Abstract and Figures

Heart rate constitutes one of the most important physiological parameters for humans and is linked to the prognosis of several diseases. Moreover, its unobtrusive measurement is necessary in several real-life applications, such as remote monitoring of elderly or infants, or even whiledriving or physical exercising. Most state-of-the art methods are able to achieve significantly high values of accuracy. Thus, focus should be given to user satisfaction, an aspect which is typically underestimated or even ignored. Therefore, in this paper we present a user evaluation study of 3 non-invasive methods for heart-rate measurement. More specifically, we evaluate an implementation of a state-of-the-art algorithm, which is based on remote video measurementstaken by a typical camera and compare it to (1) a low-cost bracelet with an integrated photoplethysmography sensor and (2) a freely available Android mobile app which uses the phone's camera and flash. All three methods combine a) low-cost, since they do not require any special kind of equipment;and b) non-invasiveness, as at most they require slight physical contact with subjects. They were all thoroughly evaluated focusing on user satisfaction. User evaluation was performed by real users using questionnaires to assess user satisfaction
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
User Evaluation of Low-cost, Non-
invasive Methods for Heart-Rate
Measurement
Eirini Mathe


 !"
"#
$%
"&'
()*+
,-#%
,*./012
-
3#*
Evaggelos Spyrou
,"
)""#
$
4."$10/1/
-
3
Abstract
5"
"#*#
".#6..
."
"#".7
#.6"#
.'."
$"#."
7##.#$"
7.#"1
..#"6
'7.""
7#.#
*# /!7
7#
# !".##7
8#9#!7
#:*#":;
#!..:
7<$7
.#""4.7
"#:
"
Keywords

.
=/04>,"#,*##
""7*"#7".##
###"'#.##"
'$7.#:'
#?"4@@??777

=/04>,"#,*##
""7*"#7".##
###"'#.##"
'$7.#:'
#?"4@@??777
3
Introduction
$##""
.#"#.##"
"." AB!$7
# >-!57.#:.
#.#"7#"
#"C"">-*",.#
B!7'#""
"6'#"7
7#&D
77
###."#
7*"##
77C"
##"#6
'7"##
E* E*)#F-/1!$#*"7
7#"<""#
##.G#
"#""!7:#7
 ,,-!;#!".##$
::#7##9>)$
###6.
""7"7
Photoplethysmography
G###
:*7(+7###.
7."*" B!
$:#.*'#D.
"##."#97.$
#7#"#$
#,,-#:7#"
7#$">)
*"7#$"9#
##&"
#":"".#
9"#*"<8'7#
.#.
Problem Statement
"#"##
"757.7#H7*"
7"###%..#
.7#""*#"#I#
..6.."
..J#
##7"7.$
."$"#"
"C"7"."
""##77"##7
##:"##"
*7##.*7
##"##7"
#""#7

$.""##"7##
7.#...7
#.??$###.
Journal of Usability Studies /0
77.###.
:"#:#""7#E
##7###
.."#"77"#"
7A
.#
:7%.##.
.#J.$"
"##..7:.#7
Heart rate measurement using pulse oximeter
G#># >-!##(###+"
#"".#.#57.
.#.7#".
7#""###.
.#"##7
."'##
..#.
.#*#.#,,-"##
'
Heart rate measurement from camera
$.*"
$<"""7@!.#
;!""";!
 <"7*
##7*#!;#!'
.D"7":;!"
7.#, ,!#
##.### KD!;#"!
#7#".
####"#
"7*#7##
"$*#C B!77#"*7
.#:6.7*",6)D#,# /!*#
".'#"".#
<".7*.####4#
#*""##-#E".#"
":77H######
 ! /LL2!$##
#"AA$" AA$!
##77##7"":"

$'7.#7#7*"E*
E*/1!*"77#
G##6'
##.#
7<#97"#7.$
("7+$#H#"7@!
"""#K FK2!;!
""# ##"#
H!;!""#&
#7"###'#.
..-#A
$* -"$!# &F$/LL2!;#!. y
!""#.*#M#
$ M$! FM#/LN/$FM#/LL/!%"
y#/)""#
";!#">-7
05H##"
"#;"!"#720#/
Journal of Usability Studies /0
? 7B0#5H!'
":".;!":7B05H#
##'0#
E7'#;!.##",#
"#";!"":"#
 .":77!''.
",
PPG-based heart rate measurement using a mobile phone
)"7#.'#".7#"7
#:D#$.:
## ,,-!7*#"
#"<8'6'
::'#"
77#8#9>) A/!$
#."7*
#: !7
77'"#

'7*7#",EC#6
/!7##'7"7.""
#*"6.##
-#M*#% //!7##"
#.7'"7"$'#
#."####
7$""#".#
">.####.#
,,-67*7"
A,O#O /0!#
"NPN"###$
#.
$#7#E7'.
"##.
77.###7
PPG-based heart rate measurement using a smart-bracelet
.#7&.#..
#"*..
#""".#
6#7"8#.
$7
":#7.>))#&.
7"H##*
"#.#H##G
"(+7"$ $!$
#."4*#"*
.'
67#.#.
.#"7%?##
#.#*C"7#?7#
##77;
7#7#Q.$,,-
;">)"*####
####
##78
Methods
$'##7#"
C7#.""
57.7"
Journal of Usability Studies /0
##"77#""7#&7##
7.#77.D
'7."7##
""4"."
7#77R/S
7#/$7""
$7###""
$"7"7@.""!
7;!,,-
#;#!,,-#6.7.
"#
""
(a) (b) (c)
Figure 1. !E7<,,-; !,'"
; !,#8:
G###"E*,10/G
#C""#"*#
* T#*FMN!6.7#&,"#
' ,#//!",7#* KCF
,/2!6.7#77 01!5)
B#00".#E7#""/NPB
1"#7##7
.#U7UBLV, N61-5H!
7/-E"6#7U00267N-E"6A
#"7###".
4".##-,7#
(##5@5F,6+/*"8
##9:8'""7#
A/!%7./06
#7# -,#.#!#."21?0
"NG#7#
% ##/07"/0!".
E#"$8#"/LP/N1
"A7.77#
6E#/&7 /0!,,- A
/!EU6"#
/@?????#W#X###
2@??777??#?
Journal of Usability Studies /0
7#""7###
8
C*7#D77#
 "7#7!#' "!$'
7J7#7."
..# N"?/!$7##?###
??D.#72##"
#.7"#7
#""#*77#" 8
7##<#".!$#""
7710 A/!$'7#"
A/!$77###"8
#7#
.#.,.7#77
",7##
*."#:#
"4#""7
##""7#A7:#/"

Results
User Satisfaction
A."7#:#.7
*7#.7#'#"
6'7#:##"7*"
)F% /LNN!## /LL/!$:##$/#.##
"8:.7"""
"#"%7#"*"./07
7"7""#"# */L1!4
"7"H.7
##"7#6.7*#
#""7#"##
:""#
A7":##"E7#
#.###. EB
7/LL17/2!G#2:.Y.#'#
.##:#.
"/0":.$7#"
Q7###$/V
Journal of Usability Studies /0
Table 1$:77#"."
#""#.#
7*
Question Answer
Usefulness 12345
/)'#
?"W
% Z Z Z Z Z 
)?
#W
% Z Z Z Z Z 
1)'#
?W
% Z Z Z Z Z 
2)'#
?"W
7 Z Z Z Z Z "
0)*
?#
.W
)' Z Z Z Z Z )'
Ease of use 1 2 3 4 5
V)*
?W
)' Z Z Z Z Z )'
B)'#
?
"#W
)' Z Z Z Z Z )'
N)'#
?#W
)' Z Z Z Z Z )'
L)*
?#
7
.W
)' Z Z Z Z Z )'
/)*
?
"#.W
)' ◯◯◯◯◯)'
Ease of learning 1 2 3 4 5
//
7?W
)' Z Z Z Z Z )'
/)#
?W
)' Z Z Z Z Z )'
Satisfaction 1 2 3 4 5
/1)*
?
W
)' Z Z Z Z Z )'
/2G##
?"#W
)' Z Z Z Z Z )'
/0'#
?W
)' Z Z Z Z Z )'
A7":##"E7#
#.###. EB
7/LL17/2!G#2:.G#
#.(##+7.##"
#."6.:.
#'#.##:
#.7/02:.$7"
Q7##$0 #!"#$V
Journal of Usability Studies /0
#!"##$B #!"

G"#A#:.#/0
":"##."##"
$A###'#D##
"# χ !X/1/1p = /!#DA#7
"7#7#G#*6*"##
7"###$#1.$
1'#D7.#7#(z
XL0/p X1!#7# z X12V2p X/!
C##D7#7
' z X//Lp XLV!$###
777#57.7
#."####
77##"#
Table 26*"##G#*
Ranks
method mean rank
camera /N
mobile app /1N
bracelet 20
Table 3G#*7"##
#7#7##
"p R0
method A
method
Bzp
mobile
app
camera L0/ 1
bracelet camera //L LV
bracelet mobile
app
12V2 /
G#&4& &4&!: E*
/LLV!&4&7##/:"##"
##.####7
7#/E&4&:'
#*#"#""
#$"7"
:##"""#"
#
,Q&4&#$BL$#
#. 7L0S'#.!"BN1N[V17B
[2B177BB0[B10E'#..#7
##77< #!%C&4&
"7#E")#"%C$/
#/1$##'D"). F 1N!X/VpR/!
$E"##'#D7#
pR/!#7# p XV!7"
#$7'#D7#
Journal of Usability Studies /0
$.#"#.&7.## &F
7/V!7.E\#
""7: ".!7#D
"&4& ".#!$##
.# "!:#"#
"H#D":#
#D$"7"7
:#D#"#
":
$.7##"]]
:":V: ])*?
W]!#1&4& ]7]!7
<%C77##.Y#6###
'D"6# F 1N!X0B1pXB!D"
Y F //L!X/NLVpX/N0!#6#Y F 1N!
X/2LpXNV!$":##7@
@C:V@20;&4&1@22
E@C:V@210;&4&1@2/0
6@C:V@20;&4&1@2
$"D"6#D"Y7'
7.#Y.#>"#.
###t$7@
$#>"7'"6
t /L!X2122pR/!
$6#E#D#' pR/!##
."pR0 t /L!X/N1/pXN1!
#E7'#D t /L!X/1pX!
$.##"#"#D
77:Q">7.
:7#'#D#
Accuracy of heart rate measurements
#"#7
*##$2$.#."
 #"7#"-6)DF*/V!7##7
"G'77"7
"G#""< "
7#!7##'."
. '*!;#"/
7E7#.7
7"#'"
Table 4>"##
%#E6 E,6!
camera mobile app
mean std mean std
error /@ 01 21 12N L1
@ B/1 0N 1/ V2
Journal of Usability Studies /0
Conclusion
7*7##."7..#"
"7&'7.#!#
:##;!7*#
##"####>);#!7
7##7:#7,,-G"#
.#"#4"
.""'".7"7.H
"7
A<##Q#
"##"<"###
."877:?"7#
%..72$#"#7
"#"$<#7*"#"
:7##"G"##
.*#7#".7
"7#J"I7#"
###"
"7##7*7#G#H
"#""#
A7####.#
7'#7##%###
.&4&
&#"7#"##$##
..7#.#77"
$"7..##7
##.##
#"7.'#7"
:#"#7#"
.""C"#.#7
###7##"';#"
'7##8#
.'7
##:G#
7#####.57."###
#"G#7.."
877".D
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7"###"7
:7""####
##.##."G"<
##."7"*#"
'#79>)7$
7:" N/!7.'
#<"&"7#
#7#J"
C#7*"#7""#
"#C"#7#:#7
#"7#7####

G#&4&"#7:
""#)7#
7"###7"".#
:"""#
"#&4&:.##"7
7#.:#<"":#
#D7"
.##":77#"
Journal of Usability Studies /0
7.H#&4&##7"7##
""##
,""7*##""#7
""&I%$>>&&<7#$
###...#'#"
"## ,M
&F&/0!6'"7#
.##"#.#
##C7*7&I%$>>&&7"
#."##"#"""
#.7"."#.#
7..
""##
""##7#.
6'"##75)
###"$#."C
7."###
7#"##
"..#:..
"#"
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!###"7
&""##*"
A###.""
##*7#*##
"*%7#:
";.##7#(.+.
6."7.##;
#####C"7#:
"#"#$.#."
.#.
Tips for Usability Practitioners
A7D."7'#"
#'7"
#?7@
7#7#.'##77
"
"77.
#"
6*""#6*#
"#"#"
G.### "
!
E""*.7*;
.*#
E#"#
"
,*7"##"
.#"7:*.7;
.#
Journal of Usability Studies /0
E#7:7'#;.#
##"#
G:# #<"!
#D
Acknowledgements
G*7#"7*<&I%$>>&&^^..$#
#"$ $!##88 6&00/!
7##^^"&).#
$&88"##C,^^.
>#.88 %&A/2!#'#-#
>4 >).A#!>6.#"#"
5A#"#. 5A!#-&"
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Physiological Measurement, 28(3)/
E*-)#AF-K /1!)"#.#
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,#C@>>>
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E#*-FM N!C@.7C
C86#
E*K /LLV!&4&:*##,K#E$FE
G# >#!4>.# /NLb/L2!#>#@$
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- B!#.#Journal of the American College
of Cardiology, 50(9)N1N1
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8.# Proceedings of the 6th IEEE International Conference on
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.HH# Proceedings of the IEEE
Conference on Computer Vision and Pattern Recognition Workshops 2V01!
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7K /LL1!6@6###D#.#'
.International Journal of Human‐Computer Interaction5(4)1N11L
7K /2!4@#c##International Journal of
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at: http://www. scipy. org
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.Proceedings of the 7th International Joint Conference on Arti>cial
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2010 Annual International Conference of the IEEE 02NN02L/!E@
>>>
,OG5FOI$ /0!>"."
Computational and mathematical methods in medicine,
0/VNV.
,-M)&>F& /0!&I%&$5&@"
""In Proceedings of the 19th Panhellenic
Conference on Informatics 21N222!-@6
,6O6)D)KF,#G /!%##
.###Optics Express, 18(10)
/BV/BB2
&KF7K /V!Quantifying the user experience: Practical statistics for user
research ##!#6@6M"
&KF$ /LL2!-#"*In Computer Society Conference on Vision
and Pattern Recognition 0L1V!&G@>>>
$*FCI B!5#Medical
Engineering & Physics, 29(8)N01N0B
$FM#$ /LL/!Detection and tracking of point features $64&
L//1!,,@64.
,FK6K 2!"#International Journal of Computer
Vision, 57(2)/1B/02
Journal of Usability Studies /0
Appendix
Appendix Table 147".""
#": :@
:!
Our Questionnaire - Heart rate measurement from camera
q user
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
110000220000002020001
21001220200020020002
321222222222020022201
402020222222002010201
500020220002002020202
620020220000022020201
712012221022222010201
8/220212222202220220
9201122002120000220
10 1202220021122022001
11 20200200002000022001
12 00000002022022202002
13 1200221220022201201
14 10202220020210022001
15 10000120020212202021
Appendix Table 247".""
"": :@:!
Our Questionnaire - Heart rate measurement using a mobile phone
q user
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
120222220200221202122
212022022200222222121
312021102211122212112
41000212211222221211
502200220022112222112
622220220022210221122
7222201/011022222212
820202112221022111121
92201210221212212102
10 1202112011211212122
11 20002222202122121122
12 00002022121121212112
13 2022220/121222112
14 20202221221121121122
15 10002221121112111122
Journal of Usability Studies /0
Appendix Table 347"."""
": :@:!
Our Questionnaire - Heart rate measurement using a smart-bracelet
q user
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
120020220000000202000
220122000202212020200
31222222002220200000
422020100122102212021
500200202102020020020
612220000122002220200
72222110200202100202
800221120120202220220
922012110222021022202
10 12220122220022022200
11 20200222200020220022
12 00000020202000000020
13 20122210122201021221
14 20002222222220020202
15 00200212222200202202
Appendix Table 4&"".""
# Y@
:.Y/@0SY@0Sb#Y1@B0SY2b!#"7
:
Heart rate measurement from camera
question
statistics i-th quartile (Qi) answer
mean median mode Q1 Q2 Q3 Q4 1 2 3 4 5
12V 0 0 2 0 0 0    2 /2
2210 0 0 2 0 0 0  /  V //
32/ 2 2 2 2 2 0    2 /2
421 2 2 2 2 0 0  / N
52V 0 0 2 0 0 0    N /
620 0 0 2 0 0 0   / N //
72 2 2 1B0 2 20 0 0 / 0
81L 2 2 2 2 2 0 / / / /1 2
920 2 0 1B0 2 0 0   1 B N
10 2 2 2 1B0 2 0 0  / 2 L V
11 2V 0 0 2 0 0 0   / V /1
12 2V 0 0 2 0 0 0    N /
13 2 2 2 1B0 2 0 0  / 2 L V
14 21 2 0 2 2 0 0   1 N L
15 20 2 0 2 2 0 0   2 B L
Journal of Usability Studies /0
Appendix Table 5&"".""
" Y@:.Y/@0S
Y@0Sb#Y1@B0SY2b!#"7:
PPG-based heart rate measurement using a mobile phone
question
statistics i-th quartile (Qi) answer
mean median mode Q1 Q2 Q3 Q4 1 2 3 4 5
12/0 2 2 2 2 20 0  /1 0
220 2 2 2 2 2 0 1 /1 2
31V0 2 1 1 2 2 0   L L 
41B 2 2 1 2 2 0  / B L 1
520 2 2 2 2 20 0 2 // 0
620 2 2 2 2 2 0 1 /1 2
71B 2 2 1 2 2 0 / / 2 // 1
81B0 2 2 1 2 2 0   N L 1
91B0 2 2 1 2 2 0 / V / 1
10 100 10 1 1 10 2 0  / L N 
11 2 2 2 2 2 2 0 2 / 2
12 1L 2 2 1 2 20 0   B N 0
13 120 2 2 1 2 2 0 / 1 2 / 
14 1B0 2 2 1 2 2 0 B // 
15 1V0 10 1 1 10 2 0  / B 1
Journal of Usability Studies /0
Appendix Table 6&"".""
" Y@:.Y/@0SY@0Sb#Y1@
B0SY2b!#"7:
PPG-based heart rate measurement using a smart-bracelet
question
statistics i-th quartile (Qi) answer
mean median mode Q1 Q2 Q3 Q4 1 2 3 4 5
12B 0 0 2 0 0 0    V /2
2210 2 2 2 2 0 0    L L
320 2 2 2 2 0 0 / / / N
420 2 2 1B0 2 0 0   0 L V
5200 0 0 2 0 0 0   / B /
6210 2 2 2 2 0 0    L L
72/ 2 2 2 2 0 0  / 1 L B
82 2 2 2 2 0 0 1 / B
920 2 2 2 2 20 0 2 // 0
10 2 2 2 2 2 0 0  / V
11 220 2 2 2 2 0 0  // L
12 2N 0 0 0 0 0 0    2 /V
13 1L 2 2 1 2 2 0 V / 2
14 210 2 2 2 2 0 0  /1 B
15 210 2 2 2 2 0 0 / // N
Appendix Table 747".""
#"&4&: :@
:!
SUS - Heart rate measurement from camera
q user
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
102220021202002001202
21///1/
321020000022220220202
41/11/11111
521222121122222121222
611//////1
702202000020220202222
8/1/1////
920002200012202122021
10 /1/11///1
Journal of Usability Studies /0
Appendix Table 847".""
""&4&: :@:!
SUS - PPG-based heart rate measurement using a mobile phone
q user
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
122222121022222122222
21111////
312222002222221222222
41//1
511111112222222211
61111111111
720202121222222222222
8212/212121/21
922222121222002012021
10 ///1/1/1/1
Appendix Table 947"."""
"&4&: :@:!
SUS - PPG-based heart rate measurement using a smart-bracelet
q user
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
122222000202020121202
21/1/////1
320202021222002222122
4121221
512112112112122222
611122221/1112
722211112121112220001
821111/////12
922021222012022222222
10 //1//
Journal of Usability Studies /0
Appendix Table 10>"#."&4&:
Method Mean Std. Error
95% Con<dence Interval
Lower Bound Upper Bound
Camera BN1B0 /12N B0001 N//LB
Mobile App B /0N VBBNB B/1
Bracelet BB0 /V21 VB1/ B2/L
Appendix Table 11"7%C7< #!
"7"&4&:6##
Source
Sum of
Squares
Degrees
of
Freedom
Mean
Square F P
Method Sphericity
Assumed
N0NL0N  2L2BL /V 
Greenhouse-
Geisser
N0NL0N /BL1 2BL// /V 
Huynh-Feldt N0NL0N /LVN 21V00L /V 
Lower-
bound
N0NL0N / N0NL0N /V 
Error
(Method)
Sphericity
Assumed
/L0N 1N 12N2
Greenhouse-
Geisser
/L0N 12V1 1N1
Huynh-Feldt /L0N 1B1N2 12V2V
Lower-
bound
/L0N /L VN/VL
Journal of Usability Studies /0
Appendix Table 12E"77#."7"
&4&:6#D#7d'0.
Method A Method B
Mean
Di?erence
A-B Std. Error Sig.
95% Con<dence
Interval
Lower
Bound
Upper
Bound
Camera Mobile
App
N1B0d /V0  2/V1 /0NB
Bracelet BV0d /B V 21 /1B
Mobile
App
Camera N1B0d /V0  /0NB 2/V1
Bracelet B0 /BVL / 01L1 1NL1
Bracelet Camera BV0d /B V /1B 21
Mobile
App
B0 /BVL / 1NL1 01L1
Journal of Usability Studies /0
About the Authors
Mathe Eirini
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Spyrou Evaggelos
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#
Journal of Usability Studies /0
Article
The global pandemic of Corona Virus Disease (Covid-19) that hit Indonesia since March 2020 has changed the face-to-face system from offline to online. Video Conference (VC) becomes an alternative choice for delivering material, both learning and working. VC is a technology that allows users to hold face-to-face meetings at their respective places simultaneously. Various VC apps are becoming increasingly popular these days, such as Google Meet, Zoom, Youtube, Webex, Skype, GoTo Meeting, and Big Blue Button (BBB). This study is intended to provide a usability test of VC applications (such as Zoom, Google Meet, and BBB) and provide recommendations for VC as an online conference media based on user preferences. The usability measurement technique used is the System Usability Scale (SUS) and the USE questionnaire. The results of this study show that Zoom has the highest usability value compared to Google Meet and BBB, Google Meet is ranked second and BBB is ranked third.
Article
Full-text available
Heart rate variability (HRV) is a useful clinical tool for autonomic function assessment and cardiovascular diseases diagnosis. It is traditionally calculated from a dedicated medical electrocardiograph (ECG). In this paper, we demonstrate that HRV can also be extracted from photoplethysmograms (PPG) obtained by the camera of a smartphone. Sixteen HRV parameters, including time-domain, frequency-domain, and nonlinear parameters, were calculated from PPG captured by a smartphone for 30 healthy subjects and were compared with those derived from ECG. The statistical results showed that 14 parameters (AVNN, SDNN, CV, RMSSD, SDSD, TP, VLF, LF, HF, LF/HF, nLF, nHF, SD1, and SD2) from PPG were highly correlated ( r > 0.7 , P < 0.001 ) with those from ECG, and 7 parameters (AVNN, TP, VLF, LF, HF, nLF, and nHF) from PPG were in good agreement with those from ECG within the acceptable limits. In addition, five different algorithms to detect the characteristic points of PPG wave were also investigated: peak point (PP), valley point (VP), maximum first derivative (M1D), maximum second derivative (M2D), and tangent intersection (TI). The results showed that M2D and TI algorithms had the best performance. These results suggest that the smartphone might be used for HRV measurement.
Conference Paper
The emerging interest in the Internet of Things (IoT) research is being motivated by the high expectations of market analysts and researchers suggesting that the IoT will transform the everyday lives of consumers, end--users, professionals, businesses, the public sector, and the global economy in the broader sense. SYNAISTHISI is an integrated platform that allows humans, systems, machines, and devices for the creation and management of services. It covers multiple tangential directions of the IoT research, enabling it to feature a wide spectrum of applicability to research and business areas. In this work, we present the architecture and an implementation of SYNAISTHISI platform based on the existing web--technologies and protocols that prevail in the current IoT ecosystem. Finally, a Smart Meeting Room use case is demonstrated.
Article
The factorization method described in this series of reports requires an algorithm to track the motion of features in an image stream. Given the small inter-frame displacement made possible by the factorization approach, the best tracking method turns out to be the one proposed by Lucas and Kanade in 1981. The method defines the measure of match between fixed-size feature windows in the past and current frame as the sum of squared intensity differences over the windows. The displacement is then defined as the one that minimizes this sum. For small motions, a linearization of the image intensities leads to a Newton-Raphson style minimization. In this report, after rederiving the method in a physically intuitive way, we answer the crucial question of how to choose the feature windows that are best suited for tracking. Our selection criterion is based directly on the definition of the tracking algorithm, and expresses how well a feature can be tracked. As a result, the criterion is optima...
Article
The philosopher of science J. W. Grove (1989) once wrote, “There is, of course, nothing strange or scandalous about divisions of opinion among scientists. This is a condition for scientific progress” (p. 133). Over the past 30 years, usability, both as a practice and as an emerging science, has had its share of controversies. It has inherited some from its early roots in experimental psychology, measurement, and statistics. Others have emerged as the field of usability has matured and extended into user-centered design and user experience. In many ways, a field of inquiry is shaped by its controversies. This article reviews some of the persistent controversies in the field of usability, starting with their history, then assessing their current status from the perspective of a pragmatic practitioner. Put another way: Over the past three decades, what are some of the key lessons we have learned, and what remains to be learned? Some of the key lessons learned are:• When discussing usability, it is important to distinguish between the goals and practices of summative and formative usability.• There is compelling rational and empirical support for the practice of iterative formative usability testing—it appears to be effective in improving both objective and perceived usability.• When conducting usability studies, practitioners should use one of the currently available standardized usability questionnaires.• Because “magic number” rules of thumb for sample size requirements for usability tests are optimal only under very specific conditions, practitioners should use the tools that are available to guide sample size estimation rather than relying on “magic numbers.”
We extract heart rate and beat lengths from videos by measuring subtle head motion caused by the Newtonian reaction to the influx of blood at each beat. Our method tracks features on the head and performs principal component analysis (PCA) to decompose their trajectories into a set of component motions. It then chooses the component that best corresponds to heartbeats based on its temporal frequency spectrum. Finally, we analyze the motion projected to this component and identify peaks of the trajectories, which correspond to heartbeats. When evaluated on 18 subjects, our approach reported heart rates nearly identical to an electrocardiogram device. Additionally we were able to capture clinically relevant information about heart rate variability.