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Reflectance pulse oximetry: Practical issues and limitations
Hooseok Lee, Hoon Ko, Jinseok Lee ∗Department of Biomedical Engineering, Wonkwang University College of Medicine, Iksan, Republic of Korea
Received 14 August 2016; received in revised form 10 October 2016; accepted 11 October 2016
Available online xxxx

AbstractThe demand for reflective-mode pulse oximetry to monitor oxygen saturation has been continuously increasing because it can be used atdiverse measurement sites such as the feet, forehead, chest, and wrists. For the wrists, in particular, pulse oximeters are easily available in the formof a band or watch. In this study, we developed a reflectance pulse oximeter and used it to measure oxygen saturation levels at the fingertips andthe wrist. We analyzed the performance of this oximeter to address the challenges and limitations associated with using reflective-mode oximetersat the wrist for clinical purposes.
c 2016 The Korean Institute of Communications Information Sciences. Publishing Services by Elsevier B.V. This is an open access article under⃝
the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Pulse oximetry; Reflective mode; Wrist; Wearable sensor

1. Introduction
Pulse oximetry is a noninvasive method for accurately estimating oxygen saturation (SaO2 ) by reading the peripheral
oxygen saturation (SpO2 ). As SaO2 and SpO2 are sufficiently
correlated and pulse oximetry has the advantages of being safe,
convenient, inexpensive, and noninvasive, this approach is clinically accepted for monitoring oxygen saturation [1,2].
Pulse oximetry is simple to carry out; it only uses two different light sources and a photodiode [3–5]. Depending on themeasurement site, either the transmissive or the reflective modecan be used. In the transmissive mode, the light sources andphotodiode are opposite to each other with the measurementsite between them. Light then passes through the site. In the reflective mode, the light sources and photodiode are on the sameside, and light is reflected to the photodiode across the measurement site.
Currently, the transmissive mode is the most commonly usedmethod because of its high accuracy and stability. Nevertheless,the demand for reflective-mode oximetry is continuously increasing because it does not require a thin measurement site. It
can be used at diverse measurement sites such as the feet, forehead, chest, and wrists. In particular, if the wrist is the available∗ Corresponding author.

E-mail addresses: gntjr2@naver.com (H. Lee), idayfly8710@gmail.com(H. Ko), gonasago@wku.ac.kr (J. Lee).
Peer review under responsibility of The Korean Institute of Communications Information Sciences.

measurement site, pulse oximeters can be conveniently used in
the form of a band or watch. To the best of our knowledge,
reflectance pulse oximetry, specifically for monitoring oxygen
saturation at the wrist, is currently not practiced clinically. In recent years, many reflectance pulse oximeters have become commercially available, but they are only for personal monitoring ofoxygen saturation and for entertainment purposes. Obviously,
the focus is on developing oximeters for medical purposes, andresearch is being carried out in this regard. Furthermore, mostresearch papers discuss only the basic principle of pulse oximetry and its utilization in smart devices; only a few papers discuss
the challenges associated with reflective pulse oximetry.
In this study, we developed a reflectance pulse oximeter andaddressed the practical issues and limitations associated withits use. Using this oximeter, we investigated the results of AC
amplitudes and DC offsets of the red and infrared signals, including modulation ratio, which are critical factors to estimate
SpO2 . We first tested our device by measuring oxygen saturation at a fingertip and wrist and analyzed its performance. Finally, we summarized all issues and discussed the feasibility ofusing the device for clinical purposes.
2. Methods
2.1. Principle of SpO2 estimation
Pulse oximetry measures arterial oxygen saturation based on
the light absorption properties of blood. When it combines with

http://dx.doi.org/10.1016/j.icte.2016.10.004
c 2016 The Korean Institute of Communications Information Sciences. Publishing Services by Elsevier B.V. This is an open access article under the2405-9595/⃝
CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

2

H. Lee et al. / ICT Express ()–Fig. 1. (a) Changes in AC R , ACIR , DC R , and DCIR with decreased SaO2 . (b)Empirical relationship between modulation ratio R and SaO2 .

oxygen, deoxyhemoglobin (Hb) changes its light absorption
characteristics. Pulse oximetry exploits the light absorption
difference between Hb and oxygenated hemoglobin (HbO2 ).
HbO2 absorbs more infrared light (660 nm wavelength)
and lesser red light (940 nm wavelength) than Hb. In thetransmissive mode, light from a pair of red and infrared lightemitting diodes (LEDs) is transmitted through a fingertip.
Then, the transmitted light is received by a photodiode on theopposite side. The transmitted light signals consist of a direct
current (DC) component and pulsatile alternating current (AC)component. Pulse oximetry calculates the modulation ratio R
by using the DC and AC components of the red and infrared
signals as follows:
R=AC R /DC R,
ACIR /DCIR(1)

where AC R and ACIR are the AC amplitudes of the red andinfrared signals, respectively [6,7]. DC R and DCIR are theDC offsets of the red and infrared signals, respectively. Then,empirically derived calibration curves are used to estimate
SaO2 based on the modulation ratio R, as seen in Fig. 1(a). AC R
and DCIR increase with decreasing SaO2 , as seen in Fig. 1(b).
On the other hand, ACIR and DC R decrease.
2.2. Reflectance pulse oximeterWe developed a reflective pulse oximeter where the LEDtransmission unit and photodiode receiving units are on theFig. 2. (a) Control and data flow. (b) Developed reflectance pulse oximeter.

same side. To switch between red and infrared LED lights,
the duty cycle was set to 50% with a switching frequencyof 500 Hz, which generated a pulse width modulation signal
through the microcontroller unit (MCU). Then, each reflected
light beam was converted to an electrical signal, which was
subsequently amplified with a gain of 100. A sample-and-holdcircuit consisting of an analog switch and operational amplifier
then separated each amplified signal with a sampling rate of
500 Hz from red and infrared LEDs. Each separated signal wasthen filtered using a low-pass filter with a cutoff frequency of
10 Hz and an additional gain of 10. For this study, we used
an MCP6004 operational amplifier, a TM4C123GHPM MCU,
an NJL5310R photodiode, an SML-LX0805SRC-TR red LED,and a KP-2012F3C infrared LED. Fig. 2 shows our design andits implementation in reflectance pulse oximetry. The maximum
LED current was 20 mA, and the light intensity was set to themaximum level of the LED capability.
To determine AC R , ACIR , DC R , and DCIR , we first detected
the pulse peak by incorporating a filter bank with variable cutoff
frequencies, spectral estimates of the heart rate, a rank-orderH. Lee et al. / ICT Express ()–3Fig. 3. Results for fingertip: (a) Reference SpO2 (%), (b) reflected infrared rawsignal, (c) reflected red raw signal, and (d) the modulation ratio R.

nonlinear filter, and decision logic [8]. Next, we calculated theaverage values of a 1-s segment signal with each detected peakas its center. Finally, DC was calculated from each averagevalue, and AC was calculated by subtracting DC from each peakvalue. This method was applied to both red and infrared signals
to determine AC R , ACIR , DC R , and DCIR .
2.3. Performance evaluation
We performed two experiments: one at a fingertip and theother at a wrist. In each experiment, the subjects breathedregularly for 40 s and then held their breath for as long as
they could. Subsequently, an additional 1 min was given for
their SpO2 levels to return to the normal state. PowerLab8/35 (ADINSTURMENTS, Sydney, Australia) was used withan oximeter pod (ADINSTRUMENTS, Sydney, Australia) tomeasure the reference SpO2 .

Fig. 4. Results from fingertip: (a) Reference SpO2 (%) measured at fingertip,(b) resultant ACIR , (c) resultant AC R , (d) resultant DCIR , and (e) resultant
DC R .

AC R increased, which sufficiently match the light absorption
characteristics described in Fig. 1(a) and (b). On the other
hand, the reflected DC components partially reflected thesecharacteristics. As SaO2 decreases, DCIR should increase andDC R should decrease. However, in the results, only DCIR
followed this pattern. In fact, DC R showed an increase. Thisbehavior can be attributed to the fact that the reflective mode
is more sensitive to pressure and ambient light sources, whichleads to DC instability.

3. Results and discussion
3.2. Results for the wrist
3.1. Results for the fingertip
Fig. 3 shows the results for the fingertip: the reference
SpO2 , reflected infrared raw signal, reflected red raw signal,and modulation ratio R. As expected, the ratio R increased
with decreasing SaO2 . Fig. 4 shows the resultant ACIR , AC R ,DCIR , and DC R . When SpO2 decreased, ACIR decreased andFig. 5 shows the results for the wrist: the reference SpO2 ,the modulation ratio R, ACIR , AC R , DCIR , and DC R . As
SpO2 decreased, the modulation ratio R increased as expected.
However, the modulation ratio R was less stable than that for
the fingertip. More specifically, as SpO2 decreased, ACIR did
not reflect the light absorption characteristics. Furthermore, the4H. Lee et al. / ICT Express ()–reflect the characteristics well. This variation may be because
the reflective mode has a low signal-to-noise ratio (SNR) and issensitive to pressure and ambient light sources.
3.3. Discussion
Based on the performance of the oximeter, monitoring SpO2
at the wrist using the reflective mode presents challenges withregard to clinical use. Another limitation is that the reflected red
and infrared pulses can only be used for specific areas, such asa radial artery; thus, most areas of the wrist are not available formonitoring. In addition, a slight position change at the measurement site significantly affects the performance of the oximeter.
Thus, the focus of research studies involving oximetry should
be on choosing appropriate measurement sites, and optimizingpressure, ambient light, and SNR.
Acknowledgment
This study was supported by a Basic Science Research
Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Science, ICT & Future
Planning: NRF-2015M3A9D7067215.
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