle  300  associated therewith is provided. In the illustrated embodiment, an optical fiber bundle  300  is secured to an exterior and/or an interior portion one of the yoke support arms  302 , but it is also within the scope of the present disclosure for a single optical fiber bundle to be associated with two yoke support arms or for separate optical fiber bundles to be associated with each yoke support arm (if more than one support arm is provided). The optical fiber bundle  300  extends between a first or lower end  304  and a second or upper end  306 . The lower end  304  is illustrated in greater detail in  FIGS. 21 and 22 , while the upper end  306  is illustrated in greater detail in  FIG. 23 . The lower end  304  is oriented in light-receiving and light-transmitting relationship to an illumination and detection assembly or optical sensor system  308 , which will be described in greater detail. The upper end  306  is directed toward the centrifuge bowl  16  of the centrifuge assembly  14   b , in light-receiving and light-transmitting relationship to the ramp  66  of the centrifuge bowl  16 . There may be an air gap between the upper and lower ends of the optical fiber bundle  300  and the centrifuge bowl  16  and illumination and detection assembly  308 , respectively, thereby avoiding the need to use an optical slip ring or fiber optic rotary joint or the like. 
     The optical fiber bundle  300  includes one or more of signal fibers  310  and one or more illumination fibers  312 , all of which are configured to transmit light between the ends  304  and  306  of the optical fiber bundle  300 . In one embodiment, the signal fibers  310  are positioned at and directly adjacent to the central axis of the optical fiber bundle  300 , while the illumination fibers  312  are positioned around the signal fibers  310 , such as in a ring or annular arrangement. This configuration is advantageous when used in combination with the particular illumination and detection assembly  308  of  FIG. 20 , but other fiber configurations (such as the mixed arrangement of signal fibers  310  and illumination fibers  312  shown in  FIG. 23A  or a configuration that positions the illumination fibers  312  at and directly adjacent to the central axis of the optical fiber bundle  300 , with the signal fibers  310  positioned around the illumination fibers  312 ) may be employed with differently configured illumination and light detection assemblies. 
     The illumination and detection assembly  308  of  FIG. 20  includes at least one light detector  314  and at least one light source  316 . The illustrated light detector(s)  314  and the light source(s)  316  are configured to correspond generally to the locations of the signal fibers  310  and illumination fibers  312 , respectively. In particular, the illustrated illumination and detection assembly  308  comprises a central photodiode  314  or other suitable light detector aligned with the central axis of the optical fiber bundle  300  at its lower end  304  (to correspond to the location of the signal fibers  310  at and directly adjacent to the central axis of the optical fiber bundle  300 ) and a plurality of light-emitting diodes or laser diodes  316  or other suitable light sources arranged in a ring around the light detector  314  (to correspond to the location of the illumination fibers  312  at the lower end  304  of the optical fiber bundle  300 ). The light source(s)  316  may be spaced away from the light detector(s)  314  to prevent the light detector(s)  314  from receiving light from the light source(s)  316 , in which case the lower end  304  of the optical fiber bundle  304  may be outwardly flared ( FIGS. 21 and 22 ) to similarly separate the signal fibers  310  from the illumination fibers  310  and to maintain the fibers in proper registration with the associated components of the illumination and detection assembly  308 . 
     In use, light is emitted by the light source(s)  316  in a direction substantially parallel to the rotational axis and received by the illumination fibers  310  at the lower end  304  of the optical fiber bundle  300 . The illumination fibers  310  transmit the light to the upper end  306  of the optical fiber bundle  300 , where it is directed onto the outer surface of the centrifuge bowl  16  in a generally radial direction, including the ramp  66  when it has rotated into light-receiving relationship with the upper end  306  of the optical fiber bundle  300 . The light source(s)  316  may be configured to be always on or to only be on when the ramp  66  is in light-receiving relationship with the upper end  306  of the optical fiber bundle  300 . Light from the illumination fibers  312  passes through the ramp  66  and the fluid thereon (as described above with respect to the embodiment of  FIGS. 1-16 ). The light is reflected back through the ramp  66  and out of the centrifuge bowl  16  (by a retroreflector or mirror or the like, as described above with respect to the embodiment of  FIGS. 1-16 ), where it is received by the signal fibers  310  at the upper end  306  of the optical fiber bundle  300 . The signal fibers  310  transmit the reflected light from the upper end  306  of the optical fiber bundle  300  to the lower end  304  of the optical fiber bundle  300 , where it is directed toward the light detector(s)  314 . The light detector(s)  314  receives the light from the signal fibers  310  and transmits the data to a processor, such as the interface command module  126 , for detecting and controlling the location of the interface on the ramp  66  and/or determining other information about the fluid on the ramp  66 . 
     According to one embodiment, a wide variety of information may be determined about the fluid processing region by providing two or more light sources  316  configured to emit light having differing wavelengths. The light sources  316  may operate simultaneously or be controlled to function separately (e.g, by switching selected light sources  316  on during one sampling session or rotation of the centrifuge bowl  16  and the switching those light sources  316  off and other light sources  316  on during another sampling session or rotation of the centrifuge bowl  16 ) to direct light of differing wavelengths into the fluid processing region, which different wavelengths may be used to determine different information about the fluid processing region (e.g., lipemia or hemolysis or the location of the interface, etc.). 
     In the illustrated embodiment, the light detector(s)  314  and the light source(s)  316  are all positioned at the same general location, which may be at a non-rotating surface of the centrifuge assembly  14   b  along the axis of rotation, but it is also within the scope of the present disclosure for the components to be located at different locations. It is also within the scope of the present disclosure for the illumination and signal fibers to be positioned at different locations. For example, the illumination fibers  312  may be positioned as shown in  FIG. 20 , while the signal fibers  310  are at least partially positioned within the centrifuge bowl  16  to directly receive light from the illumination fibers  312  after it has passed through the ramp  66  (e.g., with the upper ends of the signal fibers  301  being located on the centrifuge spool behind the ramp  66 , where the retroreflector or mirror would otherwise be to receive light transmitted through the ramp  66 ). The signal fibers  310  would then transmit the light from the illumination fibers  312  to the light detector  314 , wherever it may be located. 
     Optical sensor systems of the type illustrated in  FIG. 20  have several advantages. For example, such a design takes advantage of the proximity of the optical fiber to the fluid processing region to implement a non-imaging light collection system. This allows for more generous alignment and focusing tolerances and illumination requirements in comparison to other known optical sensor systems. Additionally, on account of the light being directed into the fluid processing region from a position that rotates in the same direction as the fluid processing region, the signal received from the fluid processing region may be longer than a signal resulting from light directed into the fluid processing region from a stationary position (e.g., on the order of twice the duration). 
     Systems of the type illustrated in  FIG. 20  may be used alone or in combination with the other aspects described herein. For example, the system of  FIG. 20  may be used in combination with the optical sensor system  70  to act as an auxiliary optical sensor system in the event that the view of the optical sensor system  70  becomes obscured or obstructed or to monitor a different aspect of the fluid on the ramp  66 . 
     Aspects of the present subject matter described above may be beneficial alone or in combination with one or more other aspects. Without limiting the foregoing description, in accordance with one aspect of the subject matter herein, there is provided a blood processing system which includes a centrifuge assembly having a light-transmissive portion, a light reflector, and a fluid processing region at least partially positioned between the light-transmissive portion and the light reflector. The blood processing system also includes an optical sensor system configured to emit a scanning light beam along a path toward the light-transmissive portion of the centrifuge assembly. The light-transmissive portion of the centrifuge is configured to transmit at least a portion of the scanning light beam to the fluid processing region and the light reflector. The light reflector is configured to reflect at least a portion of the scanning light beam toward the optical sensor system along a path substantially coaxial to the path of the scanning light beam from the optical sensor system toward the light-transmissive portion of the centrifuge assembly. 
     In accordance with another aspect which may be used or combined with the preceding aspect, the path of the scanning light beam from the optical sensor system toward the light-transmissive portion of the centrifuge assembly is substantially parallel to a radius passing through the rotational axis of the centrifuge assembly. However, the path of the scanning light beam from the optical sensor system toward the light-transmissive portion of the centrifuge assembly is oriented so as not to pass through the rotational axis of the centrifuge assembly. 
     In accordance with another aspect which may be used or combined with any of the preceding aspects, the light reflector is a retroreflector. 
     In accordance with another aspect which may be used or combined with any of the preceding aspects, the optical sensor system also includes a first light detector, a source light configured to emit a source light beam, and a beam splitter. The beam splitter is configured to receive and split the source light beam into the scanning light beam and a reference light beam. The beam splitter also directs the scanning light beam toward the light-transmissive portion of the centrifuge assembly and directs the reference light beam toward the first light detector. 
     In accordance with another aspect which may be used or combined with the preceding aspect, the beam splitter is configured to direct the scanning light beam and the reference light beam in substantially perpendicular directions. 
     In accordance with another aspect which may be used or combined with any of the preceding two aspects, the optical sensor system further includes a second light detector. The beam splitter is configured to direct the reflected scanning light beam toward the second light detector. 
     In accordance with another aspect which may be used or combined with the preceding aspect, the beam splitter is configured to direct the reflected scanning light beam in a direction substantially perpendicular to the path of the scanning light beam from the light reflector toward the optical sensor system. 
     In accordance with another aspect which may be used or combined with any of the preceding four aspects, the optical sensor system further includes a controller associated with the first light detector and the source light and configured to adjust the brightness of the source light beam based at least in part on a characteristic of the reference light beam. 
     In accordance with another aspect which may be used or combined with any of the preceding aspects, the system is configured to determine the location of an interface between separated blood components in the centrifuge assembly. 
     In accordance with another aspect, there is provided a method for monitoring fluid within a blood processing system having a centrifuge assembly. The method includes separating blood in a centrifuge assembly into at least two blood components and directing a scanning light beam along a path toward and into the centrifuge assembly so as to intersect the blood or at least one of the blood components. At least a portion of the scanning light beam is reflected after intersecting the blood or blood component, with the reflected light being directed along a path out of the centrifuge assembly that is substantially coaxial to the path of the scanning light beam toward and into the centrifuge assembly. At least a portion of the reflected light is received and analyzed. 
     In accordance with another aspect which may be used or combined with the preceding aspect, the scanning light beam is directed in a direction that is substantially parallel to a radius passing through the rotational axis of the centrifuge assembly, but that does not pass through the rotational axis of the centrifuge assembly. 
     In accordance with another aspect which may be used or combined with any of the preceding two aspects, at least a portion of the scanning light beam is reflected with a retroreflector. 
     In accordance with another aspect which may be used or combined with any of the preceding three aspects, the reflected portion of the scanning light beam is directed in a direction substantially perpendicular to the path of the scanning light beam toward and into the centrifuge assembly prior to being received and analyzed. 
     In accordance with another aspect which may be used or combined with any of the preceding three aspects, a source light beam is split into the scanning light beam and reference light beam, with the reference light beam being directed toward a light detector substantially simultaneously with the scanning light beam being directed toward and into the centrifuge assembly. 
     In accordance with another aspect which may be used or combined with the preceding aspect, the scanning light beam and the reference light beam are directed in substantially perpendicular directions. 
     In accordance with another aspect which may be used or combined with any of the preceding two aspects, at least a portion of the reference light beam is received and analyzed, with the brightness of the source light beam being adjusted based at least in part on a characteristic of the reference light beam. 
     In accordance with another aspect which may be used or combined with any of the preceding seven aspects, the reflected light is analyzed to determine the location of an interface between separated blood components in the centrifuge assembly. 
     In accordance with another aspect, there is provided an optical sensor system for use in combination with a blood processing system. The optical sensor system includes a light source, a light detector, and an optical fiber providing a light path between the light source and the light detector. 
     In accordance with another aspect which may be used or combined with the preceding aspect, the light source is at least partially positioned within a housing, the light detector is positioned outside of the housing, and the optical fiber is connected to the housing. 
     In accordance with another aspect which may be used or combined with the preceding aspect, the optical fiber is adjustably connected to the housing. 
     In accordance with another aspect which may be used or combined with the preceding aspect, a plurality of optical fibers are connected to the housing by an adjustable module configured to simultaneously adjust the position of the optical fibers with respect to the housing. 
     In accordance with another aspect which may be used or combined with any of the preceding four aspects, a beam splitter is configured to receive light from the light source and direct at least a portion of the light toward the optical fiber. 
     In accordance with another aspect which may be used or combined with the preceding aspect, the beam splitter is configured to receive light from the light source and direct portions of the light toward a plurality of optical fibers in different directions. 
     In accordance with another aspect which may be used or combined with the preceding aspect, the beam splitter is configured to direct the portions of the light in opposite directions toward the optical fibers. 
     In accordance with another aspect which may be used or combined with any of the preceding seven aspects, the optical fiber is oriented at an angle with respect to the direction of a light beam emitted by the light source. 
     In accordance with another aspect which may be used or combined with any of the preceding eight aspects, the optical fiber is oriented substantially perpendicular to the direction of a light beam emitted by the light source. 
     In accordance with another aspect, there is provided a blood processing system which includes a centrifuge assembly having a light-transmissive portion, a light reflector, and a fluid processing region at least partially positioned between the light-transmissive portion and the light reflector. The blood processing system also includes an optical sensor system having a light source configured to emit a source light beam, a light detector, and an optical fiber providing a light path to the light detector. The light-transmissive portion of the centrifuge assembly is configured to transmit at least a portion of the source light beam to the fluid processing region and the light reflector. The light reflector is configured to reflect at least a portion of the source light beam toward the optical sensor assembly. The optical fiber is configured to conduct at least a portion of the reflected source light beam toward the light detector. 
     In accordance with another aspect which may be used or combined with the preceding aspect, the light source is at least partially positioned within a housing, the light detector is positioned outside of the housing, and the optical fiber is connected to the housing. 
     In accordance with another aspect which may be used or combined with the preceding aspect, the optical fiber is adjustable connected to the housing. 
     In accordance with another aspect which may be used or combined with the twenty-eighth aspect, a plurality of optical fibers connected to the housing by an adjustable module configured to simultaneously adjust the position of the optical fibers with respect to the housing. 
     In accordance with another aspect which may be used or combined with any of the preceding four aspects, a beam splitter is configured to receive the source light beam and direct at least a portion of the source light beam toward the optical fiber. 
     In accordance with another aspect which may be used or combined with the preceding aspect, the beam splitter is configured to direct a portion of the source light beam in a direction toward the optical fiber and to receive and direct at least a portion of the reflected source light beam toward another optical fiber in a different direction. 
     In accordance with another aspect which may be used or combined with the preceding aspect, the lights are directed toward the optical fibers in opposite directions. 
     In accordance with another aspect which may be used or combined with any of the preceding seven aspects, the optical fiber is oriented at an angle with respect to the direction of the source light beam. 
     In accordance with another aspect which may be used or combined with any of the preceding eight aspects, the optical fiber is oriented substantially perpendicular to the direction of the source light beam. 
     In accordance with another aspect, there is provided a method for monitoring fluid within a blood processing system having a centrifuge assembly. The method includes separating blood in a centrifuge assembly into at least two blood components and generating a source light beam. At least a portion of the source light beam is directed into the centrifuge assembly so as to intersect the blood or at least one of the blood components. At least a portion of the source light beam is reflected after intersecting the blood or blood component and is then directed toward a light detector through an optical fiber. 
     In accordance with another aspect which may be used or combined with the preceding aspect, at least one characteristic of the reflected source light beam is detected using the light detector and a characteristic of the blood or at least one of the blood components is determined based, at least in part, on a characteristic of the reflected source light beam. 
     In accordance with another aspect which may be used or combined with any of the preceding two aspects, the reflected source light beam is directed along a path substantially perpendicular to the direction of the reflected source light beam. 
     In accordance with another aspect which may be used or combined with any of the preceding three aspects, at least a portion of the source light beam is directed toward a second light detector through a second optical fiber. 
     In accordance with another aspect which may be used or combined with any of the preceding four aspects, the optical fiber is oriented at an angle with respect to the direction of the source light beam. 
     In accordance with another aspect which may be used or combined with any of the preceding five aspects, the optical fiber is oriented substantially perpendicular to the direction of the source light beam. 
     In accordance with another aspect, there is provided an optical sensor system for use in combination with a blood processing system. The optical sensor system includes a white light source. 
     In accordance with another aspect which may be used or combined with the preceding aspect, the white light source is a light-emitting diode. 
     In accordance with another aspect which may be used or combined with any of the preceding two aspects, the white light source has a relatively high spectral power distribution in the red wavelength spectrum. 
     In accordance with another aspect which may be used or combined with any of the preceding three aspects, the white light source has a relatively high spectral power distribution in the blue wavelength spectrum. 
     In accordance with another aspect which may be used or combined with any of the preceding four aspects, a light detector is positioned adjacent to the white light source and configured to monitor the intensity of light emitted by the white light source. 
     In accordance with another aspect, there is provided a blood processing system including a centrifuge assembly and an optical sensor system. The centrifuge assembly includes a light-transmissive portion and a fluid processing region positioned at least partially adjacent to the light-transmissive portion. The optical sensor system emits a white light directed toward the light-transmissive portion of the centrifuge assembly. 
     In accordance with another aspect which may be used or combined with the preceding aspect, the optical sensor system includes a white light source comprising a light-emitting diode. 
     In accordance with another aspect which may be used or combined with the preceding aspect, the white light source has a relatively high spectral power distribution in the red wavelength spectrum. 
     In accordance with another aspect which may be used or combined with any of the preceding two aspects, the white light source has a relatively high spectral power distribution in the blue wavelength spectrum. 
     In accordance with another aspect which may be used or combined with any of the preceding three aspects, a light detector is positioned adjacent to the white light source and configured to monitor the intensity of light emitted by the white light source. 
     According to another aspect, there is provided a method for monitoring fluid within a blood processing system having a centrifuge assembly. The method includes separating blood in a centrifuge assembly into at least two blood components and generating a source light beam comprising a white light. At least a portion of the source light beam is directed toward and into the centrifuge assembly so as to intersect the blood or at least one of the blood components. At least a portion of the source light beam is reflected after intersecting the blood or blood component and at least one characteristic of the reflected source light beam is detected. 
     In accordance with another aspect which may be used or combined with the preceding aspect, a characteristic of the blood or at least one of the blood components is determined based, at least in part, on a characteristic of the reflected source light beam. 
     In accordance with another aspect which may be used or combined with any of the preceding two aspects, the intensity of the source light beam is monitored from a location adjacent to the source of the source light beam. 
     In accordance with another aspect, there is provided a blood processing system which includes a centrifuge assembly having a light-transmissive portion, a light reflector, and a fluid processing region at least partially positioned between the light-transmissive portion and the light reflector. The blood processing system also includes an optical sensor system having a light source configured to emit a source light beam and a plurality of light detectors. The light-transmissive portion of the centrifuge assembly is configured to transmit at least a portion of the source light beam to the fluid processing region and the light reflector. The light reflector is configured to reflect at least a portion of the source light beam toward the optical sensor system. The plurality of light detectors are configured to detect at least one characteristic of the reflected source light beam at different locations. 
     In accordance with another aspect which may be used or combined with the preceding aspect, a plurality of optical fibers are configured to receive different portions of the reflected source light beam and to direct the different portions of the reflected source light beam to the light detectors. 
     In accordance with another aspect which may be used or combined with the preceding aspect, an adjustable module is configured to simultaneously adjust the position of the optical fibers with respect to the reflected source light beam. 
     In accordance with another aspect which may be used or combined with any of the preceding three aspects, the different locations are in the same plane. 
     In accordance with another aspect which may be used or combined with any of the preceding four aspects, the different locations are in a plane angled with respect to the rotational axis of the centrifuge assembly. 
     In accordance with another aspect, there is provided a method for monitoring fluid within a blood processing system having a centrifuge assembly. The method includes separating blood in a centrifuge assembly into at least two blood components and generating a source light beam. The source light beam is directed toward and into the centrifuge assembly so as to intersect the blood or at least one of the blood components. At least a portion of the source light beam is reflected after intersecting the blood or blood component and at least one characteristic of the reflected source light beam is detected at a plurality of different locations. 
     In accordance with another aspect which may be used or combined with the preceding aspect, a characteristic of the blood or at least one of the blood components is determined based, at least in part, on a characteristic of the reflected source light beam. 
     In accordance with another aspect which may be used or combined with any of the preceding two aspects, the different locations at which the characteristic of the reflected source light beam is detected are simultaneously adjusted. 
     In accordance with another aspect which may be used or combined with any of the preceding three aspects, the plurality of different locations are in the same plane. 
     In accordance with another aspect which may be used or combined with any of the preceding four aspects, the plurality of different locations are in a plane angled with respect to the rotational axis of the centrifuge assembly. 
     In accordance with another aspect, there is provided a blood processing system which includes a centrifuge assembly having a rotational axis. The blood processing system also includes an optical sensor system having a light source that emits a source light beam directed along a path parallel to a radius passing through the rotational axis of the centrifuge assembly. The path of the source light beam is oriented so as to not pass through the rotational axis of the centrifuge assembly. 
     In accordance with another aspect, there is provided a method for monitoring fluid within a blood processing system having a centrifuge assembly with a rotational axis. The method includes separating blood in a centrifuge assembly into at least two blood components and generating a source light beam. At least a portion of the source light beam is directed along a path parallel to a radius passing through the rotational axis of the centrifuge assembly, but oriented so as to not pass through the rotational axis of the centrifuge assembly, and into the centrifuge assembly so as to intersect the blood or at least one of the blood components. At least a portion of the source light beam is reflected after intersecting the blood or blood component and then at least one characteristic of the reflected source light beam is detected. 
     In accordance with another aspect which may be used or combined with the preceding aspect, a characteristic of the blood or at least one of the blood components is determined based, at least in part, on a characteristic of the reflected source light beam. 
     In accordance with another aspect, there is provided a blood processing system which includes a centrifuge assembly having a rotational axis. The centrifuge assembly has a light-transmissive portion, a fluid processing region positioned radially inwardly of the light-transmissive portion, and a yoke including a first support arm configured to rotate the light-transmissive portion and the fluid processing region about the rotational axis. The blood processing system also includes an optical sensor system configured to direct a light toward the light-transmissive portion of the centrifuge assembly. The yoke is positioned between the light-transmissive portion and the optical sensor system and is configured to allow passage of at least a portion of the light through the first support arm as the light is directed toward the light-transmissive portion. 
     In accordance with another aspect which may be used or combined with the preceding aspect, the first support arm defines a window through which light from the optical sensor system may pass. 
     In accordance with another aspect which may be used or combined with any of the preceding two aspects, the yoke includes a second support arm positioned opposite the first support arm. 
     In accordance with another aspect which may be used or combined with the preceding aspect, the angular extent of the window is at least as great as the angular extent of the second support arm. 
     In accordance with another aspect, there is provided a blood processing system which includes a centrifuge assembly having a rotational axis. The centrifuge assembly has a light-transmissive portion, a fluid processing region positioned radially inwardly of the light-transmissive portion, and a yoke. The yoke includes a first support arm configured to rotate the light-transmissive portion and the fluid processing region about the rotational axis. An optical fiber bundle extends between first and second ends and is associated with the support arm of the yoke. The blood processing system also includes an optical sensor system configured to direct a light toward the first end of the optical fiber bundle. The second end of the optical fiber bundle directs the light toward the light-transmissive portion. 
     In accordance with another aspect which may be used or combined with the preceding aspect, the optical sensor system is configured to direct the light in a direction substantially parallel to the rotational axis. 
     In accordance with another aspect which may be used or combined with any of the preceding two aspects, the second end of the optical fiber bundle is configured to direct the light in a generally radial direction. 
     In accordance with another aspect which may be used or combined with any of the preceding three aspects, the first end of the optical fiber bundle has a greater outer diameter than the second end. 
     In accordance with another aspect which may be used or combined with any of the preceding four aspects, a light reflector is associated with the light-transmissive portion. At least a portion of the light directed toward the light-transmissive portion is directed to the optical fiber bundle by the light reflector. The optical fiber bundle is configured to direct at least a portion of the reflected light toward the optical sensor system. 
     In accordance with another aspect which may be used or combined with the preceding aspect, the optical fiber bundle includes at least one signal fiber configured to direct reflected light from the light reflector toward the optical sensor system and a plurality of illumination fibers configured to direct light from the optical sensor system toward the light-transmissive portion. The at least one signal fiber is positioned directly adjacent to a central axis of the optical fiber bundle and the illumination fibers are positioned radially outwardly of the at least one signal fiber. 
     It will be understood that the embodiments described above are illustrative of some of the applications of the principles of the present subject matter. Numerous modifications may be made by those skilled in the art without departing from the spirit and scope of the claimed subject matter, including those combinations of features that are individually disclosed or claimed herein. For these reasons, the scope hereof is not limited to the above description but is as set forth in the following claims, and it is understood that claims may be directed to the features hereof, including as combinations of features that are individually disclosed or claimed herein.