- Email: [email protected]
Head-coupled display systems - Research issues on health aspects
Symbiosis of Human and Artifact Y. Anzai, K. Ogawa and H. Moil (Editors) © 1995 Elsevier Science B.V. All rights reserved.
593
H e a d - c o u p l e d d i s p l a y s y s t e m s - R e s e a r c h i s s u e s on h e a l t h a s p e c t s Wolfgang Felger Fraunhofer-Institut fiir Graphische Datenverarbeitung (IGD) WilhelminenstraBe 7, D-64283 Darmstadt, Germany Phone: +49-6151-155-122, Fax: +49-6151-155-399, E-mail: [email protected] ABSTRACT Head-coupled display systems are in widespread use for virtual reality applications. This contribution introduces some technical aspects of head-coupled display systems, as well as discussing pertinent health concerns. The latter are discussed in an effort to motivate researchers conducting corresponding health studies. Finally, ten research issues on health aspects are identified. A comprehensive reference list is provided.
Keywords: 1.
head-coupled system, health aspects, HMD, 3D presentation device, virtual reality
INTRODUCTION
In concert with the great attention afforded to the field of virtual reality (VR), head-coupled display systems are becoming increasingly popular. This popularity has lead to a great variety of different head-coupled display systems. Only high system prices have restricted their wider distribution, beyond the academic and industrial research labs. Within the last few months, however, systems costing less than US$ 600 have become available. The affordability of such systems, to the mass market, may lead to a situation where they are found in everybody's work environment or home. In 1994, the Fraunhofer-IGD carried out a comprehensive market survey of head-coupled display systems [Felg-94]: commercial and research systems, as well as system components, were identified. Further to this survey, an extensive literature investigation has been performed. Surprisingly, almost no work has been found which relates health aspects to system usage. The goal of this paper is to motivate research into the possible health impact that head-coupled display systems may have on their users. This will be very important if 'careless system usage' is to be quantified and for a system's acceptance to a real VR application. Corresponding research issues are identified. The paper is structured as follows: Section 2 briefly introduces the idea of a head-coupled system, and section 3 explains main system components. Related human factors are summarized in section 4, whilst section 5 sets these in relationship to the current technology and health concerns of users. The paper concludes with an outline of open research issues relating to the health aspects of head-coupled systems usage. 2.
WHAT IS A HEAD-COUPLED DISPLAY SYSTEM ?
The idea for a head-coupled system is 30 (!) years old, having originally been proposed by Ivan Sutherland in 1965 [Suth-65]. Important developments have
594 been carried out by the military, in the 1970's [Furn-91], and by NASA Ames in the 1980's [FMHR-86]. The first commercialsystem (VPL EyePhone [Teit-90]) became available in 1989. A head-coupled display system is the classical 3D presentation device for virtual reality. It enables a user to feel immersed in a computer-generated environment. Usually, a head-coupled system presents a binocular image (a stereoscopic image with different images for the left and right eyes) to the observer: two monitors display the image pair to the eyes. Some monoscopic display systems also exist. In order to provide a wide field of view, a special optical system is applied. Most systems are directly mounted on the head (headmounted display, HMD) in the form of a helmet or goggles [CHBF-89]; others need a special mounting (e.g., BOOM [MBPF-90]). There are two main categories of head-coupled display systems: opaque systems, and see-through systems: Opaque systems block the user's view of the outside world, allowing them to concentrate on the virtual world. See-through systems superimpose computer-generated environments onto the user's real surroundings. Figure 1 shows one opaque system (left side: "Datavisor' from n-Vision) and one see-through system (right side: "i-glasses!' from Virtual I/O). Surveys of currently available systems can be found from time to time in various VR newsletters (e.g., [RTG-94]).
....i
Fig. 1: Head-coupled systems
3.
SYSTEM COMPONENTS
A head-coupled display system requires the integration of several major components [KoTa-94]" typical systems will integrate graphics displays and optical systems with devices to track head movements.
3.1.
Tracking systems Tracking systems perform head tracking (they measure the position and orientation of the head). Electromagnetic, acoustic, mechanical, optical, and inertial tracking techniques are all in general use [MeAB-92]. General purpose applications typically find the accuracy delivered by electromagnetic techniques sufficient, whilst special purpose set-ups may opt for mechanical or optical techniques. Eye tracking is a technique to provide information about the user's gaze direction, in order to detect the region of the view they find of particular interest. Accurate eye tracking is achieved using special cameras, which measure infrared light reflected off the retina.
595
3.2.
Display systems
3.3.
Optical systems
The display systems present the visual information to the user: a colour display is preferred. Display technologies include liquid crystal displays (LCD), cathode ray tubes (CRT), and fiber optics [Bola-94]. A first prototype exists, using laser technology, to scan an image directly onto the human retina [TJMF-95]. The various technologies have differing technical specifications and costs. LCDs are very economic and light-weight, but suffer from poor resolution. Typically, 150,000 primary pixels represent approximately a display resolution of 258 x 195 (horizontal x vertical). Further drawbacks are slow response times and poor contrast ratios. Best resolution (e.g., 1,280 x 960) and display characteristics can currently be achieved with CRTs, but these have the drawbacks of high price, a certain weight, and strong, high-frequency, electromagnetic emissions. Faced with these disadvantages, a few head-coupled systems physically decouple the image creation and image presentation units, using fiber optics to transport the image between both components. Optical systems typically integrate a number of lens elements. Using mirrors and/or optical lenses, the image can be magnified and a large field of view is provided [MaHH-93]. Lenses in use include: relay lenses (which relay an image to another location to increase the viewing distance), eye-piece lenses (located close to the eye and serving as simple magnifiers), and field lenses (located near the image plane and increasing the field of view) [KoTa-94]. Other elements, such as combiners or beamsplitters, have been applied to enable see-through capability. The applied optical system has an impact on image quality, focal distance, field of view, exit pupil (formed when using relay optics to produce an intermediate image plane), eye relief (distance between the last optical element and the eye entrance pupil), and also on other parameters. 4.
HUMAN FACTORS
A head-coupled system should imitate the vision characteristics of real life. In any case, their use should not impair the user's senses or body. The following main aspects have to be taken into account in the design phase [McZe-92, VeilS94]: a) Binocular vision: Depth perception enabled by binocular/stereoscopic vision should be provided. Human depth resolution is 0.47 mm. b) Field of view: The field of view of the human vision system is 180 degrees horizontal and 120 degrees vertical. The natural binocular overlap is 120 degrees. c) Resolution: The human eye is capable of discerning an element size of about 0.5 minutes of arc (this corresponds to a spatial resolution of 4,800 x 3,800). d) Focus: When viewing an object in nature, the human eyes accomodate (focus), and converge to the same point in space. With this feature, objects in a distance range from infinity to near, can all be viewed comfortably. Further characteristics to consider are: colour, brightness, contrast, and freedom from distortion and aberrations. 5.
T E C H N O L O G Y CONSTRAINTS AND HEALTH IMPACT
Besides the ergonomic aspects of head-coupled systems (such as weight, easeof-use, etc.) the health aspects of system usage are also of great importance. These aspects strongly correlate to the technology in use. By their very nature,
596 the components which make up a head-coupled system are all located very close to the human head and brain. Moreover, hygiene issues (such as transferring skin or hair infections) should not be forgetten, especially if a head-coupled system is in public use. 5.1.
Previous work Unfortunately, very little work in this area has been made openly available. The author assumes that some non-disclosed research findings (relating to military applications) must exist, because the Apache helicopter has the only operational HMD (Integrated Helmet and Display Sighting System) in service today [NeHa-94]. Adjusting a head-coupled system to individual settings can be a non-trivial task. [DeEA-94] reported that USAF aircrew members cannot obtain optimal performance from night vision goggles (NVGs), without resorting to a standard procedure (NVGs are not normally considered to be HMDs but they share many of the issues and problems). [KVBM-94] conducted a study on luning when using an HMD with partial binocular overlap displays. Luning refers to the subjective darkening that can occur in the monocular regions near the binocular overlap borders. Luning can result in fragmentation of the field of view into three phenomenally distinct regions. [KoMM-94] investigated how accomodation can be adversely affected by the use of narrowband phosphors in HMDs under dynamic conditions; i.e., the observer might accomodate inaccurately to the display if frequent changes in focus to and from the display are required, as may well be the case with see-through systems. [Kell-94] investigated depth perception with HMDs, and reported that a few subjects suffered from headaches and dizziness after the experiment. Other subjects reported perceiving double images, from time to time, during the experiment. [MoWR-93] reported on the short-term effects, of wearing a conventional HMD, on binocular stability. Subjects were examined before and after exposure to the HMD and there were clear signs of induced binocular stress for a number of subjects. [Pian-93] points out that the distance between the viewer's eyes, relative to the distance between the lenses in the HMD, can affect the visual function of most viewers. If they are improperly adjusted, optical distortion may be introduced, and subjects could become slightly esophoric, or cross-eyed, when wearing the HMD. 5.2.
O b s e r v a t i o n s at IGD Working in the field of VR since the early 1990s at IGD, in Darmstadt, we have collected a variety of observations and concerns voiced by visitors, our students, and colleagues. The most popular tracking technology uses electromagnetic fields. These are low energy fields, but the same concerns which relate to cellular phones, or high voltage cables, apply here. Furthermore, system latency and accuracy can lead to unnatural user behaviour (e.g., the user will not move their head quickly). LCD technology still has a poor resolution (e.g., 50,000 color pixels). Refresh rates and contrast need to be improved. CRTs are doing better here, but need a very high voltage to drive the cathode ray. The optical system influence the observation process. Some systems force the user to focus on a close distance, others at infinity. One head-coupled system achieves see-through capability by using one display monitor for the dominant eye, whilst allowing the other eye a clear view of the surroundings. The technology constraints just described may have an impact on human health, when users are exposed to such systems for a longer period (say, a few hours). Today, the use of such systems in research labs is mostly limited to a few
597 minutes. However, with the appearance of industrial virtual reality applications and head-coupled display systems on the consumer market, an extented usage is obvious. Large e n t e r t a i n m e n t and cable television companies have already announced their intent to support such systems. This demonstrates clearly the need for investigations on the related health aspects. 6.
RESEARCH ISSUES
The goal of this paper was to report on head-coupled systems from an engineers point of view, in order to motivate perception scientists, psychologists, and others, to perform studies on the health aspects of HMD usage~ The author is confident t h a t within the VR community, such information is eagerly sought. To conclude, the author has identified ten research issues which relate to the health aspects of HMD usage. These issues are outlined below, and can be categorised as relating to the display component (issues 1-3), the optical system (issues 4-7), and more general aspects (issues 8-10). Aspects about simulator or motion sickness are omitted, because such issues are already been well-addressed elsewhere (e.g., [Pres-92]). 1) What effects can be caused by the electromagnetic radiation of CRTs and tracking systems, used with HMDs? 2) Does a poor display resolution affect the h u m a n vision system? 3) What problems arise when the contrast and brightness is not high enough? 4) What effects can be caused by a not well-engineered optical system? How can such a bad system be identified by non-experts? 5) Can problems arise when the field of view is too narrow? Is there a lower limit? 6) Does the conflict with binocular systems -- the u n n a t u r a l behavior t h a t a viewer's eyes focus/accomodate on the screen but converge according the stereo cue -- affect the h u m a n vision system? 7) Do different effects exist for see-through systems and opaque systems, or are the problems similar? 8) What are the impacts of mid-term and long-term HMD usage (e.g., between 30 minutes and 3 hours)? 9) What individual adjustment capabilities should an HMD provide? How can a system be perfectly calibrated to the user? 10) Can the use of an opaque HMD for a certain time lead to spatial disorientation after use? Learning more about all of these issues, would enable an informed discussion on the drawbacks, and benefits, of VR applications, as well as allowing one to quantify any potential for personal risk. ACKNOWLEDGEMENTS I would like to t h a n k the Fraunhofer Society for the support in establishing the VR Demonstration Center. Special thanks to Christine Giger and Neil Gatenby for proof-reading. They are responsible for much of the readability and none of the faults. REFERENCES [Bola-94]
Bolas, M.T.: "Human Factors in the Design of an Immersive Display", in: IEEE Computer Graphics & Applications, January 1994, pp. 55-59 [CHBF-89] Chung, J.C.; Harris, M.R.; Brooks, F.P.; Fuchs, H.; Kelley, M.T.; Hughes, J.; Ouhyoung, M.; Cheung, C.; Holloway, R.L.; Pique, M.: "Exploring virtual worlds with
598 head-mounted displays", in: Proc. SPIE Vol. 1083, Three-Dimensional Visualization and Display Technology, 1989, pp. 42-52 [DeEA-94] ....DeVilbiss, C.A.; Ercoline, W.R.; Antonio, J.C.: "Visual performance with night vision goggles (NVGs) measured in USAF aircrew members", in: Proc. Helmet-and HeadMounted Displays and Symbology Design Requirements, April 5-7, 1994, Orlando (FL, USA), SPIE Vol. 2218, pp. 64-70 [Felg-94] Felger, W.: "Marktsichtung fiir kopfgebundene Darstellungssysteme", FraunhoferIGD, confidential report (in German), March 1994 J [FMHR-86] Fisher, S.S.; McGreevy, M.; Humphries, J.; Robinett, W.: "Virtual Environment Display System", in: ACM Proc. 1986 Workshop on Interactive 3D Graphics, Oct. 1986, pp. 77-87 [Furn-91] Furness III, T.A.: "Virtual Interface Technology (Virtual Reality)", Siggraph course notes #C3, 1991 [Kell-94] Kelle, O.: "Untersuchung zur Importanz dynamischer Tiefenwahrnehmungskriterien in computergenerierten interaktiven Szenen und virtuellen Environments", Dissertation, FB Sicherheitstechnik, Bergische Universit/~t-GesamthochschuleWuppertal, 1994, (in German) [KoMM-94] Kotulak, J.C.; Morse, S.E.; McLean, W.E.: "Does display phosphor bandwith affect the ability of the eye to focus?", in: Proc. Helmet- and Head-Mounted Displays and Symbology Design Requirements, April 5-7, 1994, Orlando (FL, USA), SPIE Vol. 2218, pp. 97-104 [KoTa-94] Kocian, D.; Task, H.L.: "Design and Integration Issues of Helmet-Mounted Displays", Short course notes SC05, SPIE International Symposium on Optical Engineering in Aerospace Sensing, Orlando (FL, USA), April 1994 [KVBM-94] Klymenko, V.; Verona, R.W.; Beasley, H.H.; Martin, J.S.: "Convergent and Divergent Viewing affect Luning, Visual Thresholds and Field-of-View Fragmentation in Partial Binocular Overlap Helmet Mounted Displays", in: Proc. Helmet- and Head-Mounted Displays and Symbology Design Requirements, April 5-7, 1994, Orlando (FL, USA), SPIE Vol. 2218, pp. 82-96 [MaHH-93] Ma, J.; Hollerbach, J.M.; Hunter, I.W.: "Optical Design for a Head-Mounted Display", in: Presence, Vol. 2, No. 3, Summer 1993, pp. 185-202 [MBPF-90] McDowall, I.E.; Bolas, M.; Pieper, S.; Fisher, S.S.; Humphries, J.: "Implementation and integration of a counterbalanced CRT-based stereoscopic display for interactive viewpoint control in virtual environment applications", in: Proc. SPIE Vol. 1256, 1990, pp. 136-146 [McZe-92] McKenna, M.; Zeltzer, D.: "Three Dimensional Visual Display Systems for Virtual Environments", in: Presence, Vol. 1, No. 4, Fall 1992, pp. 421-458 [MeAB-92] Meyer, K.; Applewhite, H.L.; Biocca, F.A.: "A Survey of Position Trackers", in: Presence, Vol. 1, No. 2, Spring 1992, pp. 173-200 [MoWR-93] Mon-Williams, M.; Wann, J.P.; Rushton, S.: "Binocular vision in a virtual world: visual deficits following the wearing of a head-mounted display", in: Ophthalmic & Physiological Optics, Vol. 13, No. 4, Oct. 1993, pp. 387-391 [NeHa-94] Newman, R.L.; Haworth, L.A.: "Helmet-Mounted Display Requirements: Just Another HUD or a Different Animal Altogether?", in: Proc. Helmet- and HeadMounted Displays and Symbology Design Requirements, April 5-7, 1994, Orlando (FL, USA), SPIE Vol. 2218, pp. 226-237 [Pian-93] Piantanida, T.: "Another Look at HMD Safety", in: CyberEdge Journal, Vol. 3, No. 6, Nov./Dec. 1993, pp. 9-12 [Pres-92] PRESENCE: Teleoperators and Virtual Environments, MIT Press, Vol. 1, No. 3, Summer 1992 [RTG-94] Real Time Graphics: "Head Mounted Display Survey- A comprehensive Round-Up of Products", RTG, Vol. 3, No. 2, Aug. 1994, CGSD Corp., Mountain View (CA, USA), pp. 8-12 [Suth-65] Sutherland, I.E.: "The ultimate display", in: Proc. of the IFIP Congress 2, 1965, pp. 506-508 [Teit-90] Teitel, M.A.: "The EyePhone, a head-mounted stereo display", in: in: Merritt, J.O.; Fisher, S.S. (Eds.): Stereoscopic Displays and Applications, Proc. SPIE 1256, 1990, pp. 168-171 [TJMF-95] Tidwell, M.; Johnston, R.S.; Melville, D.; Furness III, T.A.: "The Virtual Retinal Display- A Retinal Scanning Imaging System", in: Proc. of Virtual Reality World '95, Stuttgart, Feb. 21-23, 1995, Computerwoche Verlag, Munich, 1995, pp. 325-333 [VeilS-94] Veron, H.; Hezel, P.J.; Southard, D.A.: "Head Mounted Displays for Virtual Reality", in: Proc. Helmet- and Head-Mounted Displays and Symbology Design Requirements, April 5-7, 1994, Orlando (FL, USA), SPIE Vol. 2218, pp. 41-50
593
H e a d - c o u p l e d d i s p l a y s y s t e m s - R e s e a r c h i s s u e s on h e a l t h a s p e c t s Wolfgang Felger Fraunhofer-Institut fiir Graphische Datenverarbeitung (IGD) WilhelminenstraBe 7, D-64283 Darmstadt, Germany Phone: +49-6151-155-122, Fax: +49-6151-155-399, E-mail: [email protected] ABSTRACT Head-coupled display systems are in widespread use for virtual reality applications. This contribution introduces some technical aspects of head-coupled display systems, as well as discussing pertinent health concerns. The latter are discussed in an effort to motivate researchers conducting corresponding health studies. Finally, ten research issues on health aspects are identified. A comprehensive reference list is provided.
Keywords: 1.
head-coupled system, health aspects, HMD, 3D presentation device, virtual reality
INTRODUCTION
In concert with the great attention afforded to the field of virtual reality (VR), head-coupled display systems are becoming increasingly popular. This popularity has lead to a great variety of different head-coupled display systems. Only high system prices have restricted their wider distribution, beyond the academic and industrial research labs. Within the last few months, however, systems costing less than US$ 600 have become available. The affordability of such systems, to the mass market, may lead to a situation where they are found in everybody's work environment or home. In 1994, the Fraunhofer-IGD carried out a comprehensive market survey of head-coupled display systems [Felg-94]: commercial and research systems, as well as system components, were identified. Further to this survey, an extensive literature investigation has been performed. Surprisingly, almost no work has been found which relates health aspects to system usage. The goal of this paper is to motivate research into the possible health impact that head-coupled display systems may have on their users. This will be very important if 'careless system usage' is to be quantified and for a system's acceptance to a real VR application. Corresponding research issues are identified. The paper is structured as follows: Section 2 briefly introduces the idea of a head-coupled system, and section 3 explains main system components. Related human factors are summarized in section 4, whilst section 5 sets these in relationship to the current technology and health concerns of users. The paper concludes with an outline of open research issues relating to the health aspects of head-coupled systems usage. 2.
WHAT IS A HEAD-COUPLED DISPLAY SYSTEM ?
The idea for a head-coupled system is 30 (!) years old, having originally been proposed by Ivan Sutherland in 1965 [Suth-65]. Important developments have
594 been carried out by the military, in the 1970's [Furn-91], and by NASA Ames in the 1980's [FMHR-86]. The first commercialsystem (VPL EyePhone [Teit-90]) became available in 1989. A head-coupled display system is the classical 3D presentation device for virtual reality. It enables a user to feel immersed in a computer-generated environment. Usually, a head-coupled system presents a binocular image (a stereoscopic image with different images for the left and right eyes) to the observer: two monitors display the image pair to the eyes. Some monoscopic display systems also exist. In order to provide a wide field of view, a special optical system is applied. Most systems are directly mounted on the head (headmounted display, HMD) in the form of a helmet or goggles [CHBF-89]; others need a special mounting (e.g., BOOM [MBPF-90]). There are two main categories of head-coupled display systems: opaque systems, and see-through systems: Opaque systems block the user's view of the outside world, allowing them to concentrate on the virtual world. See-through systems superimpose computer-generated environments onto the user's real surroundings. Figure 1 shows one opaque system (left side: "Datavisor' from n-Vision) and one see-through system (right side: "i-glasses!' from Virtual I/O). Surveys of currently available systems can be found from time to time in various VR newsletters (e.g., [RTG-94]).
....i
Fig. 1: Head-coupled systems
3.
SYSTEM COMPONENTS
A head-coupled display system requires the integration of several major components [KoTa-94]" typical systems will integrate graphics displays and optical systems with devices to track head movements.
3.1.
Tracking systems Tracking systems perform head tracking (they measure the position and orientation of the head). Electromagnetic, acoustic, mechanical, optical, and inertial tracking techniques are all in general use [MeAB-92]. General purpose applications typically find the accuracy delivered by electromagnetic techniques sufficient, whilst special purpose set-ups may opt for mechanical or optical techniques. Eye tracking is a technique to provide information about the user's gaze direction, in order to detect the region of the view they find of particular interest. Accurate eye tracking is achieved using special cameras, which measure infrared light reflected off the retina.
595
3.2.
Display systems
3.3.
Optical systems
The display systems present the visual information to the user: a colour display is preferred. Display technologies include liquid crystal displays (LCD), cathode ray tubes (CRT), and fiber optics [Bola-94]. A first prototype exists, using laser technology, to scan an image directly onto the human retina [TJMF-95]. The various technologies have differing technical specifications and costs. LCDs are very economic and light-weight, but suffer from poor resolution. Typically, 150,000 primary pixels represent approximately a display resolution of 258 x 195 (horizontal x vertical). Further drawbacks are slow response times and poor contrast ratios. Best resolution (e.g., 1,280 x 960) and display characteristics can currently be achieved with CRTs, but these have the drawbacks of high price, a certain weight, and strong, high-frequency, electromagnetic emissions. Faced with these disadvantages, a few head-coupled systems physically decouple the image creation and image presentation units, using fiber optics to transport the image between both components. Optical systems typically integrate a number of lens elements. Using mirrors and/or optical lenses, the image can be magnified and a large field of view is provided [MaHH-93]. Lenses in use include: relay lenses (which relay an image to another location to increase the viewing distance), eye-piece lenses (located close to the eye and serving as simple magnifiers), and field lenses (located near the image plane and increasing the field of view) [KoTa-94]. Other elements, such as combiners or beamsplitters, have been applied to enable see-through capability. The applied optical system has an impact on image quality, focal distance, field of view, exit pupil (formed when using relay optics to produce an intermediate image plane), eye relief (distance between the last optical element and the eye entrance pupil), and also on other parameters. 4.
HUMAN FACTORS
A head-coupled system should imitate the vision characteristics of real life. In any case, their use should not impair the user's senses or body. The following main aspects have to be taken into account in the design phase [McZe-92, VeilS94]: a) Binocular vision: Depth perception enabled by binocular/stereoscopic vision should be provided. Human depth resolution is 0.47 mm. b) Field of view: The field of view of the human vision system is 180 degrees horizontal and 120 degrees vertical. The natural binocular overlap is 120 degrees. c) Resolution: The human eye is capable of discerning an element size of about 0.5 minutes of arc (this corresponds to a spatial resolution of 4,800 x 3,800). d) Focus: When viewing an object in nature, the human eyes accomodate (focus), and converge to the same point in space. With this feature, objects in a distance range from infinity to near, can all be viewed comfortably. Further characteristics to consider are: colour, brightness, contrast, and freedom from distortion and aberrations. 5.
T E C H N O L O G Y CONSTRAINTS AND HEALTH IMPACT
Besides the ergonomic aspects of head-coupled systems (such as weight, easeof-use, etc.) the health aspects of system usage are also of great importance. These aspects strongly correlate to the technology in use. By their very nature,
596 the components which make up a head-coupled system are all located very close to the human head and brain. Moreover, hygiene issues (such as transferring skin or hair infections) should not be forgetten, especially if a head-coupled system is in public use. 5.1.
Previous work Unfortunately, very little work in this area has been made openly available. The author assumes that some non-disclosed research findings (relating to military applications) must exist, because the Apache helicopter has the only operational HMD (Integrated Helmet and Display Sighting System) in service today [NeHa-94]. Adjusting a head-coupled system to individual settings can be a non-trivial task. [DeEA-94] reported that USAF aircrew members cannot obtain optimal performance from night vision goggles (NVGs), without resorting to a standard procedure (NVGs are not normally considered to be HMDs but they share many of the issues and problems). [KVBM-94] conducted a study on luning when using an HMD with partial binocular overlap displays. Luning refers to the subjective darkening that can occur in the monocular regions near the binocular overlap borders. Luning can result in fragmentation of the field of view into three phenomenally distinct regions. [KoMM-94] investigated how accomodation can be adversely affected by the use of narrowband phosphors in HMDs under dynamic conditions; i.e., the observer might accomodate inaccurately to the display if frequent changes in focus to and from the display are required, as may well be the case with see-through systems. [Kell-94] investigated depth perception with HMDs, and reported that a few subjects suffered from headaches and dizziness after the experiment. Other subjects reported perceiving double images, from time to time, during the experiment. [MoWR-93] reported on the short-term effects, of wearing a conventional HMD, on binocular stability. Subjects were examined before and after exposure to the HMD and there were clear signs of induced binocular stress for a number of subjects. [Pian-93] points out that the distance between the viewer's eyes, relative to the distance between the lenses in the HMD, can affect the visual function of most viewers. If they are improperly adjusted, optical distortion may be introduced, and subjects could become slightly esophoric, or cross-eyed, when wearing the HMD. 5.2.
O b s e r v a t i o n s at IGD Working in the field of VR since the early 1990s at IGD, in Darmstadt, we have collected a variety of observations and concerns voiced by visitors, our students, and colleagues. The most popular tracking technology uses electromagnetic fields. These are low energy fields, but the same concerns which relate to cellular phones, or high voltage cables, apply here. Furthermore, system latency and accuracy can lead to unnatural user behaviour (e.g., the user will not move their head quickly). LCD technology still has a poor resolution (e.g., 50,000 color pixels). Refresh rates and contrast need to be improved. CRTs are doing better here, but need a very high voltage to drive the cathode ray. The optical system influence the observation process. Some systems force the user to focus on a close distance, others at infinity. One head-coupled system achieves see-through capability by using one display monitor for the dominant eye, whilst allowing the other eye a clear view of the surroundings. The technology constraints just described may have an impact on human health, when users are exposed to such systems for a longer period (say, a few hours). Today, the use of such systems in research labs is mostly limited to a few
597 minutes. However, with the appearance of industrial virtual reality applications and head-coupled display systems on the consumer market, an extented usage is obvious. Large e n t e r t a i n m e n t and cable television companies have already announced their intent to support such systems. This demonstrates clearly the need for investigations on the related health aspects. 6.
RESEARCH ISSUES
The goal of this paper was to report on head-coupled systems from an engineers point of view, in order to motivate perception scientists, psychologists, and others, to perform studies on the health aspects of HMD usage~ The author is confident t h a t within the VR community, such information is eagerly sought. To conclude, the author has identified ten research issues which relate to the health aspects of HMD usage. These issues are outlined below, and can be categorised as relating to the display component (issues 1-3), the optical system (issues 4-7), and more general aspects (issues 8-10). Aspects about simulator or motion sickness are omitted, because such issues are already been well-addressed elsewhere (e.g., [Pres-92]). 1) What effects can be caused by the electromagnetic radiation of CRTs and tracking systems, used with HMDs? 2) Does a poor display resolution affect the h u m a n vision system? 3) What problems arise when the contrast and brightness is not high enough? 4) What effects can be caused by a not well-engineered optical system? How can such a bad system be identified by non-experts? 5) Can problems arise when the field of view is too narrow? Is there a lower limit? 6) Does the conflict with binocular systems -- the u n n a t u r a l behavior t h a t a viewer's eyes focus/accomodate on the screen but converge according the stereo cue -- affect the h u m a n vision system? 7) Do different effects exist for see-through systems and opaque systems, or are the problems similar? 8) What are the impacts of mid-term and long-term HMD usage (e.g., between 30 minutes and 3 hours)? 9) What individual adjustment capabilities should an HMD provide? How can a system be perfectly calibrated to the user? 10) Can the use of an opaque HMD for a certain time lead to spatial disorientation after use? Learning more about all of these issues, would enable an informed discussion on the drawbacks, and benefits, of VR applications, as well as allowing one to quantify any potential for personal risk. ACKNOWLEDGEMENTS I would like to t h a n k the Fraunhofer Society for the support in establishing the VR Demonstration Center. Special thanks to Christine Giger and Neil Gatenby for proof-reading. They are responsible for much of the readability and none of the faults. REFERENCES [Bola-94]
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