Visual Image Stabilization Measurements and Specifications

F. B. Bossier

The requirements for stabilizing a magnified visual image in a vibrating environment are defined by theresults of measurements of the tolerance to sinusoidal vibrations of a target. The tolerance of the ob-server to accelerations applied to the head of a stationary observer and/or seat was also measured.

I. Introduction

Seeing from moving vehicles-e.g., land vehicles, air-craft, or helicopters-differs in many ways from seeingfrom a fixed location, especially: (1) a scene viewedfrom a moving vehicle has differing relative velocitiesand accelerations of various parts of the field of view;(2) the moving vehicle will impart vibration and acceler-ation to the observer himself, interfering with bothvision and control action; (3) in a vehicle, the observer'sview of the scene is subject to restrictions and distor-tions of the windows, which frequently must compro-mise seeing against protection or aerodynamics. Asolution to some of these restrictions is the use of a peri-scope that can (1) permit angular tracking to minimizeapparent target rates, (2) provide magnification to com-pensate for loss of resolution due to the observer's vibra-tion, and (3) provide wide-angle coverage from a re-stricted or protected opening.

To provide maximum benefit, the image presented tothe observer by the periscope eyepiece, should be free ofvibration or jitter that would be deleterious to the taskof detecting or identifying objects in the scene. Inorder to specify the performance of an image stabiliza-tion system in such a periscope, it is necessary to knowquantitatively "How much vibration or motion of avisual image can be tolerated without producing somemeasurable degradation of the performance of the nor-mal eye?" One might ask slightly different questionssuch as "How much vibration or motion of an image isjust detectable?", or "How much is subjectively objec-tionable?", but it is believed that the question as firstposed above is the more significant for military pur-poses.

When a review of the literature was made, it wasfound that many investigators had considered visualtasks in vibrating or moving environments.'-6 How-

The author is with Textron's Bell Aerosystems Company,Buffalo, New York 14240.

Received 21 March 1967.

ever, all of the reported previous work was aimed at de-termining the maximum physically tolerable vibrationlimits, where severe degradation is expected and tol-erated. The region of interest here is performance atlevels of vibration only slightly over the barely percepti-ble threshold. Therefore, none of the available dataappeared usable in formulating meaningful specifica-tions. Also, no references were found that attemptedto separate the effects of body motion per se from rela-tive motion of the eye and the object, or that consideredthe effect of variations in viewing distance (accommoda-tion of the eye). The latter was found to be a criticalfactor in tests of visual response to flickering targetssuch as TV or moving pictures.7 -10 Therefore, it wasdecided to attempt a direct experimental measurement.

11. Experimental Arrangement

Seeing is affected by a great many factors, includingespecially illumination level, target size, color and con-trast, distance, and subject variables such as normaleyesight, age, and motivation. Vibration is charac-terized by direction, wave shape, amplitude, andfrequency.

Accordingly, a test was sought which would be mean-ingful without requiring replication of all of these vari-ables, and one which would be relatable to both the per-formance of the eye in a stationary environment (or tothe normal performance measures of optical instru-ments), and to the measurable parameters of vehiclevibration. These objectives appeared to be fulfilledby: (a) using unidirectional vibration, perpendicular tothe viewing direction and relatable to standard MILspecification vehicular environments; (b) using a uni-directional resolution criterion (namely a bar chart),with the bars running perpendicular to the direction ofvibration; (c) separately testing stationary subjectswith vibrating resolution targets, and vibrating subjectswith stationary resolution targets.

Most of the effort was spent on the stationary viewer,moving target case, although a few measurements ofmoving viewer, stationary target are reported below.

June 1968 / Vol. 7, No. 6 / APPLIED OPTICS 1155

4

o .Z 8

I ,

05.5 11 16 22

Frequency - cps

Fig. 1. Effect of vibration on bar chart resolution; over-allmean, min, and max vs frequency at various distances, days, and

times.

A. Test Procedure

The stationary viewer, moving target tests were madewith Bureau of Standards high contrast resolution tar-get (NBS Circular 553, 1952 resolution chart) mountedon a vibration stand and viewed by stationary observ-ers. Test were made at frequencies from 5.5 Hz to 32Hz. Ten observers (nine male, one female, aged 22 to40) were tested at a series of viewing distances. Allused one eye to view a high contrast target. Thebrightness of the white part of the target was set atabout 500 cd/mi, corresponding to the high visualacuity region of the eye. 2 Background luminance inthe laboratory was about 30-60 cd/mi; no attempt wasmade to restrict the visual field or to remove distractingperipheral vision, corresponding to a free visual task.The observers were selected for emmetropic vision, i.e.,normal eyesight, and were permitted to use their bettereye, corresponding to a monocular resolving task.Under these conditions, the minimum resolvable linepair reported by most of the observers subtended about90-120 see of arc. This value agrees with that expectedfrom the classical tests of vision under these conditions.

Each observer was seated at one of three viewing dis-tances (0.75 m, 1.5 m, or 3 m) and was asked to identifythe smallest 3-bar group on the chart which could beresolved without vibration. Vibration amplitude atone frequency was then slowly increased until the sub-ject reported loss of resoution of the just resolved target.The subject's attention was then transferred to a target(2) times as large and the motion further increaseduntil it in turn became blurred (gray). The amplitudewas then reduced to zero, and the test repeated at adifferent frequency. After all frequencies were com-pleted at one viewing distance, the distance waschanged. A complete series of tests was run in onesitting beginning with the shortest distance. A weeklater the tests were repeated, beginning with the longestdistance. This permitted evaluation of the effect ofviewing distance, the size of the bar chart, the fatigue ofthe observers during the task, and the learning of theobservers during the tests.

IV. Relation of Sinusoidal toComplex Vibrations

Before proceeding to the experimental results, a wordis in order regarding the relationship of the sinusoidalvibration used in this test to the random or gaussianvibration characterizing complex vehicular vibrations.It is tempting to apply the statistical concepts of theeffect of the amplitude distribution of the image motionas a degradation of the point spread function of theoptics. The difficulty with analytical application of thepoint spread technique to specification of visual systemshas been the definition of the effective averaging time or,conversely, frequency response of the eye to movingimages. Our results, if interpreted as showing theeffective time frequency response of the eye, might per-mit an analytical approach to be formulated except thatthe mechanism for the addition of errors by the eye andbrain is yet to be studied. This point is left open tofuture research. Until this point can be clarified, theresults of these tests with sinusoids are offered as a use-ful interim measure related to measurable engineeringquantities.

V. Experimental Results: Stationary Viewer,Moving Target

A. The Effect of Frequency of VibrationAmplitudes of motion required to produce blurring at

various frequencies are shown in Fig. 1. The abscissais the frequency of sinusoidal motion of the target.The ordinate is the criterion of resolution, i.e., peak-to-peak amplitude required to cause blur of the black andwhite bar pattern into gray. It was expected thatpeak-to-peak motion equal to the width of a black barwould cause blurring at high frequencies. However, asshown in Fig. 1, in about 50% of the observations thetolerable motion, i.e., unblurred, was about 1.3 blackbar widths, while in about 1% of the cases, the observerreported blurring with a peak-to-peak motion of only0.3 black bar widths. It was of considerable interestthat the visual system appears to follow the vibratingtarget as a first order servo with an apparent response ofabout 15 Hz for the mean observer, and about 10 Hz forthe one-percentile observer (least tolerant to vibration).These figures are considerably higher than have pre-viously been reported for moving targets, and are in therange of interesting frequencies of flickering stationarytargets.

To achieve stabilization that will give no resolutiondegradation due to image motion for 50% of the ob-servers, a design criterion of the mean curve of Fig. 1 isrecommended. Note that the allowable motion is ex-pressed in terms of the width of a (just resolvable) blackbar. In order to estimate the angular motion permissi-ble, the size of the target of interest must be considered.The size of the target and the angular motion are bothto be measured at the observer's eye; magnification ofthe optics is immaterial as long as it is enough to bringthe target above the threshold of detectability at theexisting light level.

1156 APPLIED OPTICS / Vol. 7, No. 6 / June 1968

11.6 85

-

5

I 1.. M

'0

CC a

05.5

Frequency - cps

Fig. 2. Effect of vibration on bar chart resolution; mean valuesfor three different viewing distances (ten subjects, various days

and times).

The smallest detectable vibration was approximatelya factor of two smaller than the amount of vibration re-quired to degrade the limiting resolution by one step.Therefore, for stabilization that will result in no detect-able image motion by 99% of the observers, the designcriterion should be about one-half the lower curveshown in Fig. 1.

It is believed that by normalizing the motion in rela-tion to the resolution of the stationary viewer, the com-plex and difficult question of the effects of color, shape,contrast, etc, are effectively sidestepped. Certainlyone would expect that the high contrast, high brightnesscase tested here gives the most stringent requirementsfor an image stabilization system.

B. Effect of Viewing Distance

An even more unexpected result was the effect ofviewing distance. This effect is displayed in Fig. 2which shows that the eye is more tolerant of image mo-tion when accommodated to viewing at 0.75 m (-1.33diopters) than at 1.5 m (-0.66 diopters) or 3 m (-0.33diopters). The improvement factor is about the sameat all measured frequencies and amounted to about 50%more motion tolerable at -1.32 diopters accommoda-tion than at -0.33 diopters. This result may be re-lated to the well-known factor that most observers withnormal 20/20 vision will consistently adjust telescope ormonocular eyepieces between -0.5 diopters and -1.5

diopters. This fact creates some problems when trans-lation exists between the observer's eye and the eye-piece, and also complicates the rapid transition from un-aided vision to magnified vision which can become animportant factor for fire control sighting apparatus.' 3

C. Effect of Size of Target

Half of the tests were made with targets that were atthe limit of visual resolution, 90-120 see of arc for mostobservers under conditions of these tests; the other halfof the tests were made with targets about 1.4 times as

large as the visual limit. There was no statisticaldifference between the amounts of vibration necessaryto blur the target in these two cases, when expressed interms of the ratio of peak-to-peak motion to the widthof one black bar. That is, the amount of sinusoidal mo-tion required to blur the image was related only to thesize of the target (above the visual limit) and not to thevisual limit itself. (Some exploratory tests with morecomplex vibration waveforms indicate that this conclu-sion may not be extendable from the sinusoidal case tothe complex waveform case.)

D. Effects of Fatigue of Observers

Since each set of observations required about 0.5 h,the experiments were run in one sequence, and then re-peated a week later in the opposite sequence. By com-paring the performance on similar tasks performed atthe start of a test with the same task at the end of a test,a measure of the effect of fatigue could be deduced.The results show that fatigue did not affect the data,over the 0.5 h experimental period, at a 95% level ofsignificance.

E. Effect of Learning

A similar analysis of the results of the two trials aweek apart showed that the subjects repeated their re-sults within about 10%. The difference is statisticallysignificant at the 0.05 level, but is insignificant to theconclusion on the usefulness of the data presented inFig. 1.

VI. Experimental Results: Vibrating-Observer,Stationary Target

The effect of whole body motion on visual resolutionhas been reported somewhat in the literaurre. 1-7 How-ever, the range of investigated amplitudes were gen-erally larger than those of interest here, and variationsin the method of application of the vibration to the bodyhad not been investigated. Accordingly, a brief experi-ment was set up in which vibration could be applied toan observer through his seat (through a 2.54-cm foamrubber pad), through his helmet by means of a hardhelmet rest, or directly to his head through a standardtanktype headrest. A distant target would have beenpreferred in order to minimize the angular shift due tolateral or vertical translation of the observer's head.However, since it had been found that a short viewingdistance was preferred by most observes in a vibrationviewing task, the stationary target was set at about 1.5 m(-0.66 diopters). The target was the same NBS highcontrast target at 500 cd/M2 used in the previoustesting.

The vibration machine used in this test gave approxi-mately a constant amplitude of motion over the fre-quency range of 5.5-22 Hz so that the accelerations ap-plied at the lower frequencies were smaller than 1 g byapproximately the square of the ratio of the frequencyto 22 Hz. Tests were made on three observers (male,aged 35-40).

June 1968 / Vol. 7, No. 6 / APPLIED OPTICS 1157

Target Distance -o.75m(-1.32 Diopters)

.5m (-0.66 Diopters)

. i\\ - ~~~~~3,, (-0,33 Diopters)

X I

< ~~~~~~~~~~~~~~~~ L 1x 16 22 3211

3.0

Acuity Ratio -

Under Vibration

Static

2.0

IL.

5.0

FBB - lelmetRest

tIl - eimct Rcst

IM- Forehead

Rest

FBB - Fhead Rest

FBB - N. Rest

TB - Forehead Rest

iM - No Rest

TB - Helmet Rest

TB - No Rest

Frequency cp.

Fig. 3. Blurring due to body and head vibration (three subjects,fixed target at 1.5 n).

Results are shown in Fig. 3. It was found that overthe band of 5-22 Hz, the visual resolution was not sig-nificantly degraded by vibration applied to the seat(through a '2.54-cm foam rubber pad) at levels up to ap-proximately 0.5 g (16 Hz). The same amplitude ofvibration was then applied to the head through a helmetand a standard tank type headrest. Significant degra-dation was encountered at frequencies as low as 5.5 Hzwhich corresponds to a peak acceleration of about l of ag. These preliminary test results tend to show that theheadrest must isolate the head from accelerations above-l g, while keeping the eye(s) within the exit pupil.This magnitude of tolerable acceleration correspondsroughly to the barely perceptible or noticable accelera-tion levels reported by other investigators.67 Thus,it can be expected that the headrest for any sight mustbe carefully matched to the installation.

VII. Conclusion

Engineering data on the degradation of visual imagesdue to image motion at various frequencies and viewing

distances have been presented. These data are in aform usable for the specification and design of stabilizedvisual sighting instruments.

The response of the eye appears to be much fasterthan has been previously inferred, and the response isconsiderably better when the eye is accommodated for aviewing distance of 0.75 m than at 1.5 m or 3 m.

References

1. J. C. Cuijuara, Proc. Roy. Soc. Med. 53, 92 (1960).2. R. J. Hormick, C. A. Boettcher, and A. K. Simons, "The

Effects of Low Frequency, High Amplitude Whole BodyLongitudyal and Transverse Vibration on Human Perform-ance," Bostrum Research Laboratories Final Rept. on con-tract DA-11-022-509-ORD-330, July 1961.

3. M. A. Schmitz, A. K. Simons, and C. A. Boettcher, "TheEffect of Low Frequency, High Amplitude Whole Body Ver-tical Vibration on Human Performance," Bostrum ResearchLabs Final Rept. on contract DA-49-407-MD-797, Jan-uary 1960.

4. A. 0. Radke, "The Importance of Seating in Driver Com-fort and Performance," SAE Paper 838B, Detroit, Michigan,April 1964.

5. G. M. Jones and D. H. Drazin, in Fifth Human Problems ofSupersonic and Hypersonic Flight, (Pergamon Press, NewYork, 1962), pp. 134-151.

6. D. H. Drazin and J. C. Guijuara, in Ref. 6, pp. 339-342.7. D. H. Kelly, J. Opt. Soc. Amer. 53, 422 (1961).8. D. H. Kelly, J. Opt. Soc. Amer. 51, 747 (1961).9. L. A. Riggs, F. Ratliff, J. C. Cornsweet, and T. N. Corn-

sweet, J. Opt. Soc. Amer. 43,495 (1953).10. G. J. Byram, J. Opt. Soc. Amer. 34, 718 (1944).11. W. E. K. Middleton, Vision Through the Atmosphere (U. of

Toronto Press, Toronto, 1952).12. F. B. Bossler, Ordnance Mag., Vol. 2, 283, (July-August

1967) p. 73.

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