le travail à l'écran de visualisation work with display units

ARE THERE VISUAL INTERACTIONS BETWEEN FLICKER FROM VIDEO DISPLAYTERMINALS AND FLOURESCENT AMBIENT ILLUMINATION?Peter A. Howarth, Daniel S. Greenhouse, Ian L. Bailey,Samuel M. Berroan, Gordon Heron, and Thomas W. Raasch.Lawrence Berkeley Lighting Laborabory, School of Optometry,University of California at Berkeley, CA 94 720When two light sources flicker at different frequencies, thepotential exists for them to combine and produce luminance "beats".The flicker rate of standard fluorescent lamps is dependent uponthe mains frequency, while a video display terminal (VDT) emitslight that flickers at a rate controlled by an internal oscillator,independent of the mains. When VDTs are viewed under flourescentillumination, a beat may occur between the two sources.The physical presence of these beats is easily demonstrated with aphotometer and oscilloscope display. However, for there to be aphysiological effect, the visual system must be able to respondsynchronously at the source frequencies. There is evidence fromboth animal neuro-physiological research and human electroretinogramstudies which suggests that the early visual pathwayscan respond at frequencies higher than CFF (critical fusionfrequency). The visual system therefore has the potential torespond to beats between a VDT and a flourescent source. We haveexamined two aspects of the visual system in an attempt to findsuch a response, the pupillary reflex and the temporal contrastsensitivity mechanism.The iris has previously been implicated as being the site of thediscomfort experienced under conditions of glare. If the pupilresponds to beats, then pupillary oscillations may produce symptomsof visual fatigue experienced by VDT operators. Using infra-redpupillometry, we have demonstrated that over a limited range offrequencies the pupil will follow a luminance beat. However thisrange excludes frequencies that are likely to occur naturally in anormal VDT viewing situation.Temporal adaptation in the form of a frequency-specific decrease intemporal contrast senstivity when the visual system is exposed to ahigh-modulation flickering source, has been extensively documentedin the vision literature. We were also interested to learn ifcombined flickering sources can cause adaptation at the beatfrequency. We found that exposure to a robust 8 Hz beat, createdwith a fluorescent source and stroboscope, causes beat frequencyadaptation of magnitude 7 dB for sources in the 40 Hz range,declining to 1 dB in the 60 Hz range. However, no adaptation wasfound to an 8 Hz beat created with a VDT (60 Hz refresh rate) andfluorescent source. Since the visual system is most sensitive toflicker in the 8 Hz range, it is unlikely that the beats which canoccur naturally between VDTs and fluorescent lamps cause temporaladaptation.In conclusion, we have shown that, while beats in general canproduce responses in both the pupil and the temporal contrastsenstivity mechanism of the human visual system, there is noevidence that beats between VDTs and fluorescent illuminationproduce these responses. Thus, it is unlikely that visual symptomsfrom VDT viewing can be attributed to these factors.KEYWORDS:flicker, beats, pupil, temporal contrast sensitivitySPATIAL ADAPTATION TO VIDEO TEXT DISPLAYSDaniel S. Greenhouse, Ian L. Bailey, Peter A. Howarth,School of Optometry, University of California, Berkeley;and Samuel M. Berman, Lawrence Berkeley Laboratory.The spatial contrast sensitivity function of the human visualsystem has been studied extensively in research involving boththeoretical and clinical aspects of vision. The phenomenonof spatial adaptation in the form of a spatial frequency-specificand orientation-specific reduction in contrast sensitivityfollowing exposure to various stimuli, has been documentedin numerous investigations.We have investigated spatial adaptation to text patterns presentedon a standard video display terminal (VDT). Previous reports inthe literature have described spatial after-effects related to thecoarse structure of horizontal lines of text. We are interestedspecifically in adaptation in the vertical meridian, in which theindividual letter structure and position determine the spatialprofile. Previous reports have failed to uncover adaptation inthe vertical meridian, but this may be due to use of aninsufficiently sensitive psychophysical test method, or toadaptation occuring at spatial frequencies other than the onestested.Horizontal luminance profiles of various text patterns weredetermined with a scanning photometer. Fourier analysis of thescans revealed that while upper-case text patterns have multiplepeakedspatial frequency spectra, random lower-case text exhibitsa single prominent peak at a frequency corresponding to the textcolumnar spacing. Because adaptation has been found by others toincrease with decreasing complexity of the stimulus frequencyspectrum, we conjectured that lower-case text in particular hasthe potential to produce spatial adaptation.Employing vertical sinusoidal test gratings, we measured contrastsensitivity in observers before and after adaptation to lower-casetext presented on a VDT. We used a two-spatial-alternativeforced-choice staircase paradigm with a period of re-adaptationbetween individual test trials. This method was designed to allowdetection of even rapidly-decaying effects. Four test spatialfrequencies at one-octave intervals between 1.33 and 10.67 cyclesper degree (c/d) were employed, and the adapting stimulus (VDT)was positioned to provide, on different runs, a stimulus spatialfrequency of either 2.67 or 5.33 c/d. A forehead restraint wasemployed to maintain stable viewing distances.The results for nine subjects revealed a small-magnitude (6 dB)but statistically significant frequency-specific adaptation. Byemploying a sensitive, criterion-free technique, we were able todemonstrate decreases in contrast sensitivity at spatialfrequencies predicted on the basis of analysis of the visualstimulus. We conclude that an ordinary VDT has the potentialto give rise to frequency-specific spatial adaptation.KEYWORDS:Spatial vision, contrast sensitivity, adaptation.TEV 1989 — DEUXIÈME CONFÉRENCE SCIENTIFIQUE INTERNATIONALE • MONTRÉAL • SECOND INTERNATIONAL SCIENTIFIC CONFERENCE — WWDU 1989TEV 1989 — DEUXIÈME CONFÉRENCE SCIENTIFIQUE INTERNATIONALE • MONTRÉAL • SECOND INTERNATIONAL SCIENTIFIC CONFERENCE — WWDU 198912566