Patent Publication Number: US-2022233114-A1

Title: Apparatus and method for estimating oxygen saturation

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims priority to Korean Patent Application No. 10-2021-0010051, filed on Jan. 25, 2021, and Korean Patent Application No. 10-2021-0052956, filed on Apr. 23, 2021, in the Korean Intellectual Property Office, the disclosures of which are incorporated by reference herein in their entireties. 
     BACKGROUND 
     1. Field 
     The disclosure relates to oxygen saturation estimation, and more specifically, to oxygen saturation estimation using optical signals of multiple wavelengths. 
     2. Description of Related Art 
     With the aging population, increased medical costs, and a lack of medical personnel for specialized medical services, research is being actively conducted on information technology (IT)-medical convergence technologies, in which IT and medical technology are combined. Particularly, monitoring of a health condition of a human body may not be limited to places such as hospitals, but is expanded by mobile healthcare fields that may monitor a user&#39;s health condition anywhere (e.g., at home, at the office, or in transit from one place to another place) and anytime in daily life. Some examples of bio-signals, which indicate the health condition of individuals, may include an electrocardiography (ECG) signal, a photoplethysmogram (PPG) signal, an electromyography (EMG) signal, and the like, and various bio-signal sensors are being developed to measure the bio-signals in daily life. 
     Oxygen saturation is the fraction of oxygen-saturated hemoglobin relative to total hemoglobin (unsaturated+saturated) in the blood. The human body requires and regulates a very precise and specific balance of oxygen in the blood. Normal arterial blood oxygen saturation levels in humans are 95 to 100 percent. If the level is below 90 percent, it is considered low and called hypoxemia. Arterial blood oxygen levels below 80 percent may compromise organ function, such as functions of the brain and heart, and continued low oxygen levels may lead to respiratory or cardiac arrest. In medicine, oxygen saturation measures the percentage of hemoglobin binding sites in the bloodstream occupied by oxygen. At low partial pressures of oxygen, most hemoglobin is deoxygenated, and at around 90 percent, oxygen saturation increases according to an oxygen-hemoglobin dissociation curve. A pulse oximeter relies on the light absorption characteristics of saturated hemoglobin to provide an indication of oxygen saturation. 
     SUMMARY 
     This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
     According to an aspect of an example embodiment, an apparatus for estimating oxygen saturation may include a sensor configured to measure optical signals of multiple wavelengths based on emitting multi-wavelength light onto an object; and a processor configured to: determine a section of the optical signals for estimating the oxygen saturation based on a difference between at least two optical signals among the optical signals of the multiple wavelengths; and estimate the oxygen saturation based on the optical signals of the multiple wavelengths in the section. 
     The multi-wavelength light may include a first wavelength, a second wavelength that is longer than the first wavelength, and a third wavelength that is longer than the second wavelength. The processor may determine the section of the optical signals for estimating the oxygen saturation based on the difference between a first optical signal of the first wavelength and a second optical signal of the second wavelength. The processor may determine the section of the optical signals for estimating the oxygen saturation based on the difference between the first optical signal of the first wavelength and a third optical signal of the third wavelength. 
     The processor may determine the section based on the difference being greater than or equal to a threshold. 
     The processor may be further configured to provide information that requests a user to measure additional optical signals of the multiple wavelengths based on there being no section in which the difference is greater than or equal to a threshold. 
     The first wavelength may be less than 640 nanometers. 
     The sensor may include a force sensor configured to measure a force between the object and the force sensor when the object is in contact with the force sensor, and the processor may be further configured to determine the section for estimating the oxygen saturation based on a value of the force measured by the force sensor. 
     The processor may be further configured to determine a section in which the value of the measured force is within a threshold range as the section for estimating the oxygen saturation. 
     The processor may be further configured to, based on there being no section in which the value of the measured force is within the threshold range, provide information that requests a user to measure additional optical signals of multiple wavelengths, or provide information that guides the user to apply the force within a reference range. 
     The processor may perform calibration to preset a threshold range of the force for a user. 
     The processor may acquire a first feature value of the second optical signal of the second wavelength and a second feature value of the third optical signal of the third wavelength in the section, and estimate the oxygen saturation based on the first feature value and the second feature value. 
     The processor may acquire the first feature value and the second feature value based on a point in time at which the second optical signal of the second wavelength or the third optical signal of the third wavelength includes a maximum amplitude in the section or a median point in time of the section, or is further configured to acquire the first feature value and the second feature value based on statistical values of the second optical signal of the second wavelength and the third optical signal of the third wavelength in the section. 
     According to an aspect of an example embodiment, a method of estimating oxygen saturation may include measuring optical signals of multiple wavelengths by emitting multi-wavelength light onto an object; determining a difference between at least two optical signals among the optical signals of the multiple wavelengths; determining a section of the optical signals for estimating the oxygen saturation based on the difference between the at least two optical signals; and estimating the oxygen saturation based on the optical signals of the multiple wavelengths in the section. 
     The measuring of the optical signals of the multiple wavelengths may include measuring a first optical signal of a first wavelength, a second optical signal of a second wavelength that is longer than the first wavelength, and a third optical signal of a third wavelength that is longer than the second wavelength. The determining of the difference may include calculating the difference between the first optical signal of the first wavelength and the second optical signal of the second wavelength or the difference between the first optical signal of the first wavelength and the third optical signal of the third wavelength. 
     The determining the section may include determining the section in which the difference is greater than or equal to a threshold. 
     The method may include requesting a user to measure additional optical signals of the multiple wavelengths based on there being no section in which the difference is greater than or equal to a threshold. 
     The method may include measuring a force between the object and a sensor when the object is in contact with the sensor, and the determining the section for estimating the oxygen saturation may include determining the section based on a value of the measured force. 
     The determining the section for estimating the oxygen saturation may include determining a section in which the value of the measured force is within a threshold range as the section for estimating the oxygen saturation. 
     The method may further include guiding a user to apply force within a reference range based on there being no section in which the value of the measured force is within the threshold range. 
     According to an aspect of an example embodiment, an apparatus for estimating oxygen saturation may include a sensor configured to measure optical signals of multiple wavelengths by emitting multi-wavelength light onto an object; and a processor configured to: determine a point in time at which an optical signal having a longest wavelength among the optical signals of the multiple wavelengths has a maximum amplitude based on a measurement time period of measuring the optical signals of the multiple wavelengths being longer than or equal to a predetermined time period, and estimate the oxygen saturation based on the optical signals of the multiple wavelengths at the point in time. 
     The processor may provide information that guides a user to measure the optical signals of multiple wavelengths for at least the predetermined time period based on the sensor measuring the optical signals of the multiple wavelengths. 
     Other features and aspects will be apparent from the following detailed description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features, and advantages of certain embodiments of the present disclosure will be more apparent from the following description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a block diagram illustrating an apparatus for estimating oxygen saturation according to an example embodiment; 
         FIG. 2  is a block diagram illustrating an apparatus for estimating oxygen saturation according to another example embodiment; 
         FIG. 3  is a block diagram illustrating an apparatus for estimating oxygen saturation according to still another example embodiment; 
         FIGS. 4A to 4D  are diagrams for explaining a specific process of determining a suitable section for estimating oxygen saturation; 
         FIG. 5  is a flowchart illustrating a method of estimating oxygen saturation according to an exemplary embodiment; 
         FIG. 6  is a flowchart illustrating a method of estimating oxygen saturation according to another example embodiment; 
         FIG. 7  is a flowchart illustrating a method of estimating oxygen saturation according to still another example embodiment; 
         FIG. 8  is a diagram illustrating a wearable device according to an example embodiment; and 
         FIG. 9  is a diagram illustrating a smart device according to an example embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Details of example embodiments are provided in the following detailed description with reference to the accompanying drawings. The disclosure may be understood more readily by reference to the following detailed description of example embodiments and the accompanying drawings. The disclosure may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that the disclosure will be thorough and complete and will fully convey the concept of the present disclosure to those skilled in the art, and the disclosure will only be defined by the appended claims. Like reference numerals refer to like elements throughout the specification. 
     Throughout the drawings and the detailed description, unless otherwise described, the same drawing reference numerals will be understood to refer to the same elements, features, and structures. The relative size and depiction of these elements, features, and structures may be exaggerated for clarity, illustration, and convenience. 
     It will be understood that, although the terms “first,” “second,” etc., may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. Also, the singular forms are intended to include the plural forms as well, unless the context clearly indicates otherwise. In the specification, unless explicitly described to the contrary, the term “comprise,” and variations such as “comprises” or “comprising,” will be understood to imply the inclusion of stated elements but not the exclusion of any other elements. Terms such as “unit” and “module” denote units that process at least one function or operation, and they may be implemented by using hardware, software, or a combination of hardware and software. 
     Hereinafter, example embodiments of an apparatus and method for estimating oxygen saturation will be described in detail with reference to the drawings. 
       FIG. 1  is a block diagram illustrating an apparatus for estimating oxygen saturation according to an example embodiment. 
     Various example embodiments of the apparatus  100  for estimating oxygen saturation may be mounted in various terminals, such as a smartphone, a tablet personal computer (PC), a desktop PC, a notebook PC, a wearable device, and the like. Here, the wearable device may include a watch type, a wristlet type, a wrist band type, a ring type, a glasses type, and a hair band type. However, the present disclosure is not limited thereto and the apparatus  100  may be mounted in hardware manufactured in various forms to be used in specialized medical institutions. 
     Referring to  FIG. 1 , the apparatus  100  for estimating oxygen saturation includes a sensor part (or sensor)  110  and a processor  120 . 
     The sensor part  110  may measure optical signals of multiple wavelengths including a PPG signal by emitting multi-wavelength light to an object of a user. 
     In this case, the object may be a region of a wrist surface adjacent to the radial artery, which is an upper area of the wrist where the capillary blood or venous blood passes through, or a body part with a high blood vessel density such as a finger, a toe, an earlobe, etc. 
     The sensor part  110  may include a light source configured to emit light of multiple wavelengths onto the object and a detector configured to measure a PPG signal by detecting light emitted onto the object and scattered or reflected from a body tissue of the object. In this case, the light source may include at least one of a light emitting diode (LED), a laser diode, or a phosphor, but is not limited thereto. The detector may include a photodiode, a photo transistor, a photodiode array, a photo transistor array, an image sensor (e.g., complementary metal oxide semiconductor (CMOS) image sensor), etc. 
     An electrical, mechanical, or wired/wireless connection between the processor  120  and the sensor part  110  may be established depending on the sensor part  110 . Based on receiving a request for estimating oxygen saturation, the processor  120  may control the sensor part  110  and receive optical signals of multiple wavelengths from the sensor part  110 . 
     Based on receiving the optical signals of multiple wavelengths, the processor  120  may perform preprocessing on the received optical signals, such as filtering for noise removal, amplification of the optical signals of multiple wavelengths, conversion into digital signals, or the like. For example, the processor  120  may use a bandpass filter to perform bandpass filtering of 0.4 Hz to 10 Hz, thereby removing noise from the optical signals of multiple wavelengths received from the sensor part  110 . In addition, the processor  120  may correct the optical signals of multiple wavelengths through reconstruction of the optical signals of multiple wavelengths based on fast Fourier transform. However, the present disclosure is not limited thereto, and various other preprocessing operations may be performed according to various measurement environments, such as the computing performance or measurement accuracy of the apparatus, the purpose of oxygen saturation estimation, the measurement part of the user, the temperature and humidity of the object, the temperature of the sensor part, etc. 
     The processor  120  may estimate oxygen saturation based on the received optical signals of multiple wavelengths. In this case, multiple wavelengths of light may include a first wavelength that is a shortest wavelength, a second wavelength that is relatively longer than the first wavelength, and a third wavelength that is relatively longer than the second wavelength. Here, the first wavelength that is the shortest wavelength may be less than 640 nanometers, but is not limited thereto. 
     The processor  120  may determine a suitable section for estimating oxygen saturation based on a difference between at least two optical signals among the measured optical signals of multiple wavelengths. In this case, the processor  120  may obtain normalized signals by normalizing the optical signals of multiple wavelengths and determine a suitable section for estimating oxygen saturation based on the obtained normalized signals. For example, the normalized signal may be a signal obtained by dividing an alternating current (AC) component of the optical signal of each wavelength by a direct current (DC) component of the optical signal, but is not limited thereto. Equations 1 to 3 below represent a process in which normalized signals S1, S2, and S3 are obtained by dividing an AC component of an optical signal of each wavelength by a DC component of the optical signal. 
     S 1  represents a normalized signal obtained by dividing an AC component AC 1  of an optical signal of the first wavelength by a DC component DC 1  of the optical signal, S 2  represents a normalized signal obtained by dividing an AC component AC 2  of an optical signal of the second wavelength by a DC component DC 2 , and S 3  represents a normalized signal obtained by dividing an AC component AC 3  of an optical signal of the third wavelength by a DC component DC 3 . 
         S   1 =AC 1 /DC 1   (1)
 
         S   2 =AC 2 /DC 2   (2)
 
         S   3 =AC 3 /DC 3   (3)
 
     The processor  120  may determine a suitable section for estimating oxygen saturation based on a difference between the optical signal of the first wavelength and the optical signal of the second wavelength, or a difference between the optical signal of the first wavelength and the optical signal of the third wavelength. 
     In this case, the optical signals of each wavelength may refer to the normalized signals S 1 , S 2 , and S 3 . For example, the processor  120  may determine that a section in which the difference between the optical signal of the first wavelength and the optical signal of the second wavelength, or the difference between the optical signal of the first wavelength and the optical signal of the third wavelength, is greater than or equal to a threshold is a suitable section for estimating oxygen saturation. In addition, the processor  120  may perform a calibration to preset a threshold of the difference between the optical signal of the first wavelength and the optical signal of the second wavelength or the difference between the optical signal of the first wavelength and the optical signal of the third wavelength in consideration of the user&#39;s characteristics. Moreover, the processor  120  may request the user to re-measure the optical signals of multiple wavelengths when there is no section in which the difference is greater than or equal to the threshold. 
     Further, when a measurement time period of the optical signals of multiple wavelengths is longer than or equal to a predetermined time period, the processor  120  may determine that a point at which the optical signal of the longest wavelength among the optical signals of multiple wavelengths reaches the maximum amplitude is a point in time of estimating oxygen saturation, without having to determine the suitable section as described above. In this case, a threshold of the predetermined length of time may be preset for each user. 
     The processor  120  may estimate oxygen saturation based on the optical signals of multiple wavelengths in the determined suitable section. In this case, the processor  120  may acquire a first feature value feats of the optical signal of the second wavelength and a second feature value feat 2  of the optical signal of the third wavelength at one point in time in the determined suitable section, based on the normalized signals S 1 , S 2 , and S 3  of the measured optical signals of multiple wavelengths. The processor  120  may acquire the intensities of the normalized signals S 1  and S 3  at one point in time in the determined suitable section as the first feature value feat 1  and the second feature value feat 2 , respectively. 
     In this case, the processor  120  may acquire the first feature value feat 1  and the second feature value feat 2  based on an arbitrary point in time in the determined suitable section, a point in time at which the normalized signal S 2  or S 3  of the optical signal of the second wavelength or the third wavelength signal reaches the maximum amplitude, or a median point in time of the determined suitable section. However, the present disclosure is not limited thereto, and the first feature value feat 1  and the second feature value feat 2  may be acquired based on statistical values of the optical signal of the second wavelength and the optical signal of the third wavelength in the determined suitable section. In this case, the statistical values may include an average or median value of the intensities of the optical signals in the suitable section. 
     In addition, the processor  120  may estimate oxygen saturation based on the acquired first and second feature values feat 1  and feat 2 . Equation 3 below shows how to obtain a ratio R of the first feature value feat 1  of the optical signal of the second wavelength and the second feature value feat 2  of the optical signal of the third wavelength. Equation 4 below illustrates how to obtain oxygen saturation SpO 2  through the obtained ratio R value. 
     
       
         
           
             
               
                 
                   R 
                   = 
                   
                     
                       feat 
                       1 
                     
                     
                       feat 
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
             
               
                 
                   
                     SpO 
                     2 
                   
                   = 
                   
                     110 
                     - 
                     
                       25 
                       × 
                       R 
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
       FIG. 2  is a block diagram illustrating an apparatus for estimating oxygen saturation according to another example embodiment. 
     Referring to  FIG. 2 , the apparatus  200  for estimating oxygen saturation includes a sensor part  110  and a processor  120 . 
     As illustrated, the sensor part  110  may include a multiple-wavelength optical signal measuring sensor  210  and a force sensor  220 . 
     The multiple-wavelength optical signal measuring sensor  210  may be in contact with the object and measure a multi-wavelength optical signal from the object as described above. 
     The force sensor  220  may measure a force acting between the object and the force sensor  220  when the object in contact with the force sensor  220  changes the force. The force sensor  220  may include a pressure sensor, a combination of a pressure sensor and a contact area sensor, a pressure sensor array, or the like, but is not limited thereto. 
     When the force sensor  220  of the sensor part  110  measures the force, the processor  120  may generate information for guiding an actual force acting on the force sensor  220  by the object. In this case, the guide information may include information to induce the user to gradually increase the force applied by the object while in contact with the force sensor  220 , or conversely, to induce the user to gradually decrease the force when the user initially applies the force that is greater than or equal to a predetermined threshold. 
     Based on receiving a request for estimating oxygen saturation from the user or an external device, the processor  120  may guide the user to bring the object into contact with the sensor part  110 , and when the object is in contact with the sensor part  110 , the processor  120  may guide changes in force applied by the object to the sensor part  110  during the measurement time. For example, the processor  120  may induce the user to gradually increase the force applied by the object while in contact with the sensor part  110 , or induce the user to gradually decrease the force when the user presses the sensor part  110  with a force greater than or equal to a predetermined threshold. The processor  120  may provide the user with guide information related to a force between the object and the sensor part by using, for example, a display module and/or an audio output module mounted in the apparatus  100  for estimating oxygen saturation or a display module and/or an audio output module of an external device connected to the apparatus  100 . 
     The processor  120  may determine a suitable section for estimating the oxygen saturation further based on the force value measured by the force sensor  220 . 
     If the force acting between the object and the sensor part is too low, external light is injected into the multiple-wavelength optical signal measuring sensor  210 , and venous oxygen saturation affects oxygen saturation to be measured, and thus it is not suitable for estimating the user&#39;s oxygen saturation. If the force acting between the object and the sensor part is too large, the blood vessel will be occluded, which makes it difficult to accurately measure the motion of the pulse wave. 
     For example, the processor  120  may determine that a section in which the difference between the optical signal of the first wavelength and the optical signal of the second wavelength, or the difference between the optical signal of the first wavelength and the optical signal of the third wavelength, is greater than a threshold and the force value measured by the force sensor  220  falls within a threshold range is a suitable section. 
     In addition, when there is no section in which the force value between the object and the sensor part  110  falls within the threshold range, the processor  120  may request the user to re-measure the optical signals of multiple wavelengths or guide the user to apply the force within a reference range. 
     In addition, since an appropriate force section may be different for each user according to the blood pressure, the location of blood vessels, and the mechanical and optical properties of the skin, the processor  120  may perform calibration to pre-set the threshold range of the force for each user for determining the suitable section for estimating oxygen saturation. 
     In this case, the processor  120  may obtain force values at predetermined points in time before and after the point at which the optical signal of the second wavelength or third wavelength measured by the multiple-wavelength optical signal measuring sensor  210  reaches the maximum amplitude, and may set the obtained force values as the minimum and maximum values of the threshold range of the force for determining the suitable section for estimating oxygen saturation. 
       FIG. 3  is a block diagram illustrating an apparatus for estimating oxygen saturation according to another example embodiment. 
     Referring to  FIG. 3 , the apparatus  300  for estimating oxygen saturation may include a sensor part  110 , a processor  120 , a storage  310 , an output interface  320 , and a communication interface  330 . 
     The sensor part  110  may include a multiple-wavelength optical signal measuring sensor  210  and a force sensor  220  as illustrated, but is not limited thereto and the force sensor  220  may be omitted as described with reference to  FIG. 1 . Hereinafter, since the sensor part  110  and the processor  120  are described above, description will be made with focus on the storage  310 , the output interface  320 , and the communication interface  330 . 
     The storage  310  may store various criteria for estimating oxygen saturation, the obtained multi-wavelength optical signal, an estimated oxygen saturation value, a threshold range of force set for each user, and the like. The criteria may include user information, such as a user&#39;s age, gender, occupation, health condition, and the like, and information regarding a relationship between the optical signals of multiple wavelengths and oxygen saturation, etc., but is not limited thereto. The storage  310  may include at least one storage medium of a flash memory type memory, a hard disk type memory, a multimedia card micro type memory, a card type memory (e.g., a secure digital (SD) memory, an 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, and an optical disk, and the like, but is not limited thereto. 
     The output interface  320  may provide the user with the optical signals of multiple wavelengths measured by the sensor part  110  and processing results of the processor  120 . The output interface  320  may provide the information to the user in various visual/non-visual manners using a display module, a speaker, and a haptic device mounted in the apparatus  300 . 
     In this case, a display area may be divided into two or more areas, wherein the optical signals of multiple wavelengths, the force between the object and the sensor part, and the like, may be output in various forms of graphs in a first area and the estimated oxygen saturation value may be output in a second area. In this case, if the estimated oxygen saturation value is not within a normal range, the output interface  320  may output warning information in various manners, such as highlighting an abnormal value in red, and the like, displaying the abnormal value along with a normal range, outputting a voice warning message, adjusting a vibration intensity, and the like. 
     Also, the output interface  320  may output contact pressure guide information generated by the processor  120  and a measured actual force between the object of the user and the sensor part  110  which is obtained from the force sensor  220 . For example, the output interface  320  may visually output the information by using a display module, or may output the information in a non-visual manner through voice, vibrations, tactile sensation, or the like, by using a speaker module, a haptic module, or the like. For example, the output interface  320  may visually display information on a reference range of force to be applied by the user during the measurement time and/or the actual force measured by the force sensor  220 . 
     The communication interface  330  may be connected to the external device through communication techniques under the control of the processor  120  and may receive a multi-wavelength optical signal from the external device. In this case, the external device may include, without limitation, various devices, such as smartphones, tablet PCs, wearable devices, and the like, which measure a multi-wavelength optical signal directly from the user or manage the measured multi-wavelength optical signal. Also, the communication interface  330  may transmit the processing results of the processor  120  to the external device. 
     In this case, the communication techniques may include Bluetooth communication, Bluetooth Low Energy (BLE) communication, near field communication (NFC), wireless local area network (WLAN) communication, ZigBee communication, infrared data association (IrDA) communication, wireless fidelity (Wi-Fi) direct (WFD) communication, ultra wideband (UWB) communication, Ant+communication, Wi-Fi communication, and mobile communication techniques, but are not limited thereto. 
       FIGS. 4A to 4D  are diagrams for explaining a specific process of determining a suitable section for estimating oxygen saturation. 
       FIG. 4A  is a graph illustrating normalized signals S 1 , S 2 , and S 3  obtained by normalizing an optical signal of a first wavelength, an optical signal of a second wavelength, and an optical signal of a third wavelength that are measured by the sensor part  110  over time. That is, as illustrated in  FIG. 4A , the signal of each wavelength represents the magnitude of a pulse of a signal obtained by dividing an AC component signal of the optical signal of each wavelength by a DC component signal. In this case, the first wavelength S 1  is a shortest wavelength, the second wavelength S 2  is relatively longer than the first wavelength, and the third wavelength S 3  is relatively longer than the second wavelength. T 3  represents a point in time at which the normalized signal S 2  of the optical signal of the second wavelength and the normalized signal S 3  of the optical signal of the third wavelength reach the maximum signal intensity or the maximum amplitude. 
       FIG. 4B  is a graph showing a difference between signals of two wavelengths. When signals of three different wavelengths are measured as shown in  FIG. 4A , the processor  120  may determine a suitable section by using a difference between the signal of the first wavelength that is the shortest wavelength and the signal of the third wavelength that is the longest wavelength. 
     In the graph of  FIG. 4B , the difference between the optical signal of the first wavelength and the optical signal of the third wavelength is not significant in section A (0 to T 1 ). In section A, the blood vessel on the skin surface is not properly occluded due to a small force between the object and the sensor part (see section A in  FIG. 4C ), and therefore the optical signal of the third wavelength that is a relatively long wavelength and also the optical signal of the first wavelength that is the shortest wavelength are measured. In this section A (0 to T 1 ), external light is injected into the sensor part, and also venous oxygen saturation affects blood oxygen saturation to be measured, so that blood oxygen saturation fluctuates in a large range and has an unstable value (see section A of  FIG. 4D ). 
     In the graph of  FIG. 4B , the difference between the optical signal of the first wavelength and the optical signal of the third wavelength in section B (T 1  to T 2 ) is relatively larger than that in section A. In section B, the force between the object and the sensor part is increased as compared to section A (see section B of  FIG. 4C ), and thus the blood vessel on the skin surface is first occluded so that the magnitude of the optical signal of the first wavelength that is the shortest wavelength decreases first. In this section B, oxygen saturation has a relatively stable value (see section B of  FIG. 4D ). 
     In addition, in the graph of  FIG. 4B , the difference between the optical signal of the first wavelength and the optical signal of the third wavelength in section C (T 2  to end) is relatively small as compared to that in section B. In section C, the force between the object and the sensor part is increased as compared to that in section B (see section C of  FIG. 4C ), and hence the blood vessel deep in the skin is also occluded so that the magnitude of the optical signal of the third wavelength that is a relatively long wavelength also decreases. In this section C, the oxygen saturation fluctuates in a large range and has an unstable value (see section C of  FIG. 4D ). 
     In this way, section B may be determined as the relatively most suitable section for estimating oxygen saturation. Accordingly, a threshold P to determine section B as a suitable section may be preset, and the processor  120  may determine that section B in which the difference between the optical signal of the first wavelength and the optical signal of the third wavelength is greater than or equal to the threshold P is the suitable section. Here, the threshold P may be set differently according to each user. 
       FIG. 4C  is a graph showing the force between the object and the sensor part measured while the optical signal of the three wavelengths illustrated in  FIG. 4A  are measured. 
     As described above, if the force acting between the object and the sensor part is too low or too high, it is not suitable for estimating the user&#39;s oxygen saturation. Therefore, the suitable section for estimating oxygen saturation may be determined according to a threshold range of the force between the object and the sensor part. 
       FIG. 4C  illustrates that the threshold range of the force between the object and the sensor part is set to F 1  to F 2 , and thus section B in which the range of the force is measured as F 1  to F 2  may be determined as the relatively most suitable section for estimating oxygen saturation. In this case, the threshold range F 1  to F 2  of the force between the object and the sensor part may be different from one user to another according to the user&#39;s blood pressure, location of the blood vessel, and mechanical and optical properties of the skin, and the processor  120  may perform calibration to preset the threshold range of the force for each user for determining the suitable section for estimating oxygen saturation. For example, the processor  120  may obtain force values at predetermined points in time before and after the point at which the optical signal of the second wavelength or third wavelength measured by the multiple-wavelength optical signal measuring sensor  210  reaches the maximum amplitude, and may set the obtained force values as the minimum and maximum values of the threshold range of the force for determining the suitable section for estimating oxygen saturation. 
       FIG. 4D  is a graph showing the estimated oxygen saturation. 
     As described above, in section B (T 1  to T 2 ) in which the difference between the optical signal of the first wavelength and the optical signal of the third wavelength is greater than or equal to the threshold value P (see section B of  FIG. 4B ) and the force between the object and the sensor part falls within the threshold range F 1  to F 2  (See section B of  FIG. 4C ), the estimated oxygen saturation value does not fluctuate significantly and has a stable value. 
       FIG. 5  is a flowchart illustrating a method of estimating oxygen saturation according to an example embodiment. 
     First, optical signals of multiple wavelengths are measured (operation  510 ). In this case, the multiple wavelengths include a first wavelength that is a shortest wavelength, a second wavelength that is relatively longer than the first wavelength, and a third wavelength that is relatively longer than the second wavelength. Here, the first wavelength that is the shortest wavelength may be less than 640 nanometers. 
     Then, a difference between at least two optical signals among the optical signals of multiple wavelengths is calculated (operation  520 ). Here, the two optical signals may be the optical signal of the first wavelength and the optical signal of the second wavelength, or the optical signal of the first wavelength and the optical signal of the third wavelength. 
     Next, it may be determined whether there is a section in which the calculated difference is equal to or greater than a threshold (operation  530 ). 
     Then, if there is a section in which the difference is greater than or equal to the threshold, the section may be determined as a suitable section for estimating oxygen saturation (operation  540 ). 
     If there is no section in which the difference is greater than or equal to the threshold, a user may be requested to re-measure the optical signals of multiple wavelengths. 
     Next, oxygen saturation may be estimated based on the optical signals of multiple wavelengths in the determined suitable section (operation  550 ). At this time, a first feature value of the optical signal of the second wavelength and a second feature value of the optical signal of the third wavelength may be acquired and the oxygen saturation may be estimated based on the acquired first feature value and second feature value. 
       FIG. 6  is a flowchart illustrating a method of estimating oxygen saturation according to another example embodiment. 
     First, optical signals of multiple wavelengths may be measured (operation  610 ), and force between an object and a sensor part may be measured while the optical signals of multiple wavelengths are measured (operation  620 ). 
     Then, a difference between at least two optical signals among the optical signals of multiple wavelengths may be calculated (operation  630 ). In this case, a difference between the optical signals may be calculated using normalized signals obtained by normalizing each of the optical signals of multiple wavelengths. For example, the normalized signal may be a signal obtained by dividing an AC component of an optical signal of each wavelength by a DC component, but is not limited thereto. 
     Next, it may be determined whether there is a section in which the calculated difference is equal to or greater than a threshold value (operation  640 ). At this time, if there is no section in which the difference is greater than or equal to the threshold, a user may be requested to re-measure the optical signals of multiple wavelengths. 
     Next, it is determined whether there is a section in which the measured force value falls within a threshold range (operation  650 ). When it is determined that there is no section in which the measured force value falls within a threshold range, the user may be requested to re-measure the optical signals of multiple wavelengths. 
     Here, operations  630  to  650  are not limited to the illustrated order and may be performed in reverse order, in a different order, or in parallel. 
     Next, a section in which the calculated difference is greater than or equal to the threshold value and the measured force value falls within the threshold range may be determined as a suitable section for estimating oxygen saturation (operation  660 ). 
     Next, oxygen saturation may be estimated based on the optical signals of multiple wavelengths in the determined suitable section (operation  670 ). 
       FIG. 7  is a flowchart illustrating a method of estimating oxygen saturation according to still another example embodiment. 
     First, optical signal of multiple wavelengths are measured (operation  710 ). 
     Next, it may be determined whether the measurement time period of the optical signals of multiple wavelengths is longer than a predetermined time period (operation  720 ). 
     Next, when it is determined that the measurement time period is longer than or equal to the predetermined time period, a point at which the longest-wavelength optical signal among the optical signals of multiple wavelengths reaches the maximum amplitude may be determined as a point in time of estimating oxygen saturation (operation  730 ). 
     If the measurement time period is less than the predetermined time period, the user may be guided to measure the optical signals of multiple wavelengths for longer than the predetermined time period (operation  740 ). 
     Next, oxygen saturation may be estimated based on the optical signals of multiple wavelengths at the point in time determined in operation  730  (operation  750 ). 
     Referring to  FIG. 8 , a wearable device  800  includes a main body  810  and a strap  820 . 
     The strap  820  may be made of a flexible material. The strap  820  may be connected to opposite ends of the main body  810  and may wrap around the user&#39;s wrist such that the main body  810  is in close contact with an upper portion of the wrist. In this case, air may be injected into the strap  820  or an airbag may be included in the strap  820 , so that the strap  820  may have elasticity according to a change in pressure applied to the wrist, and the change in pressure of the wrist may be transmitted to the main body  810 . 
     A battery, which supplies power to the wearable device  800 , may be embedded in the main body  810  or the strap  820 . Further, a sensor part  830  may be mounted in a rear surface of the main body  810 . The sensor part  830  may include one or more light sources and a detector. The sensor part  830  may include a multiple-wavelength optical signal measuring sensor and a force sensor. The multiple-wavelength optical signal measuring sensor may include a light source and a CMOS image sensor (CIS)-based image sensor. Further, the force sensor may include a pressure sensor, a combination of a pressure sensor and a contact area sensor, a pressure sensor array, etc. 
     A processor may be mounted in the main body  810  and may determine a suitable section for estimating oxygen saturation based on a difference between at least two optical signals among optical signals of multiple wavelengths measured by the sensor part  830 . For example, the optical signals of multiple wavelengths may include an optical signal of a first wavelength that is a shortest wavelength, an optical signal of a second wavelength that is relatively longer than the first wavelength, and an optical signal of a third wavelength that is relatively longer than the second wavelength. 
     The processor may determine a suitable section for estimating oxygen saturation based on a difference between the optical signal of the first wavelength and the optical signal of the second wavelength, or a difference between the optical signal of the first wavelength and the optical signal of the third wavelength. The processor may normalize the measured optical signals of multiple wavelengths and calculate a difference between the optical signal of the first wavelength and the optical signal of the second wavelength, or a difference between the optical signal of the first wavelength and the optical signal of the third wavelength on the basis of the normalized signals. In this case, the processor may determine that a section in which the difference is greater than or equal to a threshold and/or measured force falls within a threshold range is the suitable section for estimating oxygen saturation. 
     In addition, the processor may acquire a first feature value of the optical signal of the second wavelength and a second feature value of the optical signal of the third wavelength at one point in time in the determined suitable section based on the normalized signals of the measured optical signals of multiple wavelengths, and estimate the oxygen saturation based on the acquired first feature value and second feature value. At this time, the processor may acquire the first feature value and the second feature value based on a point at which the optical signal of the second wavelength or the optical signal of the third wavelength reaches the maximum amplitude in the determined suitable section, or the median point in time of the determined suitable section, or may acquire the first feature value and the second feature value based on statistical values of the optical signal of the second wavelength and the optical signal of the third wavelength in the determined suitable section. 
     Based on receiving a request for estimating oxygen saturation from the user, the processor may guide the user for a force and/or pressure through a display and provide the user with an estimation result through the display when oxygen saturation is estimated. The display may be mounted on the front surface of the main body  810  to output the guide information and/or the result of estimation of oxygen saturation and receive the user&#39;s touch input and transmit the touch input to the processor. 
     In addition, a storage may be mounted inside the main body  810 , and various criteria for estimating oxygen saturation and/or a processing result of the processor may be stored in the storage. 
     In addition, a manipulator  840  configured to receive a user&#39;s control command and transmit the control command to the processor may be mounted on the side of the main body  810 , and the manipulator  840  may have a function for inputting a command to turn on/off the wearable device  800 . 
     Further, a communication interface configured to transmit and receive data with the external device may be mounted in the main body  810 . The communication interface may communicate with the external device, such as the user&#39;s smartphone, an oxygen saturation measuring device, or the like, to transmit and receive various types of data related to estimation of oxygen saturation. 
       FIG. 9  is a diagram illustrating a smart device according to an example embodiment. The smart device  900  may include a smartphone, a tablet PC, etc. The smart device  900  may include the above-described various exemplary embodiments of the apparatuses  100 ,  200 , and  300  for estimating oxygen saturation. 
     Referring to  FIG. 9 , the smart device  900  may have a sensor part  930  mounted on a rear surface of a main body  910 . The sensor part  930  may include a light source  931  and a detector  932 . The sensor part  930  may be mounted on the rear surface of the main body  910  as illustrated, but is not limited thereto. For example, the sensor part  930  may be formed on a fingerprint sensor on a front surface, a part of a touch panel, or a power button or volume button mounted on the side or an upper portion of the smart device. Also, the sensor part  930  may include a sensor configured to measure force between an object and the sensor part. 
     In addition, a display may be mounted on the front surface of the main body  910  to display various types of information, such as a result of estimation of oxygen saturation, a state in which the object is in contact with the sensor part, and guide information regarding force between the object and the sensor part. 
     An image sensor  920  may be mounted in the main body  910  as illustrated, and the image sensor  920  may capture an image of a finger when the user approaches the sensor part  930  to measure optical signals of multiple wavelengths and may transmit the image to the processor. In this case, the processor may identify a relative position of the finger relative to the actual position of the sensor part  930  and guide the user for information on the relative position of the finger through the display. 
     As described above, the processor may determine a suitable section for estimating oxygen saturation based on a difference between at least two optical signals among the measured optical signals of multiple wavelengths, and estimate oxygen saturation based on the optical signals of multiple wavelengths in the determined suitable section. A detailed description thereof will be omitted. 
     The example embodiments can be implemented by computer-readable code that is stored in a non-transitory computer-readable medium and that is executed by one or more processors. Code and code segments constituting the computer program can be inferred by a computer programmer skilled in the art. The computer-readable medium includes all types of record media in which computer-readable data are stored. Examples of the computer-readable medium include a ROM, a RAM, a CD-ROM, a magnetic tape, a floppy disk, and an optical data storage. Further, the record medium may be implemented in the form of a carrier wave such as Internet transmission. In addition, the computer-readable medium may be distributed to computer systems over a network, in which computer-readable code may be stored and executed in a distributed manner. 
     A number of examples have been described above. Nevertheless, it will be understood that various modifications may be made. For example, suitable results may be achieved if the described techniques are performed in a different order and/or if components in a described system, architecture, device, or circuit are combined in a different manner and/or replaced or supplemented by other components or their equivalents. Accordingly, other implementations are within the scope of the following claims.