Since curved televisions were introduced in the market, the consumers have paid attention to this new viewing experience, which has resulted in a noticeable increase in sales [1After Years of Flattening, Curved TV Screens Leap into the Global Market, According to NPD DisplaySearch. <http://www.displaysearch.com/cps/rde/xchg/displaysearch/hs.xsl/140404_curved_tv_screens_leap_into_the_global_market.asp>. [Google Scholar],2Curved Screens Make Our Brains Light Up with Pleasure. <https://www.newscientist.com/article/mg22229714-900-curved-screens-make-our-brains-light-up-with-pleasure/>. [Google Scholar]]. Experts forecasted further market growth and predicted the expansion to other display products. More recently, leading display manufacturers launched curved monitors, and a sharp increase in sales has also been observed [3Samsung Electronics' Curved Monitor Portfolio Surpasses One Million Sales Mark. <http://news.samsung.com/global/samsung-electronics-curved-monitor-portfolio-surpasses-one-million-sales-mark>. [Google Scholar],4Samsung is the Leader in the 4 K Market … . For the Time Being. <http://4k.com/news/samsung-is-the-leader-in-the-4k-market-6784/>. [Google Scholar]].
Despite the market growth, however, there have been few studies that provide empirical evidence of the advantage that the consumers can derive by using a curved display [5K.A. Jeong, N. Na and H.J. Suk, SID. 15, 130–133 (2015). doi: 10.1002/sdtp.10314[Crossref], [Google Scholar]]. In general, it has been said that people prefer curvilinear to rectilinear objects [6O. Vartanian, G. Navarrete, A. Chatterjee, L.B. Fich, H.Leder, C. Modroño, M. Nadal, N. Rostrup and M. Skov, Proc. Nat. Acad. Sci. 110, 10446–10453 (2013). doi: 10.1073/pnas.1301227110[Crossref], [PubMed], [Web of Science ®], [Google Scholar]]. From a simple geometric form to a complex form, curvilinear objects are likely to be preferred to rectilinear objects. Based on such conception, it has been anticipated that the curved display will be preferred to the flat display [7K. Choi, H. Bae, S.-W. Ju and H.J. Suk, presented at the SID Symposium Digest, 2015. [Google Scholar]]. Indeed, the manufacturers advertise the excellence of the curved displays based on the principle of human vision; for instance, it is often quoted that the human vision system has evolved to perceive three-dimensional surroundings rather than a flat surface [8V.S. Ramachandran, Nature. 331, 163–166 (1988). doi: 10.1038/331163a0[Crossref], [PubMed], [Web of Science ®], [Google Scholar],9P.H. Winston and B. Horn, The Psychology of Computer Vision (McGraw-Hill, New York, NY, 1975). [Google Scholar]]. According to this assertion, the viewing distance can be better maintained when one watches a curved display, which may contribute to more comfortable and immersive viewing. In addition, Kelley's study demonstrated that the curved display can more effectively avoid the reflection of the ambient light source by testing the reflection performance of two of the most recently released models of the curved television [10E.F. Kelley, Inf. Disp. 30, 12–19 (2014). [Google Scholar]].
With the rising interest in the curved display, there have been a number of researches on the optimal curvature of such display, especially in relation to the curved TV [11Y. Park, J.J. Yoo, D. Kang and S. Kim, J. Soc. Inf. Disp. 23 (9), 391 (2015). doi: 10.1002/jsid.348[Crossref], [Web of Science ®], [Google Scholar],12S. Park, J. Yi, D. Choi, S. Lee, G. Kyung, B. Choi, J. E. Lee and S. Lee, presented at the SID Symposium Digest of Technical Papers, 2016. [Google Scholar]]. For instance, Park et al. proposed a model that can calculate the proper curvature, which maximizes the preference and minimizes the distortion. Still, the aforementioned researches have a limitation in that the experiments were performed with fixed radii rather than letting the users find their own optimal curvature.
Thanks to such previous studies, the superiority of the curved display was explained in terms of the human vision system, but its task efficiency was focused on less. Task efficiency is crucial in the field of curved monitor research because the users’ behavior mostly consists of creating or reading documents [13S. Park, J. Yi, D. Choi, S. Lee, G. Kyung, B. Choi, J.E. Lee and S. Lee, presented at the Proceedings of the Human Factors and Ergonomics Society Annual Meeting, 2016. [Google Scholar]]. Along with the market interest in the curved display, some recent studies provided empirical evidences that such display has a strong potential to appeal to the consumers in terms of usability and aesthetic appearance. For example, Suk et al. found that the optimal radius of curvature of a display varies depending on the display size, and may increase as the display becomes larger [14N. Na, K.A. Jeong and H.J. Suk, SPIE/SI&T Electro. Imag. Int. Soc. Opt. Photonics. 9394, 939419 (2015). [Google Scholar],15N. Na and H.J. Suk, J. Soc. Inf. Disp. 24 (1), 21–25 (2016). doi: 10.1002/jsid.411[Crossref], [Web of Science ®], [Google Scholar]]. Moreover, Ahn et al. compared a flat display TV with a curved display TV with respect to the locations of the focused points, posture, and satisfaction, and found that the study participants preferred tilted angles [16S.H. Ahn, B. Jin, S. Kwon and M.H. Yun, J. Ergon. Soc. Korea 33 (3), 191–202 (2014). doi: 10.5143/JESK.2014.33.3.191[Crossref], [Google Scholar]]. With regard to the work environment, some experts have also tried to determine the optimal ergonomic condition of a curved display based on the users’ subjective judgments [17B. Choi, S. Lee, J.E. Lee, S. Hong, J. Lee and S. Kim, J. Inf. Disp. 16 (4), 217–223 (2015). doi: 10.1080/15980316.2015.1111847[Taylor & Francis Online], [Google Scholar]]. Häkkinen et al. evaluated the users’ reading experience on curved-paper-like displays and revealed that a concave surface improves the reading performance [18J. Häkkinen, M. Pölönen, M. Salmimaa and J. Hautanen, J. Soc. Inf. Disp. 16 (11), 1099 (2008). doi: 10.1889/JSID16.11.1099[Crossref], [Web of Science ®], [Google Scholar]]. Similarly, Shupp et al. tested the task efficiency of multiple monitor interfaces and found that the curved arrangement of multiple displays enhanced the task performance compared to flat interfaces [19L. Shupp, C. Andrews, M. Dickey-Kurdziolek, B. Yost and C. North, Hum. Comput Interact. 24 (1–2), 230 (2009). doi: 10.1080/07370020902739429[Taylor & Francis Online], [Web of Science ®], [Google Scholar]].
Likewise, although previous studies tried to evaluate the task performance of the curved display in contrast to that of the flat display, and revealed its optimal ergonomic condition, there has been little consideration of the usability of the curved display in the monitor-sized display. Nevertheless, to ensure that the curved display can be used effectively and efficiently, developers should be aware of the physical and cognitive capabilities and limitations of the people who will use the product. In this given context, there is clearly a need to investigate whether consumers prefer the curved display, and if they do, what reasons underlie such preference.
In this regard, in this paper, the reading performances of the users on a monitor-sized curved display and a flat display were examined, and the two display types were compared in terms of usability. Reading performance or readability includes the following in its dictionary definition: (a) legibility; (b) reading ease; and (c) interest value of a text. As people deal with a huge amount of texts with computers [20D.C. Anastasiu, A.M. Rashid, A. Tagarelli and G. Karypis, presented at the 2015 IEEE 31st International Conference on Data Engineering, 2015. [Google Scholar]], the reading ease or legibility of the text is important. The present study has two key contributions. First, the study participants’ subjective preference for a curved monitor was investigated, and its optimal radii were analyzed. Second, it empirically investigated the advantages of the curved display by evaluating its usability.
2. Experiment plan
This study involved two experiments. In Experiment 1, the influence of each content type on the optimal radii of curvature was assessed by mounting six content types on a bendable display panel. Next, Experiment 2 was conducted to compare the readability of the flat and curved panels, and the usability of the curved display was determined based on the experiment results. The preparation of the experiment stimuli is explained in the following section.
2.1. Experimental material
A bendable display panel was devised so that the users could flexibly vary its curvature. As illustrated in Figure 1, the radius can be easily modified by pulling or pushing the bar handle. When the handle is pulled, the radius decreases up to 200 mm, and when it is pushed, the radius increases. When this tension between push and pull reaches an equilibrium state, the panel becomes flat. If the bar handle is pushed farther, the panel takes on a convex shape that may reach 1000 mm. The size of the display panel is 27 inches diagonally, with a 16:9 aspect ratio, and the display panel is labeled ‘27-inch panel’ throughout this paper.
Figure 1. Illustration of bendable display panel (above)/pulling and pushing procedure at bendable display panel to make an optimal radius of curvature to read (below).
To simulate the monitor contents displayed on the 27-inch panel, six types of contents were prepared. It is generally known that computer users spend most of their computer time surfing the Web, creating documents or presentations, reading books, watching movies, or playing games [20D.C. Anastasiu, A.M. Rashid, A. Tagarelli and G. Karypis, presented at the 2015 IEEE 31st International Conference on Data Engineering, 2015. [Google Scholar]]. Therefore, as presented in Figure 2, the contents used in this study consisted of (a) a photograph (human faces); (b) a worksheet (Microsoft Excel); (c) a computer graphic image (Microsoft Windows default background); (d) a geometric image; (e) a 3D game scene; and (f) plain text. With regard to the plain text, five variations were made, each containing 2500 Korean letters. The texts were presented with a San Serif typeface and a 12-point font size, which was cited in a previous study as adequate for comfortable reading [21M.L. Bernard, B.S. Chaparro, M.M. Mills and C.G. Halcomb, Int. J. Hum. Comput. Stud. 59 (6), 823–835 (2003). doi: 10.1016/S1071-5819(03)00121-6[Crossref], [Web of Science ®], [Google Scholar]]. The content sheet could be easily attached or detached by the experimenter.
Figure 2. Six types of contents were displayed on a 27-inch monitor: (a) photography (human faces), (b) worksheet (Microsoft Excel), (c) computer graphic image (Microsoft Windows default background), (d) geometric image, (e) 3-D game scene, and (f) plain text.