Ophlhal. Physiol. Opi. Vol. 17. No. 4. pp. :4I 244, 19971997 The Colk-gi; nf Oplomcuisls. Published h\ Elscvicr Si-icnce Ltd

in Grcal Britain47 SI7.00 t 0.0(1

PII: S0275-5408(96)00091-9

Accommodation and acuity under night-driving illumination levels

p. Arumi, K. Chauhan* aiid W. N. Charmant

Department of Optometry and Vision Sciences, UMIST, PO Box 88, Manchester M60 1QD, UK

SummaryLaboratory experiments are described in whicin the monocular changes in the refractive errorand acuity of six young, normal, adult subjects were measured as the field luminance wasreduced from approximately 100 to 10 ^cd/m^. It was found that, at luminance levels equal tothose recommended for road lighting (about 1 cd/m^), acuity fell from its photopic value of >6/6to about 6/9, with little change in the measured refraction. Marked changes in refraction, i.e.night myopia, only started to become manifest when the luminance was further reduced tobelow about 0.03 cd/m^, much less than that applying under normal night-driving conditions.Direct experiments under street-lighting conditions confirmed the absence of any significantnight myopia, it is concluded, therefore, that neural changes, rather than night myopia, normallyare responsible for the acuity loss suffered by drivers at night, i 1997 The College of Optome-trists. Pubiished by Elsevier Science Ltd.

Introduction

As Li result of a variety of sludics of night myopia inthe years during and following the second world war(e.g. Otero and Duran. 1943: Wald and Gntrm. 1947;Carreras, 1951; Koomen ci ai. 1951; Otero. 1951,1953; Campbell. 1953). the suggestion was made thatsimilar myopic shifts in refraction might occur at therelatively low light levels experienced by vehicle driversal night and that these might impair drivers" ability toacquire visual information about the road (e.g. Knoll,1952; Richards, 1967. !97S; Borish, 1970; seeCharman, 1996, for a recent review). It was hypoth-esized, therefore, that a negative 'night-driving correc-tion' might help to reduce the probability of roadaccidents for some drivers. Nighi myopia is used in thispaper to refer to myopic shifts in refraction found atscotopic luminances, in contrast to the dark focii.s.which is the refractive state measured in completedarkness.

Since typical levels of night myopia in young adultswere found to be of the order of 0.5 1 D, initial elTorts

*MBCO (lo whom Lorrc^pniulL-ncc slioiild be iiddressed).+Hon. FBCO,

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involved trials with standard corrections of about thisamount. It was suggested later that, as night myopiaand dark locus showed considerable inter-subject vari-ations (e.g. Leibowitz and Owens. 1978; Epstein ci ai.1981; McBrien and Millodot, I9S7), the correctionshould be related lo the individual's dark focus. A cor-rection equal to one-half of the full dark focus wassuggested by many authors, to take account of the factthat roads were never completely dark and, therefore,that night myopia was unlikely to be fully manifest(Owens and Leibowitz, 1976; Hope and Rubin, 1984;Arnaud and Frenette. 1996). while Kotulak e! ai(1995) suggested that the correction might be increaseduntil it equalled the full dark focus for those with lowCA C ratios.

Trials with such corrective spectacles under night-driving conditions typically have produced equivocalresults, with some drivers claiming benetUs and othersclaiming that their vision was worse with the correc-tion (Richards, 1967, 1978: Sheard, 1976; Owens andLeibowitz. 1976; Fejer, 1995). This may be becausetypical road luminance levels are much higher thanthose at which night myopia occurs. FollowingRichards (1967), Chauhan and Charman (1993) arguedthai road lighting levels (about I cd/m~) were normallyin the high mesopic range (Ur^-3edm-) rather thanin the scolopic range (10"*'-10"^ cd m') within which

291

292 Ophthai Physiol. Opt. 1997 17: No 4

night myopia developed. It appears, too, that underconditions where a binocular fixation target is avail-able, myopic shifts are much smaller than under mon-ocular conditions, since vergence input is available todrive the accommodation system towards the appro-priate level (Leibowitz et ai. 198S). Nevertheless, therecontinue to be vigorous advoeates for the policy ofproviding specific night-driving corrections (see. forexample. Davey. 1991; Fejer and Girgis. 1992; Fejer,1995).

Reeommended levels for road lighting in the UK areabout i cd m- for main roads (BS 5489. 1987). Directmeasurements confirm that most roads comply withthese recommendations, although exact levels inevita-bly vary somewhat with such conditions as the roadsurface, weather and shadhig by trees (e.g. Hargroves.1981: Chauhan and Charman. 1993).

Perhaps the most useful available laboratory studyof refractive shifts as a function of light level is that ofJohnson (1976). He measured monocular, steady-state,accommodation responses to targets in the vergencerange 0 to — 3.00 D at four luminance levels spanningthe mid-mesopic to low-photopic range. The slope ofthe response/stimulus curve was found to decrease asthe luminance was lowered, the curve pivoting aboutthe point at whieh both stimulus and response equalledthe individual dark focus. Thus, progressively greateramounts of over-accommodation occurred for distantobjects and under-accommodation for near objects asthe luminance approached scotopic levels. Johnson'smean results for accommodation to distant targets(zero vergence) are shown in Figure /(a): these effec-tively are measurements of the apparent myopia. Notethat the refractive changes between the photopic levelsof 51 cdlnr and the mesopic level of 0.51 cd m" arecomparatively small (about 0.3 D), although a substan-tial myopic shift occurs if the luminance is reducedfurther to 0.051 cd/m". It is of interest that Kotulak elai (1995) found nearly constant mean levels of accom-modation to distant targets at high mesopic lumi-nances (0.78. 0.68 and 0.77 D at luminance levels of0.04. 0.4 and 4.0 cd;m"̂ respectively), while Campbell(1954) found that for accommodation to become sub-stantially inaccurate, the luminance of a stimulus hadto fall below about 0.01 cd/m"'. depending somewhaton the spatial form of the stimulus.

Johnson also measured acuity under similar con-ditions, using sinusoidal gratings, and found that, aswould be expected, it fell with light level {Figure /(b)).Acuity in Figure 7(b) is plotted in terms of bothlogMAR and Snellen units, where logMAR is the log-arithm to base 10 of the minimum angle of resolutionin minutes of arc. logMAR zero being equivalent to 6/6 and logMAR 1.0-6/60. Changing the vergence of thetargets at each luminance level to bring them into

.01 .1 1 10Luminance (cdJnfi)

0.8

0.6

0.4

0.2

0.0

-0.2

-0.4

(b) O Direct• Corrected for

accommodation error

.01 10

Luminance (cd/tn^)

'38

'24

'18

'12

100

Figure 1. Mean data for four subjects, replofted from resultsgiven by Johnson (1976). Vertical bars give standard devi-ations, (a) Accommodation to a distant cross target (zerovergence) as a function of luminance level, (b) The logMARand Snellen acuity for sinusoidal gratings of 66% Michelsoncontrast (where the minimum angle of resolution is definedas the subtense of one-half of the period of the grating atthreshold) when the grafing is either at optical infinity (opencircles) or positioned so as to be conjugate to the retina tocompensate for the accommodation errors of Figure 1(a)and hence minimize optical blur (filled circtes). Correction ofthe 'night myopia' produces only small reductions inlogMAR, i.e. improvements in acuity.

focus on the retina while the subjects looked al a dis-tant fixation target produced only relatively minorimprovements in aeuity (about 0.1 logMAR or a factorof about 1.3 X , as shown in Figure /(b)). This suggeststhat the bulk of the fall in acuity with light level wasdue to neural factors associated with the falling retinalilluminance rather than to the optical blur from any

Accommodation and acuity: P. Arumi et al. 293

myopic shift and implies that the optical correction ofnight myopia would be of only marginal benefit.

Johnson's study has some limitations. He used alaser optometer and it has been suggested that thissometimes may lead to inaccurate results, since theneed to make a subjective judgement about the direc-tion of speckle motion may stimulate additional ac-commodation in a minority of subjects (Post ei ai.19S4; Rosentield. 1989); Bullimore el ai (I9H6), how-ever, failed to find such an effect in their study of 25subjects, possibly because they used shorter exposuretimes for the laser speckle. Perhaps the major limi-tation of Johnson's study was that it was restricted toonly four subjects and four luminanee conditions.

We have, therefore, repeated and extended thoseparts of Johnson's study which relate to night-drivingconditions, using an objective infra-red optometer torecord accommodation to a distant target and asample of six subjects, with 11 stimulus luminancelevels. The measurements were made monocularty. onthe basis that refractive changes were likely to beworse in this situation rather than under binocularconditions. Young adult subjects were used, since thisage group displays ihe highest levels of night myopiaand dark focus (Simonelli, 1983; Ramsdale andCharman. 1989; Fejer and Girgis. 1992). The labora-tory study was supplemented by observations madeunder street-lighting conditions.

Experiment I: laboratory study

.Meilnu/s

In this experiment, monocular measurements of therefractive state of the eye and visual aeuity wereobtained at 11 luminance levels with six subjects.Details of the subjects are given in Tahle I: they werecorrected as necessary with either spectacle or well-adapted contact lenses. In each case, the right eye wasused, the left eye being occluded.

The subjects sat with their chin and head stabilizedby the head rest of a Canon Auto Ref R-l infra-redautorefractor. This is an open-view instrument, havinga large dichroic beam splitter which retlects infraredmeasuring light into the subject's eyes but allows an

unobstructed view by transmitted visible light(Matsumura et ai, 1983; Berman cl ai. 1984: McBrienand Millodot. 1985). All measurements of refractionwere expressed in best-sphere form. Subjects viewed atri-phosphor computer monitor (Model JC-1403HME. Viglen Ltd) at a distanee of 6 m. Single, high-contrast (92% modulation) black-on-white Snellenletters were displayed on ihe monitor using thetJMIST Eye System (DES) computer software pro-gram. This presents letters in random sequence toavoid any learning effects, and establishes the visualacuity by a modified staircase procedure controlled bythe subject's responses (French. 1993). The back-ground luminance of the letters as viewed through theautorefractor's beam splitter was 106 cd/m" (SpectraSpotmeter. Photo Research. CA. USA). This lumi-nance is close to. but slightly lower than, the rec-ommended value of 120 cd/m" for visual acuity testing(BS 4274. 1968). The subtense of the monitor screen asseen by the subjects was 2.4 x 1,9 .

The effeetive luminance of the monitor was con-trolled by introducing calibrated neutral density filters(Melles Griot. Irving. CA. USA) just in front of theautorefraetor's beam splitter, at 0.195 m from the sub-ject's observing eye. Opaque screening ensured that allof the light reaching the subject's eye had passedthrough the neutral density filters.

As baseline measurements, visual acuity and refrac-tion were determined with the room lit to 300 lux andno screening or filter in place. All subjects recorded6/6 or better (Table I). The screening was thenreplaced, the lights turned off and. after lOmin ofdark adaptation, neutral density filters were introducedin the sequence 0. 0.5. 1. 2, 3. 4. 5, 4.5, 3.5, 1.5. 0.5. 0optical density; density is the logarithm to base 10 ofthe reciprocal of the transmittance of a filter. Thissequence was adopted to minimize effects due to driftsin attention and adaptation. Measurement of acuityand 10 measurements of refraction with each filtertypically took 3- 4 min. the whole sequence lastingabout 45 min with each subject.

The measurement session was completed with therecording of the dark focus for each subject. The subject.wearing any prescription, and autorefractor were cov-ered with a dark cloth, all room lights extinguished and

Table 1. Ocular characteristics of the six subjects

Subject Age Refractive correction (RE) Corrected Snellen acuity

ILLBLLMOSBSP

242620252324

-1.00-6.75/-0.50 X 17

04.75/ - 0.50 X 40

-0.75-0.75

6/66/4.86/4.86/6

6/4.86/4.8

294 Ophthal. Physioi Opt. 1997 17: No 4

Luminance (cd/m-l

Figure 2. The logMAR and Snellen acuity as a function oftbe effective luminance of the background of the Snellenletters for the six subjects. Upwards pointing arrows rep-resent lower limits to logMAR (logMAR 1.0 = 6/60).

the luminance of the autorefractor's television monitorturned down to minimize stray light. After approxi-mately 7 min of dark adaptation, 15 values of refractionwere recorded to give an estimate of the full dark focus.

Results

Figure 2 shows the variation in acuity (expressed inboth logMAR and Snellen terms) with luminanee levelfor each subject. At the lowest levels of luminance,only lower limits to logMAR could be established dueto the limited size of the monitor screen.

Although there are minor individual ditlerences, it isclear that, as the luminance is reduced, visual acuitygets worse throughout the luminance range studied,even al the higher levels. The levels of acuity generallyare similar to those found by Richards (1966) for sub-jects of the same age.

The corresponding changes in best-sphere refractionfrtmi the baseline photopic value (negative valuesrepresenting more myopic corrections) are shown inFigure 3. Here, individual differences are much moremarked, as would be expected from the known inter-subjecl variability in values of dark focus and nighl

10 KM) 1000

10 HKI lOOf)

001 .01 .1 I 10 100 1000 .001 .01

Luminance (cd/m^)10 100 1000

Figure 3. Change in refraction from the pbotopic, baselinelevel as a function of the luminance of the background tothe Snellen letters for the six subjects. Symbols and verticalbars represent the means and standard deviations of 10autorefractor measurements. Relative dark focus values,RDFs (i.e. the refractive cbange in complete darkness), foreach subject are as indicated.

myopia (Leibowitz and Owens. I97S; Epstein ei ai.19yi; McBrien and Millodot, 19H7). In most instances,subjects still have not reached their full dark focus atthe lowest luminance level used (0.001 cd.m"). In gen-eral, refractive changes are small until the luminancelevel falls to aboul 0.5 cd/m^. corresponding roughlyto road-lighting levels, after whieh five of the six sub-jects show a progressive shift towards the valueexpected from their dark focus.

The mean changes in acuity and refraction as afunction of luminance level are shown in Figure 4.Acuity data from Richards (1966) for 16-25-year-oldsubjects are included for comparison.

Di.svussiou

The present data suggest that, although visual acuityshows at least some chanize over the full luminance

Accommodation and acuity: P. Arumi et al. 295

.001 .01 .1 I 10 100

Luminance (cd/m^)

1000

-0.6.001 .01 .1 I 10

Luminance (cd/m-)

1000

Figure 4. Mean data for six subjects as a function of lumi-nance level, (a) The logMAR and Snellen acuity [filled sym-bols represent comparable data tor 16-25-year-old subjectsfrom Richards (1966)]. (b) Change in refraction (dioptres).

range studied, this is not true for the refractive state ofthe eye. This can be appreciated more readily if weplot mean decimal visual acuity against the meanrefractive shift at corresponding luminance levels{Figure 5). Using decimal acuity rather than logMARhas the advantage of emphasizing the changes occur-ring in the important region of higher acuity. It can beseen that decimal visual acuity has already fallen toabout 0.3 (6.18) at about O.I I cd/m'. although therehas been very little systematic change in refraction.This luminance level is lower than would be foundunder most night-driving conditions. Only at stilllower luminance levels do refractive shifts becomenoticeable. It may be concluded then that, as suggestedby Johnson's data in Figure I. the loss in visual acuity

with decrease in luminance is due largely to neural fac-tors, rather than to dioptric blur associated withrefractive shift. This implies that, for road lightingluminances, 'night vision" corrections are likely to haveonly minor effects on acuity and. by implication, otheraspects of visual performance. The data of Kotulak eiai (1995). although interpreted by these authors assupporting the concept of night vision corrections, arein fact very similar to those of Johnson (1976) and thepresent study, with minimal change in accommodationas luminance is reduced through the high mesopicrange being accompanied by a marked reduction inacuity.

It is of interest to compare the "night myopia" foundat the lowest luminance level used (0.001 cd/m") withthe full dark focus for each subject. The relevant dataare summarized in Figure 6. It can be seen that thetwo parameters are not very well correlated, possiblybecause of the variability in accommodation at lowluminances (Heath. 1962; Miller, 1978; Johnson el ai,1984; Tan and O'Leary. 1986; Gray et ai. 1993); onesubject in particular evidently had a very variabledark-focus response. However, the important result isthat, as noted earlier, the 'night myopia" is generallysubstantially less than the full dark focus, even at thevery low level of 0.001 cd/m" (i.e. at a luminance lowerby a factor of 1000 than that of a well-lit road).

Experiment 2: outdoor study

In this experiment, visual acuity and refraction weremeasured outdoors late at night under street-lightingconditions which more closely mimicked those experi-enced by drivers. All observations were monocular.and subjects and eyes were the same as those used inExperiment 1.

The subjects, again seated at the Canon Auto Ref R-l.which was now placed on the pavement outdoors.viewed a street sign at a distance of approximately60 m. The sign consisted of white lettering on a bluebackground; it subtended 68 x 25 min arc at the eyeand the letter size approximated to the equivalent ofabout 6.12 Snellen. Not all subjects could resolve theletters on the sign under the ambient night-time illumi-nation levels, but all could maintain fixation upon it.Pupil diameter under these conditions averaged6.7 ±0.6 mm.

With only a small change in fixation, the subjectscould see the monitor for acuity measurement, situatedat a distance of 6 m.

The whole area of the experiment was lit by high-pressure sodium street and other lighting to about 4-

296 Ophthal. Physioi Opt. 1997 17: No 4

-0.8 -0.6 -0.4 -0.2Change in refraction (D)

0.2

Figure 5. Mean decimal/Snellen visual acuity as a function of the corresponding change In refraction for theluminance levels (cd/m^) indicated at each data point. With reduction in luminance from photopic levels, acuityinitially falls, even in the absence of refractive change. Only at very low luminance levels (less than about0.03 cd/m^) does refraction change markedly.

5 lux, the exact level varying slightly over the exper-imental area.

Measurements of refraction first were obtained vv'henthe subject, who again wore any appropriate correc-tion, viewed the distant sign. The monitor was coveredwith black cloth to prevent it from acting as a glaresource.

The acuity monitor then was uncovered and acuityestablished by the same procedure as in Experiment I(note that the screen luminance was 106 cd/m").Refractive measurements were made while the subjects

0.5

0.0

-0.5

-1.0

-1.5

-2.0

-2.5

Regression analysisy = 0.40x - 0.02

-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5

Relative dark focu.s (D)

Figure 6. Refractive change at 0.001 cd/m^ as a function ofthe relative dark focus for the six subjects. The full line isthe least-squares regression fit to the data and the dashedline the hypothetical one-to-one relationship. All refractiveshifts are lower than the corresponding dark focus values.

maintained fixation on the smallest letters that theycould resolve. The main purpose of these essentiallyphotopic measurements was to ensure that the outdoorresults were compatible with those obtained in the lab-oratory during Experiment 1.

o -0.5 -

U

-1.0 -

-1 .5 •

-2.0Relative DF

(i)Inside 6tii

(ii)Outside 6m

(iii)Outside sign

Conditions

Figure 7. Means and standard deviations for the monocularchanges in refraction from the baseline values recorded inthe laboratory with room lights on (illuminance 300 lux) andfixation on Snellen symbols on a monitor at 6 cm distancehaving a background screen luminance of 106cd/m^, underthe following monocular (right eye) conditions of obser-vation: (i) in complete darkness in the laboratory (relativedark focus); (ii) when viewing the monitor under baselineconditions, but with the room lights off; (iii) when viewingthe monitor at 6 m in the street at night; (Iv) when viewingthe street sign at 60 m in the street at night.

Accommodation and acuity: P. Arumi et al. 297

Re.sult.s-

Figure 7 shows the mean data for refractive changes inExperiment 2. together with some comparable resultsfrom Experiment I. All the data represent changesfrom the baseline values for the subjects, as recordedin Experiment 1. As would be expected, the outdoorshifts when the relatively high-luminance monitor isfixated are small and comparable to those obtainedindoors with room lights olT. More importantly, this isalso the ease when the street sign is observed understreet lighting conditions, although the spread of theresults is slightly greater than for the higher-luminancemonitor.

Discussion

The results of Experiment 2 suggest that refractiveshifts under street-lighting conditions are likely to berelatively small for most subjects. Chauhan andCharman (1993) found similar results in a sample of20 subjects. The data are compatible with the labora-tory finding (Experiment I) of a mean shift of <0.l Dat a luminance level corresponding to that of theletters of the road sign (about 0.5 cd, m^).

General discussion

The measurements in this paper strengthen the view(Richards. 1967; Allen. 1970; Chauhan and Charman.1993) that myopic shifts in young drivers are likely tobe relatively small under night-driving conditions.More importantly, they show that the losses in acuitythat are observed under sueh conditions are due predo-minantly to neural factors and that refractive blurplays only a minor role, even at scotopic levels. Thus•night-driving" corrections can, at best, only produceminor improvements in acuity.

Studies of the present type with only limited num-bers of subjects cannot, of course, eliminate the possi-bility that the performance of some individuals mightshow markedly different behaviour, although thisseems improbable. Equally, it is possible that outdooreiTects when only vehicle lighting was available and theilluminated field was smaller might give somewhatdillerent results. This, again, seems unlikely, since suchconditions correspond quite closely to the situation inExperiment I. where the illuminated field was small.

The above conelusions. while straightforward,deserve to be highlighted in view of the emphasis thatis often put on the severity of the "problem' of nightmyopia for young drivers at night. Thus, tor example.Fejer and Girgis (1992). having measured night myo-pia with a laser speekle oplometer in a fully darkenedroom, state '4% (of suhjeels) had nighi myopia of

2.5 D which is equivaleni lo an acuity of 2(1:265 ... .This indicates that .some in this group ... if uncoirectedfor night myopia would not meel the legal requiremenl'sfor a driver's license while driving at nighl'. Clearly, ina fully darkened room, it is likely that acuity would beat a very low level not because of night myopia butbecause of the darkness. It is. of course, true that arefractive error of 2.5 D would substantially degradeacuity under photopic conditions, but this is irrelevantto the night-driving environment.

In the UK. the "number-plate" test requires that thesymbols be read under "good daylighting conditions": itis indeed highly likely that many would fail to read thesymbols of the plate at night, but these are not theconditions defined by the test. In fact. Drasdo andHaggerty (1981) found that the UK number-plate testrequired an acuity of about 6 9"" or a decimal acuityof 0.6 (logMAR 0.2). It can be seen from Figure 4that, for our young corrected subjects, this limitingacuity level occurs when the luminance falls to about1 cd/m", i.e. about the luminance level of the road atnight. However, acuity for older drivers is known tobe more su.sceptible to falling luminance (e.g.Richards, 1966), so that higher minimum luminancelevels would be required for this age-group. The faetthat most drivers would claim to be able to drivesafely at night, even though their acuity must be mark-edly reduced, has interesting implications for thedebate on desirable vision test standards for drivers.Anderson and Holliday (1995), for example, haveargued recently for the need for the introduction of avehicle licensing sight test under night-driving con-ditions, based on their observation that simulated lensopacities which have little effect on standard photopicmeasures of high-contrast acuity have a marked effecton night-time measures of contrast sensitivity for mov-ing targets.

The accommodation response under a variety of cir-cumstances becomes less accurate if the effective acuityof the visual system is impaired, because of either thecharacteristics of the stimulus or the individualtCharman. 1986). In low luminances, aecurate focus isnot required, because the retina and brain cannotmake use of the detailed information of a sharply-focused image. Thus, night myopia can be regarded asthe result of the loss in acuity that occurs as the sceneluminance is lowered, rather than its cause.

Finally, we note that the use of tinted windscreensor "night-driving" spectacles will tend to reduce theretinal illuminance under any given road conditions.Although this reduction may slightly diminish glareertects from vehicle headlights and other light sources,it also degrades most other aspects of visual perform-ance [e.g. Haber. 1955: Phillips. 1967; Clark, 1969. seealso Figure 4(a)] and tends to negate the effects of

298 Ophthai Physiol. Opt. 1997 17: No 4

improved vehicle and road lighting. Thus, .such lintscannot be recommended (Highway Code. 1993).

Acknowledgements

This work was partly supported by EPSRC LINK.TIO grant GR K56957. Author KC also is grateful forsupport from Boots Opticians Ltd. Nottingham. UK.

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