an improved visual acuity test-object

JournalOF THE

Optical Society of AmericaDEVOTED TO THEORETICAL, EXPERIMENTAL

AND APPLIED OPTICS

Volume 1 JULY, 1917 Number 4

AN IMPROVED VISUAL ACUITY TEST-OBJECTBy HERBERT E. IVES

Several years ago the present writer published an account of a newapparatus for the testing of visual acuity. 1 This consisted essentially oftwo superposed opaque line gratings on glass, arranged to rotate relativelyto each other about an axis perpendicular to their plane. When viewed bytransmitted light, at such a distance that the constituent lines of the grat-ings are below the limit of separability, parallel dark bands are seen, dueto a species of interference to be described in detail presently. Thesebands alter their separation gradually and continuously as the gratingsare rotated, from invisibility up to extreme visibility. The average bright-ness of the field presented by the crossed gratings remains constant, and thevariation of brightness from the center of a bright band to the center of theadjacent dark band is the same for all sizes of the bands.

These characteristic properties of crossed gratings are well known tophysicists, having been employed for instance to magnify small movementsof expansion and contraction, and as a regular method of testing the screwsof dividing engines. The relationship holding between the width of theinterference bands and the angle of relative rotation of the component

Electrical World, April 14, 1910, p. 939.(101)

IMPROVED VISUAL ACUITY ETC.

gratings is shown by the diagram, Figure i,

where d is the distance between the centersof the ruled grating lines, the angle of rela-tive rotation, and D the distance betweenthose regions where continuous straight linescould be drawn across the gratings withoutcrossing a clear space. These latter consti-tute the bands visible on viewing the crossedgratings at a distance. Their separation isgiven by the relation

D d2 sin 22

DI _ _

IIFig.

In Figure is shown a photomicrograph of a pair of superposed grat-ings, turned through a small angle, illustrating the principle. If this figureis viewed from a distance of several meters, the constituent lines disappear,and the interference bands alone show. At the distance of just visibilityall indication of the graduation of intensity from center to center of lightand dark bands is lost, and the bands appear merely as sharp black lines ona light field.

For use in the testing of vision the grating characteristics are correlatedwith visual acuity through the common measure of the latter, namely the'angular separation of details just visible as separate. An observer's visual

T

acuity will thus be proportional to D or to 2 sin . For small angles,D 2 whichundervisualwhich

need alone be used by proper choice of d, this becomes sin , whichthese conditions is simply . Hence the very simple relation thatacuity can be measured with this device by the angle throughthe gratings must be turned to make the interference bands just

disappear.Advantages of this form of test object over other forms, such as the

test types, are the absolutely gradual transition in the size of the detail,the possibility of substituting a decimal fraction of unity for the reading inplace of the present cumbersome fractions, such as , etc., the fact thatchildren and illiterates can be tested as accurately as other persons. Thechief disadvantages of the test types, that different letters on the same lineAre unequally difficult of recognition (for instance R, B, and I), and

102 H. E. Ives

A


that their recognition depends to a considerable extent on the patient'sfamiliarity or experience with printed letters, are eliminated by the gratingtest object.

Fig. 2 Fig. 3

Some time ago it was suggested to the writer by Mr. Max Poser, ofthe Bausch and Lomb Optical Co., that it would be desirable for the test-ing of visual acuity to have the detail of this test object extend in twodirections instead of one, that is, if possible, to have squares instead ofparallel bands. This improvement, which forms the principal subject ofthe present note, was found to be readily affected by substituting for thesingle line gratings, ones ruled with two sets of lines at right angles. Theinterference pattern due to superposing two such cross-line gratings, isshown in Figure 3, which if viewed from several meters distance, exhibits.squares in the same manner that the pattern from the single line gratingsin Figure 2 exhibit bands.

By the use of these cross-line gratings a measure of what may betermed the average visual acuity may be obtained. With the single linegratings, acuity may be measured at any angle, from which a measure ofastigmatism is obtained. Both patterns are therefore of use, and so in theinstrument as now manufactured arrangement is made for removing one ofthe cross-line gratings and putting in its place a single line one. This actsin conjunction with one set of lines of the remaining cross-line grating togive the bands peculiar to two single line gratings.

The apparatus as constructed and shown in Figs. 4 and 5 (front andback views), consists of a grating holder, a housing for the electric lampwhich provides the illumination for the gratings, and a front plate which

H. E. Ives 103

i

104 H. E. Ives IMPROVED VISUAL ACUITY ETC.

provides the surrounding field (illuminated to be of approximately thebrightness of the gratings as viewed through an aperture in it), and alsoserves to carry various other objects for visual testing.

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iI

Visual Acuity Test ObjectFront View

The grating holder, mounted directly behind the front plate, carriesthe two gratings so arranged that they may be rotated in opposite direc-tions to each other by means of a rack and pinion movement, manipulatedby turning a milled head on the pinion. The acuity scale is mounted justbelow the milled head of the rack and pinion movement and reads directlyin Snellen acuity units, that is, unit acuity corresponds to the detection ofthe separation of lines or bands whose centers are two minutes of arc apart.This is the 5/5 or 6/6 of ordinary terminology. The ordinary 6/12 or5 /IO read as 0.5 etc. The scale is marked at the side in meters of reading

11 I


distance,distance

indicating which line of the scale is to be used for any chosenbetween observer and test object.

Visual Acuity Test ObjectRear View, Showing Operating Mechanism

The grating holder, together with the rack and pinion arrangement,may be rotated about its axis so that the lines or squares appear at variousangles to the horizontal, as indicated by a pointer on a scale of degrees onon the back of the front plate. All working parts, scales, etc., are entirelyconcealed behind the front plate, so that the observer need have no knowl-edge of what is being done by the operator.

H. E. Ives 105


The operation of measuring visual acuity is very simple. First thegratings are turned so that the bands or squares are too small to be visibleto the observer. They are then rotated slowly by means of the rack andpinion movement until the observer just sees the pattern, whereupon theacuity is read from the scale. As a check on the correctness of his observa-tion, the patient may be asked in which direction the lines lie with referenceto the clock dial marked on the front plate, and this direction may be changedbetween observations so that he is in ignorance which way to expect them.Either the lines or the squares may be used for acuity testing as preferred.For finding the axis of astigmatism the lines are preferable. The gratingsare to be set so that the bands are just visible and then rotated together intheir holder until the bands appear clearest, when the corresponding angleis read from the scale marked on the back of the front plate.

In ascertaining the position of the axis of the cylinder with an ame-toropic eye, the operator may set the test object so that the black bandscoincide with the approximate position of the axis of the cylinder of thepatient's eye, found by the astigmatic chart, after which the black bandsof the test object are to be reduced in width by means of the rack and pinionmotion of the test object and after the minimum width of the black bandsjust visible to the patient has been reached, the entire screen phenomenonis slightly turned around its perpendicular axis in the one and the otherdirection, when the position in which the black bands of the test objectappear most distinct indicates the precise location of the axis of the cylin-der. This process may be repeated several times and will prove that theresults obtained check up very precisely. It is essential to keep the bright-ness of the field constant with either the phenomenon of the lines orsquares and for which purpose the lamp is fitted into an adjustable slidingsleeve so that same can be moved to and from the gratings, thus eitherincreasing or decreasing the illumination as the case may be. Interestingdata may be found when the test for visual acuity is made with the gratingsilluminated by more or less monochromatic light of the various colors ofthe spectrum. The apparatus is also useful for testing malingerers withthe aid of various color screens in conjunction with the muscle test andrailway signal test appliance.

Besides the special test object formed by the gratings, the apparatusas manufactured incorporates several standard devices which make thewhole a very complete device for visual examinations. These are carried by

106 H. E. Ives

IMPROVED VISUAL ACUITY ETC. H. E. Ives 107

the front plate and comprise a lamp with iris diaphragm for muscle testing,special colored screens for chromatic test, lamp fitted with iris diaphragm andcolor screen for railway signal tests, and smoked glass wedges by means ofwhich the effect of the colors may be dimmed or brightened as the casemay be. Below the grating aperture the front plate is provided with anoblong aperture behind which two rollers are mounted carrying a set ofSnellen test types, so arranged that only one row of letters is visible at onetime. The test letters are white on a black ground, and three rows of eachsize are provided, the letters being different in each row to prevent memor-izing. The Snellen test types are provided partly for the convenience ofthose ophthalmologists who are accustomed to their use and wish to corre-late the indications of the new test object with the types. The printedletters also form excellent tests of definition, by which to judge the successof correcting glasses, although other figures such as squares, circles, ortriangles would serve this purpose. After becoming accustomed to thenew test object the ophthalmologist may be expected to depend entirelyupon its indications, dispensing with the types.

Illumination of the front plate is by means of two long linear filamentelectric lamps. Both these and the grating lamp are provided with bluetinted glasses, so that the working illumination is midway between daylightand artificial light. If it is desired to operate the apparatus from theposition of the patient, a mirror is used. The figures on the front dial arethen reversed, and the angular scale is graduated in the direction to becorrect after the reversal introduced by the mirror.

It is expected that this test object will find its first use in universitylaboratories, where in physical, physiological and psychological studies asatisfactory test for acuity has long been needed. Later, as studentsfamiliar with its use go into private practice it is hoped the apparatus willcome into more or less general use for the everyday testing of vision.

Physical LaboratoryTHE UNITED GAS IMPROVEMENT CO.PhiladelphiaJuly, 1917

an improved visual acuity test-object

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