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Publisher : Elsevier - Science Direct (الزویر - ساینس دایرکت)
Journal : Applied Ergonomics, Volume 40, Issue 1, January 2009, Pages 69–81
An increasing demand to work with electronic displays and to use mobile computers emphasises the need to compare visual performance while working with different screen types. In the present study, a cathode ray tube (CRT) was compared to an external liquid crystal display (LCD) and a Notebook-LCD. The influence of screen type and viewing angle on discrimination performance was studied. Physical measurements revealed that luminance and contrast values change with varying viewing angles (anisotropy). This is most pronounced in Notebook-LCDs, followed by external LCDs and CRTs. Performance data showed that LCD's anisotropy has negative impacts on completing time critical visual tasks. The best results were achieved when a CRT was used. The largest deterioration of performance resulted when participants worked with a Notebook-LCD. When it is necessary to react quickly and accurately, LCD screens have disadvantages. The anisotropy of LCD-TFTs is therefore considered to be as a limiting factor deteriorating visual performance.
The private and public need for electronically displayed information has increased considerably and continuously for a fairly long time. In order to assure high productivity, it should be focused on the visual quality of electronic displays and the ease with which they allow visual information to be processed. Improvements in screen technology lead to a considerable change of the quality of electronic displays and display types. Nevertheless, the underlying ergonomic questions did not change: How can it be assured that working with electronic screens is possible without difficulty and that efficient visual processing of information is facilitated? Visual ergonomic studies were concerned with the evaluation of electronic displays and aimed at identifying possible shortcomings of current display design. The criteria for the suitability of displays for presenting information were users’ productivity in terms of speed and accuracy of visual processing as well as the emergence of visual strain (e.g. Gould et al., 1987a and Gould et al., 1987b; Dillon, 1992 and Dillon, 2004; Schlick et al., 2007; Sheedy and Bergstrom, 2002; Ziefle, 1998; Ziefle et al., 2005). Even though the studies yielded a solid visual ergonomic knowledge with respect to the displaying of electronic information, technical developments and improvements necessitate the need to continuously evaluate new technologies with respect to their actual benefit for human performance. This regards, for example, the impact of different text factors (e.g. structure, format, and breadth of electronic information) as well as display factors (e.g. contrast, resolution, image quality) (e.g. Dillon et al., 2006; Farrell, 1987; Oetjen and Ziefle, 2004 and Oetjen and Ziefle, 2007; Qin et al., 2006; Sheedy et al., 2003; Vaughan and Dillon, 2006; Ziefle et al., 2003). Also, the impact of visual and cognitive demands that are imposed by different task types and the effects of prolonged on-screen reading still receive attention (e.g. Gröger et al., 2005; Schlick et al., 2007; Stone et al., 1980; Ziefle, 1998). Another prominent research issue refers to the question, which screen type benefits or disadvantages visual performance. The comparison of different display types received special attention lately as screen technology changed. While a few years ago, the cathode ray tube (CRT) was the state-of-the-art technology and was excessively studied (Schlick et al., 2007), the liquid crystal displays (LCDs) are replacing CRTs more and more. LCDs with thin film transistor technique (LCD-TFTs) seem to overcome many disadvantages of the CRTs. Beyond others, the most significant advantage is their suitability for mobile devices. For this reason, they are not only used in small screen devices like digital cameras and mobile phones, but also play an important role in computer notebooks. Notebook computers are continuously replacing stationary desktop computers and should be focused in the ergonomic evaluation of electronic displays (Kirsch, 2004). Although distribution and purchase rates of notebook computers are increasing (BITKOM, 2007), to our knowledge no visual ergonomic study was concerned with the visual quality of Notebook-LCDs so far. Whenever notebooks are considered in ergonomic studies, mostly the specificity of hardware components (e.g. input devices, Sutter and Ziefle 2005; Armbrüster et al., 2007) or the specificity of users’ sitting posture and characteristics of work places received attention (e.g. Harbison and Forrester, 1995; Saito et al., 2000). As the trend is heading towards a steady increase in mobility, there is a considerable need to learn about the suitability of the visual conditions that are present in Notebook-LCDs. This is the topic of the present study. To achieve valuable results, a two-step procedure was realised: the first step was to physically measure the visual quality of different screen types and screen technologies. To do so, a standardised measurement procedure was applied that allows the objective, replicable and reliable comparison of the physical features of different screen types. The second step in the analysis of the suitability of Notebook-LCDs was to determine the visual discrimination performance of normal screen users. Here three screen types were compared: the screen of a Notebook-PC, an external LCD and a CRT.
نتیجه گیری انگلیسی
The present study was concerned with the ergonomic evaluation of visual performance in different screen types. A CRT was compared to the increasingly upcoming LCD-technology. A special research focus was placed upon the evaluation of the visual quality of computer notebooks. Notebook-PCs are replacing the stationary desktop computer systems more and more, because of increasing demands for mobility. The study simulated real working situations where several users are viewing onto one screen or where one operator has to survey several screens at the same time. The participants of the study worked from two sitting positions (a 0° central position where the user was placed directly in front of the screen and an off-axis condition in which the user was placed 50° to either the right or the left side). Five different viewing angles were distinguished and related to performance outcomes. In the following sections, the key results will be summarised and the impact of the outcomes will be discussed with respect to ergonomic demands of visual display work settings. Also, the optimised usage of different screen types will be a topic, focusing mainly on visual aspects. Finally, an outlook to future research issues is undertaken. In accordance with outcomes of recent studies (e.g. Gröger et al., 2003; Hollands et al., 2002; Oetjen and Ziefle, 2004 and Oetjen and Ziefle, 2007; Ziefle et al., 2003), the present study corroborated anisotropy as a major shortcoming of LCD screens. Although LCDs have many advantages, one disadvantage is that luminance measures are not homogenous over the screen surface, but vary distinctly as a function of viewing angle. Physical measurements revealed the strongest fluctuation of luminance parameters for Notebook-LCDs, followed by external LCDs. The CRT technology on the other hand was not affected by anisotropy. Performance data mirror these differences although the extent of performance deterioration is smaller than the deterioration of luminance measures. Apparently, human perceptual processes are modulating or compensating suboptimal physical conditions. When the discrimination performance of all screen positions is comprised, using the CRT led to the best performance and the Notebook-LCD to the worst. Over all screen positions, the mean difference was about 6% when CRT and external LCD were compared, and it increased to even 18% between CRT and Notebook-LCD. The strong susceptibility to off-axis viewing becomes still more evident, when only the off-axis conditions are focused. In the 56.4°-condition, the speed of visual discrimination decreased by 33% when the Notebook-LCD was compared to the traditional CRT. Nevertheless, it should be taken into account that any comparison of a Notebook-LCD and external LCDs must be “unfair” by nature, if only one dimension (visual quality) is focused. Other aspects of working contexts are also of importance in real life applications. It has to be considered that the major advantage of Notebook computers is the possibility to be mobile and to change work settings. With respect to their visual quality of screens, however, Notebook computers do not lead to the best visual performance possible. Besides these negative effects of restricted viewing angles, it should also be mentioned that for privacy reasons the effects are sometimes highly welcome (e.g. in automated teller machines (ATMs) or mobile phones). Effects of anisotropy were found in both parameters of visual performance, speed and accuracy. That reveals that both performance facets are sensitive for anisotropic effects. However, it should be noted that overall error rates were rather small, and that participants adopted a very accurate working style. From a methodological point of view, the high accuracy is especially important because the results are not affected by a speed-accuracy trade-off, which could have limited the interpretation of the effects. The procedure to record reaction times allowed the separation of two processes. The first process (reflected by discrimination times) was assumed to be mainly visual, because only in this period the target was visible. Nevertheless, it cannot be excluded completely that motor pre-programming processes are also included in the discrimination times. Anyway, at this stage encoding should be finished and the second process should indicate the pure psychomotor component. Here participants had to move the finger to one of the reaction buttons to indicate the direction of the gap. This segregation of the two reaction times could contribute to our understanding of anisotropy in LCD screens. To do so it has to be examined if anisotropic effects primarily affect the encoding system or if they also influence motor reactions. The results showed a small but significant difference between the motor reaction times for the three display types and extents of anisotropy, respectively (about 12 ms between the fastest and the slowest motor reaction time). It is unclear at this point of research if these small differences can be interpreted as visual carry-over effects. Especially as differences in visual discrimination times reached much larger amounts (up to about 430 ms). It can therefore be summarised that anisotropy mainly affects visual processing during the encoding stage (discrimination of targets). Future research will have to deal with a more precise separation of these two components. It could critically be objected that performance decrements in the off-axis viewing conditions are predominantly caused by geometric distortion effects instead of anisotropy. This objection is based on the fact that viewing objects from an extended viewing angle leads to a smaller size than viewing them from a central position. However, this argument can be ruled out. Even when geometric distortion effects cannot be excluded completely, their relative impact should apply for all screen types likewise and can be disregarded. Moreover, the influence of geometric distortion should be equally large for all screen-type comparisons. The results showed that this is not the case. The difference of discrimination times between the 0° central viewing condition and the 56.4° off-axis viewing condition differs considerably for the three screen types. The increase equals 37% for the CRT, 43% for the external LCD and even 71% for the Notebook-LCD. Thus, we can generally assume that performance decrements in the off-axis conditions are in fact caused by anisotropy. Anisotropic characteristics of screens and their visual evaluation had previously been addressed with visual search task types (Gröger et al. 2003; Hollands et al. 2002; Oetjen and Ziefle, 2004 and Oetjen and Ziefle, 2007; Yeh et al. 1999; Ziefle et al. 2003). The characteristics of this task type implicate that limitations in visual performance can be expected whenever early stages of visual processing are involved. Even though this procedure is appropriate and proves that anisotropy is a limiting factor whenever time-critical visual encoding is of interest, a cautionary note has to be taken into account. The task demand in the present study was rather simple and reflected pure visual discrimination. But it is still unknown if and to what extent anisotropy affects visual encoding and processing when cognitively more complex comparison and decision task demands are present. Such tasks are frequently necessary in traffic control contexts (air and rail environments). If, for example, an air traffic controller has to decide quickly and accurately whether two or more planes are going to collide, the task is no longer driven purely by bottom-up processes (as anisotropic effects are). This task also has a considerable cognitive component (complex comparison and decision processes and a high responsibility of the operators). It is not easy to predict how higher task complexities are modulating performance. On the one hand, the additional cognitive load present in more complex tasks could result in even stronger performance decrements. On the other hand, it is also reasonable to assume that the visual effects of anisotropy are masked by the high task complexity. In this case, task complexity would have a stronger impact on performance than visual characteristics. This should be pursued in future studies. Moreover, as it is a rather frequent task demand in real life to control several screens at the same time, future studies also have to show if performance changes when more than one screen is used simultaneously. Another important research issue for future studies is the impact of the usage of glasses in combination with anisotropic screen effects. A first investigation should address the use of conventional corrective lenses. In addition to displays’ anisotropic effects, visual performance could be further degraded when users are viewing through peripheral portions of corrective lenses or reading glasses, what might be a rather frequent viewing condition in real working contexts. Furthermore, a second experiment should focus on specific working conditions as, for example, the avionics context. To avoid anisotropic effects and to provide good visibility of the information displayed on the primary display from the other crew station across the cockpit, air crew members wear certain polarised sun glasses. Even if these kinds of glasses may help to avoid colour reversals due to anisotropy, further visual degradations could emerge from interactions between the polarised glasses with the polarisation on the LCD. The LCDs used in the present study were equipped with TN-technology. In order to extent the knowledge of the impact of different display technologies, other techniques should also be scrutinised. Two other technologies are fairly widespread and should be mentioned here. One is the MVA (multiple vertical alignment)-type, which is predominately used for screens larger than the conventional office displays. This technology is more expensive but expected to provide wider viewing angles, even though brightness losses also have to be expected within this technology. The other technology is the IPS (in-plane-switching)-type. Due to high contrast and brightness levels, it is a promising technique for office work displays. However, here slow response times (about 35 ms) are present, the displays are still fairly expensive and therefore mainly used for professional large screen displaying (Artamonov, 2004). A final consideration is concerned with application issues of the present research. Visual ergonomic approaches and results do not allow to recommend one screen type as finally and definitely better than the other one. This is because any technology has benefits and disadvantages. Recommendations for screen types should therefore be related to the specific task context they are to be used for. As human performance in off-axis viewing conditions were of central interest in the present study, there is a clear ranking of screen types for this kind of task demand. CRTs, although they have many negative characteristics, lead to the best visual performance. Considerable performance decrements have to be expected when LCDs are used, time-critical tasks have to be completed, the whole display surface is used to display the stimuli and/or extended viewing angles are present. The decrements are most pronounced in Notebook-LCDs, because their LCD technology is very susceptible to off-axis viewing conditions.