Ⅰ. Introduction

Since the invention of eyewear frames, they have been so called tailor-made items. Optometrists or ophthalmologists have prescribed eye-glasses for each patient, and opticians have dispensed individually personalized eye-glasses. Opticians have also made an effort to optimize wearing comfort by adjusting dispensed eyewear frames. An exception of such custom-made eyewear frame is sunglasses, which can be sold without optician’s fitting process. Sunglasses are manufactured with various sizes and designs, so that customers can choose most appropriately fitted ones for themselves. Various sizes of sunglasses can cover almost full size range needed by customers.

However, recent development of 3D^1,2) or smart glasses requires mass-productive eyewear frames with single or few standardized sizes instead of the tailor-made fitting process. 3D/smart glasses are mainly manufactured by IT companies, who do not prefer to diverse product size. It is because diverse product sizes may cause some problems in cost down, stock management, delivery, and so on. Recently commercialized 3D or smart glasses actually have only one size, or have few size variations at least.

This single-sized production of 3D or smart glasses induces two kinds of newly-arisen problems. One of the problems is what the best single size for mass-productive eyewear frames is. Eyewear frame of single size cannot fit into all customers, but it is supposed to satisfy as many customers as possible. This problem is not an easy one, but requires more ergonomic studies. The other problem is how to objectively assess wearing comport of the mass-productive eyewear frames. Is the adopted single size comfortable for many customers? Such assessment method of wearing comfort is getting important in manufacture industry of 3D or smart glasses.

As to the latter question, we think that electroencephalography (EEG) is one of appropriate tools for this purpose because it has successfully been utilized for the assessment of wearing comfort in the fields of apparel study and progressive addition lenses.^3-7) We have tried to utilize EEG for assessment of the wearing comfort of eyewear frames, but have not gotten obvious results in our previous study.⁸⁾ In this work, we again applied EEG to assess the wearing comfort of eyewear frames, and got some results with statistical significant.

Ⅱ. Methods

1. Subjects

This study followed the tenets of the Declaration of Helsinki. Forty Korean adult subjects were enrolled for this study. They are in their age of 20-59; 18 of them were male and 22 of them were female; 18 of them were eyeglasses wearers, 22 of them were not eyeglasses wearers. After being informed about the nature and possible consequences of the study participation, all the subjects gave their informed written consent to study participation. All of them were screened before enrolment, so that they had neither any kind of physical, mental, ocular, nor automatic-nervoussystematic illness. There was no other inclusion criterion except for the illness.

2. Study design

Subjects wore two kinds of mass-productive 3D glasses (eyewear frame-A and -B) in random sequence. The eyewear frame-A was made in 2010 by a Japanese company (S electronics) for their 3D TV (brand name X), while the eyewear frame-B was made in the same 2010 by another Japanese company (P electronics) for their 3D TV (brand name V). Wearing time for each eyewear frame was 3 minute, with 1 minute of intermission given between wearing of the two kinds of 3D glasses. For the subjects wore the eyewear frames, their brain waves were measured by EEG. In order to exclude any kind of visual interruption caused by watching displays or objects, the 3D glasses were turned off (that is, opaque) and the subjects were asked to keep their eyes closed during EEG measurement. After the measurements, we asked the subjects which eyewear frame is more comfortable.

3. Electroencephalography (EEG)

The EEG measures electrical changes of human brain generated by activation of neuronal cells, which are recorded as a form of an electronic wave function called brain wave. The brain waves are classified into several kinds by frequency, that is, the α-, β-, γ-, δ-, and θ -waves. Each of them corresponds to various brain activities, for example, it is well known that the α-wave is closely related to mental relaxation and stability.^9-12) In this study, we focused on the α-wave, assuming it represented the wearing comfort of eyewear frames because more comfortable eyewear frame would give wearers more mental relaxation and stability^8,9).

The EEG signals were measured by a Poly G-1 detector (supplied by Laxtha Inc., Korea). Its 8-channel electrodes were attached to each subject’s head, according to the international standard electrode position (the 10-20 system).¹³⁾ The positions of 8 electrodes corresponds to right and left prefrontal, frontal, temporal, and parietal lobes of the subjects’ brain, respectively.

4. Data analysis

We did not analyse wave form of the α-waves, but simply compare their time-averaged intensities to each other. Since absolute intensities of subjects’ α-waves are quite different from one another, they are not appropriate for direct comparison. Instead we adopted relative intensity of the α-wave as a variable for comparison. It is an ratio between the intensity of α-wave and the intensity of total brain waves including the α-, β-, γ-, δ-, and θ-waves. The relative intensity made it possible to compare the subjects’ α-wave intensities under normalized condition. The collected raw data were analysed using Telescan^TM software (supplied by Laxtha Inc., Korea),¹⁴⁾ which could automatically calculate the time-averaged relative intensity of brain waves. The SPSS statistics software was used for the paired t-test between data obtained from the two kinds of eyewear frames. The significant level of p-value was below 0.05.

Ⅲ. Results and Discussion

Fig. 1 is mapping diagrams that depict the time-averaged relative intensity of α-wave measured from the 40 subjects. The intensity values given in Fig. 1 is mean values averaged to the 40 subjects.

Table 1 gives mean values and standard deviations calculated from the time-averaged relative intensities of the α-waves measured from the 40 subjects. It also gives the results of paired t-test between eyewear frame-A and -B at each electrode. We can easily find the α -wave intensities of the frame-A are higher than those of the eyewear frame-B. All of the results are statistically significant with the significant level p < 0.05. Differently from our previous study,⁸⁾ statistical significance was revealed at every electrode.

It is well known that the α-waves in EEG mean comfortable state of brain.^9-11) Shown in Table 1, the time-averaged relative intensity of α-waves of the frame-A was higher than that of the frame-B with statistical significance at every electrode. It can be interpreted that, owing to the better wearing comfort, brains of the subjects wearing the frame-A generated the α -waves of higher intensity than those wearing the frame-B.

We would suggest that the time-averaged relative intensity of α-wave in EEG can be used for the assessment of wearing comfort of mass-productive eyewear frames. It is because that more comfortable frame-A resulted in higher time-averaged relative intensity of α -wave in the EEG analysis, while less comfortable frame-B resulted in lower intensity with statistical significance.

Here we premise that the eyewear frame-A has better wearing comfort than eyewear frame-B. It is because, in the oral survey after the EEG measurements, almost every subject agreed that the eyewear frame-A is more comfortable than the frame-B. It is also because that the eyewear frame-A had a function of temple-width control but the eyewear frame-B had not. Temples of the frame-A can be bent at their middle points by hinges to small, medium, or large size. The subjects were asked to adjust the temple-width of eyewear frame-A to fit to their head width before the EEG measurement. It sounds reasonable that the size-controllable eyewear frame-A is more comfortable for wearers than the single-sized eyewear frame-B.

We must also point out that absolute value of the time-averaged relative intensity of α-wave is not an index of wearing comfort itself. It so much depends on measurement conditions and subjects’ character that it would be meaningless if the measurement conditions or the subject group were different. The intensity of α-wave can be useful for wearing comfort assessment, only when two kinds of eyewear frames are compared to each another under the same measurement conditions and for the same subject group. Absolute wearing comfort cannot be digitized by experimental scheme described in this work.

Ⅳ. Conclusions

We measured the time-averaged intensities of α-waves from the subjects wearing massproductive eyewear frames. The intensities when the subjects were wearing more comfortable frame were higher with statistical significance at every electrode than the intensities when the subjects were wearing less comfortable one. Therefore, we can suggest that the timeaveraged relative intensity of α-waves can be an objective index for comparative assessment of the wearing comfort of mass-productive eyewear frames.