2011 IEEE International Conference on Robotics and Automation
Shanghai International Conference Center
May 9-13, 2011, Shanghai, China

Finger-shaped Thermal Sensor using Thermo-sensitive Paint andCamera for Telexistence
Katsunari Sato, Hiroyuki Shinoda, and Susumu Tachi,
The University of Tokyo, Keio University, Japan

Abstract— A thermal change on a fingertip is essential for
haptic perception. We have proposed a vision-based thermal
sensor using thermo-sensitive paint and a CCD camera fortelexistence. The thermo-sensitive paint is employed to measurethermal information on the basis of its color, which changes
according to its temperature. The proposed sensor can simulate
the physical interaction between a human fingertip and an
object in order to measure surface thermal information
correctly. Furthermore, because the proposed sensor can beeasily integrated with our vision-based force sensor, acomprehensive measurement device for measuring haptic
information can be realized. In this study, we constructed a
prototype of the proposed thermal sensor and experimentallyconfirmed that this sensor could measure surface thermal
information.

I. INTRODUCTION

H

APTIC sensors are essential for telexistence (ortelepresence) systems [1] involving robotic fingers;
therefore, a number of sensors have already beendeveloped. Conventional haptic sensors used in telexistencesystem mainly measure the mechanical deformation of ahuman fingertip, i.e., tactile information. For example,
Maeno et al. developed a tactile sensor [2] for thetransmission of textures of objects [3]. We have also
developed a force sensor [4] and constructed a transmission
system for spatial distribution of force [5]. However, haptics
is perceived by humans as the integrated information of thedeformation and thermal change experienced by the skin [6].
When a human touches a metal or wood left at room
temperature (approximately 20–25°C), he/she candiscriminate the difference between them from the thermalchanges; he/she perceives the metal as cooler than wood.
Such thermal information enhances the reality of theexperience in a telexistence system. For example, when a user
touches a remote person through a telexistence system, theuser feels not only the softness but also the warmth of thisperson. We consider that the user can feel even closer to thisremotepersonfromthesehapticsensesin

Manuscript received September 15, 2010.
Katsunari Sato is with the Graduate School of Information Science andTechnology, The University of Tokyo, Japan (phone: +81-3-5841-6927;
e-mail: Katsunari_Sato@ ipc.i.u-tokyo.ac.jp).
Hiroyuki Shinoda is with the Graduate School of Information Science andTechnology, The University of Tokyo, Japan (e-mail: Shino@alab.t.u-tokyo.
ac.jp).
Susumu Tachi is with the Graduate School of Media Design, Keio
University, Japan (e-mail: Tachi@tachilab.org).

978-1-61284-380-3/11/$26.00 ©2011 IEEE

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tele-communication. The haptic senses are also effective in atele-palpation system, e.g., a doctor can feel the state of aremote patient more correctly. Therefore, haptic sensors needto measure not only tactile but also thermal information.
In the telexistence system, the measured thermal
information is presented to a human in a manner that enables
him/her to perceive the thermal change as if he/she is actually
touching an object. In order to realize this, the sensor must
fulfill three requirements. First, the sensor has to measure the
distribution of temperature on its surface. Because a humanperceives the temperature of his/her skin surface, the thermal
information has to be measured on the sensor surface andpresented to the skin surface. Second, the performance of thesensor has to be better than that of a human’s fingertip. The
performance parameters are the measuring range, measuring
accuracy (resolution), spatial resolution, and time response. If
these performance parameters are not satisfactory, a human
may get the feeling that he/she is touching an object through a
glove. Finally, the physical properties of the sensor, such as
shape, compliance, and temperature, have to be similar to thatof a human fingertip to simulate the interaction caused by thecontact of an object with human skin. When a human actually
touches an object, the contact area and contact time affects tothe thermal change on the surface of his/her fingertip.
Furthermore, the thermal change is also affected by thethermal properties of the fingertip, i.e., temperature andthermal conductivity.
Some previous studies have tried to measure the thermal
along with tactile information [7][8][9]. These studies
integrated a thermal sensing element such as a Peltier elementwith a tactile sensing unit. We can mimic the shape andcompliance of human skin by placing the sensing element in
an elastic body. In this case, the thermal change in the contact
area cannot be measured, and the spatial and time responsesof the sensor decrease. As a method that does not mimic theproperties of the fingertip, Guiatni et al. proposed a thermaltransmission system using the neural network and a database[10]. This system can transmit realistic thermal information
but it requires the construction of a database by the actual
measurement of the thermal change between the fingertip andthe object. Another method of thermal measurement requires
the use of an IR camera [11]. This method can measure thesurface temperature of the fingertip when the fingertip comes
into contact with an infrared transmissive object. Therefore, it
is difficult to apply this method to the haptic sensor.
As a thermal sensor for telexistence, we have proposed avision-based thermal sensor using thermo-sensitive paint anda camera. The proposed sensor can mimic the shape,
compliance, and temperature of a human fingertip andsimulates the physical interaction between an object and afingertip. Furthermore, one can readily built a haptic sensor
that can measure both deformation and thermal change on thefinger surface by adding a vision-based force sensor [4]. In
this paper, we evaluated and discussed whether the proposed
sensor is applicable to a telexistence system.
II. VISION-BASED THERMAL SENSOR
A. Thermal sensor requirements
Before we introduce the vision-based thermal sensor, we
describe the required performance and properties of thethermal sensor for telexistence (summarized in Table 2 insection IV).
Jones and Ho [12] summarized the conventional studies
about the thermal perception in humans. The temperature
range within which a human can perceive thermal sensation is
15–45°C. If the temperature of the skin falls below 15°C orrises above 45°C, the thermal sensation changes to that ofpain. Therefore, the temperature measurement range of thethermal sensor has to cover the range of 15–45°C. The
thermal threshold of a human depends on the initial skintemperature of the skin and rate of thermal change (°C/s). In
this study, we have determined that the target measurement
accuracy of the sensor, based on the cold threshold of theindex finger, is 0.2°C measured at a rate of 2.1°C/s [13]. If themeasurement resolution is smaller than 0.2°C, then themeasured thermal information is correctly presented to ahuman. The average localization error of the warm stimuli isapproximately 19 mm [14]; therefore, the spatial resolution of
the sensor has to be smaller than 20 mm. Because we have tomeasure the surface temperature of the sensor, a high time
response is required. Ideally, the thermal change has to bemeasured exactly when the sensor comes touches the object.
Furthermore, the temperature of human skin seems to change
at the rate of 6°C/s [15]. Therefore, we have set the required
measurement rate for the thermal change to more than 6°C/s.
In order to simulate the interaction between the fingertip
and an object, the thermal and mechanical properties of thesensor and that of the fingertip have to be the equivalent. To
discuss the thermal interaction, several studies have used the“two semi-infinite bodies in contact” model. On using thismodel, the surface temperature of the skin and the objectchange instantaneously to the interface temperature Ts:

Ts 

/2
1/ 2
Tobject ,i (kc)1object,i  Tskin ,i ( kc ) skin ,i/2
1/ 2(kc)1object,i  ( kc ) skin ,iis the thermal contact coefficient [16]. Furthermore, duringcontact, the out-of the-skin heat flux q”skin is calculated asfollows:"
q skin

k skin (Tx  Tskin ,i )( skin t )1 / 2(2)

where αskin is the thermal diffusivity of the skin defined as(k/ρc)skin and t is the time. These equations suggest that thethermal contact coefficient of the skin affects the thermalchange, and therefore, it is also important for the sensor.
These thermal properties of the fingertip are summarized by
Ho and Jones [15]; the average Tskin,i is 34.6°C, k is 0.37
W/mK, ρ is 1,000 kg/m3, c is 3,770 J/kg K, and (kρc)1/2 is1,181 J/m2s1/2K. Furthermore, the sensor should be
finger-shaped, and its compliance should be the same as thatof a human fingertip. The compliance is denoted by Young’s
modulus, which is 1.36 × 105 Pa for the epidermis, 8.0 × 104
Pa for the dermis, and 3.4 × 104 Pa for the subcutaneous fat[17]. These mechanical properties are also important for theaccurate measurement of tactile information.
B. Thermal measurement using thermo-sensitive paint andcameraTo realize a thermal sensor that satisfies theabovementioned requirements, we have proposed avision-based thermal sensor using thermo-sensitive paint anda camera. Thermo-sensitive paint changes its color
corresponding to thermal change. This paint can visualize
thermal information and hence, it is applied for temperature
management of products such as food or industrial machines.
Thermo-sensitive paint is also applied in scientificinvestigations such as the analysis of fluids [18]. In this study,
we have used the thermo-sensitive paint to measure thethermal change on the surface of the haptic sensor fortelexistence.
Figure 1 represents the configuration of the proposed
vision-based thermal sensor. This sensor consists of an elastic
seat with thermo-sensitive paint, a transparent elastic body, acamera, a heat source, and a light source. The
thermo-sensitive paint is printed on the inner side of thesensor surface so that its color changes corresponding to thethermal change on the sensor surface, i.e., the interface
temperature Ts in Eq. 1. The camera detects the color of thethermo-sensitive paint and converts it to the temperature of(1)

In this equation, k is the thermal conductivity, ρ is the density,
c is the specific heat, Tobject,i is the initial temperature of theobject, Tskin,i is the initial temperature of the skin, and (kρc)1/2

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Fig. 1. The configuration of the proposed vision-based thermal sensor.

the sensor surface. We ensure that the compliance of theelastic body is the same as that of a human fingertip.
Therefore, it can mimic the deformation of the skin caused by
the contact. The heat source maintains the temperature of
sensor and that of the human fingertip at the same level. The
temperature of the sensor surface has to be controlled to
mimic the temperature of the fingertip.
To convert color to temperature, we use the hue of thecaptured image. In a previous study [18], it was indicated thatthe hue change corresponds to the thermal change. Therefore,
we consider that the temperature of the sensor surface can becalculated from an equation developed for converting hue, h,to temperature, T:T  f (h)(3)

In this equation, f(•) denotes the conversion function from thehue of the captured thermo-sensitive paint to its temperature.
Note that the color change of the normal thermal-sensitive
paint finishes when the temperature changes more than 5°C
or 10°C. In order to cover the temperature measurement rangeof 15–45°C, we have to use several paints that have different
temperature ranges.
C. Integration with vision-based force sensor
The haptic sensor needs to measure the thermal along withtactile information, i.e., distribution of force applied to thesensor surface.
The basic configuration of the proposed thermal sensor isthe same as that of vision-based force sensor [4]; therefore,
we consider that these sensors can be easily integrated. Figure
2 represents the force measurement theory of the vision-based
force sensor. It consists of a transparent elastic body with
two-layered markers and a CCD camera. When a force isapplied to the sensor, the markers are displaced. The CCD

Fig. 2. (a) Basic theory of vision-based force sensor [4] and (b)configuration of vision-based force and thermal sensors.

camera captures the displacement of the markers and thencalculates the distribution of the applied force from these
displacements. A haptic sensor that can measure bothdeformation and thermal change is basically achieved byincorporating the markers in the vision-based thermal sensor.
Because we calibrate the initial marker positions, we caneasily arrange two layered markers by hand. The markers
have to be uniformly arranged not to eliminate overlap witheach marker.
III. EXPERIMENTThe proposed vision-based thermal sensor can measure thetemperature on its surface. Furthermore, the physicalproperties of the sensor can be the same as that of the humanfingertip when the elastic material is properly selected. Here,the question is “whether the performance of the proposed
sensor is adequate.” Therefore, we have evaluated thecapability of the proposed thermal sensor for telexistence interms of its performance.
A. Prototype implementation
Figure 3a represents the constructed prototype. For
thermo-sensitive paint, we have used a thermo-sensitive
liquid sheet (chromatic liquid, Japan Capsular Products Inc.).
The seat is composed of a paper that is harder than humanskin. Therefore, the nine 5.0 × 5.0 mm2 blocks of the liquidseat are arranged on the elastic seat to maintain the elasticityof the sensor. The thickness of the elastic seat is 0.5 mm. The
shape of the elastic body is partly spherical and cylindrical;
hence, it resembles a human fingertip. The size of the elastic
body is 18 × 9 × 19 mm3. The elastic seat and transparent
elastic body are composed of urethane gel (Human Skin Gel,
Exseal Corp.) because their Young’s modulus (5.0×104 Pa)
resembles that of the human fingertip. However, the contact
coefficient of this material (urethane rubber; 400~600

Fig. 3. (a) Constructed prototype and (b) image captured by the camera.

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J/m2s1/2K) differs from that of human skin. In the future, wemust appropriately select the material of the elastic body inorder to consider its thermal properties.
Figure 3(b) shows the image captured by the camera. The
frame rate and resolution of the CCD camera (ViewPlus, Inc.)
are 15 Hz and 640 × 480 (VGA), respectively. The camera
image is captured as a RGB format; therefore, we have
converted it to the HSV format using the OpenCV (Open
Source Computer Vision) library. The light source is eightLEDs. The temperature of the sensor surface is maintained at
approximately 35°C by the heat from the LEDs when thesurrounding air temperature is 26°C. We fix the amount of
current that applies to LEDs to adequately maintain both theblitheness of the camera image and the temperature of thesensor. To eliminate the spatial noise of the image, we havedivided the image into 10 × 10 pixel blocks and calculated theaveraged hue within the blocks. In the image captured by thecamera, the length of the thermal calculation block (10 pixels)
is almost equivalent to 0.5 mm. Therefore, in this case, thetemperature of the sensor surface could be measured at 0.5
mm intervals.
B. Calibration
Using the constructed prototype, we have created an
equation to convert the hue of the captured image to thetemperature of the sensor surface. Figure 4 represents thecalibration setup. The sensor was fixed by the jig connectedto the 1-axis slider. The center of the sensor surface is incontact with a cool plate (NCP-2215, Nissinrika Corp.). The
temperature of the cool plate is controlled from 31 to 36°C
with 0.5°C intervals. After the temperature of the cool plate
was fixed at the target value, the contact between the sensor
and plate was maintained for more than 120 s. Then, the hueat the center of the contact area was recorded. Ten values arerecorded for each temperature.
Figure 5 represents the result of the calibration. From theFig. 5. The relationship between the hue of the captured
thermo-sensitive liquid seat and its temperature. Error bar represents
the standard deviation.

graph, we can confirm that the function of the hue andtemperature is monotonically decreasing in the temperature
range of 31.5–35°C. Therefore, the proposed sensor cancalculate the temperature from the hue of the captured image
within this range. Using these results, we have set up theconversion functions of equation (3) as a linear function with
0.5°C intervals.

T  ah b(4)

The calculated coefficient a, and intercept b are shown in
Table 1. Based on the conversion functions, we have also
estimated the measurement resolution of the temperature. We
defined the resolution as a product of the absolute value of |a|and the hue standard deviation in each temperature interval.
From the table, the resolutions of the prototype differ
according to the temperature of the sensor.
C. Thermal measurement
Using the conversion equations created in the above
subsection, we have measured the change in the temperature
distribution on the surface of the sensor. At first, the sensor
surface was not in contact with any object except air. The airtemperature was maintained at approximately 26°C.
Therefore, the temperature of the prototype is approximately
35°C. Then, the sensor surface is pressed to the surface of thecool plate maintained at 32°C. The temperature distribution
was calculated at 15 Hz, and visualized on the display using
OpenGL at 30 Hz.
Table 1. Calculated conversion function and resolution.
Temperature(°C)
31.5~32.0
32.0~32.5
32.5~33.0
33.0~33.5
33.5~34.0
34.0~34.5
34.5~35

Fig. 4. Setupss for calibration and evaluation of time response.

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a

b

-0.06
-0.03
-0.03
-0.04
-0.05
-0.08
-0.24

36.7
34.6
34.5
34.9
35.3
36.1
39.0

standard
deviation of h
0.44
0.82
0.40
0.32
0.35
0.25
0.18

Resolution
(°C)
0.03
0.02
0.01
0.01
0.02
0.02
0.04

Figure 6 represents the time variation of the temperature
distribution of the sensor. In this figure, the temperature isrepresented by the color and height of the displayed surface;
the color continuously changes from red at 36°C to green at
34°C to blue at 32°C. The temperature of the center decreases
when the sensor surface contacts the cooler plate. Then, thetemperature of the whole contact area gradually decreases.
From this figure, we can confirm that the prototype sensor
can measure the spatial and temporal changes in thetemperature distribution.
D. Time response
To evaluate the time response of the sensor, we recorded
the thermal change of the prototype when it was pressed to analuminum board (Fig. 4). The thickness of the board was 2
mm. To approximate the moment of contact, we put afilm-type force sensor (FlexiForce, Nitta Corp.) between thealuminum board and the cool plate. The force was measured
at 200 Hz. The temperature at the center of the contact area
was recorded at 0.2 s intervals after contact detection by theforce sensor. Before the prototype touched the aluminumboard at each temperature, we left it in air at 26°C.
At first, we evaluated the rate of thermal change. The threetemperatures of the aluminum board were 20, 25, and 30°C.
Figure 7 represents the recorded thermal change in the sensor
at each temperature of the aluminum board. When thetemperature of the aluminum board is low, the rate of thermalchange becomes large. The largest rate of the thermal change
is approximately 2°C/s.
Furthermore, we evaluated the delay of the sensor after
contact. The sensor was pressed to the aluminum board that
was at 30°C for 4 s. Then, we released the sensor from thealuminum board and recorded the temperature change. Figure
8 represents the results. In Figure 7 and 8, the delay of theinitial response of the sensor is less than 0.2 s. However, inFigure 8, the sensor showed a delay of 1 s after the sensor was
released from the aluminum board; same thermal changeremained approximately 1 s after the contact.

Fig. 6. Measurement of the thermal information.

Fig. 7. The thermal change rate.

IV. DISCUSSION
We discuss in this section whether the proposed sensor isapplicable to telexistence systems based on the experimental
results (summarized in Table 2).
The temperature range of the thermo-sensitive liquid sheet
is 31.5–35°C; therefore, the measurement range is also
31.5–35°C. Because the temperature range of a standard
thermo-sensitive ink is approximately 5–10°C, to cover themeasurement range of 15–45°C, we must use multiple
thermo-sensitive paints that have different temperature
ranges. Because the spatial resolution of a human to thermal
stimulation is relatively low (approximately 20 mm), we
consider of arranging several rectangular blocks ofthermo-sensitive paint in a reticular pattern over an area of 20
× 20 mm2. When we identify the position of eachthermo-sensitive block and calculate the temperature of thereticular pattern area, the temperatures in the range of
15–45°C can be measured. The position of eachthermo-sensitive block can be easily detected with markers
put on the corner of the blocks. In Table 1, we see that themeasurement accuracy seems to satisfy the requirement of
0.2°C. In the evaluation, the spatial resolution of the sensor
was not quantitatively evaluated. However, from Figure 6,
the spatial resolution of the sensor seems to be sufficiently
small compared to that of human skin (19 mm).
The time response of the sensor is critical for the proposed
sensor. The small rate of thermal change and delay of theFig. 8. Sensor delay after contact.

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material to accurately simulate the thermal interaction
between an object and the human fingertip. Then, we will
integrate the vision-based force sensing method and realize atelexistence system that can translate realistic haptic
information.

Table 2. Performance and physical properties of human skin of thefinger [12] [13] [14] [15] [17] and prototype of the proposed sensor.
Performances or properties
Measurement range (°C)
Measurement accuracy (°C)
Spatial resolution (mm)Rate of thermal change (°C/s)
Delay (s)Initial temperature (°C)
Temperature contact
coefficient (J/m2s1/2K)
Young’s modulus (Pa)

Human skin
15–45
0.2
19
6
0 (Instant)
34.6

Prototype
32–35
0.01–0.07
0.5
2
1
35

1,181
5

3.4 × 10 –1.36 × 10

ACKNOWLEDGMENT
This work was partly supported by Grant-in-Aid for
CREST and JSPS Fellows (20•10009).

400–600
55.0×104

REFERENCES

prototype seems to be caused mainly by two factors: one is
the thermal contact coefficient of the elastic material, and theother is the thickness of the elastic seat. Because we mimic
the shape and compliance of a human fingertip, the prototype
sensor simulates the mechanical interaction between thefingertip and the object. Therefore, the thermal distribution
seems to be measured correctly (Fig. 6). However, thethermal contact coefficient of the elastic body is smaller thanthat of the skin and therefore, the rate of the thermal change inthe sensor is lower than that of human skin. To improve thetime response and accurately measure the thermal
information of a human, the material for the elastic body not
only has to possess the similar mechanical but also the similar
thermal properties as the human skin. The thickness of theelastic seat also blocks the thermal change in thethermo-sensitive liquid seat. To increase the time response of
the sensor, the thermo-sensitive paint should be painted as
close to the surface of the sensor as possible. We believe thatthese improvements in the time response can be easily
implemented. Furthermore, the 1 s delay of the prototype
sensor after release (Figure 8) seems to be caused by the lowtemperature contact coefficient (approximately 5.5 J/m2s1/2K)of the air.
Based on the abovementioned evaluation, we conclude
that the proposed method can be applied to the haptic sensors
for telexistence by further improving its measurement range
and time response.
V. CONCLUSION
In this study, we have proposed a vision-based thermal
sensor using thermo-sensitive paint and a camera fortelexistence. The proposed sensor can not only mimic theperformance and physical properties of human skin but also
be integrated with a vision-based force sensor. Therefore, thissensor can measure the haptic information adequately andcorrectly. Furthermore, the most important benefit of thevision-based haptic sensor is its simple structure; we canreadily build it. From the results of the evaluation carried out
using the prototype, we concluded that the proposed sensor
could be applied to telexistence systems.
However, the conventional prototype has problems such as
the measurement range and time response. In the future, wewill select the appropriate thermo-sensitive paint and elastic

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