卒業論文 2008年度 (平成 20年度)

LEDs as Photoelectric Receivers:Performance Assessment and Applications

to Biometric Technology

慶應義塾大学 環境情報学部

氏名：津田 尚明

指導教員慶應義塾大学 環境情報学部

村井 純徳田 英幸中村 修楠本 博之高汐 一紀三次 仁植原 啓介重近 範行中澤 仁

Rodney D. Van Meter III

January 29, 2009

Abstract of Bachelor’s Thesis

Academic Year 2008

LEDs as Photoelectric Receivers:

Performance Assessment and Applications

to Biometric Technology

The purpose of this study is to examine the actual performance of Light emitting diodes(LED) as photodiodes (PD) and their applications for photoplethysmography (PPG) ofarteriolar blood volume (aBV).

PPG sensors which are for measuring aBV, are one of the most important applicationexamples of PDs. Because aBV is a source of rich information on biometric technology,it has a wide range of applications, such as pulse oximetry, stress scale, blood vesselvaluation, individual identification, and so on.

Although it is little known, LEDs are bidirectional. An LED can work as both alight emitter and a light sensor. That is, LEDs are fundamentally PDs available at a fewpercent of the cost of normal PDs.

One problem is, however, that little is known about the practicability of LEDs as lightsensors. If LEDs provide enough performance in light detection for PPG, we could useLEDs as low-cost multifunctional alternatives to PDs.

In order to assess the performance of LEDs as photoelectric receivers, three exper-iments were conducted. First, the author compared the generated electromotive forcesof LEDs and PD by a krypton lamp as the light source where the distance between thepoint light source and each light detector was variable (Experiment 1). Second, the authorcompared the generated electromotive forces of LEDs and PD by a set of 7 red LEDs at624nm and a set of 7 infrared LEDs (IR-LED) at 940nm as the light sources where theeach of 7 LEDs of both sets was independently controllable by a microcontroller (Exper-iment 2). Third, the author compared the time response characteristics of LEDs and PDto an artificial light pulse generated by the same device in Experiment 2 (Experiment 3).

The result of Experiment 1 showed that the generated voltages of half of LEDs werenealy triple of that of PD at their peak bands but much lower at the others. The result ofExperiment 2 showed that both visible LEDs (V-LED) and IR-LED detected the 624nmradiation but V-LEDs did not detect the 940nm radiation while IR-LED detected it.The result of Experiment 3 showed that there were no differences in the time responsecharacteristics of LEDs and PD at frequency of 100Hz except that V-LEDs did not detectthe 940nm pulses.

The results suggest that although there are some limitations, LEDs are useful lightsensors in various application fields including biometrics. LEDs have advantages over PDsin terms of cost and multifunctionality.

Keywords :1. arteriolar blood volume (aBV), 2. bidirectionality, 3. biometrics,4. light emitting diode (LED), 5. photodiode (PD), 6. photoplethysmography (PPG),7. pulse oximetry

Keio University, Faculty of Environment and Information Studies

Naoaki Tsuda

iii

論文要旨 2008年度 (平成 20年度)

受光器としての発光ダイオード：性能評価および生体計測への応用

本研究の目的は，発光ダイオード (LED)を受光器，つまりフォトダイオード (PD)として用いたときの性能評価を行い，特に，LEDの，動脈血液量 (aBV)を計測するための光電式体積変動記録器 (PPG)への応用可能性について考察することである．

PPGセンサーは，非常に重要なPDの応用技術のひとつである．PPGによって計測される aBVの変動は，多くの生体情報を含んでいるため，その応用範囲は多岐に渡る．具体例としては，パルスオキシメトリー，ストレス尺度，血管年齢推定，個体認証等が挙げられる．一般的にはほとんど知られていないが，LEDには双方向性があり，発光器としてだけ

でなく，受光器としても用いることができる．つまり，LEDは本質的には PDでもあり，また，そのコストは通常の PDの数パーセントである．問題となるのは，LEDを受光器として用いたときの実際的性能が不明であることだ．

もし，LEDがPPG用受光器として十分な性能を示すのであれば，低コストかつ多機能なLEDは，PDの優れた代替品となり得る．

LEDの受光器としての性能を評価するため，著者は３つの実験を行った．実験１では，クリプトン電球を点光源とし，受光器である LEDおよび PDと，点光源との距離を変化させた際，受光器の起電圧がどのように変化するかを比較した．実験２では，７つの赤色LED(波長 624nm)と７つの赤外線LED(波長 940nm)をそれぞれ一束にした２種の光源を用い，マイクロコントローラ制御により７段階に発光するそれら光源２種に対し，受光器である LEDおよびPDの起電圧が各々どのように変化するかを比較した．実験３では，実験２と同様の光源発生装置を用い，人工的な光パルスを発生させ，受光器である LEDおよび PDの応答性を比較した．実験１の結果，半数のLEDの起電圧は，ピーク域でPDの約３倍を記録したが，他の

領域では PDを大きく下回った．実験２の結果，波長 624nmの赤色光に対しては，受光器である可視光 LEDおよび赤外線 LEDの両者共に反応があったが，波長 940nmの赤外光に対しては，赤外線 LEDは反応し，可視光 LEDは反応しないことがわかった．実験３の結果，可視光 LEDが赤外光のパルスに反応しなかった点を除き，本実験においてはLEDと PDの応答性に違いは見られなかった．実験結果は，生体計測を含む様々な応用分野において，LEDは一部制限はあるが有用

な受光器であることを示唆している．また，LEDはPDに対して，低コストである点，多機能である点において勝っている．

キーワード1．光電式体積変動記録, 2．生体計測, 3．双方向性, 4．動脈血液量, 5．発光ダイオード,6．パルスオキシメトリー, 7．フォトダイオード

慶應義塾大学 環境情報学部津田 尚明

Acknowledgments

I am grateful to my thesis supervisor, Assistant Professor Kenji Saito (Research In-stitute for Digital Media and Content, Keio University) for his guidance, suggestions,encouragement, and revisions throughout the process of writing this thesis.

I would like to thank Professor Osamu Nakamura for permitting my admission to theInternet Research Laboratory in spite of the fact that I am in the final semester.

I am also grateful to Professor Yoshiyasu Takefuji. He introduced to me the novelidea of bidirectionality of LEDs, which is the main theme of this thesis. I have learned alot about how to build microcontroller devices necessary for this study from his inspiringlectures.

A huge thanks to all the members of the Internet Research Laboratory, especially themembers of Sotsuron-gakari for their supports. Thanks also to my company, Gen Minekiand Megumi Nakasato for their guidance to deal with many little problems in the workingprocess.

Lastly, and most importantly, I am grateful to my wife for everything.

Contents

1 Introduction 1

1.1 Purpose of This Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Contribution of This Study . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.3 Organization of This Thesis . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2 Background 3

2.1 Photoplethysmography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.1.1 Pulse Oximetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.1.2 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.2 LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.3 PDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3 Bidirectionality of LED 5

3.1 LEDs as PDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3.2 Related Works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

4 Methods 6

4.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4.1.1 Experimental Materials . . . . . . . . . . . . . . . . . . . . . . . . . 6

4.2 Experiment 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

4.3 Experiment 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4.4 Experiment 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4.4.1 Experiment 3-a: R-LED pulse . . . . . . . . . . . . . . . . . . . . . 14

4.4.2 Experiment 3-b: IR-LED pulse . . . . . . . . . . . . . . . . . . . . 14

5 Results 16

5.1 Experiment 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

5.2 Experiment 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

5.2.1 Experiment 2-a . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

5.2.2 Experiment 2-b . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

5.3 Experiment 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

6 Discussion 23

6.1 Experiment 1 and Experiment 2 . . . . . . . . . . . . . . . . . . . . . . . . 23

6.2 Experiment 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

i

CONTENTS

7 Conclusions 257.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257.2 Future Works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

ii

List of Figures

2.1 Optical Absorption of Hemoglobin . . . . . . . . . . . . . . . . . . . . . . . 4

4.1 Optical Spectrum of Krypton Lamp . . . . . . . . . . . . . . . . . . . . . . 84.2 Experimental Setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94.3 Digital Multimeter MS8221C (MASTECH) . . . . . . . . . . . . . . . . . . 94.4 Circuit Diagram of Experiment 1 . . . . . . . . . . . . . . . . . . . . . . . 104.5 USB LED Flasher and 7 R-LEDs . . . . . . . . . . . . . . . . . . . . . . . 114.6 USB LED Flasher and 7 IR-LEDs . . . . . . . . . . . . . . . . . . . . . . . 124.7 Lightproof Cylinder for Light Sources and Light Detectors . . . . . . . . . 134.8 Circuit Diagram of Experiment 2 . . . . . . . . . . . . . . . . . . . . . . . 134.9 Timing Chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144.10 Reverse Bias Amplifier Circuit . . . . . . . . . . . . . . . . . . . . . . . . . 144.11 Circuit Diagram of Experiment 3 . . . . . . . . . . . . . . . . . . . . . . . 154.12 USB Oscilloscope PS40M10 (USB Instruments) . . . . . . . . . . . . . . . 15

5.1 The distance from the light source (cm) and the generated voltage of eachlight detector (mV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

5.2 The number of Flashed R-LEDs and the Generated Voltages of Light De-tectors (mV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

5.3 The number of Flashed IR-LEDs and the Generated Voltages of LightDetectors (mV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

5.4 Timing Chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205.5 Recorded pulses in Experiment 3-a . . . . . . . . . . . . . . . . . . . . . . 215.6 Recorded pulses in Experiment 3-b . . . . . . . . . . . . . . . . . . . . . . 22

7.1 Optical Absorption of Hemoglobin . . . . . . . . . . . . . . . . . . . . . . . 26

iii

List of Tables

4.1 List of LEDs and PD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74.2 Light Source Krypton Lamp of Experiment 1 . . . . . . . . . . . . . . . . . 8

5.1 The distance from the light source (DFLS cm) and the generated voltageof each light detector (mV) . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

5.2 The number of Flashed R-LEDs and the Generated Voltages of Light De-tectors (mV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

5.3 The number of Flashed IR-LEDs and the Generated Voltages of LightDetectors (mV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

5.4 The number of flashed R-LEDs and current voltages (mV) of light detectors 205.5 The number of flashed IR-LEDs and current voltages (mV) of light detectors 21

iv

Chapter 1

Introduction

In this chapter, the author introduce the overview of this thesis. First, the purposeof this study is described. Then, the contribution of this study is stated. At last, theorganization of this thesis is described.

1.1 Purpose of This Study

The purpose of this study is to examine the actual performance of Light emitting diodes(LED) as photodiodes (PD) and their applications for photoplethysmography (PPG).

PPG sensors which are for measuring arteriolar blood volume (aBV), are one of themost important application examples of PDs. Because aBV is a source of rich informationon biometric technology, it has a wide range of applications, such as pulse oximetry, stressscale, blood vessel valuation, individual identification, and so on.

Although it is little known, LEDs are bidirectional. An LED can work as both alight emitter and a light sensor. That is, LEDs are fundamentally PDs available at a fewpercent of the cost of normal PDs.

One problem is, however, that little is known about the practicability of LEDs as lightsensors. Because all LED manufacturers only suppose that their LEDs are used as lightemitters, the use of LEDs as light detectors is entirely unexpected for them. Therefore,every single datasheet of LEDs does not contain the information about the performancein light detection.

If LEDs provide enough performance in light detection for various applications of PDs,we could use LEDs as low-cost multifunctional alternatives to PDs. For this reason, it isvery significant to conduct the performance assessment of LEDs as PDs.

1.2 Contribution of This Study

This study contributes to understanding basic characteristics of LEDs as PDs. Inorder to assess the performance of LEDs as photoelectric receivers, three experiments willbe conducted.

Experiment 1 Comparison test of the generated electromotive forces by a krypton lamp

1

1.3. ORGANIZATION OF THIS THESIS

Experiment 2 Comparison test of the generated electromotive forces by red LEDs andinfrared LEDs

Experiment 3 Comparison test of the time response characteristics to an artificial lightpulse

The results of these experiments will suggest how we should apply LEDs to biometricsand other fields and raise further questions we should ask.

1.3 Organization of This Thesis

In Chapter 2, the background knowledge of this study about PPG, aBV, LED, andPD is introduced. Chapter 3 describes the detail of bidirectionality of LEDs. In addition,related works of the bidirectional LEDs are introduced. Chapter 4 explains the experi-mental methods of this study. In Chapter 5 and Chapter 6, the results are discussed .Chapter 7 addresses the area for future study and concludes the thesis.

2

Chapter 2

Background

In this chapter, the author would like to introduce basic information about PPG, PDs,and LEDs.

2.1 Photoplethysmography

Photoplethysmography (PPG) is an optical volumetric monitoring method. It is usedto monitor arteriolar blood volume (aBV). Because blood stream influences the changesin aBV, PPG can measure blood pulse wave (BPW).

Although PPG has a wide range of applications, the most common example is pulseoximetry. Because a pulse oximeter, which is the device for pulse oximetry is often servesas another device for other PPG applications, the author would like to explain aboutpulse oximetry first in this section. And then, other applications of PPG are introduced.

2.1.1 Pulse Oximetry

Pulse oximetry is a non-invasive medical method for monitoring the oxygenation of apatient’s hemoglobin. Because one’s need for oxygen is one of the most essential elementsto life, the data of pulse oximetry is necessary in various situations such as intensive care,critical care, emergency department areas of a hospital, and home care.

How Pulse Oximeter Works

The principle of pulse oximetry is based on the difference between oxygenated anddeoxygenated hemoglobin in the red (R) and infrared (IR) light absorption characteristics.IR light is more absorbed than R light by oxygenated hemoglobin. On the contrary, Rlight is more absorbed than IR light by deoxygenated hemoglobin. [1] aaaa [2]

The sensor unit of pulse oximeter, which has two light sources (R and IR) and aphotodetector, is usually placed on a patient’s fingertip or earlobe.

The R and IR light signals pass through the patient’s anatomy are received by thephotodetector. The R/IR ratio is calculated and converted to an SpO2 (Saturation ofPeripheral Oxygen) value.

3

2.2. LEDS

600 700 800 900 1000

1e+

025e

+02

5e+

035e

+04

Wavelength (nm)

Mol

ar E

xtin

ctio

n C

oeffi

cien

t (cm

−1/

M)

Hb02

Hb

660 nm 940 nm

Figure 2.1: Optical Absorption of Hemoglobin

Light Source

An early prototype of pulse oximeter used a halogen lamp as a light source, and thebroad band wavelength were filtered and divided into two narrow band wavelengths: Rand IR.

All current pulse oximeters use a combination of a R LED and an IR LED as thelight sources because of their narrow bandwidth, high efficiency, low temperature, highswitching speed, stable peak wavelength, inexpensiveness, and safety.

Light Detector

Although some semiconductor sensors have been used as the photodetectors of pulseoximeters since the earliest stage, current pulse oximeters usually use a single photodiode(PD) to detect the each radiation from both R and IR LEDs.

Phototransisters (PT) are more sensitive than PDs but are more noisy and less re-sponsive. Because of the low noise characteristic and the high switching speed, PDs aremore suitable for pulse oximeters than PTs.

2.1.2 Applications

2.2 LEDs

2.3 PDs

4

Chapter 3

Bidirectionality of LED

3.1 LEDs as PDs

3.2 Related Works

[3][4]

5

Chapter 4

Methods

4.1 Overview

First, the author compares the generated electromotive forces of LEDs and PDs by akrypton lamp as the light source where the distance between the point light source andeach light detector is variable (Experiment 1). The purpose of this experiment is to knowthe response of them to a light source which has a wide range of wavelengths.

Second, the author compares the generated electromotive forces of LEDs and PDs bya set of 7 red LEDs at 624nm and a set of 7 infrared LEDs (IR-LED) at 940nm as thelight sources where the each of 7 LEDs of both sets is independently controllable by amicrocontroller (Experiment 2). The purpose of this experiment is to know the responseof them to the two narrow bandwidths which are very important for PPG.

Third, the author compares the time response characteristics of LEDs and PDs toan artificial light pulse generated by the same device used in Experiment 2 in order toexamine whether LEDs are suitable for dynamic measurement.

4.1.1 Experimental Materials

In these experiments, 7 Visible LEDs (V-LED), 1 IR-LED and 1 PD were selected asthe experimental materials and each was assigned its ID as Table 4.1.1.

6

CHAPTER 4. METHODS

Table 4.1: List of LEDs and PD

ID Model Manufacture Wavelength Color Luminous 50% PowerIntensity Angle

25R OSHR3131P OptoSupply 624 (nm) Red 25000 (mcd) 30 (deg)18R OSHR5161P OptoSupply 624 (nm) Red 18000 (mcd) 60 (deg)0.5R OSDR3133A OptoSupply 624 (nm) Red 500 (mcd)25Y OSYL3131P OptoSupply 590 (nm) Yellow 25000 (mcd) 30 (deg)0.6Y OSYL3133A OptoSupply 590 (nm) Yellow 600 (mcd)0.5G OSNG3133A OptoSupply 525 (nm) Green 500 (mcd)12B OSUB3131P OptoSupply 470 (nm) Blue 12000 (mcd) 30 (deg)IR OSIR5113A OptoSupply 940 (nm) Infrared 15 (deg)PD TPS703 Toshiba

7

4.2. EXPERIMENT 1

4.2 Experiment 1

The purpose of this experiment is to know the basic characteristics of LEDs and PD(Table 4.1.1) in light detection of a light source which includes a wide range of wavelengthssuch as natural light, room lights, and their mixtures.

Table 4.2: Light Source Krypton Lamp of Experiment 1Model Manufacturer Rated Power Total

Consumption Luminous Flux

LSD100V36W Panasonic 36 W 490 lm

A krypton lamp (Table 4.2) was selected as the point light source because of the easyavailability and the optical characteristic (Figure 4.1).

Wavelength (nm)

Spe

ctra

l Dis

trib

utio

n (%

)

Figure 4.1: Optical Spectrum of Krypton Lamp

In this experiment, the light source was placed on top of a vertical pole on which ameasuring tape was attached and each light detector was fixed on the pole by a plasticclip (Figure 4.2).

The experiment was conducted in a darkroom whose temperature was kept at 15 Cby air conditioning so that the light detectors were not affected by any lights except forthe point light source and the characteristics of the light detectors were not also affectedby the room temperature.

8

CHAPTER 4. METHODS

Figure 4.2: Experimental Setting

The distance between the light source and each light detector was changed at intervalsof 5 cm (from 0 to 100) and the generated voltages at both ends of the light detector weremeasured by a digital multimeter (Figure 4.3) in the way of Figure 4.4.

Figure 4.3: Digital Multimeter MS8221C (MASTECH)

9

4.2. EXPERIMENT 1

1

2

A B C

1

2

A B C

LAMP 40W100V PD DCV-METERMS8221C

Figure 4.4: Circuit Diagram of Experiment 1

10

CHAPTER 4. METHODS

4.3 Experiment 2

The purpose of this experiment is to compare the basic characteristics of LEDs andPD (Table 4.1.1) in light detection of two important bandwidths in PPG, especially pulseoximetry: red (R) and infrared (IR).

To control the quantity of light of the light sources, a set of 7 R-LEDs (0.5R in Table4.1.1) and a set of 7 IR-LEDs (IR in Table 4.1.1) were bundled respectively (Figure 4.5and 4.6) and both sets were driven by a USB LED Flasher controlled by a microcomputer(Atmel Attiny2313).

Figure 4.5: USB LED Flasher and 7 R-LEDs

The experiment was conducted in the same room at the same temperature as those ofExperiment 1. The both sets of light sources and each light detector were placed on theboth inside ends of a lightproof cylinder in order to exclude other light emissions (Figure4.7).

The number of flashed LEDs were changed from 1 to 7 in both R (Experiment 2-a)and IR (Experiment 2-b) conditions. In each level of the flash, the generated voltagesat both ends of the light detector were measured by the same digital multimeter used inExperiment 1 (Figure 4.3) in the way of Figure 4.8.

11

4.3. EXPERIMENT 2

Figure 4.6: USB LED Flasher and 7 IR-LEDs

12

CHAPTER 4. METHODS

Figure 4.7: Lightproof Cylinder for Light Sources and Light Detectors

1

2

A B C

1

2

A B C

OUTIN GND TA48M033F1.5uFGND GNDD-VCCD+GNDUSB

GND

1.5K

22pF 22pF

68

68GND12MHz PD1/TXD13 PD0/RXD2

XTAL24 XTAL15 PD2/INT06 PD3/INT17 PD4/T08 PD5/T19

RESET1

GND10 ICP/PD6 11AIN0/PB0 12AIN1/PB1 13PB2 14OC1/PB3 15PB4 16MOSI/PB5 17MISO/PB6 18SCK/PB7 19VCC 20

TINY2313GND 2K2K2K2K2K2K2K

100100100100100100100LED LEDLEDLEDLED LED LED

GND GND GND GND GND GND GND

NPN NPN NPN NPN NPN NPN NPNPDDCV-METERMS8221C

Figure 4.8: Circuit Diagram of Experiment 2

13

4.4. EXPERIMENT 3

4.4 Experiment 3

The purpose of this experiment is to compare the responsiveness of LEDs and PD toa light pulse in order to examine whether LEDs have enough performance as PDs.

The experiment was conducted in the same room at the same temperature as those ofExperiment 1 and Experiment 2.

4.4.1 Experiment 3-a: R-LED pulse

Based on the results of Experiment 2 (Chapter 5), 2 R-LEDs (25R and 18R), IR-LEDand PD were selected as the subjects in this condition because of their sensitiveness toR-LEDs.

An artificial pulse wave consisting of 2 of 7 R-LEDs (Figure 4.5), whose emission pat-tern is as shown in Figure 4.9 was generated by the same USB Flasher used in Experiment2.

Figure 4.9: Timing Chart

The pulse wave was detected through a reverse bias amplifier circuit (Figure 4.10) inthe way of Figure 4.11 (the resistance value was 10M Ω) and the data was recorded by aUSB Oscilloscope (Figure 4.12) at sampling frequency of 100 Hz.

Figure 4.10: Reverse Bias Amplifier Circuit

4.4.2 Experiment 3-b: IR-LED pulse

Based on the result of Experiment 2 (Chapter 5), IR-LED and PD were selected asthe subjects in this condition because of their sensitiveness to IR-LEDs.

14

CHAPTER 4. METHODS

1

2

A B C

1

2

A B C

OUTIN GND TA48M033F1.5uFGND GNDD-VCCD+GNDUSB

GND

1.5K

22pF 22pF

68

68GND12MHz PD1/TXD13 PD0/RXD2

XTAL24 XTAL15 PD2/INT06 PD3/INT17 PD4/T08 PD5/T19

RESET1

GND10 ICP/PD6 11AIN0/PB0 12AIN1/PB1 13PB2 14OC1/PB3 15PB4 16MOSI/PB5 17MISO/PB6 18SCK/PB7 19VCC 20

TINY2313GND 2K2K2K2K2K2K2K

100100100100100100100LED LEDLEDLEDLED LED LED

GND GND GND GND GND GND GND

NPN NPN NPN NPN NPN NPN NPNTL071

PD

GNDBIAS-V

GND OscilloscopePS40M10

Figure 4.11: Circuit Diagram of Experiment 3

Figure 4.12: USB Oscilloscope PS40M10 (USB Instruments)

The pulse wave was the same as that of Experiment 3-a except that it consisted of 2of 7 IR-LEDs (Figure 4.6). The resistance value of the amplifier circuit (Figure 4.10 and4.11) was changed from 10M Ω to 1M Ω to adjust the sensitivity of the light detectors tothe IR-LED pulse. The data was recorded in the same way as that of Experiment 3-a.

15

Chapter 5

Results

5.1 Experiment 1

Here is the result of Experiment 1. Figure 5.1 was plotted from the Table 5.1.

0 20 40 60 80 100

050

010

0015

00

Distance

Vol

tage

(m

V)

25R

18R

0.5R

25Y

0.6Y

0.5G

12B

IR

PD

Figure 5.1: The distance from the light source (cm) and the generated voltage of eachlight detector (mV)

16

CHAPTER 5. RESULTS

Table 5.1: The distance from the light source (DFLS cm) and the generated voltage ofeach light detector (mV)

DFLS 25R 18R 0.5R 25Y 0.6Y 0.5G 12B IR PD

0 (cm) 1526 1482 646 1627 1010 1450 205 923 5545 (cm) 1480 1474 218 1532 266 312 84 891 470

10 (cm) 1416 1453 86 1036 112 128 36 874 39815 (cm) 1029 1396 40 502 85 68 15 841 36020 (cm) 672 1263 23 320 68 38 7 805 35325 (cm) 424 961 10 218 42 27 3 781 32630 (cm) 294 786 6 158 35 17 2 766 32035 (cm) 233 603 5 118 33 12 1 740 31540 (cm) 181 482 3 87 29 9 1 728 30645 (cm) 142 401 2 71 23 6 0 707 30350 (cm) 115 362 1 56 18 4 0 686 29855 (cm) 93 281 1 45 14 3 0 642 29460 (cm) 86 262 0 40 11 2 0 583 29265 (cm) 71 245 0 36 10 2 0 520 28970 (cm) 53 203 0 32 7 1 0 416 28875 (cm) 51 184 0 28 6 1 0 350 28780 (cm) 46 166 0 24 5 0 0 284 28285 (cm) 45 126 0 20 5 0 0 269 28290 (cm) 44 119 0 17 4 0 0 238 28195 (cm) 42 104 0 15 2 0 0 200 281

100 (cm) 38 98 0 11 0 0 0 179 281

17

5.2. EXPERIMENT 2

5.2 Experiment 2

Here are the results of Experiment 2-a and 2-b.

5.2.1 Experiment 2-a

Figure 5.2 was plotted from the Table 5.2.1.

1 2 3 4 5 6 7

050

100

150

200

250

300

Number of Flashed R−LEDs

Vol

tage

(m

V)

25R

18R

0.5R

25Y

0.6Y

0.5G

12B

IR

PD

Figure 5.2: The number of Flashed R-LEDs and the Generated Voltages of Light Detectors(mV)

Table 5.2: The number of Flashed R-LEDs and the Generated Voltages of Light Detectors(mV)

No 25R 18R 0.5R 25Y 0.6Y 0.5G 12B IR PD

7 183 289 19 2 0 0 0 115 1816 157 213 15 2 0 0 0 96 1755 133 161 14 1 0 0 0 79 1674 108 140 12 1 0 0 0 69 1653 69 74 9 1 0 0 0 38 1532 45 34 6 1 0 0 0 26 591 15 13 3 1 0 0 0 14 30

5.2.2 Experiment 2-b

Figure 5.3 was plotted from the Table 5.2.2.

18

CHAPTER 5. RESULTS

1 2 3 4 5 6 7

020

040

060

080

0

Number of Flashed IR−LEDs

Vol

tage

(m

V)

25R

18R

0.5R

25Y

0.6Y

0.5G

12B

IR

PD

Figure 5.3: The number of Flashed IR-LEDs and the Generated Voltages of Light Detec-tors (mV)

Table 5.3: The number of Flashed IR-LEDs and the Generated Voltages of Light Detectors(mV)

No 25R 18R 0.5R 25Y 0.6Y 0.5G 12B IR PD

7 0 0 0 0 0 0 0 891 3436 0 0 0 0 0 0 0 890 3395 0 0 0 0 0 0 0 889 3334 0 0 0 0 0 0 0 881 3253 0 0 0 0 0 0 0 873 3142 0 0 0 0 0 0 0 869 3021 0 0 0 0 0 0 0 814 277

19

5.3. EXPERIMENT 3

5.3 Experiment 3

Here are the results of Experiment 3-a and 3-b.

Table 5.4: The number of flashed R-LEDs and current voltages (mV) of light detectorsClock N. of 25R 18R IR PDCount LEDs (mV) (mV) (mV) (mV)

0 0 0.04 -0.04 -0.03 0.091 1 0.35 0.9 0.19 0.492 1 0.41 0.79 0.14 0.433 1 0.35 0.9 0.19 0.54 1 0.43 0.79 0.14 0.435 1 0.34 0.9 0.19 0.496 2 0.8 1.46 0.34 0.827 2 0.75 1.56 0.29 0.918 2 0.8 1.46 0.34 0.829 2 0.73 1.56 0.3 0.910 2 0.8 1.46 0.34 0.8211 2 0.74 1.56 0.29 0.912 1 0.41 0.79 0.2 0.4213 1 0.34 0.9 0.13 0.514 1 0.42 0.79 0.19 0.4315 1 0.35 0.89 0.13 0.4916 1 0.41 0.79 0.19 0.4317 0 -0.02 0.06 -0.02 0.09

Table 5.4 is the raw data of Experiment 3-a and Table 5.5 is the raw data of Experiment3-b. ”N. of LEDs” means the number of flashed R-LEDs or IR-LEDs.

• When both LED 1 and LED 2 are off, the value is 0.

• When LED 1 is on and LED 2 is off, the value is 1.

• When both LED 1 and LED 2 are on, the value is 2.

(See Figure 5.4)

Figure 5.4: Timing Chart

20

CHAPTER 5. RESULTS

Table 5.5: The number of flashed IR-LEDs and current voltages (mV) of light detectorsClock N. of IR PDCount LEDs (mV) (mV)

0 0 0 01 1 0.43 0.362 1 0.43 0.363 1 0.43 0.364 1 0.42 0.355 1 0.43 0.366 2 0.82 0.657 2 0.82 0.658 2 0.82 0.659 2 0.82 0.6410 2 0.82 0.6511 2 0.82 0.6512 1 0.43 0.3613 1 0.43 0.3614 1 0.43 0.3715 1 0.43 0.3716 1 0.43 0.3617 0 0.01 0

Figure 5.5 was plotted from Table 5.4 and Figure 5.6 was plotted from Table 5.5. Thevalues of ”N. of LEDs” are not actual voltages but they are plotted on the same graphfor the descriptive purpose.

0 5 10 15

0.0

0.5

1.0

1.5

2.0

Clock Count (x10ms)

Cur

rent

−V

olta

ge (

mV

)

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

N. of Flashed R−LEDs

R25

R18

IR

PD

Figure 5.5: Recorded pulses in Experiment 3-a

21

5.3. EXPERIMENT 3

0 5 10 15

0.0

0.5

1.0

1.5

2.0

Clock Count (x10ms)

Cur

rent

−V

olta

ge (

mV

)

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

N. of Flashed IR−LEDs

IR

PD

Figure 5.6: Recorded pulses in Experiment 3-b

22

Chapter 6

Discussion

In this chapter, the author would like to consider all the factors of the results of theexperiments on the whole.

6.1 Experiment 1 and Experiment 2

Figure 5.1 shows the differences in the characteristics in light detection of a broadbandpoint source. We see from the figure three points: (a) the relation between the peakvoltages and the luminous intensities in the same wavelength (b) the relation between thedown curves of the voltages and the 50 % power angles in the same wavelength (c) therelation between the down curves of the voltages and the wavelengths

In regard to (a), at the length of 590nm and 624nm, one whose luminous intensity ishigher generated a higher voltage at the peak (25R,18R,0.5R and 25Y,0.6Y). Therefore,we can infer that the peak voltages and the luminous intensities in the same wavelengthare proportional.

In regard to (b), at the length of 624nm, one whose 50% power angle is greater hadmore gently down curve of the voltages (25R,18R). Therefore, we can infer that thegentleness of the down curve of the voltages and the measure of the 50% power angles areproportional.

In regard to (c), one whose wavelength is longer had more gently down curve of thevoltages (All LEDs). Therefore, we can infer that the gentleness of the down curve of thevoltages and the wavelengths are proportional.

Figure 5.2 and Figure 5.3 show the differences in the characteristics in light detectionof two narrowband light sources. We see from the figures two points: (d) the relationbetween the peak voltages and the 50 % power angles in the same wavelength (e) therelation between the wavelength of emitters and detectors

In regard to (d), at the length 624nm, one whose 50% power angle is greater generateda higher voltage at the peak (25R,18R) in R-LEDs detection. Therefore, we can infer thatthe peak voltages and the measure of the 50% power angles in the same wavelength areproportional.

Although the inferences based on (a) and (d) are seemingly incompatible, this is how itmight be. The light sources and the space around the light detectors in Experiment 1 andExperiment 2-a are totally different. Because krypton lamps are less directive than LEDs,

23

6.2. EXPERIMENT 3

the positional relations between the krypton lamp and the light detectors are not critical.However, in detection of LEDs, the positional relations are one of the most importantfactors because if the face of a light detector is not within the range of the maximumpower angle of the LED, the detector lost the greater part of the quantity of light emittedfrom the LED. That is, the detected quantity of light from each LED of 7 by the lightdetector are different. In addition to that, the lightproof cylinder (4.7) reflects the lightinside. That is why 18R, which has a greater power angle than 25R, might have detectedmuch more quantity of light and generated greater voltages than those of 25R from N. ofLEDs 3 to N. of LEDs 7 in Experiment 2-a.

In regard to (e), one whose wavelength is nearer to that of the light source detectedthe light more (25R,18R,0.5R in Experiment 2-a, IR in Experiment 2-b) and one whosewavelength is higher than that of the light source detected the light source (IR in Exper-iment 2-a) but the lower almost did not. Therefore, we can infer that the light emissionwhose wavelength is higher than that of the light detector is hardly detected. The in-ference raises a question about whether a light emission whose wavelength is lower thanthat of the light detector is always detectable or not.

To verify the preferences in this chapter, we need to blueprint more advanced experi-ences.

6.2 Experiment 3

Figure 5.5 and Figure 5.6 show the time response characteristics of LEDs and PD indetecting a light pulse. From the figures, we can see no difference in terms of the timingof the changes of the current voltages at the sampling frequency of 100Hz.

The differences of experimental conditions between Experiment 3-a and 3-b are thelight sources and the resister value of the circuit. A higher resistance value amplifythe signals more greatly at the cost of noise reduction. That is why the waveforms inFigure 5.5 have zigzag lines compared to those in Figure 5.6. Considering the fluctuationby noise, however, the detected pulses are coincident with each other except that theabsolute values are different. The results show the frequency of 100Hz are too low toclarify the differences in responsiveness.

24

Chapter 7

Conclusions

7.1 Conclusions

The result of Experiment 1 showed that the generated voltages of half of LEDs werenearly triple of that of PD at their peak bands but much lower at the others. The result ofExperiment 2 showed that both visible LEDs (V-LED) and IR-LED detected the 624nmradiation but V-LEDs did not detect the 940nm radiation while IR-LED detected it.The result of Experiment 3 showed that there were no differences in the time responsecharacteristics of LEDs and PD at frequency of 100Hz except that V-LEDs did not detectthe 940nm pulses.

At first glance, the fact that V-LEDs does not work in detecting IR-LEDs at 940nmimplies that it is impossible to use them as light detectors of pulse oximeters. However,there is a possibility that some R-LEDs whose wavelength are over 700nm might be ableto detect some IR-LEDs whose wavelengths are shorter than 940nm as is the case withExperiment 2-a where 25Y slightly detected R-LEDs, whose length (624nm)is longer thanthat of 25Y (590nm).

In addition to that, Figure 7.1 shows that it is not always necessarily to use IR-LEDsas one of light emitters of pulse oximeters because there is another band except the IRband where the red line of HbO2 is above the blue line of Hb around 470nm, which is thewavelength of blue LEDs.

Although The first pulse oximeter was invented in 1970s, the practical blue LEDs werenot available in the next few decades. As a result, most manufacturer adopted R-LEDsat 660nm and IR-LEDs at 940nm because of the availability of them in those day.

Considering that some pulse oximeters have blue LEDs as the light emitters insteadof IR-LEDs in these days, the disability of V-LEDs to detect IR-LEDs does not seem tobe critical. However, we need to confirm whether V-LEDs can detect blue light around470nm or not.

In regard to the time response characteristics of LEDs, the author would now like togo on to develop the experimental methods and devices in order to conduct more strictperformance assessments.

At this point, the results of this study suggest that although there are some limi-tations, LEDs are useful light sensors in various application fields including biometrics.What is more, it is clear that LEDs have advantages over PDs in terms of cost and

25

7.2. FUTURE WORKS

multifunctionality.

400 600 800 1000

1e+

021e

+03

1e+

041e

+05

1e+

06

Wavelength (nm)

Mol

ar E

xtin

ctio

n C

oeffi

cien

t (cm

−1/

M)

Hb02

Hb470 nm

660 nm 940 nm

Figure 7.1: Optical Absorption of Hemoglobin

7.2 Future Works

The future direction of this study will be one that encompasses both measurementand implementation approaches.

In the measurement approach, the author would like to go on to a more detailedexamination of the performance of LEDs as PDs. In the implementation approach, theauthor would like to develop an actual PPG for evaluation of LEDs.

26

References

[1] Moyle, John TB, “Pulse Oximetry Second edition”, London: BMJ, 2002.

[2] Webster, J G, “Design of Pulse Oximeters”, New York: Taylor & Francis Group, 1997.

[3] Prahl, Scott, “Optical Absorption of Hemoglobin”, [Online]http://omlc.ogi.edu/spectra/hemoglobin/

[4] Dietz, Paul et al., Very Low-Cost Sensing and Communication Using BidirectionalLEDs, UbiComp 2003, Seattle, Washington, October 12-15, 2003.

27