Abstract:
An optical sensor for measuring the volume of an object, the object having a top and a side. The optical sensor comprises a source of light and a light sensor adapted to measure the amount of light reflected off the side and off the top of the object, wherein the measured amount of the light reflected off the side and the top of the object correlates to a height and a diameter of the object. At least one optical device is adapted to direct light reflected off the side of the object to the light sensor, and at least one optical device is adapted to direct light reflected off the top of the object to the light sensor.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to volume measurement devices, and, more particularly, to an optical volume sensor for measuring the volume of a drop of blood. 
     BACKGROUND OF THE INVENTION 
     It is often necessary to quickly and inexpensively measure the volume of an object. One example of a need for volume measurement is in connection with a blood glucose monitoring system where it may be necessary to measure the volume of a drop of blood. 
     Those who have irregular blood glucose concentration levels are medically required to regularly self-monitor their blood glucose concentration level. An irregular blood glucose level can be brought on by a variety of reasons including illness such as diabetes. The purpose of monitoring the blood glucose concentration level is to determine the blood glucose concentration level and then to take corrective action, based upon whether the level is too high or too low, to bring the level back within a normal range. The failure to take corrective action can have serious implications. When blood glucose levels drop too low—a condition known as hypoglycemia—a person can become nervous. shaky, and confused. That person&#39;s judgment may become impaired and that person may eventually pass out. A person can also become very ill if their blood glucose level becomes too high—a condition known as hyperglycemia. Both conditions, hypoglycemia and hyperglycemia, are both potentially life-threatening emergencies. 
     One method of monitoring a person&#39;s blood glucose level is with a portable, hand-held blood glucose testing device. A prior art blood glucose testing device  100  is illustrated in FIG.  1 . The portable nature of these devices  100  enables the users to conveniently test their blood glucose levels wherever the user may be. The glucose testing device contains a test sensor  102  to harvest the blood for analysis. The device  100  contains a switch  104  to activate the device  100  and a display  106  to display the blood glucose analysis results. In order to check the blood glucose level, a drop of blood is obtained from the fingertip using a lancing device. A prior art lancing device  120  is illustrated in FIG.  2 . The lancing device  120  contains a needle lance  122  to puncture the skin. Some lancing devices implement a vacuum to facilitate the drawing of blood. Once the requisite amount of blood is produced on the fingertip, the blood is harvested using the test sensor  102 . The test sensor  102 , which is inserted into a testing unit  100 , is brought into contact with the blood drop. The test sensor  102  draws the blood to the inside of the test unit  100  which then determines the concentration of glucose in the blood. Once the results of the test are displayed on the display  106  of the test unit  100 , the test sensor  102  is discarded. Each new test requires a new test sensor  102 . 
     One problem associated with some lancing devices is that the requisite amount of blood for accurate test results is not always obtained. Roughly thirty percent of lances to do not produce enough blood for accurate analysis. The amount of blood obtained from each lance varies between zero and ten microliters (“μl”). For an accurate result, at least two μl of blood must be obtained. If less than this amount is produced, the test results may be erroneous and a test sensor is wasted. More serious an issue, however, is that the user may be relying on inaccurate results. Obviously, because of the serious nature of the medical issues involved, erroneous results are not preferred. 
     Another problem associated with conventional lancing devices is that there is no mechanism to let the user know whether the correct amount of blood has been obtained for accurate analysis. Typically, the test units come with instructions containing a graphical illustration of the actual size of the blood drop required for accurate testing. However, this visual comparison is subjective and often produces inconsistent results. To insure the requisite amount of blood is produced, users often overcompensate by squeezing or otherwise manipulating their fingers to produce larger than necessary drops of blood. However, this adds more time to the overall testing process and also results in an increased amount of wasted blood. 
     The inconsistent results produced by conventional lances has impeded the integration of the lancing device, the harvesting device, and the blood glucose analysis device into a single unit. Because the analysis may begin even though the requisite amount of blood has not been obtained, it appears problematic to combine the lancing with the actual harvesting due to the potentially inaccurate results. 
     SUMMARY OF THE INVENTION 
     According to one embodiment of the present invention, there is an optical sensor for determining the volume of an object. One application of the optical sensor is for use in a blood glucose monitoring system which integrates the lancing device. the harvesting device, and the blood glucose analysis device into a single unit. In accordance with the present invention, the optical sensor comprises a source of light and a light sensor adapted to measure an amount of light reflected off the side and off the top of a drop of blood, wherein the measured amount of the light reflected off the side and the top correlates to a height and a diameter of the blood drop. At least one optical device is adapted to direct light reflected off the side of the object to the light detector, and at least one optical device is adapted to direct light reflected off the top of the object to the light detector. 
     The above summary of the present invention is not intended to represent each embodiment, or every aspect, of the present invention. Additional features and benefits of the present invention will become apparent from the detailed description, figures, and claims set forth below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and advantages of the invention will become apparent upon reading the following detailed description in conjunction with the drawings in which: 
     FIG. 1 is a top view of a prior art blood glucose testing device; 
     FIG. 2 is a top view of a prior art lance; 
     FIG. 3 is an optical design for a optical volume sensor wherein light ray traces are shown illuminating a blood drop according to one embodiment of the present invention; 
     FIG. 4 is an optical design for an optical volume sensor wherein light ray traces are shown reflected off a blood drop according to one embodiment of the present invention; 
     FIG. 5 is a plot of the intensity distribution of the light reflected off the side and off the top of a blood drop according to one embodiment of the present invention; 
     FIG. 6 is a plot of the modeled volume measurements of an optical volume sensor versus the actual modeled volumes according to one embodiment of the present invention; 
     FIG. 7 is an optical design for an optical volume sensor wherein light ray traces are shown reflected off a blood drop according to an alternative embodiment of the present invention; and 
     FIG. 8 is a perspective view of an integrated glucose monitoring device according to one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 3, a design for an embodiment of an optical volume sensor  200  is shown. The volume of a drop of blood  202  is determined by illuminating the blood drop  202  and measuring the amount of light reflected off one side  204  of the blood drop and off a top  206  of the blood drop  202 . The blood drop  202  is illuminated by reflecting light from a light source  208  through a series of imaging optics, along light paths  210 ,  212  onto the side  204  and the top  206  of the blood drop  202 . The light directed along the light path  210  illuminates the side  204  of the blood drop  202 . The light directed along the light path  212  illuminates the top  206  of the blood drop  202 . The side illumination light path  210  has edges  210   a ,  210   b  and the top illumination light path  212  has edges  212   a ,  212   b.    
     The source of light  208  has a wavelength of about 800 nanometers (“nm”). A source of light having a wavelength greater than 750 nm is desirable to avoid significant variation in blood and skin reflectance seen at visible wavelengths from 450 to 750 nm. Utilizing a source of light  208  having a wavelength greater than 750 nm results in a more consistent amount of light reflected off the blood drop  202 . The light source  208  is an incandescent light source but can also be one or more light emitting diodes (“LEDs”). 
     Light emitted from the light source  208  is reflected off a beam splitter  214  down through a side view lens  216  and a top view lens  218 . In one embodiment of the present invention, the beam splitter  214  is a fifty percent beam splitter  214  causing approximately half of the incoming light to be transmitted through the beam splitter  214  and the remaining approximately half of the incoming light to be reflected by the beam splitter towards the side view lens  216  and the top view lens  218 . Thus, in FIG.  3 , half of the light incoming from the source of light  208  passes through the beam splitter  214  and the other half of the light is reflected downward along the side illumination light path  210  and the top illumination light path  212 . The light transmitted though the beam splitter  214  is labeled with reference number  220 . 
     The light reflected by the beam splitter  214  that is directed along the side illumination light path  210  passes through the side view lens  216  to a mirror  222  which directs the light onto the side  204  of the blood drop  202 . The side view lens  216  expands the light so that the light when directed off the mirror  222  over-illuminates the blood drop  204  causing some of the light to be cast upon a white surface  238  disposed adjacent to the blood drop  202 . 
     The light reflected by the beam splitter  214  that is directed along the top illumination light path  212  passes through the top view lens  218  and a wedge lens  224  onto the blood drop  202 . The wedge lens  224  directs the light onto the top  206  of the blood drop  202 . Similar to the side view lens  216 , the top view lens  218  expands the light so that the light when directed though the wedge lens  224  over-illuminates the blood drop  202  causing some of the light to be cast upon an area of skin  236  upon which the blood drop has formed. 
     When the light comes into contact with the blood drop  202  a portion of that light is absorbed by the blood drop  202  while a portion of the light is reflected off the blood drop  202 . Accordingly, the light reflected off the blood drop  202  is less intense than the light illuminating the blood drop  202 . The light not coming into contact with the blood drop  202  due to over-illumination is reflected off the skin  236  and off the white surface  238 . The white surface  238  has reflectance properties similar to the skin  238 . Both the skin  236  and the white surface  238  are more reflective than the blood drop  202 . Due to the absorption by the blood drop  202 , the light reflected off the blood drop  202  is less intense than the light reflected off the skin  236  and the white surface  238 . The blood drop  202  absorbs approximately fifteen percent more light than the skin  236  and the white surface  238 . Therefore, the light reflected off the blood drop  202  is approximately fifteen percent less intense than the light reflected off the skin  236  and the white surface  238 . It is this amount of the less-intense light reflected off the blood drop  202  which is indicative of the height and the diameter of the blood drop  202 . 
     Referring now to FIG. 4, the light paths  230 ,  232  of the light reflected off the side  204  and off the top  206  of the blood drop  202 , respectively, are illustrated. The side reflected light path  230  has edges  230   a ,  230   b  and the top reflected light path  232  has edges  232   a ,  232   b . The light reflected off the side  204  and off the top  206  of the blood drop  202  is directed along the side reflected light path  230  and the top reflected light path  232 , respectively, to a light sensor  234 . The side reflected light path  230  has edges  230   a ,  230   b  and the top reflected light path has edges  232   a ,  232   b.    
     The light reflected off the side  204  of the blood drop  202  and off the white surface  238  is directed by the mirror  222  back through the side view lens  216 . The side view lens  216  brings the side reflected light into focus and images the side reflected light onto the light sensor  234 . The side view lens  216  also prevents any scattering of the light directed along the side reflected light path  230 . In an alternative embodiment of the present invention, the side view lens  216  can be excluded. 
     The light reflected off the top  206  of the blood drop  202  and off the skin  236  is directed by the wedge lens  224  through the top view lens  218  onto the light sensor  234 . The function of the top view lens  218  is similar to the side view lens  216  in that it brings the top reflected light into focus and images the top reflected light onto the light sensor  234 . The top view lens  218  also prevents any scattering of the top reflected light. In an alternative embodiment of the present invention, the side view lens  218  can be excluded. 
     The light directed along the side and top reflected light paths  230 ,  232  is transmitted through the beam splitter  214  to the light sensor  234 . The beam splitter  214  transmits a portion of the reflected light to the light sensor  234 , while reflecting a portion of the light. In the embodiment wherein the beam splitter  214  is a fifty percent beam splitter, about half of the reflected light is transmitted to the light sensor  234 . 
     The light sensor  234  measures the intensity of the reflected light and communicates this information to a processor (not shown). The light reflected off the blood drop  202 , the skin  236 , and the white surface  238  as well as any external light will be detected by the light sensor  234 . The intensities of the light reflected off the blood drop  202 , the skin  236 , and the white surface  238  are a function of the intensity of the light source  208  and the absorptivity of the blood  202 , the skin  236 , and the white surface  238 . Preferably, there is significant contrast between the light reflected off the blood drop  202  and the light reflected off the skin  236  and/or the white surface  238  due to the skin  236  and the white surface  238  being more reflective than the blood drop  202 . Specifically. in the embodiment of the optical volume sensor  200  wherein the light source  234  is an approximately 800 nm light source, the light reflected off the blood drop  202  is approximately fifteen percent less intense than the light reflected off the skin  236  and the white surface  238 . Any external light detected by the sensor  234  is expected to have an intensity much less than the light reflected off the blood drop  202 , the skin  236 , and the white surface  238 . The light falling within the expected range of light reflected off the blood drop  202  will be indicative of the height and diameter of the blood drop  202 . 
     In the present invention, the light sensor  234  is a 1×128 pixel line array light detector. Each pixel of the line array light detector individually measures the intensity of light. In operation, the two light paths  230 ,  232  are directed onto the line array light detector  234 . Both light paths  230 ,  232  will contain light reflected off the blood drop  202  along with light reflected off the skin  236  or the white surface  238  on either side. Accordingly, the less intense light (reflected off the blood drop  202 ) is surrounded by the more intense light (reflected off the skin  236  and the white surface  238 ). The width of the less intense light that is reflected off the side  204  and off the top  206  of the blood drop  202  is indicative of the height and diameter of the blood drop  202 , respectively. Each pixel correlates to a fixed distance. Accordingly, the more pixels which detect light having an intensity of light reflected off the blood drop  202 , the larger the blood drop  202  is. In the embodiment of the optical volume sensor  200  illustrated in FIGS. 3 and 4, the spatial resolution for one pixel viewing the blood drop is 25 micrometers (“μm”) for the height and 50 μm for the diameter. For example, if thirty pixels detect light reflected off the side  204  of the blood drop  202 , the blood drop  202  has a height of approximately 750 μm or 0.75 millimeters (“mm”), and if 60 pixels detect light reflected off the top  206  of the blood drop  202 , the blood drop  202  has a diameter of 3000 μm or 3 mm. 
     The design for the optical volume sensor shown in FIGS. 3 and 4 was modeled with LightTools software, manufactured by Optical Research Associates located in Pasadena, Calif. The blood drop  202  was modeled as a spherical lambertian. The light source  208  was modeled as a 800 nm light source. 
     FIG. 5 shows the intensity distribution of a two μl blood drop on the line array detector. The side view (blood drop height) is shown on the left-hand side of the plot and the top view (blood drop diameter) is shown on the right-hand side of the plot. The drop in intensity on both the left and right side of the plot correlates to the less intense light reflected off the side  204  and off the top  206  of the blood drop  202 . The magnitude of each drop in intensity represents the difference in intensities between the light reflected off the blood drop  202  and the light reflected off the skin  236  or the white surface  238 . 
     Once the height and diameter of the blood drop are determined, the approximate volume of the blood drop  202  is calculated using the following algorithm: 
     
       
         Volume=½(Height)×(Diameter) 2   
       
     
     Under the above example where the height is 0.75 mm and the diameter is 3 mm the volume of the blood drop is approximately 3.4 μl. 
     Using the above algorithm, the optical volume sensor was also modeled with LightTools software for a number of blood drops having volumes ranging from 0.5 to 4.5 μl. FIG. 6 is a plot of the volumes calculated using the above algorithm versus the actual modeled blood drop volumes. FIG. 6 shows that the modeled optical volume sensor was able to determine the blood volume with good correlation to the actual modeled volume. 
     An alternative embodiment of the optical volume sensor  200  is illustrated in FIG.  7 . In the embodiment illustrated in FIG. 7, the light source  208  is disposed above the blood drop  202 . Disposing the light source  208  obviates the need for the beam splitter  208  (FIGS. 3 and 4) because it is not necessary to reflect the illuminating light (FIG. 3) or to transmit the reflected light (FIG.  4 ). 
     Referring now to FIG. 8, one application of the present invention is in an integrated blood glucose monitoring system  300  which integrates a lance  302 , a test sensor  304  for blood harvesting, and a blood glucose analyzer into a single instrument. The lance  302  comprises a needle which is used to puncture a user&#39;s skin in order to obtain a drop of blood. The test sensor  304  is used to harvest the blood drop from the user&#39;s fingertip for analysis. The blood glucose monitoring system  300  is activated with a switch  306 . After the user&#39;s skin is lanced using the lancing component  302  of the system  300 , the volume of the blood on the user&#39;s skin is measured with an optical volume sensor  300  (FIGS. 3 and 4) to insure the requisite amount of blood is obtained before analysis begins. Once a sufficient amount of blood has been obtained, the test sensor  304  harvests the blood so that the blood glucose level may be analyzed. The results of the analysis are communicated to the user via a display  308 . 
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and will be described in detail herein. It should be understood, however, that it is not intended to limit the invention to the particular forms disclosed, but, to the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the appended claims.