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,
2,
3,
4,
5,
6,
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.
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,
37,
38,
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,
41,
42,
43,
44,
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 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,
53,
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,
53,
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.
4. Experimental Results
We first verify the validity of our thermal model.
Figure 20 shows a histogram in temperature differences between results obtained by our model and a thermal solver [
62]. The absolute errors were almost within a few degrees. This is the result under the conditions that five heat generating components are placed at one side only and the power consumptions are uniform and 5 W in the total.
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 both sides. Those are examples of conditions in which the total power consumption is 5 W and the power consumption is uniform.
Figure 22 shows differences in temperatures when the device body was set to (a) one side only and to (b) both sides. In the figure, “HG” means the highest temperature in heat generating components, “DB” means the temperature on the surface of the device body, “Temple” means the temperature in the center of the temple frame, “Ear” means the temperature of the frame surface where the glasses frame is on the ear, and “Nose” means the temperature of a nose pad. In their parts, the temperature of the “Ear” and “Nose” are very important since they directly touch the skin. For example, under the condition of the total power consumption of 5 W, when the device body was set to one side only, the temperatures on the back side of the temple and the ear are 60.6 and 51.0 °C, respectively. On the other hand, when the device body was set to both sides, the temple and ear temperatures were 48.6 and 43.6 °C, respectively. By distributing the device body (heat generating components), the temperature rising to the ambient temperature of 25 °C can be reduced by 33.6% and 29.4%. The difference of the ear temperature from the core temperature of 36.6 °C is reduced by 51.4%. As seen from
Figure 22, the temperature growths can be significantly suppressed by distributing heat sources regardless of total power consumption.
Figure 23 illustrates the conditions in which power consumptions are not uniform and heat sources were placed at one side only.
Figure 24 shows temperature results at each part when power consumptions are uniform, in descending order, and in ascending order. The temperatures of “Ear” were 51.0, 49.4, and 52.7 °C (temperature differences from the core temperature of 36.6 °C were 14.4, 12.8, and 16.1 °C) for uniform, descending order, and ascending order. The descending order can reduce a temperature rise by 11.1% compared with uniform power consumptions. Therefore, the temperature of the part near the ear that directly touches the skin can be reduced when power consumptions of heat generating components are placed in descending order.
Figure 25 illustrates the position on the temple in the x direction.
Figure 26 shows the temperatures at each position when power consumptions are uniform in descending order and in ascending order. For example, at 55.8 mm, temperatures for a uniform, descending order, and an ascending order are 58.6, 56.1, and 61.4 °C. The descending order is superior in that it can reduce the temperature near the ear.
Figure 27 illustrates differences in the frame materials, where (a) is the all-Al frame, (b) is the all-CA frame, and (c) is the CA temples and temple tips and Al rims.
Figure 28 depicts that Al greatly reduces temperatures of “HG,” “DB,” and “Temple,” whereas CA extremely raises the temperatures. The reason is thermal conductivity of Al is much higher than that of CA. However, the temperature of “Ear” is highest (51.0 °C) when Al was used but lowest (41.6 °C) when Al and CA were used. The temperature differences from the core temperature of 36.6 °C were 14.4 and 5.0 °C. The combination can suppress a temperature rise by 65.3% compared with Al only. For parts directly contacting the body, we found that a combination of CA temples and temple tips and Al rims is the best. The result obtained by a combination of Al and CA can satisfy the limit temperature (43 °C), which does not cause a low temperature burn [
32]. Therefore, the method will be effective for the physical health safety and comfortable use of future smart glasses.