Determining limits to avoid double vision in an autostereoscopic display:Disparity and image element width

Jukka Häkkinen (SID Member)Jari TakataloMarkku KilpeläinenMarja Salmimaa (SID Member)Göte Nyman

Abstract — The purpose of this study was to investigate the diplopia limits for three-dimensionalstereoscopic content and to determine the main methodological issues when the limits are studiedwith an autostereoscopic display. One of the main issues regarding stereoscopic content is the struc-tural features that enable the user to see stereo image as a single image. If the depth of the content isnot within certain limits, the perceiver cannot see the images three dimensionally and the viewing isuncomfortable. On the other hand, if these limits are followed, the user can stereoscopically fuse theimages and see the resulting three-dimensional image correctly. Some of these limits were tested andguidelines for proper depth values for stereoscopic images will be presented.

Keywords — Autosteroscopic displays, human factors, diplopia, stereoscopic vision.

DOI # 10.1889/JSID17.5.433

1 IntroductionStereoscopic movies or games can produce a superior userexperience because the three-dimensional contents makethe visual experience more immersive and life-like.1–5 Thiseffect is created with a stereoscopic display which is basedon binocular fusion, where slightly differing informationchannels are combined in the visual system. When thiseffect is produced in electronic displays, this dual-channelarrangement must be replicated with adequate quality.Methods for this have been investigated and developed formore than 150 years.6,7 Stereoscopic viewing has tradition-ally required users to wear specialized viewing devices, suchas glasses with polarizing or color filters. For example, dur-ing the early 1900s, stereoscopic images were shown atmovie theaters with anaglyph film that consisted of over-printed red and green recorded images.8 Later, this stereo-image creation technology was largely replaced bypolarization techniques.

Stereoscopic content can also be realized without anyviewing aids. These display devices are called autostereo-scopic displays. The invention of autostereoscopic technolo-gies arose in the early 1900s, when the parallax-barriertechnique and stereo mechanism based on lenticular lenseswere developed.8 Both of these techniques are based onspatial interlacing, where the display pixels are divided forthe left-and right-eye information.9

Modern parallax barrier displays are based on an addi-tional layer of transmissive and non-transmissive columns inthe display structure. The non-transmissive columns pre-vent the flow of right-eye pixel information from the left eyeand vice versa (Fig. 1). With parallax-barrier displays, theswitching between 2-D and 3-D modes is relatively easy toimplement, but usually the 3-D mode suffers from lowerluminance compared to the 2-D mode of the same display.

As mentioned earlier, autostereoscopic displays utiliz-ing spatial interlacing can also be implemented by using anoptical-lens structure designed especially for the purpose ofdelivering stereoscopic content. This is done by applying athin sheet of plastic on top of an ordinary display structure.This sheet has narrow cylindrical lenses molded on one side(Fig. 2).

The lens structure directs the light from the displaypixels (or subpixels) to the user’s eyes so that each eye willsee only its appropriate image. Autostereoscopic displaysbased on lenticular lens structure are available not only as atwo-view version but also as a multi-view version. One of themain advantages of the lenticular lens technique is that itmaintains the luminance of the base panel behind the lensstructure.

Recently, the overall technological developments inautostereoscopic displays have made stereo viewing withoutany additional viewing devices a viable alternative for the

J. Häkkinen and M. Salmimaa are with Nokia Research Center, P.O. Box 407, Nokia Group, 00045, Tampere, Finland;telephone +358-5048-39483, fax +358-7180-37290, e-mail: jukka.hakkinen@nokia.com.

J. Takatalo, M. Kilpeläinen, and Göte Nyman are with the Department of Psychology, University of Helsinki, Helsinki, Finland.

© Copyright 2009 Society for Information Display 1071-0922/09/1705-0433$1.00

FIGURE 1 — Principle of the parallax-barrier stereoscopic display.

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creation of a better user experience.10,11 Firstly, the imagequality in autostereoscopic displays is currently at an accept-able level. This is a result of developments in the underlyingdisplay technologies, where increased resolutions havedecreased one of the main drawbacks of autostereoscopicdisplays, the resolution drop caused by the required two (ormore) information channels, and the principle of imple-menting this by using spatial interlacing. Secondly, therehave been developments in mechanisms that allow the opti-mization of the viewing position. This reduces the problemof optimal viewing position significantly. The third driver forcommercialization has been the support for electricalswitching between 2-D and 3-D modes.12,13 And finally, asthe autostereoscopic feature does not greatly increase theprice of the product, it is possible that everyday consumerelectronics, such as home-entertainment devices or mobiledevices will soon have the ability to show stereoscopic content.This has already happened to a certain extent, as mobilephones and computer displays with an autostereoscopic dis-play as an option have appeared on the market. The firstmobile-phone product, NTT DoCoMo model SH251iS, waslaunched in November 2002.14 This device featured a2-D/3-D switchable display using a parallax barrier as a ste-reo mechanism. Another mobile device with the same fea-ture but slightly larger display size, the SH505i, followed inJune 2003. 3-D monitors are available from several manu-facturers using both the parallax-barrier and lenticular-lenstechniques for stereo creation. Also some laptops withautostereoscopic screens, such as the Sharp R3-D3 used inour experiments, are on the market.14,15

Although technology seems to be developing rapidly,significant content design issues remain that should besolved. When the third dimension is added to the viewingsituation, the requirements for content design becomemore complex. With stereoscopic displays there are numer-ous ways of enhancing user experience in terms of bothentertainment as well as stereoscopic user interface andvisualizations.16 However, the visual structures that are used

to convey the three-dimensional effect must conform to theconstraints that are related to the physiology of the visualsystem and the neural processing of stereoscopic depth.These structural principles constitute an important require-ment for the subjective image quality of three-dimensionalstereoscopic content because they assure easy and effortlessviewing of the 3-D content.

In this study, we will focus on stereoscopic depthmagnitude, which is probably one of the most importantparameters affecting user experience of stereoscopic con-tent. In content production, the use of as large depth aspossible is a tempting alternative for maximizing user expe-rience. However, too much depth can produce viewingproblems because it exceeds the limits of human ability tostereoscopically fuse images. The viewing limits are mani-fested in several different ways when depth magnitude isincreased. First of all, in a stereoscopic display the normalrelationship between the convergence and accommodationsystems of the visual system is disrupted and a convergence-accommodation conflict occurs.17 The severity of this con-flict is directly proportional to the amount of depth presentin the display and is a significant factor that restricts the useof large depth values. Although the convergence-accommo-dation conflict at small and medium depth levels does notproduce any direct perceptual consequences, long-termviewing of the image might produce eye strain, feelings ofnausea, and headache. If depth value is further increased,the user might experience visual blurring and with the larg-est values might be unable to binocularly fuse the images. Inthe latter case, the user sees the stereoscopic pair as doubleimages, i.e., the image component seen in double is diplopic.In this study, we will focus on diplopia thresholds that areless well known than the eye strain caused by stereoscopicstimuli.18–23 Diplopia can be annoying to the user becausethe three-dimensional perception is disrupted, the two half-images alternate in visual perception and even eye strain isoften experienced. So, avoiding diplopia is one of the keyissues that the content or user interface designer must takeinto account when stereoscopic content is created. In thisstudy we conducted experiments on basic image parametersrelated to diplopia with an autostereoscopic display.

The parameters that produce stereoscopic fusion ordiplopia are related to the way stereoscopic images are pro-duced. A stereoscopic image consists of an image pair – thehalf-images – and the three-dimensional illusion is createdby showing the left half-image to one eye and the right half-image to the other. The conventional way of creating theimages is to produce perspective differences for the images.6,7

When the perspective differences in the half-images imitatethose seen by the left and right eye in the natural world, fusingthe images produces the perception of three dimensionality.

This three-dimensional perception is formed by com-paring local differences in the views of the left and righteye.24–30 These local differences can include differences insize,31,32 form,33 place,25 or occlusion.34–37 The most importantlocal stereoscopic cue is horizontal disparity, which refers to

FIGURE 2 — Principle of the lenticular-lens stereoscopic display.

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the difference in the horizontal location of correspondingimage elements visible to the left and right eyes. The corti-cal cells sensitive to stereoscopic depth are mostly sensitiveto horizontal disparity29,38–40; accordingly, the most significantstructural rules can be defined in terms of horizontal disparities.

As horizontal disparity directly determines the amountof depth perceived in the image, disparity limit is in fact thelimit for maximum perceived depth. The maximum depththat can be used without diplopia is called Panum’s fusionalarea and its size is estimated to be 6–27 minutes of arc ofvisual angle.41–46 However, it is not clear whether theresults of classic psychophysics experiments can be directlyapplied to the use of modern display technologies and view-ing situations. There are three issues that might limit theapplicability of the principle above. Firstly, these resultshave been mostly acquired with Wheatstone-type mirrorstereoscopes with a viewing distance of more than 1 m. Inthe modern context, the autostereoscopic display offers acompletely different image environment with more cros-stalk, shorter viewing distance – at least with the hand-helddevices and computer displays – and more cues for the eyes’accommodation system because of crosstalk and other arti-facts on the display. Secondly, presentation time in the psy-chophysics experiments is typically short, usually between100 and 200 msec. The purpose of this short presentationtime has been to eliminate the artifacts produced by eyemovements. However, with any stereoscopic content or userinterface most of the objects on the display are probablyshown for long periods of time, which changes diplopiathresholds significantly.47,48 A third issue is related to theimage size, which has typically been very small in psycho-physical experiments because using a thin line or a gray-scale distribution based on the Gabor function are necessaryfrom the neurophysiological point of view. However, in astereoscopic game or user interface the objects can have anysize; consequently, a diplopia limit obtained with a thin lineis probably not applicable to them. To conclude, the resultsof classic psychophysics give the general directions of theparameter limits related to stereoscopic structures, but fur-ther measurements should be conducted in accordance withthe development and adoption of new display technologies.

In the present study, we report some diplopia thresh-old findings that were acquired with an autostereoscopicdisplay with a short viewing distance. We measured theeffect of the horizontal width of the image to find outwhether it significantly affects the diplopia thresholds. Thepurpose of these experiments was to outline a research strat-egy, which can be used to define structural guidelines forstereoscopic content design. We also report practical obser-vations related to the diplopia threshold measurements withautostereoscopic displays.

2 MethodsThe experiments had five participants, of whom three wereexperienced psychophysics subjects and two were inxperi-

enced. The latter two participants did not know the purposeof the experiments. However, these participants were notable to learn the experimental task and after extensive train-ing attempts did not participate in the main experiment.The primary reason for their failing was that they could notlearn to differentiate diplopia from the small image-widthdifferences produced by crosstalk in the autostereoscopicdisplay. Thus, the three experienced participants were thesubjects of the main experiments. Although the small par-ticipant number can be considered a problem for gener-alizability of the results, it should be noted that eachparticipant underwent a large number of experiments(4000–12,000 stimulus presentations), so the results do sug-gest general characteristics of the diplopia limits. Further-more, using a small number of participants is commonlyaccepted in the vision-science community because it isassumed that a large number of stimulus presentations andthe use of psychophysics methodology reduces the individ-ual differences in the results.31,33

2.1 Visual criteria for participationThe participants were tested for horizontal heterophoria,interpupillary distance, and stereo acuity. The purpose ofthis testing was to exclude any participants who might some-how differ from the normal population.

2.2 Horizontal near heterophoriaOne of the main purposes of the muscle system that movesthe eyes is maintaining the singleness of the vision when theperceiver or objects of interest move. However, mosthumans have small defects in the muscle system and thiscauses the eyes to fix on slightly wrong positions in the visualfield. These defects can cause both horizontal and verticalmisalignment, but horizontal misalignment, which is calledhorizontal heterophoria, is much more common. Hetero-phoria can affect stereoscopic perception, so we excludedpersons who had an abnormally high level of heterophoria.The exclusion limits were seven diopters esophoria, i.e., het-erophoria toward the direction of the nose, and 13 dioptersexophoria, i.e., heterophoria toward the temples.49 The par-ticipants’ heterophoria values are shown in Table 1.

TABLE 1

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2.3 Interpupillary distanceInterpupillary distance is the distance between the centersof the observer’s pupils. Interpupillary distances between 61and 66 mm were accepted for the experiment because theyrepresent 95% of the population.50 Persons with much lessor much higher interpupillary distance might get signifi-cantly different diplopia thresholds because of different vis-ual geometry between the eyes and the display. Theparticipants’ interpupillary distance values are shown inTable 1.

2.4 Stereoscopic acuityStereoscopic vision is the ability to perceive small depth dif-ferences. Acuity was tested with a Randot stereo test. Par-ticipants with stereo acuity worse than 60 arc seconds wereexcluded from the experiment.49 The reason for the exclu-sion is evident: if the participant does not have sufficientstereoscopic ability, she/he probably cannot properly detectdiplopia thresholds. The participants’ stereo acuities areshown in Table 1.

2.5 TrainingCrosstalk between left and right views in the display madethe discrimination of diplopic and non-diplopic stimuli dif-ficult, so extensive training was necessary. Each participantwas trained for approximately 1–3 hours (1-hour periods).During the training, the experiment leader checked theresults and talked with the participant to ensure that his/heranswering criteria were relevant to the purpose of the experi-ment.

2.6 Stimuli and procedure

2.6.1 GeneralThe stereoscopic stimuli were produced with a ScionImageprogram and were presented with Presentation® software(Neurobehavioral Systems, http://www.neurobs.com/). Weused a Sharp R3-D3 laptop as the presentation device. Thelaptop used a parallax barrier technique for stereo creation,and the resolution of the 15-in. display was 1024 × 768 pixels(XGA). The stimuli were red (RGB 220,0,0) pixels on ablack (RGB 0,0,0) background. Red was chosen as thestimulus color because subjective evaluation indicated thatcrosstalk was least visible with this color combination. View-ing distance was 40 cm. A chinrest was used to keep thehead stationary. The vertical position of the head wasadjusted so that the angle between the visual axis of the par-ticipant and the display was 90°. The horizontal position waschosen subjectively so that the participant was able to seethe stereo image as well as possible.

2.6.2 Experiment 1In Experiment 1, we measured the relation betweendiplopia threshold and the horizontal size of the image, asprevious findings indicate that such a relationship mightexist. The stereo image consisted of a single vertical red linein the middle of the screen. Stereoscopic depth was createdby changing the horizontal position, i.e., the horizontal dis-parity of the lines in the left and right image.

At the beginning of each trial the participant saw afixation cross (28.2 × 28.2 arcmin, 11 × 11 pixels; Fig. 3) inthe middle of the screen for 1000 msec. The stimulus imagewas shown for 100 or 1000 msec depending on the experi-mental condition. The stimulus was a line having a length of3.6° of visual angle (83 pixels). The stimulus could have ninedifferent widths, shown in the first column of Table 2, andcould have a disparity value that was one of nine possiblevalues between 0 and 41 arcmin (16 pixels). Because wewanted to restrict the size of the experiment and as the exactposition of the diplopia threshold was different for each par-ticipant, the disparity values used were different for eachparticipant. During training, the experimenter chose thedisparity values for each participant so that at least onedepth value would be so small that it could always bebinocularly fused and at least one stimulus would have adepth value large enough that it would always be perceived

TABLE 2

FIGURE 3 — Stimulus sequence.

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as diplopic. The other values were chosen between thesetwo extremes.

2.6.3 Experimental procedureAn experimental session consisted of one stimulus width andnine possible disparity values. Each stimulus was repeated 20times, thus a single session consisted of 180 stimuli. In thenear-depth experiment, participant MK underwent testswith nine different stimulus widths and participants SY andLN with four stimulus widths. All participants underwentthe experiments with two presentation times (100 or 1000msec). Participant MK repeated each experimental condi-tion four times, so the total number of experimental sessionswas 72 (2 presentation time types × 9 stimulus widths × 4repeats). The total number of stimulus presentations forparticipant MK was 12960. Participants SY and LN under-went 24 experiments (2 presentation time types × 4 stimuluswidths × 3 repeats) and viewed a total of 4320 stimuli. Theorder of experimental sessions was randomized.

The task of the participants was to detect whether thetarget image looked diplopic or not (test sequence shown inFig. 3). The participants signaled the answer by pressingone of two possible keys on a computer keyboard.

In Experiment 1, all of the disparities were crossed,i.e., they were perceived as in near depth in front of thedisplay. Experiment 2 was otherwise similar to Experiment1, but all the stimuli had uncrossed disparity, i.e., the barswere perceived as being at far depth further away from thedisplay.

3 ResultsDiplopia thresholds were calculated from the values derivedfrom experiments with a single stimulus width. For exam-ple, Fig. 4 shows the diplopia values for participant MK forthe stimulus width of 10.2 arcmin in one experimental ses-sion with a long presentation time. Each point representsthe percentage of “I see it fused” answers in the experiment.These percentages have been plotted as a function of dispar-

ity and a cumulative normal distribution has been fitted tothe results with the least squares method. The diplopiathreshold is defined as the point at which the fitted functioncrosses the 50% line of the figure. Each data point inFigs. 5–8 is a threshold value that has been calculated in asimilar manner.

Figure 5 shows the diplopia thresholds for participantMK. Each data point represents the threshold depth valuefor perceiving diplopia. In other words, it is the maximumacceptable depth value that should be used in order to avoiddiplopia. The results clearly show that diplopia thresholdschange significantly as a function of the stimulus width.There is also a clear difference between short and long pres-entation times, as the latter case leads to larger acceptabledepth magnitudes. A logarithmic function has been fitted tothe results, and both curves show a fairly linear relationbetween the stimulus width and diplopia thresholds at thesmallest stimulus widths. However, when the stimulus widthis more than 10 arcmin, the linear increase of the diplopia

FIGURE 4 — Percentage of stimuli seen as binocularly fused as afunction of disparity.

FIGURE 5 — Stereoscopic diplopia thresholds as a function of stimuluswidth. Near-depth results from participant MK.

FIGURE 6 — Stereoscopic diplopia thresholds as a function of stimuluswidth. Near-depth results from participant SY.

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threshold starts to diminish and the thresholds stop increas-ing around a diplopia threshold of 45 arcmin for 1000 msecand 35–40 arcmin for 100 msec.

The diplopia thresholds for participant SY (Fig. 6) andparticipant LN (Fig. 7) have a similar logarithmic form, butslightly smaller diplopia thresholds were acquired. As well,the difference between short and long presentation time isnot so clear with these participants.

Figure 8 shows the diplopia thresholds for participantMK from the far-depth experiment. It can be seen that thediplopia thresholds were lower in the far disparity than inthe near disparity. The far-depth experiment proved to betoo difficult for the two less experienced participants, so thefar-depth results were obtained only from the most experi-enced participant, MK.

Based on the results, we can draw approximate recom-mendations for the maximum suitable depths that should beused with stimuli with different horizontal widths (Table 2).The values were calculated based on the results of MK andwhen other data were available, as a mean of the three par-ticipants.

4 ConclusionsThe easiest way to produce stereoscopic content that can becomfortably fused is to keep the disparities of the imagecomponents fairly small. However, this is not always desirablebecause larger depth values are needed either to increasethe efficiency of use or to enhance the entertainment expe-rience. In these situations, the largest temporarily accept-able depth changes as a function of the horizontal width ofthe object. Table 2 contains the relevant disparity values foran autostereoscopic display and with a short viewing dis-tance of 40 cm. As can be seen, the maximum disparity val-ues start at around 17 arcmin and increase until 40–45arcmin for larger objects. Values for short viewing times(100 msec in this example) are slightly lower. The small valuesthat were acquired with the narrow stimulus of 5.1 arcmincorrespond fairly well to the typical values of Panum’s

fusional area that are cited in the literature. However, whenthe stimulus width increases, the maximum depth valuesalso increase until the function saturates at slightly over 40arcmin of visual angle with long (1000-msec) presentationtime and slightly less than 40 arcmin of visual angle withshort (100-msec) presentation time. These values give someindication of the suitable scale and dynamics of depth valuesfor objects of different sizes in stereoscopic user interfaces,stereoscopic games, and stereoscopic movies.

The difference between long and short presentationtime represents the role of eye movements in stereoscopicfusion. When eye movements were possible due to thelonger presentation time, they were used to decreasediplopia, which leads to larger depth tolerance. The differ-ence is important in user interface and content designbecause it indicates that smaller depth values should beused for objects that appear and disappear quickly.

The fact that eye movements can reduce diplopia canbe exploited in stereoscopic content design because gradualincrease in depth can make the tolerable disparities muchlarger at least for shorter periods of time. This effect, stereohysteresis, is related to the motor fusion of the visual fieldsconducted by the vergence movements which try to keepthe left and right visual images locked when the location ofobjects and the location of the perceiver constantly changeduring perception.47,48,51,52 Thus, a depth value that is 2–3times larger than the diplopia threshold can be temporarilyused if the maximum depth value is reached by increasingthe depth gradually. However, when the diplopia thresholdis exceeded, eye strain may become a significant problem,so long-term use of the large depth magnitudes should stillbe avoided. These effects should be further tested withrealistic game and movie content.

The final conclusion that can be derived from theseexperiments is that as small disparities as possible should beused in stereoscopic content. This would certainly reducethe risk of eyestrain53,54 and diplopia, but it is not certainhow this would affect user experience. To our knowledge,there are no studies about the differences in user experience

FIGURE 7 — Stereoscopic diplopia thresholds as a function of stimuluswidth. Near-depth results from participant LN. FIGURE 8 — Stereoscopic diplopia thresholds as a function of stimulus

width. Far-depth results for participant MK.

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with large and small depth magnitudes. There is a clearneed for this kind of study because it would give an indica-tion of the amount of depth reduction that can be done toreduce the adverse effect of depth.

4.1 Other perceptual constraintsDisparity magnitude is not the only variable that is relevantto stereoscopic content design. In visual psychophysics, sev-eral research themes are clearly relevant to the formation ofstereoscopic depth perception. Firstly, diplopia thresholdsdepend on disparity gradients between adjacent depthstructures,41,55,56 with too steep gradients leading to diplopia.The relevance of these results to content presented in newdisplay technologies should be clarified. Secondly, periodicdepth structures can produce depth illusions, which lead toerroneous depth perceptions.57–59 Thirdly, textured andnon-textured surfaces form different three-dimensionalperceptions in a way which suggests that the edges ofobjects do not always determine the perceived three-dimen-sional structure of the surfaces.60–62 These are examples ofresearch questions that should be addressed when theguidelines for proper stereoscopic structures in stereo-scopic content are formulated.

Measuring diplopia threshold with an autostereo-scopic display is a difficult task, especially for untrained par-ticipants. Three main problem types should be taken intoaccount when diplopia thresholds are measured withautostereoscopic displays. Firstly, crosstalk can make thedifferentiation between diplopia and interocular ghostingdifficult. Secondly, wide stimuli might be problematicbecause the vertical edges that are critical in diplopiadetection might be difficult to detect with non-foveal areasof the eye. Thirdly, objects at far depth appear qualitativelyvery different from near objects, which forces the partici-pant to change the perceptual criteria for diplopia detectionthresholds.

4.2 Perceptual decision criteria for diplopiaAlthough crosstalk is not clearly visible in gaming or videoviewing, it causes problems in difficult psychophysics experi-ments, such as ours. It was difficult for the participants toestablish the decision criteria for diplopia when the presen-tation time was short (100 msec). The experimental task wasfairly difficult, as the stimuli were often near the sensorythreshold for diplopia. Because of this difficulty, inexperi-enced participants very easily reacted to the apparent widthof the line. The apparent width of the bar changes if thedisparity is more than zero, and there are some problemswith fusion and some crosstalk in the display. The solutionto this problem is twofold: the participants must practiseenough and the decision criteria should be discussed withthem. The participants should be encouraged to use theproper diplopia criteria even if it might be very difficult atthe beginning of the experimentation. In our experiment

two participants were not able to differentiate betweenwidth differences and diplopia and thus were excluded fromthe main experiment. It seems that experience even in non-stereoscopic psychophysics helps a subject to form decisioncriteria.

4.3 Stimulus width and diplopia detectionThe wide stimuli were difficult to evaluate. When the stim-uli were over 20 pixels wide, it was difficult to evaluate thediplopia with the short presentation time (100 msec), whichmade it impossible to move the fovea of the eye to the edgesto evaluate the diplopia. Because the edges of the widerrectangles were projected to the non-central areas of theeye that have lower visual resolution, the task was fairly dif-ficult. There are two possible solutions to this problem.Firstly, practice is needed before good results can beobtained with a short presentation time and wide stimulus.Secondly, it might be useful to design the experiment so thatthe edge of a wider stimulus bar would always be located atthe central area of the display that the participant looks atduring the experiment.

4.4 Perceptual decision criteria in far depthUsing far depth locations was difficult because the partici-pants differed significantly in their ability to differentiatethe stimuli from each other. This probably occurs becausethe stereoscopic system computes the apparent depth of thefar diplopic stimuli in a different way compared to neardiplopic stimuli. At near depth, the diplopic stimuli arelocalized to the display level and thus are easier for the par-ticipant to notice. However, at far depth the diplopic stimuliseem to remain at that depth, so it is much more difficult todifferentiate between diplopia and fusion. Consequently,sufficient precautions should be taken if far depths are usedin diplopia experiments.

AcknowledgmentsWe thank Maria Olkkonen for her help in producing thestereoscopic stimuli, Toni Järvenpää for some of the graphi-cal images, and Marja Liinasuo for her comments on themanuscript.

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440 Häkkinen et al. / Avoid double vision in an autostereoscopic display

Jukka Häkkinen received his Ph.D. degree in experi-mental psychology in 2007 from the Departmentof Psychology, University of Helsinki, Finland. Heworked at the Department of Psychology, Univer-sity of Helsinki, from 1992 to 2000. In 2000, hejoined Nokia Research Center in Helsinki. Cur-rently, he works as a visiting scientist at the Uni-versity of Helsinki and as a principal researcher atNokia Research Center. His research consists ofbasic research related to stereoscopic vision and

applied research related to human factors with emerging display tech-nologies, such as virtual displays, stereoscopic displays, and paper-likedisplays. Currently, his main research focus is in defining subjectiveimage quality of stereoscopic contents.

Jari Takatalo is a researcher in the Psychology ofEvolving Media and Technology (POEM) researchgroup at the Department of Psychology, Univer-sity of Helsinki. He has several years of experi-ence in developing a psychological theory andmeasurement method to understand experiencesin various computer-generated environments. Hisresearch interests concern both work as well asentertainment applications with various displaytechnologies (for example, head-mounted and

stereo displays and CAVE’s). Currently, he is finishing his Ph.D. in userexperience measurement model for digital games.

Marja Salmimaa works in the Immersive Commu-nication Team of the Media Laboratory of NokiaResearch Center as a Principal Member of theengineering staff. She earned her M.Sc. in 1997from the Tampere University of Technology, Elec-tronics Laboratory. She has been working withnovel display technologies since 1996 and hasbeen working and led several projects on displaysystems at the Nokia Research Center since 1998.Before that, she was working in research positions

in Electronics Laboratory at Tampere University of Technology. Herresearch interests include emerging display technologies, immersivecommunications, and related user interface and interaction issues.Marja Salmimaa works in the Immersive Communication Team of theMedia Laboratory of Nokia Research Center as a Principal Member ofthe engineering staff. She earned her M.Sc. in 1997 from the TampereUniversity of Technology, Electronics Laboratory. She has been workingwith novel display technologies since 1996 and has been working andled several projects on display systems at the Nokia Research Centersince 1998. Before that, she was working in research positions in Elec-tronics Laboratory at Tampere University of Technology. Her researchinterests include emerging display technologies, immersive communica-tions, and related user interface and interaction issues.

Göte Nyman is a professor of Psychology at theUniversity of Helsinki. He is the leader of the researchprogram on Psychology of Evolving Media andTechnology (POEM). His research acitivities inPOEM concern especially high visual quality indigital and print technology, 3-D imaging, gameexperiences, virtual communication and collabo-ration, and magazine reading experiences. Hehas published altogether approximately 170 sci-entific papers and three books.

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