Thermal Model and Countermeasures for Future Smart Glasses

sensors
Article
Thermal Model and Countermeasures for Future
Smart Glasses †
Kodai Matsuhashi, Toshiki Kanamoto and Atsushi Kurokawa *
 Graduate School of Science and Technology, Hirosaki University, Aomori 036-8560, Japan;
 ms19518@eit.hirosaki-u.ac.jp (K.M.); kana@hirosaki-u.ac.jp (T.K.)
 * Correspondence: kurokawa@eit.hirosaki-u.ac.jp
 † This paper is an extended version of Matsuhashi, K.; Kurokawa, A. Thermal Countermeasures of Glass
 Wearable Devices published in the Proceedings of ICCE-TW 2019, Yilan, Taiwan, 20–22 May 2019 and
 Matsuhashi, K.; Kanamoto, T.; Kurokawa, A. Thermal Resistance Model and Analysis for Future Smart
 Glasses, published in Proceedings of IMPACT 2019, Taipei, Taiwan, 23–25 October 2019.
 



 Received: 17 January 2020; Accepted: 5 March 2020; Published: 6 March 2020 


 Abstract: The market for wearable devices such as smart watches and smart glasses continues to
 grow rapidly. Smart glasses are attracting particular attention because they offer convenient features
 such as hands-free augmented reality (AR). Since smart glasses directly touch the face and head,
 the device with high temperature has a detrimental effect on human physical health. This paper
 presents a thermal network model in a steady state condition and thermal countermeasure methods
 for thermal management of future smart glasses. It is accomplished by disassembling the state by
 wearing smart glasses into some parts, creating the equivalent thermal resistance circuit for each
 part, approximating heat-generating components such as integrated circuits (ICs) to simple physical
 structures, setting power consumption to the heat sources, and providing heat transfer coefficients of
 natural convection in air. The average temperature difference between the thermal network model
 and a commercial thermal solver is 0.9 ◦ C when the maximum temperature is 62 ◦ C. Results of
 an experiment using the model show that the temperature of the part near the ear that directly touches
 the skin can be reduced by 51.4% by distributing heat sources into both sides, 11.1% by placing higher
 heat-generating components farther from the ear, and 65.3% in comparison with all high conductivity
 materials by using a combination of low thermal conductivity materials for temples and temple tips
 and high conductivity materials for rims.

 Keywords: thermal management; wearable device; thermal modeling; smart glasses; thermal analysis

1. Introduction
 Wearable devices have become popular as state-of-the-art electronic devices, such as smart
watches, smart glasses, smart clothing, and fitness trackers, have been made commercially available
for consumer and industrial uses. Currently, smart watches are the wearable device with the largest
market size. However, smart glasses have also been released by many companies [1–7] and can be used
for various purposes such as medical care, health, learning/education, and entertainment. Differences
in the uses between the smart glasses and the wrist-worn wearables such as smart watches and fitness
trackers come from the differences between wrists and eyes. Smart glasses have advantages that
users can look at various things such as maps (e.g., current location) and movies with augmented
reality (AR) through a display, and their eye and facial movements can be recognized for medical
care, health monitoring, and dozing prevention. In the future, heat issues will become more serious
because smart glasses will require faster central processing unit (CPU) and larger memory to deal with
enormous amounts of data. Therefore, thermal design is becoming one of the key technologies for
future wearable devices.

Sensors 2020, 20, 1446; doi:10.3390/s20051446 www.mdpi.com/journal/sensors

Sensors 2020, 20, 1446 2 of 20

 Various techniques for utilizing smart glasses have been developed by many researchers.
For medical uses, techniques have been presented for clinical and surgical applications [8], medical
emergency situations [9], and disaster medicine [10]. For recognition and interface, techniques have
been presented for head gestures [11], face detection [12], eye movements [13], user authentication [14],
estimation of respiration rate [15], speech interaction with eye blinking detection [16], context-aware
lightning control [17], distance learning [18], and indoor localization [19], drowsiness and fatigue
detection to increase road safety [20], countermeasures to phishing attacks [21], gait aid for Parkinson’s
disease patients [22], contextually-aware learning in physics experiments [23], and guiding for visually
impaired users [24]. Some techniques regarding thermals have been presented such as 3D thermal
model reconstruction based on image-based modeling using smartphone sensors [25], design of an oven
utilizing radiative heat transfer for smart phone panels [26], thermal management systems for civil
aircraft engines [27], and thermal properties of glasses [28]. A system for low-power smart glasses has
been presented [29]. However, there have been only a few technical reports about the heat of smart
glasses [30,31]. Smart glasses directly touch human skin. The heat of the smart glasses is capable
of causing burns of the skin. Thermal management of smart glasses is essential for physical health
safety and comfortable use. Current smart glasses consume 1 to 3 W under various workloads [30].
We presented thermal countermeasures of smart glasses [31] and discussed only the maximum
temperature in the integrated circuits that generate heat. Only a heat generating component was used
and was placed on a limited space. Moreover, the temperature of the device surface touching skin that
may cause a low-temperature burn even at 43 ◦ C [32] was not analyzed. In Reference [33], we presented
a thermal network model for thermal designs of future smart glasses. In this paper, we provide more
detailed resistance models for all parts of devices, thermal properties that were used in the analysis,
a difference in temperatures due to the position on the temple, more detailed explanations for each
figure, motivations for this work, and discussions.
 Smart glasses mainly comprise the electronic device body, and display a liquid crystal on the
silicon (LCOS) device [34] and battery. The device body consists of many heat generating electronic
components, including processors, memory, wireless modules, and power management integrated
circuits (ICs). In accordance with a design concept, the device body is mounted to various places [35].
We simplify the device body as follows: IC packages are reconstructed by using three layers (heat
generating, upper, and lower) in the vertical direction, and several packages are arranged horizontally
on a printed circuit board (PCB). Thereby, the simplified device body can be re-sized and consume
power in a non-uniform manner. By making a thermal resistance model of the state wearing smart
glasses, temperatures at each part can be calculated. The thermal model can be used for various types
of smart glasses such as glasses’ structures, materials, heat sources, and layouts of the components.
 Smart glasses include various functions such as a camera, video, map, translation, weather
information, and search in real-world environments, augmented reality, and virtual reality (VR). As the
demand for higher precision and higher speed increases, power consumption also increases. Moreover,
power density increases with higher integration (including 3D ICs). Therefore, thermal management is
very important for future smart glasses. Systems and design methodologies of smart glasses have
been proposed [29,36–39]. Among them, thermal management has become one of the crucial issues
in AR and VR processing [30,31] where image screens as well as image sensors have been equipped.
Even the current high definition (HD) smart glasses consume 1 to 3 W [30]. Upcoming advanced
features including 4K/8K resolution processing are expected to need additional power to render the
images [40–45]. This will require more organized thermal management with overviewing packages,
boards, and systems as well as heating processor chips [46]. Motion detection is another power
consuming factor. Even the current artificial intelligence (AI)-based moving object identification from
the sensor images also requires up to 3W of power. Additionally, the expected features in the near
future such as human detection will impose an extra power expense. Furthermore, promising smart
glasses need to communicate with external networks and transfer large amounts of the processed data.
The leading 5G communications technologies reduce transmission power in exchange for consuming

Sensors 2020, 20, 1446 3 of 20

additional circuit power due to the required hardware expansions including signal processing to
establish low-power mmWave communications at extremely high frequencies [47].
 In the viewpoint of the safety and comfort of smart glasses, there are several issues such as
weight [48,49], battery [50,51], AR/VR [52–54], and heat [31,33]. In Reference [48], regarding weight
issues, the effect of weight balance for shutter glasses in terms of subjective discomfort and physical
load on the nose has been investigated [48] and a user discomfort on the different wearing mode glasses
with different support points has been presented [49]. With respect to battery safety, an extremely
safe and wearable solid-state zinc ion battery has been fabricated [50] and a quasi-solid-state aqueous
rechargeable lithium-ion battery with outstanding stability, flexibility, safety, and breathability toward
various wearable electronics has been reported [51]. AR/VR include many problems such as display
size, resolution, computing capability, negative effects in some people with autism spectrum disorder,
and architecture/server/network issues [52–54]. Moreover, it is necessary to ensure the safety and
comfort due to heat generation [31,33] since smart glasses are worn on the face/head of the human
body for use.
 The main sources of heat generation of smart glasses come from power consumptions of ICs.
Thermal countermeasures within IC chips must be mainly low power designs and have a limit.
For smart glasses, the temperature not only in chips for circuit operation but also on the device
surface touching the skin is important for preventing low temperature burns. Thermal management of
smart glasses is required to determine various conditions such as arrangements of heat generating
components and materials of parts. Although a commercial thermal solver can obtain high accuracy
results, it has the disadvantages of a complex structure input, long processing time, and unsuitability
for parameter optimization. Therefore, we have developed a thermal network model to improve
design efficiency. The model has been devised for not only current products but also future products.
To deal with as wide a variety of smart glasses as possible, the entire thermal network is divided into
several parts and is expressed by a block diagram (as described in Section 3.1). In addition, each block
is removable and replaceable. We use virtual smart glasses composed of some parts but not real smart
glasses so that the entire thermal network can be applied to various types of smart glasses.
 Additionally, using the proposed model, we present thermal countermeasures of smart glasses
for ensuring the health safety and comfortable use. We clarify the following facts: (1) If high thermal
conductivity materials like Al are used for a grasses frame, the whole temperature can be reduced,
but a low temperature burn may be caused near an ear. (2) If low conductivity materials like cellulose
acetate (CA) plastic are used, temperature near an ear can be reduced, but the surface temperature
of the device body rises. (3) When Al is used, by locating higher power density, ICs near the lens,
temperature at the ear decreases but not sufficiently. (4) When the device body is divided and placed
on both sides, temperatures decrease as a whole. From these results, we found that the best solution
is to use plastic for the temples and temple tips for hanging on the ears and Al for the other parts of
the frame in order to locate the device body to the lens side and divide it into both sides as much
as possible.
 The rest of the paper is organized as follows. Section 2 describes the details of smart glasses
assumed in this work. Section 3 presents thermal network models for the smart glasses. Section 4
shows experimental results for thermal countermeasures. Section 5 presents discussions of this work.
Section 6 concludes this paper.

2. Physical Structural Model of Smart Glasses
 In this section, the smart glasses assumed in this work are described. First, an overview of the
smart glasses is shown. Next, a physical structural model with dimensions is presented. Lastly, the heat
generating components are discussed.

Sensors 2020, 20, 1446 4 of 20

2.1. Overview of Smart Glasses
 Figure 1 shows an overview of a basic structure of the smart glasses used in this study. In the basic
structure of smart glasses, batteries are connected to device bodies. Device body cases are mounted on
the temples of both sides. Electronic components are installed in the right device body, and displays
 Sensors 2019, 19, x FOR PEER REVIEW 4 of 20
are Sensors
 set in 2019,
 front19,ofxlenses.
 FOR PEER REVIEW 4 of 20

 Figure 1. Overview of smart glasses used in this paper.
 Figure
 Figure 1. Overview
 1. Overview of smart
 of smart glasses
 glasses usedused in this
 in this paper.
 paper.

 2.2.Structure
2.2. StructureofofSmart
 SmartGlasses
 Glasses
 2.2. Structure of Smart Glasses
 Table11 lists
 Table lists thermal
 thermal properties
 properties of the smart glasses used in our basic analysis. FigureFigure
 2 shows
 Table 1 lists thermal propertiesofofthe
 thesmart
 smart glasses used
 glasses used in our
 in our basic
 basic analysis.
 analysis. 2
 Figure 2 shows
 dimensions
shows of a
 dimensions face/head
 of a model.
 face/headFor a face
 model. model,
 For a we referred
 face model, to a
 we model of
 referred human
 to a thermoregulation
 model of human
 dimensions of a face/head model. For a face model, we referred to a model of human thermoregulation
 [55] and used a simpler
thermoregulation andmodel. a The skinmodel.
 thickness
 The was 2thickness
 mm. Thewas core and ambient
 core temperatures
 [55] and used [55]a simpler used simpler
 model. The skin thickness skin
 was 2 mm. The core 2 mm.andThe
 ambientand ambient
 temperatures
 were set
temperaturesto 36.6 and 25 °C, respectively.
 ◦ Figure 3 shows dimensions of the smart glasses.
 were set towere
 36.6 set
 andto2536.6
 °C,and 25 C, respectively.
 respectively. Figuredimensions
 Figure 3 shows 3 shows dimensions of the
 of the smart smart glasses.
 glasses.
 Table1.1.Thermal
 Table Thermalproperties
 propertiesofofsmart
 smartglasses.
 glasses.
 Table 1. Thermal properties of smart glasses.
 Parts Material Abbreviation Thermal Conductivity (W/mK)
 Parts Parts Material
 Material Abbreviation
 Abbreviation Thermal Thermal Conductivity
 Conductivity (W/mK)
 (W/mK)
 Aluminum Al 236
 Frame Aluminum
 Aluminum Al 236
 Frame Frame Cellulose acetate CAAl 0.2 236
 Cellulose
 Cellulose acetate
 acetate CA
 CA 0.2 0.2
 Display Polycarbonate PC 0.19
 Display Display Polycarbonate
 Polycarbonate PC
 PC 0.190.19
 Lens Polycarbonate PC 0.19
 Lens Lens Polycarbonate
 Polycarbonate PC
 PC 0.190.19
 Nose pad Cellulose
 Nose padNose padCellulose propionate
 propionate CP CP 6
 Cellulose propionate CP 6 6
 BatteryBattery
 Battery —---
 ---
 ---—
 ---
 15
 15 15

 (a) (b)
 (a) (b)
 Figure2.2.Overview
 Figure Overviewofofsmart
 smartglasses
 glassesused
 usedin
 inthis
 thispaper:
 paper:(a)
 (a)front
 frontview
 viewand
 and(b)
 (b)top
 topview.
 view.
 Figure 2. Overview of smart glasses used in this paper: (a) front view and (b) top view.

 (a) (b) (c)
 (a) (b) (c)

(a) (b)
Sensors 2020, 20, 1446 5 of 20
 Figure 2. Overview of smart glasses used in this paper: (a) front view and (b) top view.

 Sensors 2019, 19, x FOR PEER REVIEW 5 of 20
 (a) (b) (c)
Sensors 2019, 19, x FOR PEER REVIEW 5 of 20
 2.3. Heat Generating
 Figure
 Figure 3. Components
 3. Dimensions (in
 (in mm)
 mm) of
 of smart
 smart glasses:
 glasses:(a)
 (a)front
 frontview,
 view,(b)
 (b)side
 sideview,
 view,and
 and(c)
 (c)top
 topview.
 view.
2.3. Heat Generating
 In Generating
 general, heatComponents
 generating components of smart glasses include processors, memories (e.g.,
2.3. Heat Components
 DDR4 SDRAM, and NAND
 In general, heat generating flash), audio ICs,of
 components wireless
 smart modules, power processors,
 glasses include management ICs, and (e.g.,
 memories LCOS
 In
 devices. general, heat
 Figureand generating
 4 illustrates components
 an example of smart glasses include processors, memories (e.g.,
DDR4 SDRAM, NAND flash), audio of a cross-sectional
 ICs, wireless modules,structure
 powerofmanagement
 an IC package ICs,with
 andthe
 LCOS flip-
DDR4
 chip SDRAM, and
 technology. NAND
 Table 2 flash),
 lists an audio ICs,
 example of wireless
 the thermalmodules,
 propertypower
 and management
 thickness of ICs,layer.
 each and LCOS
 Figure
devices. Figure 4 illustrates an example of a cross-sectional structure of an IC package with the flip-
devices. Figure
 5 illustrates an4 example
 illustratesofana example of a cross-sectional
 cross-sectional structure structure of an IC package withantheexample
 flip-chipof
chip technology. Table 2 lists an example of the thermalofproperty
 an LCOSand device. Tableof3 each
 thickness lists layer. Figure
technology.
 the thermal Table 2
 property lists
 andan example
 size. The of
 heatthe thermal
 generating property and
 components thickness
 are composedof each
 of layer.
 various Figure
 structures5
5 illustrates an example of a cross-sectional structure of an LCOS device. Table 3 lists an example of
illustrates
 and an
 thermal example of
 properties. a cross-sectional structure of an LCOS device. Table 3 lists an example of the
the thermal property and size. The heat generating components are composed of various structures
thermal property and size. The heat generating components are composed of various structures and
and thermal properties.
thermal properties.

 Chip substrate Package
 Bumps
 Device layer
 Package substrate
 Metal layer Balls
 Copper plane
 PCB
 Figure 4. IC package (flip-chip package).

 Figure 4.
 Figure 4. IC
 IC package
 package (flip-chip
 (flip-chip package).
 package).
 Table 2. Thermal properties and size of IC package.
 Table 2. Thermal properties and size of IC package.
 Table 2. Thermal
 Component Thermalproperties and size(W/mK)
 Conductivity of IC package.
 Thickness (mm)

 Component
 Component
 Mold Thermal Conductivity
 Thermal (W/mK)
 Conductivity
 0.88 (W/mK) Thickness--- (mm) (mm)
 Thickness
 Metal
 MoldMoldlayer 98
 0.880.88 0.003
 --- —
 Metal layer
 Device layer 14998 0.0010.003
 Metal layer 98 0.003
 DeviceSi layer
 substrate 149
 149 0.1 0.001
 Device layer
 Si substrate 149149 0.001 0.1
 Bumps 60 0.08
 Si substrate
 Bumps 149 60 0.1 0.08
 Package substrate 149 0.2
 Bumps
 Package substrate 60 149 0.08 0.2
 Balls 33 0.35
 Package
 Balls substrate 149 33 0.2 0.35
 PCB 13 0.8
 PCB Balls 33 13 0.35 0.8
 Copper plane 401 0.03
 Copper PCBplane 13 401 0.8 0.03
 Copper plane 401 0.03

 Glass substrate
 Liquid crystal layer
 Chip

 PCB
 5. 5.Liquid
 Figure
 Figure Liquidcrystal
 crystalononthe
 thesilicon
 silicon(LCOS) device.
 (LCOS) device.

 Figure 5. Liquid crystal on the silicon (LCOS) device.
 Table 3. Thermal properties and size of liquid crystal on the silicon (LCOS) device.

 Table 3.Component
 Thermal propertiesThermal
 and size Conductivity
 of liquid crystal on the siliconx,(LCOS)
 (W/mK) device.
 y, z (mm)
 Glass substrate 0.8 8, 8, 0.5
 Component Thermal Conductivity (W/mK) x, y, z (mm)
 Liquid crystal layer 0.15 8, 8, 0.005
 Glass substrate 0.8 8, 8, 0.5
 Copper plane 401 14, 13, 0.03
 Liquid crystal layer 0.15 8, 8, 0.005
 PCB 13 14, 13, 0.8
 Copper plane 401 14, 13, 0.03

Sensors 2020, 20, 1446 6 of 20

 Table 3. Thermal properties and size of liquid crystal on the silicon (LCOS) device.

 Component Thermal Conductivity (W/mK) x, y, z (mm)
 Glass substrate 0.8 8, 8, 0.5
 Liquid crystal layer 0.15 8, 8, 0.005
 Copper plane 401 14, 13, 0.03
 PCB 13 14, 13, 0.8

 In this scenario, we model the heat generating components by applying them to various types of
 Sensors 2019, 19, x FOR PEER REVIEW 6 of 20
smart glasses. The simple physical model for an equivalent circuit of one heat generating component
is approximated with
 Sensors 2019, 19, x FORupper and lower layers, as
 PEER REVIEW Airshown in Figure 6. 6 of 20
 Heat Upper layer
 generating Air Heat generation
 component
 Heat Upper layer
 generating Lower layer
 Heat generation
 component PCB Copper plane
 Lower layer
 PCB Copper plane
 Figure 6. Simple model of the heat generating component.

 Figure 6. Simple model ofofthe heat generatingcomponent.
 component.
 In this work, in theFigure
 device6. body
 Simple atmodel
 one side, thewe
 heat generating
 assumed the use of four IC packages shown in
 InFigure 4 with conditions in Table 2 and one LCOS device shown in Figure 5 with conditions in Table
 thisInwork,
 this in the
 work, in device body at one side, we
 we assumed theuse
 use
 ofAof four IC packages shown in
 3. Figure 7 shows thethe device
 arrangementbody of at one
 five side,
 heat assumed
 generating the
 components. four IC packages
 thermal resistanceshown in
 network
FigureFigure
 4 with4conditions in Table
 with conditions 2 and
 in their
 Table one
 2 and LCOS
 one LCOS device
 deviceshown
 shown inFigure
 in Figure5 5with
 with conditions in Table 3.
 for the device body with components is constructed by using the simpleconditions in Table
 physical model in
Figure 3.7Figure
 shows
 Figure6.7the arrangement
 shows the of
 arrangement five
 of heat
 five generating
 heat generatingcomponents.
 components. AA thermal
 thermal resistance
 resistance network
 network for
the device
 for thebody with
 device theirwith
 body components is constructed
 their components by using
 is constructed the simple
 by using physical
 the simple model
 physical in Figure
 model in 6.
 Figure 6.

 Figure
 Figure 7. Layout
 7. Layout ofofheat
 heatgenerating
 generating components.
 components.

2.4. Structures and Materials
 2.4. Structures of Components
 and Materials Figure 7. Layout of heat generating components.
 of Components
 In2.4.
 theStructures
 future,
 In the smart
 future, glasses
 smart ofwith
 and Materials glasses various
 Components shapes, size,
 with various materials,
 shapes, and arrangements
 size, materials, of components
 and arrangements of
must becomponents
 produced. must be produced.
 Size and materialsSize and materials
 are basically are basically
 modifiable because modifiable
 they canbecause theyby
 be applied can be
 changing
 In the future, smart glasses with various shapes, size, materials, and arrangements of
thermalapplied by changing
 resistance values.thermal resistance values.
 components must be produced.
 Although this paper uses rectangular Size and lithium
 materials are basically
 polymer modifiable
 (LiPo) batteries, thebecause
 shapes andtheymaterials
 can be
 Although
 applied this
 by paper uses
 changing thermalrectangular
 resistance lithium
 values. polymer (LiPo) batteries, the shapes and materials of
 of batteries are not greatly restricted because a battery is modeled simply. In this case, thermal
batteries are not greatly
 Although restricted
 this paper because a battery is modeled simply. In this case, thermal resistance
 resistance circuits for a uses rectangular
 cylindrical lithium
 battery polymer
 are discussed. (LiPo) batteries,
 Figure 8a showsthe shapes
 a crossand materials
 section of a
circuitsof for a cylindrical
 batteries
 cylindrical are not battery
 battery. greatly
 A are isdiscussed.
 restricted
 cylinder because
 expressed Figure
 with 8a thermal
 a battery
 three shows aresistances
 is modeledcross section
 simply. In of
 [56,57]. aThe
 this cylindrical
 case, thermal
 inner battery.
 liquid
 resistance
A cylinder is circuits
 expressed for a
 with cylindrical
 three battery
 thermal are discussed.
 resistances Figure
 [56,57]. The 8a shows
 inner a
 liquid cross
 fluid in a battery is covered by a frame. The heat conduction resistance value for the internal cylinder section
 fluid in a of a
 battery is
covered cylindrical
 by a battery.
 frame. The A cylinder
 heat is
 conductionexpressed with
 resistance
 (inner liquid fluid) can be calculated from the equation below.three
 valuethermal
 for resistances
 the internal [56,57].
 cylinder The inner
 (inner liquid
 liquid fluid)
can be fluid in a battery is covered by a frame. The heat conduction resistance value for the internal cylinder
 calculated from the equation below. 
 (inner liquid fluid) can be calculated from the equation = below. (1)
 2 
 ln (r1 )
 where r1 is the internal radius, k is the thermal R1 =conductivity
 = of the material, and l is the length of (1)a (1)
 2 
 2πkl
 cylinder. The heat conduction resistance value for an outer frame can be calculated from the equation
 where r1 is the internal radius, k is the thermal conductivity of the material, and l is the length of a
where rbelow.
 1 is the internal radius, k is the thermal conductivity of the material, and l is the length of a cylinder.
 cylinder. The heat conduction resistance value for an outer frame can be calculated from the equation
The heat conduction resistance value for an outer frame ⁄can be calculated from the equation below.
 below. = (2)
 2 
 
 ln(r2 /r ⁄ 
 where r2 is the outer radius. The heat R =
 convection 1)
 resistance value from a frame surface can (2) be (2)
 2 = 2 
 calculated from Equation (3) below. 2πkl
 where r2 is the outer radius. The heat convection resistance value from a frame surface can be
 r2 is the outer
where calculated 1 value from a frame surface can be calculated
 fromradius.
 Equation The
 (3) heat
 below.convection resistance
 = (3)
from Equation (3) below. ℎ 2 
 1
 where hc is the heat transfer coefficient. In this = all the necessary resistances can be obtained.(3)
 way, By
 ℎ 2 
 assigning them the model of rectangular batteries (i.e., by converting a circle into a rectangle as shown
 where hc is8b),
 in Figure thea heat transferbattery
 cylindrical coefficient.
 is alsoInapplicable.
 this way, all the necessary resistances can be obtained. By
 assigning them the model of rectangular
 For modeling heat generating components, we batteries (i.e., by converting
 used a circle
 the PCB with a flipinto a rectangle
 chip-ball as shown
 grid array (FC-
 in Figure 8b), a cylindrical battery is also applicable.
 BGA) package shown in Figure 4 as an example. However, types of packages (e.g., wafer level
 For modeling
 package (WLP)) and heat boards
 generatingare components, we used the
 not greatly restricted PCB with
 because a flipgenerating
 a heat chip-ball grid array (FC-is
 component

Sensors 2020, 20, 1446 7 of 20

 1
 Rc = (3)
 hc 2πr2 l
 where hc is the heat transfer coefficient. In this way, all the necessary resistances can be obtained.
 By assigning them the model of rectangular batteries (i.e., by converting a circle into a rectangle as
 shown in Figure 8b), a cylindrical battery is also applicable.
 For modeling heat generating components, we used the PCB with a flip chip-ball grid array
Sensors 2019, 19,package
 (FC-BGA) x FOR PEER shown
 REVIEWin Figure 4 as an example. However, types of packages (e.g., wafer 7 oflevel
 20
 package (WLP)) and boards
 Sensors 2019, 19, x FOR PEER REVIEWare not greatly restricted because a heat generating component is modelled
 7 of 20
 very simply
modelled veryassimply
 a structure shown in shown
 as a structure Figure 6.in The idea6.ofThe
 Figure theidea
 thermal modeling
 of the thermalpresented
 modelingin this paper
 presented
 modelled
incan
 thisuse not very
 paper onlyuse
 can simply
 rigid as a structure
 notboards
 only but also
 rigid shown
 boards butinalso
 flexible Figure 6. The
 substrates
 flexible idea of
 [58–60] the[58–60]
 such
 substrates thermal
 as modeling
 polyimide
 such asand presented
 polyethylene
 polyimide and
 in this paper can
 terephthalateterephthalate
polyethylene use not
 (PET). However, only rigid boards
 if such
 (PET). but also flexible
 flexibleifcircuits
 However, substrates
 are used,
 such flexible [58–60]
 the model
 circuits such as
 of device
 are used, polyimide
 body parts
 the model and
 should
 of device
 bepolyethylene
body replaced with
 parts should
 terephthalate
 abemore (PET). However,
 appropriate
 replaced thermal
 with a more
 if such flexible circuits are used, the model of device
 model.
 appropriate thermal model.
 body parts should be replaced with a more appropriate thermal model.

 (a)(a) (b)
 (b)
 Figure
 Figure
 Figure8. 8.
 Thermal
 8.Thermalmodeling
 Thermal modeling
 modeling inin
 cross sections
 cross for
 sections
 sections forcylindrical
 for cylindrical
 cylindrical battery:
 battery:
 battery: (a)(a)
 thermal resistance
 thermal
 thermal circuit
 resistance
 resistance and
 circuit
 circuit andand
 (b) conversion
 (b)(b)conversion of the circle
 conversionofofthe to
 thecircle
 circletorectangle.
 to rectangle.
 rectangle.

3.3.Thermal
 3.Thermal
 ThermalNetwork
 Network
 Network Model
 Model
 Model
 InInthis
 Inthis
 thissection,
 section,
 section, wewe
 wepresent
 present
 present a thermal
 thermal
 a thermal network
 network
 network model
 model
 model for
 for
 for smart
 smart
 smart glasses
 glasses
 glasses inin
 in a steady
 aasteady
 steady state
 state
 state condition.
 condition.
 condition.
 Based
 Based on ona block
 a blockdiagram
 diagram for
 for an entire
 entire thermal
 thermal network,
 network, thermal
 thermal models
 models forfor each
 each block
 block are
Based on a block diagram for an entire thermal network, thermal models for each block are presented.are presented.
 presented.

 3.1. Block Diagram for Entire Thermal Network
3.1.3.1. Block
 Block Diagram
 Diagram forfor Entire
 Entire ThermalNetwork
 Thermal Network
 AAblock
 block diagram
 diagram for the
 theentire
 entirethermal
 thermal network of smart glasses is shown in Figure 9.
 A block diagram forforthe entire thermal network
 network of smart
 of smart glasses
 glasses is
 is shown
 shown ininFigure
 Figure9.9.ByBy
 Byrepresenting
 representing each
 each block
 block byby thethe equivalent
 equivalent thermal
 thermal resistance
 resistance circuit,
 circuit, ourour model
 model cancan
 be be applied
 applied to to
representing each block by the equivalent thermal resistance circuit, our model can be applied to
 varioustypes
 various typesofofsmart
 smartglasses.
 glasses. Designers
 Designerscan canremove
 removeororreplace blocks
 replace blockswhen
 when necessary. In this
 necessary. paper,
 In this paper,
various types of smart glasses. Designers can remove or replace blocks when necessary. In this paper,
 a thermalmodel
 a thermal modelofofeach
 eachblock
 block is is constructed
 constructed by
 by aa representative
 representativeexample.
 example. ByBychanging
 changingthethe
 thermal
 thermal
a thermal model of each block is constructed by a representative example. By changing the thermal
 modelofofeach
 model eachblock,
 block,smart
 smartglasses
 glasses under various
 variousconditions
 conditionscancanbebeexpressed.
 expressed.
model of each block, smart glasses under various conditions can be expressed.

 Battery Battery
 Battery Temple Temple Battery
 Temple
 Tip Temple
 Tip
 Tip Tip
 Face
 Face
 Temple Temple
 Device Device
 Temple Temple
 Body
 Device Body
 Device
 Body Body
 Lens Lens
 and and
 Lens
 Rim Lens
 Rim
 and and
 Rim Rim

 Figure 9. Block diagram for entire thermal network of smart glasses.
 Figure 9. Block diagram for entire thermal network of smart glasses.

 Figure 9. Block diagram for entire thermal network of smart glasses.
 3.2. Basic Thermal Resistance Model

3.2. BasicFor a thermal
 Thermal resistance
 Resistance model for heat conduction of one cell (called a thermal cell), we basically
 Model
 use the three-dimensional (3D) equivalent resistance model shown in Figure 10a. The heat conduction
 For a thermal
 resistance value resistance modelcan
 in each segment forbeheat conduction
 calculated from of
 theone cell (called
 equation below.a thermal cell), we basically

Sensors 2020, 20, 1446 8 of 20

3.2. Basic Thermal Resistance Model
 For a thermal resistance model for heat conduction of one cell (called a thermal cell), we basically
use the three-dimensional (3D) equivalent resistance model shown in Figure 10a. The heat conduction
resistance value in each segment can be calculated from the equation below.
Sensors 2019, 19, x FOR PEER REVIEW 8 of 20
 l
 R= (4)
 where l is the length of a heat transfer path, k is thekS thermal conductivity of the material, and S is the
 cross-sectional
where area. The
 l is the length of a natural convection
 heat transfer path,heat transfer
 k is the thermal coefficient value for
 conductivity air can
 of the be calculated
 material, and S isfrom
 the
 the equation below.
cross-sectional area. The natural convection heat transfer coefficient value for air can be calculated
from the equation below. .
 × × × ! 0.25 .
 ℎ = k × g × β × Pr
 4 ×  ∆T
 × 0.25 (5)
 hc = × K × (5)
 η2 L
 where g is the acceleration of gravity, β is the air thermal expansion coefficient, Pr is the Prandtl number,
where g is the acceleration of gravity, β is the air thermal expansion coefficient, Pr is the Prandtl number,
 η is the air kinematic viscosity, K is the coefficient in the vertical or horizontal direction, ΔT is the
η is the air kinematic viscosity, K is the coefficient in the vertical or horizontal direction, ∆T is the
 temperature difference, and L is the characteristic length [61]. The heat convection resistance value can
temperature difference, and L is the characteristic length [61]. The heat convection resistance value can
 be calculated from the equation below.
be calculated from the equation below.
 11
 Rc = = (6)
 (6)
 hcℎS 
 where SS is
where is the heat dissipation
 dissipationarea.
 area.The
 Theconvection
 convection heat
 heat transfer
 transfer coefficients
 coefficients for for
 the the
 top,top, bottom,
 bottom, and
and
 sideside are distinguished,
 are distinguished, as shown
 as shown in Figure
 in Figure 10b. The10b. The thermal
 thermal resistance
 resistance model formodel for each
 each part part by
 is created is
created by setting thermal cells in heat transfer paths. The number of thermal cells depends
 setting thermal cells in heat transfer paths. The number of thermal cells depends on heat flow rates on on heat
flow ratestransfer
 the heat on the path.
 heat transfer
 The heatpath. The heat
 convection convection
 resistances areresistances
 connected are connected
 to the thermal to theinthermal
 cells contact cells
 with
in contactair.
 ambient withTheambient
 ambientair. The ambient
 temperature temperature
 is connected to aiscircuit
 connected to a circuit ground.
 ground.

 hc,T

 hc,S

 hc,B
 (a) (b)
 Figure10.
 Figure 10. Thermal
 Thermal resistance
 resistance circuits:
 circuits: (a)
 (a) 3D
 3D heat
 heat conduction
 conduction resistance
 resistance model
 model for
 for one
 one thermal
 thermal cell,
 cell,
 and(b)
 and (b)heat
 heatconvection
 convectionresistance
 resistancemodel
 modelwith withconvection
 convectionheat
 heattransfer
 transfercoefficients.
 coefficients.

3.3.
3.3. Thermal
 Thermal Model
 Model of
 of the
 the Temple
 Temple
 AAtemple
 templeofof thethe
 glasses is divided
 glasses is dividedinto four
 intocells
 fourtocells
 enabletothe connections
 enable with four heat
 the connections withgenerating
 four heat
components of a device body (see Figure 11). In this case, the method to derive
 generating components of a device body (see Figure 11). In this case, the method to derive resistance resistance values of the
thermal model is described in detail. Figure 12 shows thermal resistance
 values of the thermal model is described in detail. Figure 12 shows thermal resistance circuits withcircuits with the structure
of
 thea structure
 glasses’ temple. Thetemple.
 of a glasses’ temple The is divided
 temple is into four thermal
 divided into fourcells.
 thermalA thermal cell consists
 cells. A thermal of the
 cell consists
3D heat conduction resistance model in Figure 10a, which has six
 of the 3D heat conduction resistance model in Figure 10a, which has six resistances in threeresistances in three dimensions
from the center
 dimensions of the
 from the thermal
 center ofcell.
 theFor simplification,
 thermal series resistance
 cell. For simplification, is expressed
 series resistance byisone resistance.
 expressed by
Heat conduction resistance values are calculated by substituting the length,
 one resistance. Heat conduction resistance values are calculated by substituting the length, thermal thermal conductivity of
the material, and
 conductivity areamaterial,
 of the into Equation
 and area(4). into EquationR(4).
 For example, 3 in For
 the vertical
 example, direction
 R3 in the of vertical
 one cell direction
 is 0.25 K/W of
from l = is
 one cell 2.50.25
 mm, K/Wk =from
 236 W/mK,
 l = 2.5 mm,and kS ==236 × 2 mm
 85/4W/mK, 2
 and. STable 4 summarizes
 = 85/4 × 2 mm2. Table the4heat conduction
 summarizes the
resistance values resistance
 heat conduction for a temple. values for a temple.
 The
 The heat dissipations are
 heat dissipations aresetset to
 to three
 three points
 points at at the
 the back,
 back, top,
 top, and
 and bottom
 bottom of of each
 each cell
 cell of
 of aa temple.
 temple.
Heat convection resistance values are calculated by using Equations (5) and
 Heat convection resistance values are calculated by using Equations (5) and (6). The thermal profile(6). The thermal profile for
air shown in Table 5 is used for Equation (5). The characteristic lengths for heat
 for air shown in Table 5 is used for Equation (5). The characteristic lengths for heat dissipation areas dissipation areas use
use the short side for the horizontal surface and height for the vertical surface. The temperature
difference is obtained by iterating the temperature calculation. The number of iterations used in this
paper is three. As a concrete example to calculate the resistance in the vertical direction in one cell of
a temple, the characteristic length is 2 mm, the heat dissipation area is 21.25 mm × 2 mm, and K is
0.52. When the temperature difference is 10 °C, the heat convection resistance becomes Rc = 2.14 × 103

Sensors 2020, 20, 1446 9 of 20

 the short side for the horizontal surface and height for the vertical surface. The temperature difference
 is obtained by iterating the temperature calculation. The number of iterations used in this paper is
 three. As a concrete example to calculate the resistance in the vertical direction in one cell of a temple,
 the characteristic length is 2 mm, the heat dissipation area is 21.25 mm × 2 mm, and K is 0.52. When the
 temperature difference is 10 ◦ C, the heat convection resistance becomes Rc = 2.14 × 103 K/W. Table 6
 summarizes the heat convection resistance values of a temple. The top and bottom of a temple are
 connected
Sensors
 Sensors2019, with
 2019,19,
 19, thePEER
 xxFOR
 FOR bottom
 PEER of a temple tip and with the rim, respectively.
 REVIEW
 REVIEW 99 ofof 20
 20

 Figure
 Figure11. Thermal model of temple (at right side).
 Figure 11.
 11. Thermal
 Thermal model
 model of
 of temple
 temple (at
 (at right
 right side).
 side).

 Figure
 Figure12.
 12.Thermal
 Thermalresistance
 resistancecircuits
 circuitswith
 withaaphysical
 physicalstructure
 structureof
 ofthe
 thetemple.
 temple.
 Figure 12. Thermal resistance circuits with a physical structure of the temple.
 Table
 Table4.4.Heat
 Heatconduction
 conductionresistance
 resistancevalues
 valuesof
 ofthe
 thetemple.
 temple.
 Variable
 Variable
 Variable lll(mm)
 (mm)
 (mm) SS(mm
 S(mm
 (mm
 2)2
 )2 ) Thermal
 ThermalConductivity
 ThermalConductivity(W/mK)
 Conductivity (W/mK) Thermal
 (W/mK) ThermalResistance
 Resistance
 Thermal (K/W)
 (K/W)
 Resistance (K/W)
 RR1,1,RR7 7 10.625
 10.625 10
 10 236
 236 4.50
 4.50
 R1 , R7 10.625 10 236 4.50
 RR2 2 11 106.25
 106.25 236
 236 0.04
 0.04
 R2 1 106.25 236 0.04
 R3 2.5 42.5 236 0.25
 R3R3 2.5
 2.5 42.5
 42.5 236
 236 0.25
 0.25
 RR4,4,RR5,5,RR6 6 21.25
 21.25 10
 1010 236
 236 9.00
 9.00
 R ,R ,R
 4 5 6 21.25 236 9.00

 Table
 Table5.5.Thermal
 Thermalprofile
 profilefor
 forair.
 air.
 Table 5. Thermal profile for air.
 Parameter
 Parameterof ofAir
 Air Value
 Value
 Thermal Parameter of Air Value −2−2
 Thermalconductivity
 conductivity(W/mK)
 (W/mK) 2.625
 2.625××1010
 Acceleration of
 ofgravity
 gravity(m/s
 Thermal conductivity
 Acceleration (m/s 2
 (W/mK) )2) 2.625 × 10−2
 9.80665
 9.80665
 Acceleration 2) 9.80665
 Thermal
 Thermal expansionof
 expansion gravity (m/s
 coefficient
 coefficient (1/K)
 (1/K) 3.247
 3.247 ××10
 10−3−3
 Thermal expansion coefficient (1/K) −3
 Prandtl
 Prandtl number
 number 7.268×××10
 3.247
 7.268 10
 10−1−1
 Prandtl number 7.268 × 10 −1
 Kinematic
 Kinematicviscosity
 viscosity(m(m/s)
 2 2/s) 1.655
 1.655××10 −5
 10−5
 Kinematic viscosity (m 2 /s) 1.655 × 10 −5
 KKin
 inthe
 thevertical
 verticaldirection
 direction 0.56
 0.56
 KKin K in the vertical direction 0.56
 inthe
 thelower
 lowerhorizontal
 horizontaldirection
 direction 0.26
 0.26
 KKinK in the lower horizontal direction 0.26
 inthe
 theupper
 upperhorizontal
 horizontaldirection
 direction 0.52
 0.52
 K in the upper horizontal direction 0.52

 Table
 Table6.6.Heat
 Heatconvection
 convectionresistance
 resistancevalues
 valuesof
 ofthe
 thetemple.
 temple.
 Variable
 Variable ll(mm)
 (mm) SS(mm
 (mm2)2) KK Thermal
 ThermalResistance
 Resistance(K/W)(K/W)
 RR8,8,RR1111, ,RR1414, ,RR1717 55 106
 106 0.56
 0.56 1.0
 1.0××10
 103 3
 RR9,9,RR1212, ,RR1515, ,RR1818 22 42.5
 42.5 0.26
 0.26 4.29
 4.29××10
 103 3
 RR1010, ,RR1313, ,RR1616, ,RR1919 22 42.5
 42.5 0.52
 0.52 2.14 × 10
 2.14 × 103 3

3.4.
 3.4.Thermal
 ThermalModel
 ModelofofElectronic
 ElectronicDevice
 DeviceBody
 Body
 Figure
 Figure13
 13shows
 showsaathermal
 thermalmodel
 modelaround
 aroundaaheating
 heatingcomponent,
 component,which
 whichisiscomposed
 composedof
 ofthermal
 thermal

Sensors 2020, 20, 1446 10 of 20

 Table 6. Heat convection resistance values of the temple.

 Variable l (mm) S (mm2 ) K Thermal Resistance (K/W)
 R8 , R11 , R14 , R17 5 106 0.56 1.0 × 103
 R9 , R12 , R15 , R18 2 42.5 0.26 4.29 × 103
 R10 , R13 , R16 , R19 2 42.5 0.52 2.14 × 103

3.4. Thermal Model of Electronic Device Body
 Figure 13 shows a thermal model around a heating component, which is composed of thermal
resistances for each layer in the vertical direction and a heat source connected in the center of a heat
generation layer. The heat source is given by power dissipated by a heat generating component.
A power consumption value (in Watt) is given to the heat source. A thermal model of an electronic
device body that incorporates five heat generating components is shown in Figure 11. It is a model
 Sensors
 Sensors 2019,
 19, 19, x FOR PEER REVIEW 10 10 of 20
for the2019,
 device x FOR
 body PEER
 of REVIEW
 the right side. The device body is composed of heat generating components, of 20

PCBs, copper planes, device body cases, and a projector. Their parts are replaced by thermal cells.
 PCBs,
 PCBs, copper
 copper planes,
 planes, device
 device body
 body cases,
 cases, andand a projector.
 a projector. Their
 Their parts
 parts areare replaced
 replaced byby thermal
 thermal cells.
 cells.
ForForexample,
 example,
 a PCB
 a PCB
 is is
 replaced
 replaced
 with
 with
 four
 four
 thermal
 thermal
 cells.
 cells.
 The
 The
 thermalresistance
 thermal
 resistancevalues
 valuesinineach
 eachthermal
 thermal
 For example, a PCB is replaced with four thermal cells. The thermal resistance values in each thermal
cell are
 cellcell
 calculated
 areare calculated
 calculated
 from
 from fromthe length,
 length,
 thethe length,thermal
 thermal
 thermal
 conductivity,
 conductivity,
 conductivity,
 and
 andandcross-sectional
 cross-sectional
 cross-sectional
 area
 area
 area
 of aacell,
 of aofcell,
 cell,as
 as as
 shown
 shownin
 shown
Equation
 in Equation
 in Equation (4). The
 (4).(4).
 The heat
 Theheat dissipations
 heat dissipations
 dissipations are set
 areare to
 setset four
 to four
 to four points
 points
 points at the
 at the
 at the front,
 front,
 front, back,
 back,
 back, top,
 top,
 top, and and
 and bottom
 bottom
 bottom ofofeach
 of each each
heat
 heat generating
 heat generating
 generating component.
 component.
 component. AAheat
 Aheat
 heat generating
 generating
 generating component
 component
 component represented
 represented
 represented inFigure
 in Figure
 in Figure 6 is66modelled
 isismodelled
 modelled toto
 to the thethe
equivalent
 equivalent
 equivalent thermal
 thermal
 thermal resistance
 resistance circuit
 resistancecircuit in
 circuit Figure
 in in 14.
 Figure
 Figure The
 14.14.
 Theheat
 The source
 heat
 heat is
 source
 source located in
 is located
 is located the
 in in center
 thethe with
 center
 center a current
 with
 with a a
source
 current symbol.
 current source
 source Five
 symbol. heat
 symbol. Fivegenerating
 Five heat
 heat components
 generating
 generating componentsare arranged
 components areare in theinbody
 arranged
 arranged in the
 the case.
 body
 body case.case.

 Figure
 Figure
 Figure 13.13.
 13. Thermal
 Thermal
 Thermal model
 model
 model around
 around the
 thethe
 around heating
 heating component.
 component.
 heating component.

 Figure
 Figure 14. Thermal
 14.14.
 Figure Thermal model
 Thermal
 model of of
 model ofthe
 the the device
 device
 device body
 body (on
 (on(on the
 thethe right
 right
 right side).
 side).
 side).

3.5.3.5. Thermal
 Thermal Model
 Model of Temple
 of Temple TipTip
 A temple
 A temple tiptip of the
 of the glasses
 glasses is modelled
 is modelled as as
 oneone thermal
 thermal cell.
 cell. Figure
 Figure 15 15 shows
 shows a thermal
 a thermal model
 model forfor
 a a
 temple
temple tiptip of the
 of the right
 right side.
 side. TheThe temple
 temple tiptip is expressed
 is expressed by by
 thethe3D3D equivalent
 equivalent resistance
 resistance model
 model shown
 shown
in in Figure
 Figure 10a.
 10a. TheThe heat
 heat dissipations
 dissipations to to
 an an
 airair
 areare
 setset
 to to three
 three points
 points at the
 at the top,
 top, bottom,
 bottom, andand back
 back of of
 a a

Sensors 2020, 20, 1446 11 of 20
 Figure 14. Thermal model of the device body (on the right side).

 3.5.
 3.5. Thermal
 Thermal Model
 Model of
 of Temple
 Temple Tip
 Tip
 A
 A temple tip of the glasses is modelled
 modelled as as one
 one thermal
 thermalcell.
 cell.Figure
 Figure1515shows
 showsaathermal
 thermalmodel
 modelfor
 fora
 atemple
 templetip
 tipofofthe
 theright
 rightside.
 side.The
 Thetemple
 templetip
 tipisisexpressed
 expressed byby the
 the 3D
 3D equivalent resistance model shown
 shown
 in
 in Figure
 Figure 10a. The
 The heat
 heat dissipations
 dissipationsto
 toan
 anair
 airare
 areset
 settotothree
 threepoints
 pointsatatthe
 thetop,
 top,bottom,
 bottom,and
 andback
 backofofa
 atemple
 templetip.
 tip.

 Sensors2019,
Sensors 2019,19,
 19,x xFOR
 FORPEER
 PEERREVIEW
 REVIEW
 Figure 15. Thermal
 Figure 15. Thermal model
 model of
 of temple
 temple tip
 tip (on
 (on the
 the right
 right side).
 side). 1111ofof2020

 3.6. Thermal
 3.6.Thermal
3.6. Model
 Modelofof
 ThermalModel Battery
 ofBattery
 Battery
 For
 Foraaabattery
 For battery model,
 batterymodel,
 model,the the inner
 theinner liquid
 innerliquid fluid
 liquidfluid and
 fluidand outer
 andouter frame
 outerframe
 frameare arerepresented
 are representedby
 represented by thermal
 bythermal resistances.
 thermalresistances.
 resistances.
 Figure
 Figure 16
 16 shows
 shows thermal
 thermal resistance
 resistance circuits
 circuits of
 of aa battery
 battery structure.
 structure. R
 R 1 is
 is the
 the resistance
 resistance
Figure 16 shows thermal resistance circuits of a battery structure. R1 is the resistance to connect with
 1 to
 to connect
 connect with
 with
 a device
aadevice
 devicebody,body, R to R are
 body,RR222totoRR888are the
 arethe resistances
 theresistances of a frame,
 resistancesofofaaframe, R to R
 frame,RR999totoRR1111 are the
 arethe
 11are resistances
 theresistances of
 resistancesofofthethe inner
 theinner parts,
 innerparts,
 parts,
 and
 and RR12 to
 to R
 R 15 are
 are the
 the heat
 heat convection
 convection resistances.
 resistances. Figure
 Figure 17
 17 shows
 shows the
 the
and R12 to R15 are the heat convection resistances. Figure 17 shows the thermal model of the battery
 12 15 thermal
 thermal model
 model of
 of the
 the battery
 battery
 on the
 onthe
on right
 theright side.
 rightside.
 side.TheThe number
 numberofof
 Thenumber thermal
 ofthermal
 thermalcellscells used
 cellsused for
 usedforforaaabattery
 battery
 batteryin in this
 inthis paper
 thispaper isisseven.
 paperis seven.
 seven.The The heat
 Theheat
 heat
 dissipations
 dissipationsare
dissipations are set
 areset to five
 settotofive points
 fivepoints at the
 pointsatatthe front,
 thefront, back,
 front,back, top,
 back,top, bottom,
 top,bottom,
 bottom,and and
 andtiptip of each
 tipofofeach cell
 eachcell of a battery.
 cellofofaabattery.
 battery.

 (a)
 (a) (b)
 (b) (c)
 (c)
 Figure 16.Thermal
 Figure Thermal resistancecircuits
 circuits withaaphysical
 physical structureofofthe
 the battery:(a)
 (a) zydirection,
 direction, (b)zx
 zx
 Figure16.
 16. Thermalresistance
 resistance circuitswith
 with a physicalstructure
 structure of thebattery:
 battery: (a)zyzy direction,(b)
 (b) zx
 direction, and(c)
 direction, (c) xydirection.
 direction.
 direction,and
 and (c)xy
 xy direction.

 Figure 17.
 Figure17.
 Figure Thermal
 17.Thermal model
 modelofof
 Thermalmodel the
 ofthe battery
 thebattery (on
 battery(on the
 (onthe right
 theright side).
 rightside).
 side).

 3.7.Thermal
3.7. ThermalModel
 ModelofofLens
 Lensand
 andRim
 Rim

 AAlens
 lensisisrepresented
 representedbybyaatwo-dimensional
 two-dimensionalthermal
 thermalresistance
 resistancemodel,
 model,andandthe
 therim
 rimofofthe
 thelens
 lens
 frame is represented by a one-dimensional thermal resistance model. The lens and rim
frame is represented by a one-dimensional thermal resistance model. The lens and rim are divided are divided
 intothree
into threeparts:
 parts:lens,
 lens,upper
 upperrim,
 rim,and
 andlower
 lowerrim.
 rim.Three
 Threethermal
 thermalcells
 cellsare
 areused.
 used.Figure
 Figure1818shows
 showsthethe
 model of the lens and rim of the right side. The heat dissipations are set to eight points at
model of the lens and rim of the right side. The heat dissipations are set to eight points at the frontthe front

Sensors 2020, 20, 1446 12 of 20

3.7. Thermal Model of Lens and Rim
 A lens is represented by a two-dimensional thermal resistance model, and the rim of the lens
frame is represented by a one-dimensional thermal resistance model. The lens and rim are divided
into three parts: lens, upper rim, and lower rim. Three thermal cells are used. Figure 18 shows the
model of the lens and rim of the right side. The heat dissipations are set to eight points at the front
and back of a lens and the upper front, back, and top and the lower front, back, and bottom of a rim.
The temperature
Sensors 2019, 19, x FORofPEER
 a nose pad on the nose is represented by T109 . The node (T109 ) is connected
 REVIEW 12 with
 of 20
a Sensors
 face part.
 2019, 19, x FOR PEER REVIEW 12 of 20
 Sensors 2019, 19, x FOR PEER REVIEW 12 of 20

 Figure 18. Thermal model of the lens and rim (on the right side).
 Figure Thermal
 18. Thermal model of thelens
 lens and rim
 rim (on the right side).
 Figure 18. Thermalmodel
 Figure18. modelofofthe
 the lensand
 and rim(on
 (onthe
 theright
 rightside).
 side).
3.8. Thermal
3.8. Thermal Model of the
 ModelModel
 3.8. Thermal of theofFace
 Face PartPart
 Part
 the Face
 3.8. Thermal Model of the Face Part
 FigureFigure
 Figure 19 shows
 19 shows aa thermal
 19 shows thermal
 a thermalmodel
 modelmodel ofof
 of aa face
 aface for
 facefor smart
 forsmart glasses.
 smart glasses. Theface
 glasses. The
 The facepart
 face part is expressed
 is is
 part expressed
 expressed by one
 by one
 by one
thermal
thermal cell. The
 Figure
 thermal
 cell. The
 19cell. center
 shows node
 anode
 The center
 center is set
 thermal
 node
 is set to 36.6
 ismodel
 set
 to 36.6
 to of◦°C.
 36.6aC.°C.RR
 face
 R 1 1and
 for RR22 represent
 andsmart glasses.
 represent the
 the thermal
 The resistances
 face resistances
 thermal part of the
 is expressed
 of the byface
 face one
 1 and R2 represent the thermal resistances of the face
skin. The
 thermal
 skin.side
 cell.
 The and
 The
 side bottom
 center
 and of
 node
 bottom the
 ofis face
 set
 the topart
 face 36.6
 partare
 °C.
 are connected
 R 1 and R2with
 connected the
 represent
 with the temple
 the
 temple tip
 thermal
 tip
skin. The side and bottom of the face part are connected with the temple tip nose pads, respectively. nose
 nose pads, respectively.
 resistances
 pads, of the
 respectively. face
 skin. The side and bottom of the face part are connected with the temple tip nose pads, respectively.

 Figure 19. Thermal model of the face part.
 Figure 19. Thermal
 Figure 19. Thermal model
 model of
 of the
 the face
 face part.
 part.
 4. Experimental Results Figure 19. Thermal model of the face part.
4. Experimental Results
4. Experimental Results
 We first verify the validity of our thermal model. Figure 20 shows a histogram in temperature
 We first verify
 4. Experimental
 differences the validity
 Results
 between of our thermal
 results obtained by our model.
 model and Figure 20 shows
 a thermal solvera [62].
 histogram in temperature
 The absolute errors
 We first
differences verify
 between the validity
 results of ourby
 obtained thermal
 our model.
 model and Figure
 a 20 shows
 thermal solvera histogram
 [62]. The in temperature
 absolute errors
 were almost within a few degrees. This is the result under the conditions that five heat generating
differences
 We
were almost between
 first verify
 within results
 the obtained
 validity of by
 our our
 thermal model and
 model. a thermal
 Figure 20 solver
 shows a [62]. The
 histogram absolute
 in errors
 temperature
 components areaplaced
 few degrees. This
 at one side is the
 only and result under
 the power the conditions
 consumptions that five
 are uniform heat
 and 5W generating
 in the
were almost
 differences
components within
 between
 total. are
 a results
 placed few degrees.
 at one obtained
 side Thisand
 only byis our
 the result under
 the model
 power and a the conditions
 thermal
 consumptions solver
 are that
 [62].five
 uniform Theheat
 and generating
 5 absolute
 W in the errors
 total.
components
 were almostare placed
 within at one
 a few side only
 degrees. This and theresult
 is the power consumptions
 under are uniform
 the conditions that fiveandheat5generating
 W in the
total.
 components are placed at one side only and the power consumptions are uniform and 5 W in the
 total.

 Figure
 Figure 20. Error
 20. Error distribution
 distribution forforallallnodes
 nodesof
 of the
 the thermal
 thermalnetwork
 networkmodel.
 model.

 Next, we perform thermal analysis using the proposed model. Some countermeasures to reduce
 temperaturesFigure
 are shown in this
 20. Error section. Itfor
 distribution is all
 important
 nodes ofto reduce
 the thenetwork
 thermal surface model.
 temperature of smart
 glasses for more physical health safety and comfortable use.
 Figure 20. Error distribution for all nodes of the thermal network model.
 Next,Figure 21 illustrates
 we perform thermalthatanalysis
 heat sources
 using (a)the
 were placed onmodel.
 proposed one side only countermeasures
 Some and (b) were divided
 tointo
 reduce
 both sides. Those are examples of conditions in which the total power consumption is 5 W and the

Sensors 2020, 20, 1446 13 of 20

 Next, we perform thermal analysis using the proposed model. Some countermeasures to reduce
 temperatures are shown in this section. It is important to reduce the surface temperature of smart
 glasses for more physical health safety and comfortable use.
 Figure 21 illustrates that heat sources (a) were placed on one side only and (b) were divided into
Sensors 2019, 19, x FOR PEER areREVIEW 13 of 20the
 Sensors sides.
 both 2019, 19,Those
 x FOR PEER examples
 REVIEW of conditions in which the total power consumption is 5 W and 13 of 20
 power consumption is uniform. Figure 22 shows differences in temperatures when the device body
waswas
 was setsettoto
 set (a)(a)
 to one
 one side only
 side only
 onlyand
 and
 and toto(b)
 to (b)both
 (b) both
 both sides.
 sides.InIn
 sides. Inthe
 the
 thefigure,
 figure,
 figure, “HG”
 “HG”
 “HG” means
 means
 means the
 the
 thehighest
 highest
 highest temperature
 temperature
 temperature in
ininheat
 heat
 heat generating
 generating
 generating components,
 components,
 components, “DB”
 “DB” means
 “DB” means the temperature
 the temperature
 means the temperature on
 on theon the
 surface surface of
 of the device
 the surface the device
 of thebody, device body,
 “Temple”
 body,
“Temple”
 means the
 “Temple” means
 means the
 temperature
 thetemperature
 temperature inin
 in the center the ofcenter
 the the ofof
 the
 temple
 center the temple
 frame,
 temple frame,
 “Ear” “Ear”
 means
 frame, “Ear” means
 the the
 temperature
 means thetemperature ofof
 of the frame
 temperature
the
 theframe
 surface
 frame surface
 where
 surface where
 the where the
 glassesthe glasses
 frame frame
 is onframe
 glasses is
 the ear, on the
 andthe
 is on ear,
 “Nose” and
 ear, and “Nose”
 means means the
 the temperature
 “Nose” temperature
 of a nose pad.
 means the temperature of a nose
 of In their
 a nose
pad.
 pad. In their
 parts,Inthe theirparts,
 temperaturethe temperature
 parts, the of the “Ear” and
 temperature of the
 of the “Ear”
 “Nose” and
 “Ear”areand “Nose”
 very
 “Nose” are
 important very
 are very important
 since since
 they directly
 important they
 sincetouch directly
 the skin.
 they directly
touch
 For the
 touch the skin.
 example, skin.For
 under
 For example,
 example, under
 the condition
 under the
 ofthe condition
 the ofof
 total power
 condition the total
 consumption
 the totalpower
 power consumption
 of 5 W, when the
 consumption ofof 5device
 W,
 5 W, when
 bodythe
 when was
 the
device
 set to body
 one was
 side set
 only,to one
 the side only,
 temperatures the temperatures
 on the back side on of the
 theback side
 temple
 device body was set to one side only, the temperatures on the back side of the temple and the ear are of
 and the
 the temple
 ear are and
 60.6 the
 and ear are
 51.0 ◦ C,
60.6 and 51.0
 respectively.
 60.6 °C,
 and 51.0 °C, respectively.
 On respectively.
 the other hand,On the
 On when other
 the other hand,
 the hand, when
 devicewhen the
 bodythe device
 wasdevice body
 set tobody was
 both was set
 sides, to
 setthe both
 to temple sides,
 both sides, the
 and theear
temple and
 temperatures ear temperatures
 were 48.6 and were
 43.6 ◦48.6
 C, and 43.6
 respectively. °C, Byrespectively.
 distributing
 temple and ear temperatures were 48.6 and 43.6 °C, respectively. By distributing the device body Bythedistributing
 device body the device
 (heat body
 generating
(heat
 (heat generating
 components),
 generating components),
 the temperaturethe
 components), thetemperature
 rising to the ambient
 temperature rising totothe
 temperature
 rising theambient
 of 25 ◦temperature
 ambient Ctemperature
 can be reduced ofof2525by°C°C can
 canbe
 33.6% and
 be
reduced
 29.4%. The
 reduced by 33.6% and
 by difference 29.4%.
 33.6% andof29.4%. The
 the earThe difference
 temperature
 differencefromof the
 of the ear
 the ear temperature
 coretemperature from
 temperaturefrom the
 of 36.6 core
 ◦
 theCcore temperature
 is reduced
 temperature ofof
 by 51.4%.
36.6
 As°C
 36.6 seen
 °Cis is
 reduced
 from
 reduced byby
 Figure 51.4%.
 22, the As
 51.4%. seen
 temperature
 As seen from
 from Figure
 growths
 Figure 22,
 can22,the
 be
 the temperature
 significantly
 temperature growths
 suppressed
 growths can bybedistributing
 can significantly
 be significantlyheat
suppressed by
 sources regardless
 suppressed distributing heat
 of total power
 by distributing sources regardless
 consumption.
 heat sources of total power consumption.
 regardless of total power consumption.

 (a)(a) (b)(b)
 Figure 21. Illustration of heat sources: (a) one side and (b) both sides (example of total power of 5 W).
 Figure
 Figure 21.21.Illustration
 Illustrationofof heat
 heat sources:
 sources: (a)(a) one
 one side
 side and
 and (b)(b) both
 both sides
 sides (example
 (example of of total
 total power
 power of of 5 W).
 5 W).

 (a)(a) (b)(b)
 Figure 22.22.
 Figure Temperature differences
 Temperature of of
 differences heat sources:
 heat (a)(a)
 sources: oneone
 side and
 side (b)(b)
 and both sides.
 both sides.
 Figure 22. Temperature differences of heat sources: (a) one side and (b) both sides.

 Figure 23 illustrates
 Figure the conditions in which power consumptions are not uniform and heat
 Figure2323illustrates
 illustratesthetheconditions
 conditionsininwhich
 whichpower
 powerconsumptions
 consumptionsare arenot
 notuniform
 uniformandandheat
 heat
 sources
sources were
 were placed
 placed at
 atat
 oneone side
 side only.
 only. Figure
 Figure 24
 2424 shows
 shows temperature
 temperature results
 results at
 atat each
 each part
 part when
 when power
 power
 sources were placed one side only. Figure shows temperature results each part when power
 consumptions
consumptions are are uniform,
 uniform, in in descending
 descending order,
 order, andandin in ascending
 ascending order.
 order. The
 The temperatures
 temperatures of of “Ear”
 “Ear”
 consumptions are uniform, in descending order, and in ascending order. The temperatures of “Ear”
 ◦ C (temperature differences from the core temperature of 36.6 ◦ C were 14.4,
 were
were 51.0,
 51.0, 49.4,
 49.4, and and 52.7
 52.7 °C°C(temperature differences from the core temperature ofof
 36.6 °C°C
 were 14.4,
 were 51.0, 49.4, and 52.7 (temperature differences from the core temperature
 ◦ C) for uniform, descending order, and ascending order. The descending order can reduce
 36.6 were 14.4,
 12.8,
12.8, and16.1
 16.1°C)
 12.8, and 16.1 °C) for uniform, descending order, and ascending order. The descending ordercan
 and for uniform, descending order, and ascending order. The descending order can
 a temperature
reduce a rise byrise
 temperature 11.1%by compared
 11.1% with uniform
 compared with power consumptions.
 uniform power Therefore,Therefore,
 consumptions. the temperature
 the
 reduce a temperature rise by 11.1% compared with uniform power consumptions. Therefore, the
 of the part near the
temperature ear that directly touches the skin can be reduced when power consumptions of
 temperatureofofthe thepart
 partnear
 nearthe theear
 earthat
 thatdirectly
 directlytouches
 touchesthe theskin
 skincan
 canbebereduced
 reducedwhenwhenpower
 power
 heat generating
consumptions of components
 heat generating arecomponents
 placed in descending
 are placed order.
 in descending order.
 consumptions of heat generating components are placed in descending order.