Patent Publication Number: US-2019167118-A1

Title: Method and apparatus for measuring blood pressure

Description:
TECHNICAL FIELD 
     The present disclosure relates to a medical technology, and more particularly, to the field of pressure measurement. 
     BACKGROUND ART 
     Blood pressure is generally measured using a sphygmomanometer. Invasive measurement through an arterial wall is not common, and blood pressure measurement is usually limited to hospital settings. Non-invasive auscultatory and oscillometric measurements are simpler than invasive measurements, are user-friendly, have no limitation in terms of applications thereof, are easy to use, and cause no pain for a patient, but have low accuracy and there are small systematic differences in measurement values. 
     Auscultatory blood pressure measurement is performed using a stethoscope and a sphygmomanometer. Auscultatory blood pressure measurement includes a cuff that is connected to a mercury manometer or an aneroid manometer and is arranged around a forearm at almost the same height as the heart. 
     The mercury manometer, regarded as standard, provides an absolute result needing no calibration by measurement of the height of a mercury column, causing neither error nor drift of calibration. Use of the mercury manometer is often required for clinical measurement and clinical investigation of high blood pressure of high-risk patients such as pregnant women. 
     The oscillometric measurements involve observation of vibration of a manometer cuff pressure caused by vibration (a pulse) of a bloodstream. Generally, an electronic version of the oscillometric measurements is used for long-term measurements. The oscillometric measurements use not only a manometer cuff like the auscultatory measurements, but also an electronic device that automatically interprets automatic expansion and contraction of an electronic pressure sensor (converter) and a cuff for observing the vibration of a cuff pressure. In this case, the pressure sensor has to be periodically calibrated to maintain the accuracy of measurement. 
     Recently, new technologies based on a pulse wave velocity (PWV) principle have been developed. The technologies based on the PWV principle use the fact that the velocity of a pulse moving along arteries depends on blood pressure, and provide an indirectly estimated blood pressure by changing a PWV value into a blood pressure value. The technologies based on the PWV principle do not need to expand a brachial cuff and can continuously measure a PWV value without medical supervision. 
     DESCRIPTION OF EMBODIMENTS 
     Technical Problem 
     Some embodiments provide a method and wearable device capable of measuring a user&#39;s blood pressure anywhere without using a cuff. 
     Some embodiments provide a method and device capable of accurately measuring a user&#39;s blood pressure by reducing noise caused by skin, a capillary, or user&#39;s movement. 
     Some embodiments provide a method and device capable of providing an accurate blood pressure value by determining a calibration method using a user&#39;s blood pressure value measured by another blood pressure measurement device. 
     Solution to Problem 
     According to an aspect of the present disclosure, a device irradiates light to a target region of a user, receives scattered light scattered from a bloodstream under the target region, and determines a blood pressure value of the user based on an intensity of the received scattered light. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates a method of monitoring biometric information regarding a user&#39;s bloodstream, according to an embodiment. 
         FIG. 2  is a flowchart of a method, performed by a device, of determining a user&#39;s blood pressure value, according to an embodiment. 
         FIG. 3  illustrates a device that detects biometric information regarding a user&#39;s bloodstream, according to an embodiment. 
         FIG. 4  illustrates a contact plate included in a device, according to an embodiment. 
         FIG. 5  illustrates a polarization filter included in a device, according to an embodiment. 
         FIGS. 6A through 6C  illustrate an optical circuit of a device, according to an embodiment. 
         FIG. 7  illustrates a method, performed by a device, of displaying biometric information regarding a user&#39;s bloodstream, according to an embodiment. 
         FIG. 8  illustrates a method, performed by a device, of displaying a guide to wear a device, according to an embodiment. 
         FIG. 9  is a flowchart of a method, performed by a device, of determining a user&#39;s blood pressure value, according to another embodiment. 
         FIG. 10  illustrates a user interface image for receiving a user input for selecting a blood pressure measurement point in time, performed by a device, according to an embodiment. 
         FIG. 11  is a flowchart of a blood pressure value calibration method, performed by a device, based on a reference blood pressure value, according to an embodiment. 
         FIGS. 12A and 12B  illustrate a method, performed by a device, of obtaining a reference blood pressure value, according to an embodiment. 
         FIG. 13  illustrates a method, performed by a device, of obtaining a reference blood pressure value, according to another embodiment. 
         FIG. 14  illustrates a method, performed by a device, of visualizing a bloodstream based on speckle-correlation analysis, according to an embodiment. 
         FIG. 15  is a flowchart of a method, performed by a device, of determining a user&#39;s blood pressure value, according to an embodiment. 
         FIGS. 16A and 16B  illustrate a time-space averaging algorithm of a speckle and a weighting factor with respect to a Gaussian window, according to an embodiment. 
         FIGS. 17 and 18  are block diagrams of a device according to an embodiment. 
     
    
    
     BEST MODE 
     According to a first embodiment of the present disclosure, a wearable device includes a light source configured to irradiate light to a target region of a user, a light receiver configured to receive scattered light scattered from a bloodstream under the target region, a controller configured to determine a blood pressure value of the user based on an intensity of the received scattered light, and a display unit on which the determined blood pressure value is displayed. 
     The wearable device may be a smart watch worn on a wrist of the user by using a strap, and the light source and the light receiver may be arranged on the strap. 
     The controller may be further configured to determine an image indicating an intensity of the received scattered light and to determine a blood pressure value of the user from data of the determined image based on a filter in a Gaussian averaging window form. 
     The wearable device may further include a contact glass plate for compressing the target region when the wearable device is worn on the user&#39;s wrist. 
     The contact glass plate may include a mirror that reflects light emitted from the light source such that the light is directed toward the target region. 
     The wearable device may further include a first polarization filter arranged in an output unit of the light source and a second polarization filter arranged in an input unit of the light receiver, in which the first polarization filter and the second polarization filter are optically orthogonal to each other. 
     The wearable device may further include a light guide configured to guide light emitted from the light source such that the light is directed toward the target region. 
     The wearable device may further include a user interface configured to receive a user input to input a reference blood pressure value of the user, in which the controller is further configured to calibrate the determined blood pressure value based on the input reference blood pressure value. 
     The wearable device may further include a user interface configured to receive a user input to set a blood pressure measurement time, in which the controller is further configured to irradiate light to a target region of a wrist of the user at the set blood pressure measurement time. 
     The display unit may be further configured to display a guide image for guiding the light receiver such that the light receiver is located on a radial artery of the wrist of the user. 
     According to a second embodiment of the present disclosure, a method of measuring a blood pressure may include irradiating, by a wearable device, light to a user&#39;s target region, receiving scattered light scattered from a bloodstream under the target region, determining a blood pressure value of the user based on an intensity of the received scattered light, and displaying the determined blood pressure value. 
     The determining of the blood pressure value of the user based on the intensity of the received scattered light may include determining an image indicating the intensity of the received scattered light and determining the blood pressure value of the user from data of the determined image based on a filter in a Gaussian averaging window form. 
     The method may further include receiving a user input to input a user&#39;s reference blood pressure value and calibrating the determined blood pressure value based on the input reference blood pressure value. 
     The method may further include receiving a user input to set a blood pressure measurement time, and the irradiating the light to the target region of the wrist of the user includes irradiating light to the target region of the wrist of the user at the set blood pressure measurement time. 
     The method may further include displaying a guide image for guiding the light receiver of the wearable device such that the light receiver is located on a radial artery of the wrist of the user. 
     MODE OF DISCLOSURE 
     Terms used herein will be described in brief, and the present disclosure will be described in detail. 
     Although terms used in the present disclosure are selected with general terms popularly used at present under the consideration of functions in the present disclosure, the terms may vary according to the intention of those of ordinary skill in the art, judicial precedents, or introduction of new technology. In addition, in a specific case, the applicant voluntarily may select terms, and in this case, the meaning of the terms is disclosed in a corresponding description part of the disclosure. Thus, the terms used in the present disclosure should be defined not by the simple names of the terms but by the meaning of the terms and the contents throughout the present disclosure. 
     Throughout the entirety of the specification of the present disclosure, when it is assumed that a certain part includes a certain component, the term ‘including’ means that a corresponding component may further include other components unless a specific meaning opposed to the corresponding component is written. The term used in the embodiments such as “unit” or “module” indicates a unit for processing at least one function or operation, and may be implemented in hardware, software, or in a combination of hardware and software. 
     Herein, an image indicating the intensity of scattered light may be referred to as a speckle-modulated image. In addition, a bright region appearing as a small spot in the speckle-modulated image may be referred to as a speckle. 
     When scattered particles move, a change occurs in interference and the intensity of received scattered light also changes. Thus, statistics of time and a space of a speckle pattern in the speckle-modulated image may provide information about movement of the scattered particles. For example, as movement of red corpuscles in a blood vessel becomes fast, the speckle pattern becomes blurry and a device determines the degree of blurring to determine a location or velocity of bloodstream. Herein, a value quantitatively indicating the degree of blurring in the speckle-modulated image may be mentioned as a contrast. As the scattered particles move faster, the speckle pattern becomes blurry, as which the contrast may decrease. 
       FIG. 1  illustrates a method of monitoring biometric information regarding a user&#39;s bloodstream, according to an embodiment. 
     Referring to  FIG. 1 , a device  1000  may detect scattered light scattered by user&#39;s bloodstream and detect biometric information about the user&#39;s bloodstream based on the intensity of the detected scattered light. The biometric information about the user&#39;s bloodstream may include, but not limited to, for example, a blood pressure value, a bloodstream velocity, the amount of bloodstream, a pulse wave, microcirculation, and a motion of a cardiovascular system. 
     The device  1000  may include a light source  1910  and a light receiver  1930 . 
     The light source  1910  may irradiate light to a user&#39;s target region. The light source  1910  may be a device that generates monochromatic light such as a laser. Depending on an embodiment, a light source may be configured to operate in a continuous mode as well as a pulse mode. 
     According to an embodiment, the light source  1910  may be a device that generates light in a near-infrared spectrum range. When compared to a laser in a visible light range, the laser in the near-infrared range (around a wavelength of about 980 nm) is substantially scattered by red corpuscles and noise scattering from an outer layer occurs less. Thus, by using the light in the near-infrared spectrum range, the device  1000  may accurately receive information about a bloodstream in a blood vessel located deeper than skin or a capillary and may be less affected by the skin or the capillary. 
     The near infrared laser generated from the light source  1910  may be irradiated onto skin on a radial artery  10  in a user&#39;s wrist. Once monochromatic light such as a laser is irradiated to a space where scattered particles (e.g., red corpuscles  5 ) are located, diffused reflection occurs due to the scattered particles, and the scattered light that mutually interfere due to the diffused reflection may be received by the light receiver  1930 . 
     The light receiver  1930  may include an optical sensor. The optical sensor may be, but not limited to, e.g., a charge coupled device (CCD), a complementary metal-oxide semiconductor (CMOS), a linear image sensor, an array silicon-type image sensor, or an InAsGa sensor. The light receiver  1930  may change the intensity of the scattered light into a digital signal. 
     As the light receiver  1930  receives the scattered light, the device  1000  may determine the speckle-modulated image indicating the intensity of the scattered light. The speckle-modulated image indicating the intensity of the scattered light may indicate the speckle pattern including small bright dots. 
     Depending on an embodiment, the device  1000  may include a blend  1905  for blocking external light. The device  1000  may include a lens  1920  for increasing light emitted from the light source  1910  to a size of a contact zone of a contact plate (not shown) and collimating the light. The device  1000  may also include a lens  1937  for receiving the scattered light reflected from the radial artery  10  through the light receiver  1930 . 
     In an embodiment, the device  1000  may be a wearable device that is wearable by the user. For example, the device  1000  may be a watch-type device or a bracelet-type device. The device  1000  may also be a smart watch having mounted thereon a function provided by a smart device such as a smart phone, etc. 
     To remove Fresnel reflection from the skin, the device  1000  may include a polarization filter arranged in an input part of the light source  1910  or an output part of the light receiver  1930 . 
     For example, the light emitted from the light source  1910  may be linearly polarized through a first polarization filter  1942 . When completely polarized light is emitted from the light source  1910  (e.g., the laser is used), at least one second polarization filter  1944  may be included in an input to the light receiver  1930 . 
     A sensing region of a sensor in the light receiver  1930  may be covered with two linear polarization filters (not shown) oriented orthogonal to each other. 
     In addition, the first polarization filter  1942  may be arranged in an output part of the light source  1910  and a second polarization filter  1944  may be arranged in an input part of the light receiver  1930  such that polarization directions of light output from the light source  1910  and light incident to the light receiver  1930  are orthogonal to each other, in which the polarization directions of the first polarization filter  1942  and the second polarization filter  1944  may be orthogonal to each other. 
     Laser light output from the light source  1910  may be delivered to a target region through a light guide plate (not shown) that is a contact glass plate contacting a body organ. 
     The device  1000  may determine a diameter of the blood vessel and the velocity of the bloodstream based on the speckle-modulated image, and determine the user&#39;s blood pressure value based on the determined diameter of the blood vessel and the determined velocity of the bloodstream. 
     The method of determining the diameter of the blood vessel and the velocity of the bloodstream of the user from the speckle-modulated image and the method of determining the user&#39;s blood value from the determined diameter of the blood vessel and the determined velocity of the bloodstream will be described with  FIGS. 14 through 16B . 
       FIG. 2  is a flowchart of a method of determining a user&#39;s blood pressure value by the device  1000 , according to an embodiment. 
     In operation S 210 , the device  1000  may irradiate light to a user&#39;s target region. 
     The device  1000  may be a wearable device worn on a user&#39;s wrist. When the device  1000  is the wearable device worn on the user&#39;s wrist, the target region may be a wrist&#39;s radial artery. 
     Depending on an embodiment, the target region may be a finger, a toe, a nose bridge, or an earlobe. In this case, the device  1000  may be a wearable device worn on the finger, the toe, the noise bridge, or the earlobe. 
     The light may be visible light or near infrared light. The light emitted from the light source  1910  in the device  1000  may be irradiated to a target region on the user&#39;s wrist by a holographic optical element such as a mirror. The first polarization filter  1942  may be provided in the output part of the light source  1910 . 
     In operation S 220 , the device  1000  may receive the scattered light scattered from the bloodstream under the target region. 
     The light irradiated to a radial artery under the user&#39;s wrist may be scattered by the scattered particles such as red corpuscles in the radial artery. As the light is scattered by the scattered particles, the light receiver  1930  of the device  1000  may receive the scattered light. The second polarization filter  1944  may be provided in the input part of the light receiver  1930 . 
     In operation S 230 , the device  1000  may determine a user&#39;s blood pressure value based on an intensity of the received scattered light. 
     The device  1000  may generate a speckle-modulated image indicating the intensity of the received scattered light. The device  1000  may determine a position and a velocity of the bloodstream based on speckle-modulated images over time. The device  1000  may determine a diameter of the radial artery based on the determined position of the bloodstream. The device  1000  may also determine the user&#39;s blood pressure value based on the diameter of the radial artery and the velocity of the bloodstream in the radial artery. The method of determining the user&#39;s blood pressure value based on the received intensity of the scattered light will be described in detail with reference to  FIGS. 14 through 16B . 
     In operation S 240 , the device  1000  may display the determined blood pressure value. 
     The device  1000  may display the blood pressure value by controlling a display unit in the device  1000 , and depending on an embodiment, may send a signal instructing another device connected with the device  1000  to display the blood pressure value to the another device. 
       FIG. 3  illustrates the device  1000  that detects biometric information regarding a user&#39;s bloodstream, according to an embodiment. 
     Referring to  FIG. 3 , the device  1000  may be a watch worn on a user&#39;s wrist  30 . For example, the device  1000  may be a smart watch such as Galaxy Gear™, or an analog or digital watch that provides only a watch function. 
     The radial artery  10 , which is generally palpated in pulse examination, is located inside the wrist  30 . In the wrist  30  are located an ulna bone  64 , a radial bone  62 , and the radial artery  10 . Thus, when the device  1000  is implemented as a watch type, a module that receives biometric information regarding the radial artery  10  may be located on a strap  1850  of a watch as a separate module from a watch module that shows time. 
     When the device  1000  is a watch or a smart watch, the device  1000  may include a main module  1800  having a function provided by an existing watch or smart watch and an optical module  1900  that receives biometric information regarding the user&#39;s radial artery  10 . The optical module  1900  and the main module  1800  may communicate with each other. For example, the optical module  1900  and the main module  1800  may communicate using a communication line included in the strap  1850  or using a short-range communication technology. 
     When the main module  1800  is an analog watch, the main module  1800  may be implemented with only hardware; when the main module  1800  is a digital watch or smart watch, the main module  1800  may be implemented with hardware and software. The optical module  1900  may be implemented with only hardware or both hardware and software. The optical module  1900  may be controlled by the main module  1800  and may operate independently of the main module  1800 . The optical module  1900  may include a display device. 
     The optical module  1900  may be arranged as a part of the strap  1850  of the watch or a part of a bracelet. For example, when the user wears the watch on the wrist  30  to locate the main module  1800  on the back of the wrist  30 , the optical module  1900  may be arranged such that the optical module  1900  or a light receiver in the optical module  1900  are located on the skin on the radial artery  10  inside the wrist  30  of the user. 
     The optical module  1900  may be embedded in the strap  1850  of the device  1000  and may be docked with the strap  1850  of the device  1000 . The optical module  1900  may be arranged and fixed in a predefined position of the strap  1850 , and may be moved by the user in the strap  1850 . 
     Although it is shown in  FIG. 3  that the device  1000  is separated into the main module  1800  and the optical module  1900 , depending on an embodiment, the device  1000  may be implemented as one module including both a watch function and a function for receiving biometric information about the radial artery  10 . Depending on an embodiment, the device  1000  may provide only a healthcare function such as user&#39;s biometric information and health care information, etc., without providing the watch function. 
       FIG. 4  illustrates a contact plate included in the device  1000 , according to an embodiment. 
     Since the radial artery is under an outer layer of the wrist, a signal detected from the scattered light may much noise. The noise may occur due to microcirculation of skin  30  and user&#39;s artificial movement. 
     When the skin  30  is mechanically compressed, microcirculation of the skin  30  may be suppressed and the outer layer may become optically transparent. Referring to  FIG. 4 , the electronic device  1000  may include a contact glass plate  1960  compressed on the skin  30 . 
     The contact glass plate  1960  may be compressed on the skin  30  of a target region. The contact glass plate  1960  compressed on the skin  30  may suppress movement of the skin  30  that vibrates due to pulse wave propagation. 
     The contact glass plate  1960  may be arranged in the device  1000  in such a way to be compressed on the skin  30  on the user&#39;s radial artery when the user wears the device  1000 , and light emitted from the light source may be irradiated to the target region through the contact glass plate  1960 . 
     Depending on an embodiment, the contact glass plate  1960  may serve as a light guide that transmits the light emitted from the light source to the target region. For example, the contact glass plate  1960  may include a mirror therein, and the traveling direction of the light emitted from the light source may be changed to be directed to the target region. 
       FIG. 5  illustrates a polarization filter included in the device  1000 , according to an embodiment. 
     Since the radial artery is located deeper than the skin and the capillary, the scattered light scattered from the radial artery to the light receiver  1930  may include noise caused by the surface of the skin and noise caused by the capillary. 
     Referring to  FIG. 5 , the device  1000  may include a polarization filter for canceling the noise caused by the surface of the skin and noise caused by the capillary. For example, the device  1000  may include the first polarization filter  1942  in the output of the light source  1910  and the second polarization filter  1944  in the input of the light receiver  1930 , in which the polarization directions of the first polarization filter  1942  and the second polarization filter  1944  may be orthogonal to each other. As the optically orthogonal polarization filters are included in the output of the light source  1910  and the input of the light receiver  1930 , respectively, the noise caused by the surface of the skin and noise caused by the capillary may be canceled. 
     Depending on an embodiment, two polarization filters having orthogonal polarization directions may be included in the input of the light receiver  1930 . 
     As the polarization filter is included in the device  1000 , the device  1000  may obtain, from the received scattered light, information about the velocity of the bloodstream in the radial artery located deeper than the capillary and information about a location of the radial artery, improve a signal-to-noise ratio, and improve the accuracy of blood pressure value measurement. 
       FIGS. 6A through 6C  illustrate an optical circuit of the device  1000 , according to an embodiment. 
     The device  1000  may include at least one holographic optical element for collimating light emitted from the light source  1910  to the target region  30  or collecting scattered light received from the radial artery through the light receiver  1930 . The holographic optical element may include, but not limited to, a lens, a mirror, a grating, a prism, and a splitter. 
     For example, referring to  FIG. 6A , the device  1000  may include a light guide  1970  for collimating the light emitted from the light source  1910  to the target region  30 . The light guide  1970  may include a mirror  1980  as a holographic optical element and may be configured such that the light emitted from the light source  1910  is incident to the mirror  1980  in the light guide  1970 . The light guide  1970  may include a plurality of mirrors which may be arranged to collimate the light incident from the light source  1910  to the target region  30 . Thus, even when the light emitted from the light source  1910  is not directly oriented to the target region  30  of the user, the light may arrive at the target region  30  through the light guide  1970 . 
     As the light incident to the target region  30  is scattered by an artery under the target region  30 , the scattered light may be received in the light receiver  1930  by the lens  1937 . 
     Depending on an embodiment, the device  1000  may include at least two lenses for transmitting light to the target region  30 . 
     In another embodiment, referring to  FIGS. 6B and 6C , the device  1000  may include the collimating cylindrical lens  1920  and the light guide  1970 . 
     The light source  1910  may be a laser diode. The collimating cylindrical lens  1920  may be made of optical glass or plastic. The light guide  1970  may be made of optical glass or plastic and may have the shape of a parallelepiped. The light guide  1970  may also have the shape of a parabolic cylinder in which a side  1972  of the light guide  1970  is inclined at an angle with respect to another side. The inclined side  1972  of the light guide  1970  may be provided with mirror coating. Depending on an embodiment, the light guide  1970  may serve as a contact plate that contacts the skin  30  of the wrist of the user to pressurize the skin  30 . 
     The device  1000  may also include a projection type lens  1937 , a polarization filter  1944 , a multi-pad light receiver (charge coupled device (CCD), complementary metal oxide semiconductor (CMOS))  1930 . 
     The laser light emitted from the light source  1910  may be collimated to a surface of the light guide  1970  by the cylindrical lens  1920  (a collimator). 
     The light emitted from the light source  1910  may be introduced into the light guide  1970  through a surface of the light guide  1970  and may propagate to an opposite surface (the inclined side  1972 ). Since the opposite surface includes a parabolic mirror, the light may be collimated and a direction thereof may be changed toward a side where the user&#39;s wrist is situated, and the light may be emitted from the light guide  1970  and pass through the skin  30  of the wrist. The light passing through the skin  30  of the wrist may be diffused and reflected in the radial artery, and the diffused and reflected light may be readjusted toward the transparent lens  1937 . 
     As the device  1000  includes at least one holographic optical element for collimating the light emitted from the light source  1910  to the target region, the size of the device  1000  may be reduced. For example, as shown in  FIG. 6A , a distance from the target region to the light receiver  1930  may be reduced to 5 mm or less, and a widthwise length of the light guide  1970  that is an element contacting the target region among elements of the device  1000  may be reduced to about 10 mm or less. 
     The light guide  1970  may not only collimate the light to the skin  30 , but also serve as a contact glass plate that pressurizes the skin  30 , thereby improving the accuracy of blood pressure value measurement. 
     The light guide  1970  in a direction from the user&#39;s skin  30  to the transparent lens  1937  is the same as a plane-parallel plate, such that the light emitted from the user&#39;s skin  30  may pass through the light guide  1970  and be incident to the transparent lens  1937 . The transparent lens  1937  may focus the light passing through the light guide  1970  onto the light receiver  1930 . Thus, the device  1000  may obtain a reversed real image of the radial artery in the wrist. For further polarization analysis, the device  1000  may include the polarization filter  1944 . 
     Since the monochromatic light is emitted from the light source  1910 , the collimating cylindrical lens  1920  and the transparent lens  1937  may be formed as diffractive or holographic optical elements. 
       FIG. 7  illustrates a method of displaying biometric information regarding a user&#39;s bloodstream by the device  1000 , according to an embodiment. 
     Referring to  FIG. 7 , the device  1000  may display biometric information on a screen. 
     The device  1000  may provide a blood pressure menu for displaying a measured blood pressure value, and may display the blood pressure value upon receiving a user input to select the blood pressure menu. The displayed blood pressure value may include a systolic blood pressure value  710  and a diastolic blood pressure value  720 . The device  1000  may display biometric information such as a heart rate, blood sugar, temperature, etc., measured together during measurement of the blood pressure value, together with the measured blood pressure value. 
     According to an embodiment, the device  1000  may display a determined blood pressure value immediately after measurement of the blood pressure of the user, without a separate user input. For example, the device  1000  may include a hardware button  700  for measuring a blood pressure. Upon receiving a user input to select the hardware button  700 , the device  1000  may determine the user&#39;s blood pressure value and display the determined blood pressure value. 
     The device  1000  may also display a measured pulse wave  730  of the user. 
     According to an embodiment, the device  1000  may transmit measurement data to a device of which short-range communication connection with the device  1000  is set up. In this case, the measurement data may be displayed on a screen of the device of which short-range communication connection is set up. 
       FIG. 8  illustrates a method of displaying a guide to wear the device  1000  by the device  1000 , according to an embodiment. 
     To accurately measure biometric information about a user&#39;s bloodstream, it may be crucial for the light source or the light receiver of the device  1000  to be positioned on the user&#39;s radial artery. Thus, the device  1000  may display a guide image  810  for guiding the light source or the light receiver to be positioned on the user&#39;s radial artery. 
     On the device  1000  may be marked an indicator  820  indicating a position of a hardware component for measuring biometric information about a bloodstream, e.g., a position of the light source or the light receiver in the device  1000 . 
     For example, as shown in  FIG. 8 , when the device  1000  is a clock or a smart watch, the hardware component for measuring the biometric information about the bloodstream may be included in a part of a strap or bracelet, and thus, the indicator  820  may be marked in a part of a strap or bracelet of the smart watch. 
     Depending on an embodiment, when the hardware component for measuring the biometric information about the bloodstream is a main component of the device  1000 , the hardware component for measuring the biometric information about the bloodstream may be configured as one module with a display screen, and in this case, an indicator may be marked in a bezel region of the device  1000 . 
       FIG. 9  is a flowchart of a method of determining a user&#39;s blood pressure value by the device  1000 , according to another embodiment. 
     In operation S 910 , the device  1000  may determine whether it is time to perform blood pressure measurement. 
     The device  1000  may include a button for starting blood pressure measurement. The device  1000  may display a menu for starting blood pressure measurement. Upon receiving a user input to select the button or menu for starting blood pressure measurement, the device  1000  may start blood pressure measurement. 
     The time to perform blood pressure measurement may be previously set by the user. For example, as shown in  FIG. 10 , the device  1000  may provide a menu for selecting a measurement time period and a menu for setting a particular measurement time. Depending on an embodiment, the device  1000  may provide the menu for selecting the time to perform blood pressure measurement based on a user&#39;s activity or a menu for starting blood pressure measurement at an arbitrary timing. 
     The device  1000  may also determine a time when the user wears the device  1000  as the time to perform blood pressure measurement. For example, the device  1000  may determine using a sensor of the device  1000  whether the device  1000  is buckled up, and may start blood pressure measurement when determining that the device  1000  is buckled up. 
     Depending on an embodiment, the device  1000  may start blood pressure measurement upon receiving a user&#39;s blood pressure value from a blood pressure measurement device with which short-range wireless communication connection of the device  1000  is set up. The device  1000  may store the received blood pressure value as a reference blood pressure value and compare the reference blood pressure value with the measured blood pressure value to determine a blood pressure value calibration method. 
     Depending on an embodiment, when the device  1000  is a wearable device, the device  1000  may receive a blood pressure measurement start command from a mobile device of which short-range wireless communication connection with the device  1000  is set up. 
     In operation S 920 , the device  1000  may determine whether the user is in a state appropriate for blood pressure measurement. 
     The blood pressure may be measured when user&#39;s mind and body are stable. For example, when the user is exercising or is in a strained or excited state, the user&#39;s blood pressure value may temporarily increase. Since the blood pressure measured in the unstable state of the mind and body may be meaningless to the user, the determination of whether the user&#39;s mind and body are stable needs to be made. 
     Thus, the device  1000  may determine using the sensor thereof whether the user is in a state appropriate for blood pressure measurement and measure the user&#39;s blood pressure when the user is in the appropriate state for blood pressure measurement. 
     For example, the device  1000  may start blood pressure measurement for the user when the user&#39;s motion is less than or equal to a reference value for a preset time. To this end, the device  1000  may determine the degree of the motion of the user by using a motion detecting sensor thereof, for example, a geomagnetic sensor, an acceleration sensor, an altimeter, a gyro sensor, etc. 
     The device  1000  may start blood pressure measurement for the user when a biometric value indicated by biometric data of the user is within a preset value. For example, the device  1000  may start blood pressure measurement for the user when a user&#39;s heartrate satisfies a preset value range. To this end, the device  1000  may determine the user&#39;s heartrate by using an electrocardiogram sensor thereof. For example, the device  1000  may start blood pressure measurement for the user when a user&#39;s stress value or strain value satisfies a preset value range. To this end, the device  1000  may determine user&#39;s stress value, strain value, or excitement value by using a galvanic skin response (GSR) sensor thereof. 
     When the device  1000  determines that it is the time to perform the blood pressure measurement in operation S 910  and determines that the user is not in the appropriate state for the blood pressure measurement in operation S 920 , then the device  1000  may measure again the user&#39;s state after a preset time. 
     In operation S 930 , the device  1000  may inform the user that blood pressure measurement is to start. 
     The device  1000  may output a sound, an image, or vibration to inform the user of start of the blood pressure measurement, before starting the blood pressure measurement. For example, the device  1000  may display a countdown image indicating that the blood pressure measurement is to start. The device  1000  may also output a buzzer sound or guide speech indicating that the blood pressure measurement is to start. The device  1000  may also output a vibration pattern indicating that the blood pressure measurement is to start. By informing in advance the user that the blood pressure measurement is to start, the user&#39;s motion may be avoided. 
     In operation S 940 , the device  1000  may measure the user&#39;s blood pressure. 
     A method of measuring the user&#39;s blood pressure may refer to the description made with reference to  FIG. 2 . The device  1000  may display the determined blood pressure value of the user. 
       FIG. 10  illustrates a user interface image for receiving a user input for selecting a blood pressure measurement point in time by the device  1000 , according to an embodiment. 
     Referring to  FIG. 10 , the device  1000  may display a menu for selecting the time to perform blood pressure measurement. 
     The device  1000  may provide a menu  1010  for setting a period of the blood pressure measurement. As the device  1000  receives a user input to select the menu  1010  for selecting the period, the device  1000  may display a graphic user interface for setting a time period. The time period may be set to, without limited to, 10 minutes, 30 minutes, one hour, etc., and may be set together with start time and end time of the time period. 
     The device  1000  may provide a menu  1020  for setting specific time to perform blood pressure measurement. As the device  1000  receives a user input to select the menu  1020  for selecting the specific time, the device  1000  may display a graphic user interface for setting specific time. 
     The device  1000  may display a menu, although not shown, for selecting the time to perform blood pressure measurement based on a user&#39;s activity. For example, the device  1000  may provide a menu for measuring the blood pressure after an elapse of a time set by the user from the wake-up time of the user. To this end, the device  1000  may include a sensor for detecting a user&#39;s motion. The device  1000  may determine using a sensor thereof whether the user wakes up, and start the blood pressure measurement after an elapse of the time set by the user from the wake-up time of the user. 
       FIG. 11  is a flowchart of a blood pressure value calibration method based on a reference blood pressure value by the device  1000 , according to an embodiment. 
     In operation S 1110 , the device  1000  may determine a user&#39;s bloodstream parameter. 
     The user&#39;s bloodstream parameter may include, but not limited to, a diameter of a radial artery and a velocity of the bloodstream. A method of determining the user&#39;s bloodstream parameter may be described with reference to  FIGS. 14 and 15 . 
     In operation S 1120 , the device  1000  may obtain a user&#39;s reference blood pressure value. 
     For example, the device  1000  may receive a user input to input the reference blood pressure value by using a menu for inputting the reference blood pressure value. The user may measure the blood pressure using a different type of a manometer and input a measured blood pressure value to the device  1000  by using the menu for inputting the reference blood pressure value, provided in the device  1000 . An embodiment where the reference blood pressure value is received using the menu will be described later with reference to  FIG. 12 . 
     Depending on an embodiment, the device  1000  may directly receive the reference blood pressure value from a different type of a blood pressure measurement device connected to the device  1000 . An embodiment where the reference blood pressure value is received from the different type of the blood pressure measurement device will be described later with reference to  FIG. 13 . 
     In operation S 1130 , the device  1000  may determine a blood pressure value calibration method based on user&#39;s bloodstream parameter and reference blood pressure value. 
     The device  1000  may determine a calibration system such that the reference blood pressure value is determined as an output of the calibration system, when a bloodstream parameter measured from the user&#39;s bloodstream is determined as an input of the calibration system. 
     The calibration system may be implemented as software, and may be updated each time when the bloodstream parameter and the reference blood pressure value are received. For example, the device  1000  may determine a coefficient of a neural network to determine the reference blood pressure value as an output of the neural network by using a neural network algorithm, when the velocity of the bloodstream and the diameter of the blood vessel of the user are determined as an input of the neural network. 
     According to another embodiment, the device  1000  may determine the calibration system such that the reference blood pressure value is determined as the output of the calibration system, when the measured blood pressure value determined based on the user&#39;s bloodstream parameter is determined as the input of the calibration system. 
     As the calibration system is determined, the device  1000  may determine the output of the calibration system as a final blood pressure value of the user. For example, when the device  1000  determines the user&#39;s blood pressure value after the calibration system is determined, the device  1000  may determine, as the user&#39;s final blood pressure value, the output of the calibration system determined by substituting the user&#39;s bloodstream parameter into the input of the calibration system. 
       FIGS. 12A and 12B  illustrate a method of obtaining a reference blood pressure value by the device  1000 , according to an embodiment. 
     Referring to  FIG. 12A , the device  1000  may display a user&#39;s blood pressure list  1215 . For example, as the device  1000  receives a user input to select a menu for viewing a measured blood pressure value, the device  1000  may display the user&#39;s blood pressure list  1215 . In this case, the device  1000  may sequentially display a user&#39;s blood pressure list. 
     Referring to  FIG. 12B , in response to a user input to select one from a displayed blood pressure list, the device  1000  may display a graphic user interface for inputting a reference blood pressure value with respect to the selected blood pressure value. The device  1000  may display the blood pressure value selected by the user and time when the selected blood pressure value is measured. Upon receiving a user input to input a systolic reference blood pressure value  1225  and a diastolic reference blood pressure value  1235 , the device  1000  may determine the calibration system such that the reference blood pressure value input by the user is the output of the calibration system, when the selected blood pressure value or the bloodstream parameter of the selected blood pressure value is the input of the calibration system. 
       FIG. 13  illustrates a method of obtaining a reference blood pressure value by the device  1000 , according to another embodiment. 
     Referring to  FIG. 13 , the device  1000  may receive a user&#39;s reference blood pressure value from a blood pressure measurement device  2000  connected to the device  1000 . 
     Short-range wireless communication connection may be set in advance between the device  1000  and the blood pressure measurement device  2000 . For example, when the device  1000  and the blood pressure measurement device  2000  have Bluetooth functions, the user may add the found blood pressure measurement device  2000  to a Bluetooth connection menu of the device  1000 . 
     As the user measures the blood pressure value using the blood pressure measurement device  2000 , the blood pressure measurement device  2000  may transmit the measured blood pressure value to the device  1000  by using short-range wireless communication. 
     As the device  1000  receives the measured blood pressure value from the blood pressure measurement device  2000 , the device  1000  may store the blood pressure value received from the blood pressure measurement device  2000  as the reference blood pressure value. 
     As the device  1000  receives the reference blood pressure value from the blood pressure measurement device  2000 , the device  1000  may start blood pressure measurement for the user. The device  1000  may determine the blood pressure calibration method based on the reference blood pressure value of the user received from the blood pressure measurement device  2000  and the user&#39;s blood pressure value or bloodstream parameter measured by the device  1000  at the time of reception of the reference blood pressure value. The device  1000  may display the reference blood pressure value on the screen. 
       FIG. 14  illustrates a method of visualizing bloodstream based on speckle-correlation analysis by the device  1000 , according to an embodiment. 
     The device  1000  may include a light receiver  1930 , a light source  1910 , and a dozer  1985 . 
     When a laser, e.g., coherent light is irradiated, an image about a fragment of a sample may be recorded. Since a final subject of a test is a hidden bloodstream, speckle imaging may be implemented using a single-mode near-infrared laser diode having a wavelength of 980 nm. Although a visible wavelength range (e.g., flash light similar to phone camera flash) including predictable degradation may be used for an image quality of a received image, substantial scattering of probing radiation occurs due to red corpuscles and noise scattering from an outer layer may be minimized, at a wavelength of 980 nm. 
     In  FIG. 14 , a near-infrared laser beam (e.g., having a wavelength of 980 or 1300 nm) may be irradiated to a target  1415  simulating an artery under the skin layer by the light source  1910 . The speckle-modulated image of the irradiated region may be recorded by the light receiver  1930 , e.g., a monochromatic CMOS camera (656×491 pixels and a pixel size of 9.9×9.9 micron). 
     To perform measurement in real time, speckle-modulated images may be averaged for a time of 0.1 to 20 ms. As an averaging time increases, a contrast reduction rate in a recorded speckle may differ with an average time during which a scattering center in a target volume moves a distance that is equal to a wavelength of the probing radiation and the number of average times a scattering event occurs during radiation propagation in the target volume. 
     By analyzing a local contrast value with respect to a region including a given number of speckles at fixed time, regions where the velocity in the scattering center is different from an average of all the probed volumes may be visualized. The contrast value may be calculated by processing speckle-modulated images of an analyzed part of a tissue surface. 
     For movement of a fluid through the target  1415 , the dozer  1985  electronically controlled to form a flow of the fluid by a preset parameter is used, in which the flow of the fluid may be registered in the light receiver  1930  by reception of light, emitted from the light source  1910  and reflected from the target  1415 , by the light receiver  1930 . 
     To improve a signal-to-noise ratio and the accuracy of blood pressure measurement, an additional technique may be used. To increase an initial contrast and exclude skin&#39;s Fresnel reflection, a polarizing imaging method may be used. To optically transparentize the outer layer and vivify a hidden blood vessel, mechanical compression of the skin (application of a contact glass plate compressed to the skin) may be used. 
     The device  1000  may determine the blood pressure value by tracing a parameter determined by bloodstream characteristics. According to an embodiment, the device  1000  may determine a user&#39;s blood pressure value based on a linear velocity of the bloodstream (systolic and diastolic) and a diameter of an artery (systolic and diastolic) in the user&#39;s target region. Contrast dynamic (value increase or reduction) is inversely proportional to a flow velocity V of red corpuscles, and a spatial distribution of the contrast indicates an inner diameter D of an artery. Thus, the device  1000  may determine the linear velocity of the bloodstream based on the contrast value with respect to the user&#39;s target region. The device  1000  may determine the diameter of the artery based on the spatial distribution of the contrast. For example, the device  1000  may determine the velocity of the bloodstream from obtained experiment data (speckle contrast data obtained during simultaneous pressure measurement by using a blood pressure gauge or manometer) by using a “lookup table” indicating a relationship between a velocity and a contrast. 
     Since the device  1000  estimates the user&#39;s blood pressure value based on the contrast with respect to the target region, the blood pressure value determined based on the contrast may differ from a blood pressure value measured by a known pressure measurement technique within an error range. The device  1000  may determine a calibration method by comparing the blood pressure value measured by the known pressure measurement technique with the blood pressure value determined based on the contrast. According to an embodiment, before initial use, a user&#39;s actual blood pressure value obtained by the known pressure measurement technique may be input to the device  1000 , and the blood pressure value may be calibrated. According to an embodiment, the device  1000  may calibrate the blood pressure value by using the neural network algorithm. For example, the device  1000  may train the neural network to determine the blood pressure value obtained by the known pressure measurement technique as an output when the bloodstream parameter determined by the user&#39;s contrast is selected as an input. 
       FIG. 15  is a flowchart of a method of determining a user&#39;s blood pressure value by the device  1000 , according to an embodiment. 
     In operation S 1510 , the device  1000  may capture a speckle dynamic with respect to the bloodstream in the radial artery. 
     In operation S 1520 , the device  1000  may determine a speckle pattern image by using a speckle analysis algorithm. 
     In operation S 1530 , the device  1000  may dynamically determine the velocity of the bloodstream and the diameter of the radial artery based on the speckle pattern image. 
     In operation S 1540 , the device  1000  may obtain calibration data. In operation S 1550 , the device  1000  may obtain measurement data in a body. 
     In operation S 1560 , the device  1000  may determine a user&#39;s blood pressure value by comparing calibration data with the measurement data in the body. 
     A flow visualizing technique using speckle analysis may be based on contrast calculation of dynamic speckles time-averaged based on an exposure time in registration of a speckle-modulated image. Local estimation of a contrast V k  with respect to a fixed exposure time, which is performed in a region having a given number of speckles, may make it possible to visualize a region of a tissue where scattered particles have different velocities. 
         V   k =σ Ik /( Ī ) k   (1)
 
     In Equation (1), k indicates the number of frames in a sequence of speckle-modulated images, and (I) k  and σ Ik  indicate a scattered light intensity averaged with respect to an analyzed frame and a root-mean-square value of a fluctuating component of a pixel brightness, respectively. 
     
       
         
           
             
               
                 
                   
                     
                       〈 
                       I 
                       〉 
                     
                     k 
                   
                   = 
                   
                     
                       ( 
                       
                         1 
                         / 
                         MN 
                       
                       ) 
                     
                      
                     
                       
                         ∑ 
                         
                           m 
                           = 
                           1 
                         
                         M 
                       
                        
                       
                         
                           ∑ 
                           
                             n 
                             = 
                             1 
                           
                           N 
                         
                          
                         
                           
                             I 
                             k 
                           
                            
                           
                             ( 
                             
                               m 
                               , 
                               n 
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
             
               
                 
                   
                     σ 
                     Ik 
                   
                   = 
                   
                     
                       
                         ( 
                         
                           1 
                           / 
                           MN 
                         
                         ) 
                       
                        
                       
                         
                           ∑ 
                           
                             m 
                             = 
                             1 
                           
                           M 
                         
                          
                         
                           
                             ∑ 
                             
                               n 
                               = 
                               1 
                             
                             N 
                           
                            
                           
                             
                               { 
                               
                                 
                                   
                                     I 
                                     k 
                                   
                                    
                                   
                                     ( 
                                     
                                       m 
                                       , 
                                       n 
                                     
                                     ) 
                                   
                                 
                                 - 
                                 
                                   
                                     〈 
                                     I 
                                     〉 
                                   
                                   k 
                                 
                               
                               } 
                             
                             2 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     In Equation (2) and Equation (3), M and N indicate the number of pixels in a column and a row of an analyzed region of a frame, respectively. I k (m,n) indicates a brightness of a pixel ((m,n)-pixel) located in an m th  row and an n th  column of a k th  frame (k-frame). 
     A problem in quantitative velocity measurement may be associated with understanding of a correlation between a contrast of speckles and a velocity of scattering centers (or velocity distribution). With respect to a fixed exposure time, as the velocity of the scattering center increases, fluctuation in the intensity of light reflected and received from the user is accelerated and the contrast of speckles may be measured as a low value. A correlation between a contrast and a temporal autocorrelation function of fluctuation in the intensity of light may be described as follows: 
     
       
         
           
             
               
                 
                   
                     
                       σ 
                       s 
                       2 
                     
                      
                     
                       ( 
                       T 
                       ) 
                     
                   
                   = 
                   
                     
                       1 
                       T 
                     
                      
                     
                       
                         ∫ 
                         0 
                         T 
                       
                        
                       
                         
                           
                             
                               g 
                               ~ 
                             
                             2 
                           
                            
                           
                             ( 
                             τ 
                             ) 
                           
                         
                          
                         d 
                          
                         
                             
                         
                          
                         τ 
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     σ s   2 (T) indicates a dispersion of spatial intensity variation. T indicates an exposure time. {tilde over (g)} 2 (τ) indicates a covariation function of a temporal intensity fluctuation of each speckle which is an analogue of an autocorrelation coefficient. 
     
       
         
           
             
               
                 
                   
                     
                       g 
                       2 
                     
                      
                     
                       ( 
                       τ 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         
                           G 
                           _ 
                         
                         2 
                       
                        
                       
                         ( 
                         τ 
                         ) 
                       
                     
                     
                       
                         
                           G 
                           _ 
                         
                         2 
                       
                        
                       
                         ( 
                         0 
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       
                         G 
                         _ 
                       
                       2 
                     
                      
                     
                       ( 
                       τ 
                       ) 
                     
                   
                   = 
                   
                     
                       〈 
                       
                         
                           [ 
                           
                             
                               I 
                                
                               
                                 ( 
                                 t 
                                 ) 
                               
                             
                             - 
                             
                               
                                 〈 
                                 I 
                                 〉 
                               
                               t 
                             
                           
                           ] 
                         
                          
                         
                           [ 
                           
                             
                               I 
                                
                               
                                 ( 
                                 
                                   t 
                                   + 
                                   τ 
                                 
                                 ) 
                               
                             
                             - 
                             
                               
                                 〈 
                                 I 
                                 〉 
                               
                               t 
                             
                           
                           ] 
                         
                       
                       〉 
                     
                     t 
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     Equations (4) through (6) may determine a correlation between a full-field speckle-correlometry and a method of using fluctuation in the intensity of laser light scattered by a moving object or particle. A basic method uses speckle-modulation in a far-field region, whereas the full-field speckle-correlometry uses speckle-modulation in a region of an image. 
     For the full-field speckle-correlometry, several additional assumptions may be made to refine a relationship between a measured speckle contrast (defined as σs/ I ) and a correlation time τc. According to a type of a studied motion, different models may be used. For Lorentz velocity distribution, an equation may take the following form: 
     
       
         
           
             
               
                 
                   
                     
                       σ 
                       s 
                     
                     
                       〈 
                       I 
                       〉 
                     
                   
                   = 
                   
                     
                       [ 
                       
                         
                           
                             τ 
                             c 
                           
                           
                             2 
                              
                             T 
                           
                         
                          
                         
                           { 
                           
                             1 
                             - 
                             
                               exp 
                                
                               
                                 ( 
                                 
                                   - 
                                   
                                     
                                       2 
                                        
                                       T 
                                     
                                     
                                       τ 
                                       c 
                                     
                                   
                                 
                                 ) 
                               
                             
                           
                           } 
                         
                       
                       ] 
                     
                     
                       1 
                       / 
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     Equation (7) associates a speckle contrast for the given T with the correlation time τc. The full-field speckle-correlometry may encounter a problem all frequency-temporal methods face, that is, a problem that evaluation of correlation time is affected by the form of a velocity distribution of scattered particles, multi-scattering, the size of scattered particles (in this case, red corpuscles), the shape of scattered particles, the non-Newtonian flow of the fluid, non-Gaussian statistics occurring due to a small number of scattered particles, etc. Due to uncertainty caused by the listed factors, proper calibration, rather than absolute measurement, may be performed using a dynamic phantom of a tissue. 
     To determine temporal statistics of the speckle pattern, a strength of each speckle may be traced. In this case, a light-receiving region (aperture) of the light receiver needs to be smaller than an average speckle size; otherwise, spatial averaging may occur and first-order statistics may be disturbed. The full-field speckle-correlometry means calculating a local contrast of a speckle by using a pixel set, and the number of pixels may be controlled by a manipulator. As a larger area is processed, higher-quality statistics may be received. However, not only a large number of pixels, but also many speckles need to be processed. When a speckle size is much greater than a pixel size, a smaller number of speckles may be processed. This situation means that there may be constraints in finding a speckle of a proper size. When the speckle is too small, each pixel may include one or more speckles, which may introduce averaging and reducing a measured contrast. On the other hand, when the speckle is too large, the number of speckles is too small, failing in providing good-quality statistics. Thus, the speckle size has to be carefully controlled, and an image aperture of an optical system determines the speckle size, such that the speckle size may be implemented by selecting the image aperture. On the other hand, since a camera shutter exposure time is designated by a measured velocity range, selection of the image aperture may limit the possibility of controlling light flux coming to a camera. When a dynamic range of the camera is not broad, the incapability of controlling the light flux coming to the camera may be a limitation and a light flux level suitable for the light receiver may be provided using a neutral filter. 
     Another problem is that it is difficult to experimentally obtain a contrast change across the entire range disclosed by the theory. Theoretically, a contrast of a stationary user should be 1 (unity, σ=&lt;I&gt;). A contrast of a completely smeared speckle pattern generated for fast movement of scattered particles should be 0. For example, with a Lorentz model, a dependency of a contrast σ&lt;T&gt; with respect to a ratio τc/T may be predicted. For a given exposure time T, the dynamic range of contrast measurement from 0.1 to 0.9 should match the size of about 2.5 order with respect to τc (thus, velocity). 
     To improve a signal-to-noise ratio (SNR), a polarization imaging function may be added to a speckle processing algorithm. The principle of polarization discrimination may be based on the effect of gradual reduction of a polarization degree of polarized light undergoing a scattering event of a random sequence. In a spatial scale in a transverse coherence length (a distinctive speckle size in a view plane) level, the effect of multi-scattering on polarization characteristics of a speckle-modulated scattered field may be produced as production of a local polarization structure associated with a static or dynamic speckle pattern. In a speckle pattern, each speckle may be caused by a local polarization ellipse in which an azimuth (with respect to a polarization direction of a linearly polarized incident beam) and an eccentricity have random values. The azimuth and the eccentricity may be arbitrarily various from one speckle to another speckle, and spatial averaging of speckle strength for a speckle pattern using individual detection of co-polarized and cross-polarized multi-scattered light enables estimation of the residual linear polarization degree of the multi-scattered light as follows: 
     
       
         
           
             
               
                 
                   
                     P 
                     L 
                   
                   = 
                   
                     
                       
                         I 
                         II 
                       
                       - 
                       
                         I 
                         ⊥ 
                       
                     
                     
                       
                         I 
                         II 
                       
                       + 
                       
                         I 
                         ⊥ 
                       
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
     In Equation (8), I II  indicates an average of the intensity of light having a polarization direction in the direction of the incident beam, and I ⊥  indicates an average of the intensity of the light having an orthogonal polarization direction. 
     In a frame of a phenomenological model of polarization attenuation caused by multi-scattering, the co-polarized and cross-polarized intensities may be determined as follows: 
     
       
         
           
             
               
                 
                   
                     
                       I 
                       II 
                     
                     = 
                     
                       
                         I 
                         2 
                       
                        
                       
                         
                           ∫ 
                           0 
                           ∞ 
                         
                          
                         
                           
                             ( 
                             
                               1 
                               + 
                               
                                 exp 
                                  
                                 
                                   [ 
                                   
                                     
                                       - 
                                       s 
                                     
                                     / 
                                     ξ 
                                   
                                   ] 
                                 
                               
                             
                             ) 
                           
                            
                           
                             ρ 
                              
                             
                               ( 
                               s 
                               ) 
                             
                           
                            
                           ds 
                         
                       
                     
                   
                   , 
                   
                     
 
                   
                    
                   
                     
                       I 
                       ⊥ 
                     
                     = 
                     
                       
                         I 
                         2 
                       
                        
                       
                         
                           ∫ 
                           0 
                           ∞ 
                         
                          
                         
                           
                             ( 
                             
                               1 
                               - 
                               
                                 exp 
                                  
                                 
                                   [ 
                                   
                                     
                                       - 
                                       s 
                                     
                                     / 
                                     ξ 
                                   
                                   ] 
                                 
                               
                             
                             ) 
                           
                            
                           
                             ρ 
                              
                             
                               ( 
                               s 
                               ) 
                             
                           
                            
                           ds 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
     In Equation (9), I indicates a total intensity of light coming from a scattering medium, ρ(s) indicates a probability density function of a path length distribution with respect to partial waves forming a multi-scattered speckle-modulated field, and ξ indicates a depolarization length determined by a used wavelength, scattering anisotropy of the medium, scattering anisotropy of a probed medium, a probed medium illumination, and a method of detecting scattered light. 
     In the worst case where probe light diffuses and propagates in the medium, when the average path length &lt;s&gt; of the propagating waves significantly exceeds the depolarization length, I II →I ⊥  and the output light is almost completely depolarized. 
     On the other hand, when light is weakly scattered in an almost transparent medium, ρ(s)→δ(s), and a co-polarized component of the scattered light is dominant over a cross-polarized component (outgoing light is polarized strongly). Due to such characteristics, only a cross-polarized speckle is obtained, such that speckle-based detection of dynamic inhomogeneities inherent in a multi-scattered medium needs much improvement. Such improvement is expected for reasons provided below. 
     First, formation of a linearly polarized speckle in detection may increase a value of a speckle contrast. An orthogonal polarized component of a speckle pattern is excluded, and a non-coherent strength sum for two non-correlated orthogonal polarized random speckle patterns may be excluded. 
     Second, blocking of the co-polarized component of the multi-scattered light may exclude contribution from a short-range partial wave propagating in a probed medium at a shallow depth. This may increase a fraction of a deep-depth component in a detected signal, increasing an SNR during speckle-based characterization of the inherent dynamic inhomogeneities. 
     The influence of scattering upon the determined depolarization length may be determined by the scattering anisotropy of the probed medium. For a medium having small scattering anisotropy (Rayleigh systems), a residual linear polarization degree may be high in a backscattering mode and low in a forward scattering mode. By contrast, for a Mie scattering procedure, backscattering is characterized by almost complete depolarization of outgoing light. Forward scattering may strongly maintain linear polarization of propagating light. 
     This characteristic should be considered when the SNR is estimated in speckle-based measurement of the inherent dynamic inhomogeneities, and may be modified into Equations (9) and (10). 
     
       
         
           
             
               
                 
                   
                     
                       I 
                       II 
                     
                     = 
                     
                       
                         I 
                         2 
                       
                        
                       
                         { 
                         
                           
                             
                               ∫ 
                               0 
                               
                                 s 
                                 ′ 
                               
                             
                              
                             
                               
                                 ( 
                                 
                                   1 
                                   + 
                                   
                                     exp 
                                      
                                     
                                       [ 
                                       
                                         
                                           - 
                                           s 
                                         
                                         / 
                                         
                                           ξ 
                                           f 
                                         
                                       
                                       ] 
                                     
                                   
                                 
                                 ) 
                               
                                
                               
                                 ρ 
                                  
                                 
                                   ( 
                                   s 
                                   ) 
                                 
                               
                                
                               ds 
                             
                           
                           + 
                           
                             
                               ∫ 
                               
                                 s 
                                 ′ 
                               
                               ∞ 
                             
                              
                             
                               
                                 ( 
                                 
                                   1 
                                   + 
                                   
                                     exp 
                                      
                                     
                                       [ 
                                       
                                         
                                           - 
                                           s 
                                         
                                         / 
                                         
                                           ξ 
                                           b 
                                         
                                       
                                       ] 
                                     
                                   
                                 
                                 ) 
                               
                                
                               
                                 ρ 
                                  
                                 
                                   ( 
                                   s 
                                   ) 
                                 
                               
                                
                               ds 
                             
                           
                         
                         } 
                       
                     
                   
                   , 
                   
                     
 
                   
                    
                   
                     
                       I 
                       ⊥ 
                     
                     = 
                     
                       
                         I 
                         2 
                       
                        
                       
                         { 
                         
                           
                             
                               ∫ 
                               0 
                               
                                 s 
                                 ″ 
                               
                             
                              
                             
                               
                                 ( 
                                 
                                   1 
                                   - 
                                   
                                     exp 
                                      
                                     
                                       [ 
                                       
                                         
                                           - 
                                           s 
                                         
                                         / 
                                         
                                           ξ 
                                           f 
                                         
                                       
                                       ] 
                                     
                                   
                                 
                                 ) 
                               
                                
                               
                                 ρ 
                                  
                                 
                                   ( 
                                   s 
                                   ) 
                                 
                               
                                
                               ds 
                             
                           
                           + 
                           
                             
                               ∫ 
                               
                                 s 
                                 ′ 
                               
                               ∞ 
                             
                              
                             
                               
                                 ( 
                                 
                                   1 
                                   - 
                                   
                                     exp 
                                      
                                     
                                       [ 
                                       
                                         
                                           - 
                                           s 
                                         
                                         / 
                                         
                                           ξ 
                                           b 
                                         
                                       
                                       ] 
                                     
                                   
                                 
                                 ) 
                               
                                
                               
                                 ρ 
                                  
                                 
                                   ( 
                                   s 
                                   ) 
                                 
                               
                                
                               ds 
                             
                           
                         
                         } 
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
     When a value s′ is determined by a distinctive propagation path of the probed light required for reaching dynamic inhomogeneities hidden in a medium, values ξ f  and ξ b  indicate depolarization lengths for forward scattering and backward scattering, respectively. For rough estimation of fractions of the co-polarized component and the cross-polarized component from the detected speckle-modulated signal, Equation (10) may be re-written into Equation (10′). 
     
       
         
           
             
               
                 
                   
                     
                       I 
                       II 
                     
                     = 
                     
                       
                         I 
                         2 
                       
                        
                       
                         { 
                         
                           
                             
                               ∫ 
                               0 
                               
                                 s 
                                 ′ 
                               
                             
                              
                             
                               
                                 ( 
                                 
                                   1 
                                   + 
                                   
                                     exp 
                                      
                                     
                                       [ 
                                       
                                         
                                           - 
                                           s 
                                         
                                         / 
                                         
                                           ξ 
                                           f 
                                         
                                       
                                       ] 
                                     
                                   
                                 
                                 ) 
                               
                                
                               
                                 ρ 
                                  
                                 
                                   ( 
                                   s 
                                   ) 
                                 
                               
                                
                               ds 
                             
                           
                           + 
                           
                             
                               ∫ 
                               
                                 s 
                                 ′ 
                               
                               
                                 S 
                                 cutoff 
                               
                             
                              
                             
                               
                                 ( 
                                 
                                   1 
                                   + 
                                   
                                     exp 
                                      
                                     
                                       [ 
                                       
                                         
                                           - 
                                           s 
                                         
                                         / 
                                         
                                           ξ 
                                           b 
                                         
                                       
                                       ] 
                                     
                                   
                                 
                                 ) 
                               
                                
                               
                                 ρ 
                                  
                                 
                                   ( 
                                   s 
                                   ) 
                                 
                               
                                
                               ds 
                             
                           
                         
                         } 
                       
                     
                   
                   , 
                   
                     
 
                   
                    
                   
                     
                       I 
                       ⊥ 
                     
                     = 
                     
                       
                         I 
                         2 
                       
                        
                       
                         { 
                         
                           
                             
                               ∫ 
                               0 
                               
                                 s 
                                 ″ 
                               
                             
                              
                             
                               
                                 ( 
                                 
                                   1 
                                   - 
                                   
                                     exp 
                                      
                                     
                                       [ 
                                       
                                         
                                           - 
                                           s 
                                         
                                         / 
                                         
                                           ξ 
                                           f 
                                         
                                       
                                       ] 
                                     
                                   
                                 
                                 ) 
                               
                                
                               
                                 ρ 
                                  
                                 
                                   ( 
                                   s 
                                   ) 
                                 
                               
                                
                               ds 
                             
                           
                           + 
                           
                             
                               ∫ 
                               
                                 s 
                                 ′ 
                               
                               
                                 S 
                                 cutoff 
                               
                             
                              
                             
                               
                                 ( 
                                 
                                   1 
                                   - 
                                   
                                     exp 
                                      
                                     
                                       [ 
                                       
                                         
                                           - 
                                           s 
                                         
                                         / 
                                         
                                           ξ 
                                           b 
                                         
                                       
                                       ] 
                                     
                                   
                                 
                                 ) 
                               
                                
                               
                                 ρ 
                                  
                                 
                                   ( 
                                   s 
                                   ) 
                                 
                               
                                
                               ds 
                             
                           
                         
                         } 
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   
                     10 
                     ′ 
                   
                   ) 
                 
               
             
           
         
       
     
     In Equation (10), a cutoff value S cutoff  may be set to remove a cause for a very long path for analytic and numeric simulation. By introducing a weight of a component 1±exp└−s/ξ f,b ┘, Equation (10′) may be re-written into Equation (10″). 
     
       
         
           
             
               
                 
                   
                     
                       I 
                       II 
                     
                     = 
                     
                       
                         I 
                         2 
                       
                        
                       
                         { 
                         
                           
                             
                               〈 
                               
                                 1 
                                 + 
                                 
                                   exp 
                                    
                                   
                                     [ 
                                     
                                       
                                         - 
                                         s 
                                       
                                       / 
                                       
                                         ξ 
                                         f 
                                       
                                     
                                     ] 
                                   
                                 
                               
                               〉 
                             
                             
                               0 
                               , 
                               
                                 s 
                                 ′ 
                               
                             
                           
                           + 
                           
                             
                               〈 
                               
                                 1 
                                 + 
                                 
                                   exp 
                                    
                                   
                                     [ 
                                     
                                       
                                         - 
                                         s 
                                       
                                       / 
                                       
                                         ξ 
                                         b 
                                       
                                     
                                     ] 
                                   
                                 
                               
                               〉 
                             
                             
                               
                                 s 
                                 ′ 
                               
                               , 
                               
                                 S 
                                 cutoff 
                               
                             
                           
                         
                         } 
                       
                     
                   
                   , 
                   
                     
 
                   
                    
                   
                     
                       I 
                       ⊥ 
                     
                     = 
                     
                       
                         I 
                         2 
                       
                        
                       
                         
                           { 
                           
                             
                               
                                 〈 
                                 
                                   1 
                                   - 
                                   
                                     exp 
                                      
                                     
                                       [ 
                                       
                                         
                                           - 
                                           s 
                                         
                                         / 
                                         
                                           ξ 
                                           f 
                                         
                                       
                                       ] 
                                     
                                   
                                 
                                 〉 
                               
                               
                                 0 
                                 , 
                                 
                                   s 
                                   ′ 
                                 
                               
                             
                             + 
                             
                               
                                 〈 
                                 
                                   1 
                                   - 
                                   
                                     exp 
                                      
                                     
                                       [ 
                                       
                                         
                                           - 
                                           s 
                                         
                                         / 
                                         
                                           ξ 
                                           b 
                                         
                                       
                                       ] 
                                     
                                   
                                 
                                 〉 
                               
                               
                                 
                                   s 
                                   ′ 
                                 
                                 , 
                                 
                                   S 
                                   cutoff 
                                 
                               
                             
                           
                           } 
                         
                         . 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     10 
                     ″ 
                   
                   ) 
                 
               
             
           
         
       
     
     Thus, in a speckle-correlation-based device that performs polarization discrimination, the residual linear polarization degree of the outgoing light may be expressed as in Equation (11). 
     
       
         
           
             
               
                 
                   
                     P 
                     L 
                   
                   = 
                   
                     
                       2 
                        
                       
                         { 
                         
                           
                             
                               ∫ 
                               0 
                               
                                 s 
                                 ′ 
                               
                             
                              
                             
                               
                                 exp 
                                  
                                 
                                   [ 
                                   
                                     
                                       - 
                                       s 
                                     
                                     / 
                                     
                                       ξ 
                                       f 
                                     
                                   
                                   ] 
                                 
                               
                                
                               
                                 ρ 
                                  
                                 
                                   ( 
                                   s 
                                   ) 
                                 
                               
                                
                               ds 
                             
                           
                           + 
                           
                             
                               ∫ 
                               
                                 s 
                                 ′ 
                               
                               
                                 S 
                                 cutoff 
                               
                             
                              
                             
                               
                                 exp 
                                  
                                 
                                   [ 
                                   
                                     
                                       - 
                                       s 
                                     
                                     / 
                                     
                                       ξ 
                                       b 
                                     
                                   
                                   ] 
                                 
                               
                                
                               
                                 ρ 
                                  
                                 
                                   ( 
                                   s 
                                   ) 
                                 
                               
                                
                               ds 
                             
                           
                         
                         } 
                       
                     
                     == 
                     
                       2 
                        
                       
                         
                           { 
                           
                             
                               
                                 〈 
                                 
                                   exp 
                                    
                                   
                                     [ 
                                     
                                       
                                         - 
                                         s 
                                       
                                       / 
                                       
                                         ξ 
                                         f 
                                       
                                     
                                     ] 
                                   
                                 
                                 〉 
                               
                               
                                 0 
                                 , 
                                 
                                   s 
                                   ′ 
                                 
                               
                             
                             + 
                             
                               
                                 〈 
                                 
                                   exp 
                                    
                                   
                                     [ 
                                     
                                       
                                         - 
                                         s 
                                       
                                       / 
                                       
                                         ξ 
                                         b 
                                       
                                     
                                     ] 
                                   
                                 
                                 〉 
                               
                               
                                 
                                   s 
                                   ′ 
                                 
                                 , 
                                 
                                   S 
                                   cutoff 
                                 
                               
                             
                           
                           } 
                         
                         . 
                       
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
     As such, a processed image may be a result of subtracting two images having different polarization states. 
     Since a flow in an artery is hidden in an outer layer, a detected signal may include a lot of noise. The noise may occur due to microcirculation of the skin and user&#39;s artificial movement. To suppress microcirculation of the skin, optical removal using a mechanical compression approach may be used. The contact glass plate may be compressed on the skin in a region of a target part (e.g., the wrist or the arm) of a tissue. The contact glass plate compressed on the skin may suppress movement of the skin after pulse wave propagation. By using the contact glass plate compressed on the surface of the tissue of the body, in comparison to a space interface having no tissue, detection of a dynamically scattered component of the outgoing light is affected by partial matching of refractive indices between a transparent medium of the glass plate and the body tissue and changes in an optical delivery parameter of the body tissue and a geometry of the body tissue, caused by compression using the contact glass plate. 
     The partial matching of the refractive indices between the probed tissue and the transparent medium of the glass plate may suppress multi-inner reflection of diffusing light in the boundary and reduce an average propagation path of light in the tissue. 
     On the other hand, compression of the tissue may increase a transport mean free path, resulting in reduction of an effective value of an inhomogeneity depth. A part (fraction) of the dynamically scattered component in the detected signal is very sensitive to the inhomogeneity depth, and may be approximated to an extended exponential function. Thus, the effect of a change of an optical transport parameter (especially, the transport mean free path) may be prior to a negative role of refractive index matching. 
       FIGS. 16A and 16B  illustrate a time-space averaging algorithm of a speckle and a weighting factor with respect to a Gaussian window, according to an embodiment. 
     To normalize a final value by considering scattering characteristics of the skin of the user and reduce speckle noise caused by microcirculation of the skin, the device  1000  may use a Gaussian averaging window. 
     Blood pressure study based on analysis of artery bloodstream dynamics may be conducted using a full-field speckle-correlometry technology because the study is a non-invasive contactless method capable of visualizing a red corpuscle flow in real time without scanning a laser beam. The full-field speckle-correlometry may solve a problem of artery blood imaging through analysis of spatial statistics of time-averaged speckles, especially, analysis of a spatial contrast of a speckle pattern. 
     When compared to a method of analyzing a temporal contrast of a speckle pattern calculated using a set of images where contrast values are continuously obtained, a method of analyzing a spatial contrast of a speckle pattern has a higher time resolution, making it possible to measure time-dependent scattering from the user having complex dynamics. 
     For flow visualization of a deep tissue of a target part, a data processing application having a finer window than a Dirichlet window to suppress high-frequency noise may be used. For point (point-to-point or voxel-to-voxel) contrast determination, Gaussian spatial-temporal windows and median-based estimates may be used with respect to an average and a standard deviation of speckle strengths. 
     Referring to  FIG. 16A , the most general equation for contrast determination in a 3D space (averaged for a 3D box having two spatial dimensions and one time dimension) may be as below. 
     
       
         
           
             
               
                 
                   
                       
                   
                    
                   
                     
                       
                         
                           
                             〈 
                             I 
                             〉 
                           
                           
                             m 
                             , 
                             n 
                           
                           j 
                         
                         = 
                         
                           
                             ∑ 
                             
                               i 
                               = 
                               
                                 
                                   - 
                                   
                                     ( 
                                     
                                       N 
                                       - 
                                       1 
                                     
                                     ) 
                                   
                                 
                                 / 
                                 2 
                               
                             
                             
                               i 
                               = 
                               
                                 
                                   ( 
                                   
                                     N 
                                     - 
                                     1 
                                   
                                   ) 
                                 
                                 / 
                                 2 
                               
                             
                           
                            
                           
                             
                               ∑ 
                               
                                 k 
                                 = 
                                 
                                   
                                     - 
                                     
                                       ( 
                                       
                                         N 
                                         - 
                                         1 
                                       
                                       ) 
                                     
                                   
                                   / 
                                   2 
                                 
                               
                               
                                 k 
                                 = 
                                 
                                   
                                     ( 
                                     
                                       N 
                                       - 
                                       1 
                                     
                                     ) 
                                   
                                   / 
                                   2 
                                 
                               
                             
                              
                             
                               
                                 ∑ 
                                 
                                   l 
                                   = 
                                   
                                     
                                       - 
                                       
                                         ( 
                                         
                                           L 
                                           - 
                                           1 
                                         
                                         ) 
                                       
                                     
                                     / 
                                     2 
                                   
                                 
                                 
                                   l 
                                   = 
                                   
                                     
                                       ( 
                                       
                                         L 
                                         - 
                                         1 
                                       
                                       ) 
                                     
                                     / 
                                     2 
                                   
                                 
                               
                                
                               
                                 
                                   a 
                                   ik 
                                 
                                  
                                 
                                   b 
                                   l 
                                 
                                  
                                 
                                   L 
                                   
                                     
                                       m 
                                       - 
                                       i 
                                     
                                     , 
                                     
                                       n 
                                       - 
                                       k 
                                     
                                   
                                   
                                     j 
                                     - 
                                     l 
                                   
                                 
                               
                             
                           
                         
                       
                       ; 
                     
                      
                     
                       
 
                     
                      
                     
                       
                         σ 
                         
                           l 
                           
                             m 
                             , 
                             n 
                           
                         
                         j 
                       
                       = 
                       
                         
                           
                             
                               ∑ 
                               
                                 i 
                                 = 
                                 
                                   
                                     - 
                                     
                                       ( 
                                       
                                         N 
                                         - 
                                         1 
                                       
                                       ) 
                                     
                                   
                                   / 
                                   2 
                                 
                               
                               
                                 i 
                                 = 
                                 
                                   
                                     ( 
                                     
                                       N 
                                       - 
                                       1 
                                     
                                     ) 
                                   
                                   / 
                                   2 
                                 
                               
                             
                              
                             
                               
                                 ∑ 
                                 
                                   k 
                                   = 
                                   
                                     
                                       - 
                                       
                                         ( 
                                         
                                           N 
                                           - 
                                           1 
                                         
                                         ) 
                                       
                                     
                                     / 
                                     2 
                                   
                                 
                                 
                                   k 
                                   = 
                                   
                                     
                                       ( 
                                       
                                         N 
                                         - 
                                         1 
                                       
                                       ) 
                                     
                                     / 
                                     2 
                                   
                                 
                               
                                
                               
                                 
                                   ∑ 
                                   
                                     l 
                                     = 
                                     
                                       
                                         - 
                                         
                                           ( 
                                           
                                             L 
                                             - 
                                             1 
                                           
                                           ) 
                                         
                                       
                                       / 
                                       2 
                                     
                                   
                                   
                                     l 
                                     = 
                                     
                                       
                                         ( 
                                         
                                           L 
                                           - 
                                           1 
                                         
                                         ) 
                                       
                                       / 
                                       2 
                                     
                                   
                                 
                                  
                                 
                                   
                                     a 
                                     ik 
                                   
                                    
                                   
                                     
                                       
                                         b 
                                         l 
                                       
                                        
                                       
                                         ( 
                                         
                                           
                                             I 
                                             
                                               
                                                 m 
                                                 - 
                                                 i 
                                               
                                               , 
                                               
                                                 n 
                                                 - 
                                                 k 
                                               
                                             
                                             
                                               j 
                                               - 
                                               l 
                                             
                                           
                                           - 
                                           
                                             
                                               〈 
                                               I 
                                               〉 
                                             
                                             
                                               m 
                                               , 
                                               n 
                                             
                                             j 
                                           
                                         
                                         ) 
                                       
                                     
                                     2 
                                   
                                 
                               
                             
                           
                           ; 
                         
                       
                     
                      
                     
                       
 
                     
                      
                     
                         
                     
                      
                     
                       
                         V 
                         
                           m 
                           , 
                           n 
                         
                         j 
                       
                       = 
                       
                         
                           σ 
                           
                             l 
                             
                               m 
                               , 
                               n 
                             
                           
                           j 
                         
                         / 
                         
                           
                             
                               〈 
                               I 
                               〉 
                             
                             
                               m 
                               , 
                               n 
                             
                             j 
                           
                           . 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   12 
                   ) 
                 
               
             
           
         
       
     
     I m,n   j  indicates a brightness of an (m, n) th  pixel in a j th  frame, a ik  indicates a weight value in the space domain, and b l  indicates a weight value in the time domain. Herein, a 3D box having the same dimension in a basic (XY) direction and used in determination of a single voxel contrast may be considered. 
     Values of N and L may be odd numbers to allocate the calculated contrast value to the center of the 3D box. Application of the 3D box associated with Equation (12) may cause time delay in calculation of a contrast value for a currently captured frame. A delay value may be equal to L/2 (a half of a width of a time-domain window). Application of an asymmetric time-domain window suitable for on-line data processing may be separately considered. 
     When an average is calculated using a rectangular (Dirichlet) window in the space domain and the time domain, all weight values may have the same value (a ik =1/N 2 ; b 1 =1/L). A spectral function for such a window is described as a sinc function, such that a calculated sequence of contrast values may have vibration and sign change characteristics that cause high-frequency artifacts. 
     Referring to  FIG. 16B , the Gaussian window that is smoothly attenuated in the space (or time) domain is characterized by smooth attenuation in the frequency domain, thus completely suppressing high-frequency noise. 
     The weight value of the Gaussian window may be expressed as below. 
     
       
         
           
             
               
                 
                   
                     a 
                     
                       i 
                       , 
                       k 
                     
                   
                   = 
                   
                     C 
                      
                     
                         
                     
                      
                     
                       exp 
                        
                       
                         [ 
                         
                           
                             - 
                             4.5 
                           
                            
                           
                             { 
                             
                               
                                 
                                   ( 
                                   
                                     i 
                                     
                                       N 
                                       - 
                                       1 
                                     
                                   
                                   ) 
                                 
                                 2 
                               
                               + 
                               
                                 
                                   ( 
                                   
                                     k 
                                     
                                       N 
                                       - 
                                       1 
                                     
                                   
                                   ) 
                                 
                                 2 
                               
                             
                             } 
                           
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   ( 
                   13 
                   ) 
                 
               
             
           
         
       
     
     In Equation (13), a regularization coefficient C may be calculated under a regularization condition as provided in Equation (14). 
     
       
         
           
             
               
                 
                   
                     C 
                      
                     
                       
                         ∑ 
                         
                           i 
                           = 
                           
                             
                               - 
                               
                                 ( 
                                 
                                   N 
                                   - 
                                   1 
                                 
                                 ) 
                               
                             
                             / 
                             2 
                           
                         
                         
                           i 
                           = 
                           
                             
                               ( 
                               
                                 N 
                                 - 
                                 1 
                               
                               ) 
                             
                             / 
                             2 
                           
                         
                       
                        
                       
                         
                           ∑ 
                           
                             k 
                             = 
                             
                               
                                 - 
                                 
                                   ( 
                                   
                                     N 
                                     - 
                                     1 
                                   
                                   ) 
                                 
                               
                               / 
                               2 
                             
                           
                           
                             k 
                             = 
                             
                               
                                 ( 
                                 
                                   N 
                                   - 
                                   1 
                                 
                                 ) 
                               
                               / 
                               2 
                             
                           
                         
                          
                         
                           exp 
                            
                           
                             [ 
                             
                               
                                 - 
                                 4.5 
                               
                                
                               
                                 { 
                                 
                                   
                                     
                                       ( 
                                       
                                         i 
                                         
                                           N 
                                           - 
                                           1 
                                         
                                       
                                       ) 
                                     
                                     2 
                                   
                                   + 
                                   
                                     
                                       ( 
                                       
                                         k 
                                         
                                           N 
                                           - 
                                           1 
                                         
                                       
                                       ) 
                                     
                                     2 
                                   
                                 
                                 } 
                               
                             
                             ] 
                           
                         
                       
                     
                   
                   = 
                   1. 
                 
               
               
                 
                   ( 
                   14 
                   ) 
                 
               
             
           
         
       
     
     Similarly, Gaussian weighting in the time domain may be expressed as shown in Equation (15). 
     
       
         
           
             
               
                 
                   
                     b 
                     l 
                   
                   = 
                   
                     
                       C 
                       ′ 
                     
                      
                     
                       exp 
                        
                       
                         [ 
                         
                           
                             - 
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     In Equation (15), the regularization coefficient C may be calculated under a regularization condition as provided in Equation (16). 
     
       
         
           
             
               
                 
                   
                     
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     By using the Gaussian window in the space window and the time window, a sampling volume may indicate a form of an “apodized” oval having a rotational axis aligned along time coordinates. 
     With the principle of median filtering, noise may be reduced in contrast estimation for voxels. A median-based algorithm may be applied to pixel brightness data selected by a 3D rectangular sampling box. A median value of a data sample may be an estimate of an average value of data samples. A process of contrast determination based on median filtering may include the following operations:
         resorting data samples in an ascending order (descending order) and allocating a median Ĩ as a value in the center of the resorted samples;   calculating (I i,k   l −Ĩ) 2  for the data samples;   resorting (I i,k   l −Ĩ) 2  in the ascending order (descending order) and allocating the median  (I i,k   l −Ĩ)   2  as a value in the center of the resorted sequences; and   calculating a median-based contrast {tilde over (V)} with       

     
       
         
           
             
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     Blood pressure monitoring may have a continuous or a single-shot mode. In the continuous monitoring mode, the blood pressure value may be traced over time by the device  1000 . In the single-shot monitoring mode, the device  1000  may display the blood pressure value once on a display screen of the device  1000 , e.g., a smartwatch screen. 
     The device  1000  may determine the velocity of the bloodstream and the diameter of the artery from spatial and temporal distributions of speckles by using a Siegert ratio as expressed in Equation (17). 
         g   2 (τ)=β g   1 (τ) 2   (17)
 
     In Equation (17), β indicates a constant determined by a radiation reception condition, g 2 (τ) indicates a normalized autocorrelation function of light intensity fluctuations, g 1 (τ) and indicates a module of the normalized autocorrelation function of fluctuation of a scattered field at a random point on the surface of the medium. The device  1000  may obtain an image of a part of the body including an artery. For example, the device  1000  may calculate a pixel value as given by Equation (18) from a measurement value of a field for each element (pixel) of the image. 
     
       
         
           
             
               
                 
                   
                     
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     (pixel value in a monochromatic image) 
     The device  1000  may determine the velocity of the bloodstream and the diameter of the artery by using a neural network trained based on calibration data. 
     The device  1000  may determine the systolic blood pressure value and the diastolic blood pressure value of the artery by substituting the velocity of the bloodstream, the systolic and diastolic artery diameter, and a preliminarily introduced calibration pressure value into Poiseuille Equation (17). 
     
       
         
           
             
               
                 
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     In Equation (19), Q indicates a volumetric bloodstream, and may be calculated as V*ρ*S (V—the velocity of the bloodstream, ρ—the density of the blood vessel, S—the cross-sectional area of the blood vessel). d indicates a diameter of the blood vessel, and η indicates the viscosity of the blood. I indicates the length of the blood vessel corresponding to a coverage area of the light receiver (optical sensor), and may be a constant. ΔP may be calculated as (p2−p1), p2 indicates a calibration pressure, and p1 indicates a user&#39;s pressure (pressure of interest to be obtained). 
     Puayzel formula may most completely describe a hydrodynamic system in a blood vessel including all required parameters. 
     Thus, a main result is calculation of final systolic and diastolic pressures based on calibration (reference) data and Poiseuille Equation (17), and the device  1000  may use the neural network. Due to discontinuity and complexity of processes occurring in the artery, a volumetric database having values and conditions that generally require a large amount of computational resources and time has to be collected. 
     To address such a defect, the neural network may consider a combination of numerous parameters during a short time unit, such that the device  1000  may use the neural network. The number and time of processing variations in the neural network do not affect a consumed time. 
     Training is performed by inputting measurement data and indicating a desired output result. Redistribution of coupling (training) between neurons (weighting factors) is selection of a common factor that fixes a statistical structure of unknown coupling probability distribution between observed variables. 
     To train the neural network, as a general method for blood pressure measurement, correct calibration reference data obtained using a cuff manometer (aneroid or digital manometer) may be used. 
       FIGS. 17 and 18  are block diagrams of the device  1000  according to an embodiment. 
     The device  1000  may be a bracelet-type device structurally wearable on a wrist, which includes an integrated electronic module. The electronic module may include the light source  1910 , the light receiver  1930 , and the controller  1300  that are structurally and functionally connected to one another through a communication line. The light source  1910  may be arranged in the device  1000  to irradiate light on a part of the skin on the artery. 
     In one of the embodiments, the device  1000  of the bracelet type includes a contact plate (not shown), especially, a contact glass plate, for compression of the skin to allow light emitted from the light source  1910  to pass throughout the skin, thereby improving an SNR and thus the accuracy of blood pressure measurement. 
     As shown in  FIG. 17 , the device  1000  according to an embodiment may include the light source  1910 , the light receiver  1930 , a user input unit  1100 , an output unit  1200 , and a controller  1300 . However, all of the elements shown in  FIG. 17  are not mandatory elements of the device  1000 . More elements or less elements than those shown in  FIG. 17  may be used to implement the device  1000 . 
     For example, the device  1000  according to some embodiments may include the light source  1910  and the light receiver  1930 . 
     The device  1000  according to some embodiments may include the controller  1300 , the light source  1910 , and the light receiver  1930 . 
     The device  1000  according to some embodiments may include the controller  1300 , a communication unit (not shown), the light source  1910 , and the light receiver  1930 . 
     Although not shown in  FIGS. 17 and 18 , the device  100  according to some embodiments may further include a light guide, a polarization filter, a lens, a mirror, etc., as well as the light source  1910  and the light receiver  1930 . 
     For example, as shown in  FIG. 18 , the device  1000  according to some embodiments may further include a communication unit  1500 , a sensing unit  1400 , an audio/video (A/V) input unit  1600 , and a memory  1700  in addition to the communication unit  1500 , the sensing unit  1400 , the A/V input unit  1600 , and the memory  1700 . 
     The user input interface  1100  is a means through which a user inputs data for controlling the device  1000 . For example, the user input interface  1100  may include, but not limited to, a keypad, a dome switch, a touch pad (a capacitive overlay type, a resistive overlay type, an infrared beam type, a surface acoustic wave type, an integral strain gauge type, a piezoelectric effect type, etc.), a jog wheel, a jog switch, etc. The user input unit  1100  may include a physical blood pressure measurement button included in the device  1000 . The user input unit  1100  may receive a user input to input a user&#39;s reference blood pressure value. The user input unit  1100  may receive a user input to set blood pressure measurement time. 
     The output interface  1200  outputs an audio signal, a video signal, or a vibration signal, and may include a display unit  1210 , an audio output interface  1220 , and a vibration motor  1230 . 
     On the display unit  1210 , information processed by the device  1000  is displayed. For example, a user&#39;s blood pressure list may be displayed on the display unit  1210 . A guide image for guiding the light receiver may be displayed on the display unit  1210  such that the light receiver is located on the radial artery of the wrist of the user. 
     When the display unit  1210  and a touch pad are constructed as a touch screen in a layer structure, the display unit  1210  may be used as an input device as well as an output device. 
     The audio output interface  1220  outputs audio data received from the communication unit  1500  or stored in the memory  1700 . The vibration motor  1230  outputs a vibration signal. 
     The controller  1300  controls an overall operation of the device  1000 . For example, the controller  1300  may control in overall the user input unit  1100 , the output unit  1200 , the light source  1910 , and the light receiver  1930  by executing programs stored in the memory  1700 . The controller  1300  may execute a function of the device  1000  disclosed in  FIGS. 1 through 16B  by executing the programs stored in the memory  1700 . 
     The controller  1300  may be implemented with hardware that is known in related techniques such as a processor, a controller, a microcontroller, an application specific integration circuit, a circuit, etc., and is capable of executing a specific function. 
     A software part of the device  1000  indicating instructions or commands instructing the device  1000  to perform a particular function may be stored in an internal or external memory of the controller  1300 , in which the internal or external memory may be a volatile memory, a non-volatile memory, random access memory (RAM), read only memory (ROM), a register, a flash memory, a read only memory on an optical or magnetic recording medium, or another storage medium that is known in this field and is suitable for storing, recording, and reading the instructions or commands. 
     The controller  1300 , especially, the processor is a digital means for manipulating information according to an algorithm programmed in advance stored in the memory, and may have functionality enough to process particular data obtained from the light receiver (to execute a speckle-contrast analysis technique and perform calculation based on the obtained blood pressure value analysis result). 
     More specifically, the controller  1300  may control the light source  1910  to irradiate light to the user. The controller  1300  may control the light source  1910  to irradiate coherent light from the coherent light source  1910  such as a laser to a target region of the body like a part of the skin of the wrist under which an artery is located. The device  1000  may further include a lens system (not shown) or a flat optics (not shown) that controls light irradiated to the target region or received by the device  1000  to satisfy previously determined parameters. 
     The controller  1300  may control the light receiver  1930  to receive scattered light from the user. The light receiver  1930  may be a camera  1610  included in the device  1000 . Since a result determined from the received scattered light is a monochromatic image obtained by a coherent emitter, a blood pressure value determined finally may not be affected by a change such as sweat, a temperature, etc. An image determined from the received scattered light (after processed using the Siegert equation) may indicate an artery having a variable diameter and a bloodstream velocity. 
     Depending on an embodiment, the device  1000  may include two polarization filters, e.g., two polarization films (not shown), polarization directions of which are shifted by 90° with respect to each other. Since surface reflection is filtered by crossed polarization filters, the output of the device  1000  may not be affected even when a condition for light reflection from the surface of the skin is changed. A characteristic time of change in a chemical composition is much longer than a characteristic time of blood pressure value fluctuation, and thus dynamic characteristics of a speckle field may not be affected by the change in the chemical composition. 
     Moreover, the controller  1300  may obtain spatial and temporal distributions of speckles by processing the light received by the light receiver  1930  using a laser speckle contrast analysis technique and processing the speckles by using a Gaussian window that averages a 3D box having two space dimensions and one time dimension. 
     The controller  1300  may also determine the velocity of the bloodstream and the diameter of the artery from the spatial and temporal distributions of the speckles by using the Siegert ratio as expressed in Equation (17). 
     The controller  1300  may determine a blood pressure value calibration method by using the user&#39;s reference blood pressure value measured by an existing blood pressure measurement device. For example, the controller  1300  may determine a coefficient of a neural network to calculate the reference blood pressure value as an output when a user&#39;s blood pressure parameter determined by the controller  1300  is an input. In this case, a type of the neural network may be a standard back-propagation method in which once a known result is delivered as an output and related data reaches the input of the neural network, then weight coefficients of the neural network are selected. 
     The sensing unit  1400  senses a state of the device  1000  or a state around the device  1000 , and delivers sensed information to the controller  1300 . 
     The sensing unit  1400  may include, but not limited to, at least one of a magnetic sensor  1410 , an acceleration sensor  1420 , a temperature/humidity sensor  1430 , an infrared sensor  1440 , a gyroscope sensor  1450 , a positioning sensor (e.g., a global positioning system (GPS))  1460 , a pressure sensor  1470 , a proximity sensor  1480 , and a red/green/blue (RGB) sensor (or an illuminance sensor)  1490 . A function of each sensor may be intuitively construed from a name of each sensor by those of ordinary skill in the art, and thus will not be described in detail. 
     The communication unit  1500  may include a short-range wireless communicator  1510 , a mobile communicator  1520 , and a broadcasting receiver  1530 . 
     The short-range wireless communicator  151  may include, but not limited to, a Bluetooth Low Energy (BLE) communicator, a near field communication (NFC) unit, a wireless local area network (WLAN) (WiFi) communicator, a ZigBee communicator, an infrared Data Association (IrDA) communicator, a WiFi Direct (WFD) communicator, an ultra-wideband (UWB) communicator, and an Ant+communicator. 
     The mobile communicator  1520  transmits and receives a radio signal to and from at least one of a base station, an external terminal, and a server over a mobile communication network. Herein, the radio signal may include various forms of data corresponding to transmission/reception of a voice call signal, a video communication call signal, or a text/multimedia message. 
     The broadcasting receiver  1530  receives a broadcast signal and/or broadcasting-related information from an external source through a broadcasting channel. The broadcasting channel may include a satellite channel and a terrestrial channel. According to implementation examples, the device  1000  may not include the broadcasting receiver  1530 . 
     The memory  1700  stores programs for processing and control of the controller  1300  and data input to or output from the device  1000 . 
     The memory  1700  may include a storage medium of at least one type of a flash memory type, a hard disk type, a multimedia card micro type, a card type memory (e.g., a secure digital (SD) or extreme digital (XD) memory, etc.), a random access memory (RAM), a static random access memory (SRAM), a read-only memory (ROM), an electrically erasable programmable read-only memory (EEPROM), a programmable read-only memory (PROM), a magnetic memory, a magnetic disk, an optical disk, and so forth. 
     The programs stored in the memory  1700  may be classified into a plurality of modules depending on a function thereof, e.g., a user interface (UI) module  1710 , a touch screen module  1720 , a notification module  1730 , and so forth. 
     The UI module  1710  provides a specialized UI or graphic UI (GUI) interworking with the device  1000  for each application. The touch screen module  1720  senses a touch gesture of a user on a touch screen and delivers information about the touch gesture to the controller  1300 . The touch screen module  1720  according to some embodiments recognizes and analyzes a touch code. The touch screen module  1720  may be configured with separate hardware including a controller. The main module  1800  shown in  FIG. 3  may include the user input unit  1100 , the output unit  1200 , and the controller  1300  shown in  FIG. 17 . The main module  1800  shown in  FIG. 3  may include the sensing unit  1400 , the communication unit  1500 , the A/V input unit  1600 , and the memory  1700  shown in  FIG. 18  as well as the user input unit  1100 , the output unit  1200 , and the controller  1300 . 
     In an embodiment, all of the measured data may be input to the neural network that outputs a result of blood pressure value determination, and training of the neural network may be performed using calibration data. 
     An embodiment is not limited to the description made herein, and other embodiments not departing from the gist and scope of the present disclosure will be apparent to those of ordinary skill in the art based on information included in the specification as well as the knowledge in this technical field. In addition, an element mentioned in a singular form does not exclude plural elements unless mentioned specially. 
     Operable connection between elements should be understood as a relationship that allows these elements to correctly interact with each other and to implement functions of the elements. A specific example of the operable connection may be connection appropriate for exchange of information, connection appropriate for transmitting electric current, connection appropriate for delivering mechanical movement, connection appropriate for delivering light, sound, electromagnetism, mechanical vibration, etc. A special form of the operable connection is determined by a method of the interaction between the elements, and unless indicated otherwise, is provided by a well-known means using a principle known in this field. 
     A method disclosed herein may include one or more steps or operations for achieving the disclosed method. The steps and/or operations of the method may be interchangeably used without departing from the scope of claims. In other words, unless a particular order of a step or operation is defined, the order and/or use of the step and/or operation may be modified without departing from the claims. 
     Although the present application does not specify particular hardware and software for implementing blocks in the drawings, the gist of the present disclosure is not limited to implementation of the particular hardware or software, and thus it would be understood by a person skilled in the art that proper hardware and software known in this technical field may be used to implement the present disclosure. 
     Therefore, the hardware may be implemented with one or more on-demand integrated circuits, a digital signal processor, a digital signal processing device, a programmable logic device, an on-site programmable gate array, a processor, a controller, a microcontroller, a microprocessor, an electronic device, another electronic device adopted to implement a function described herein, a computer, or a combination thereof. 
     Although example embodiments have been described in detail and illustrated in the attached drawings, various other modifications may be apparent to those of ordinary skilled in the art, and thus such embodiments are merely intended for an exemplary purpose and do not limit the claimed disclosure, and the present disclosure is not confined to the illustrated and exemplified layout and design. 
     The features mentioned in different dependent claims and embodiments disclosed in other parts of the description may be combined to achieve useful effects even when the possibility of such combination is not explicitly disclosed. 
     A numerical value disclosed in the specification or drawings is intended to include any value including lower and higher values than the numerical value when at least two unit element intervals exist between the lower and higher values.