|Year : 1963 | Volume
| Issue : 1 | Page : 19-22
Zeiss screen-type visual balance tester
|Date of Web Publication||28-Jan-2008|
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|How to cite this article:|
Beyer E. Zeiss screen-type visual balance tester. Indian J Ophthalmol 1963;11:19-22
In the prescription of spectacles the testing of the wearers' binocular vision plays an ever more important part. Similarly as defects in refraction are being established in monocular eye testing procedures, new binocular vision tests and the respective instrumentation are being devised with the aid of which it will he possible to comprehend in a measurable way and to correct all those defects that are likely to give rise to disorders in binocular vision and to asthenopic complaints.
What is aimed at is to ensure from the very outset the results the prescribed aid is expected to give, i.e., to attain a general visual balance or, in other words, to impart to spectacle wearers uncomplicated binocular vision which, free from complaint, affords good stereoscopic fusion together with maximum visual acuity.
In cases of pronounced binocular imbalance, double images will appear with one or the other being usually suppressed, or one eye being excluded altogether. Minor defects are frequently overcome either temporarily or even permanently, but the incurred effect often leads to general complaints, such as headaches, fatigue, exhaustion and discomfort.
The cause of visual disturbance occasioned in binocular vision is attributable to differences which occur between the visual impressions produced by the left and the right eye and which either prevent or complicate the perfect fusion of the two images into a single one.
The differences in question may refer to the sharpness or the contrast as well as to the relative positions and sizes of the perceived images.
They are either of anatomically functional nature or are caused by unilaterally slight residual defects of refraction (refractive and accommodative imbalance, latent heterophoria due to accommodative convergence), or attributable to correction lenses of highly differing powers aniseikonia with anisometropia) and to highly astigmatic correcting lenses with oblique principal section positions in opposite directions (optical cyclophoria). Differences, also including those between the two visual sensations, can be ascertained, measured and-if at all possible-corrected quite accurately by simultaneous comparisons. In order therefore to be able to compare the images perceived with the left and right eyes simultaneously it is required to separate them from each other.
With older methods, especially for the heterophoria tests, the simultaneous visual sensations are dissociated in shape and colour or by their relative positions, thus eliminating fusion altogether if possible.
One of these methods is to test with `MADDOX-cylinder' .where a point of light is seen by one eye directly and by the other one through a (usually red tinted) MADDOX cylinder, as a drawn out red line. or the GRAFEE test in which by a vertically deflecting prism, double images are created which cannot be fused so that in their corresponding positions lateral deviations may be ascertained.
Practice has shown that values obtained with these tests may not be fully used as correction values, nor that corrections obtained from measured values according to ampirical rules may be in every instance correct. This is most probably due to the binocular visual conditions not being normal.
With the more recent methods, which all of us fall back upon, the so-called "TURVILLE Infinity Balance" (TIB), the attempt is made to create testing conditions that approximate the natural binocular viewing process. The 'test is carried out in a bright room with juxtaposed test marks that are distinguishable but as physiologically equivalent as possible and separately perceivable for the left and the right eye.
Whilst the test signs do not produce any fusion stimulus in the central field of vision, fusion is stimulated peripherally in indirect vision by the binocular, mutually visible surrounding area, intensified by its border lines.
The original TIB-method utilised an opaque separator at half the testing distance between the chart signs and he patient. When reading by mirror the separator lies directly on the mirror. The values determined by means of the TURVILLE method may be in general used directly as correction values, excepting the exophoria values which, due to the accommodation and the resulting convergence towards the separator, are too high. TURVILLE's experiment with an invisible mirror separator (imaging via two mirrors tilted at a small angle to each other) did not lead to any marked improvement, since accommodation may be effected on the mirror itself. What must be regarded as a drawback in both the methods is the fact that a rather meticulous alignment between the patient's eye and the separator is called for.
These disadvantages can be avoided when separation is effected in polarized light. A pre-requisite for this however is the use of steroprojection or of instruments which are relatively complicated to produce.
Furthermore the polarizing equipment of the analyzers fixed in front of the eyes would need to be very accurately aligned relative to the polarizers on test objects.
With the Zeiss Screen-Type Visual Balance, separated test marks are presented to both eyes, same as with the TIB-method, yet the separation of the test marks is effected in a novel way suggested by UEBERSCHAAR in accordance with the screen method resorted to in some stereo-film methods. The advantage of this process is that the separator is firmly connected with the test chart and lies close to its plane. It is not therefore possible to simulate an esophoria by accommodative convergence (as with a separator arranged in closer proximity) and there is no need for an accurate alignment of any axillary aids as is called for in separation by polarization. Binocular vision is thus accomplished unhampered under practically normal conditions.
The use of physiologically equivalent test-marks, viz., the digits 5 and g, permits on the one hand a highly accurate comparison of the visual sensation for the left and right eyes when testing the refractive balance, and on the other hand the questioning and answering of questions during all tests is facilitated thus reducing the testing time.
The instrument [Figure - 1] consists of a housing containing two glass plates spaced a few centimeters apart, the outer one of which contains a fine screen of vertical lines and the inside one the test marks which are likewise of screen pattern [Figure - 2]. Both discs are transluminated via an opal glass disc by means of 350 watt bulb behind which a reflector is situated. A pivot serves for fixing the instrument on a wall bracket.
The width of the bright and dark strips of the screen is so narrow that for the testing distance of 5 m no resolution takes place, thus producing the impression of a uniformity transluminated area. The screen-patterned test marks- are arranged on the rear disc relative to the front disc in such a way that by the left eye only the one and by the right eye only the other can be seen simultaneously [Figure - 3]. The figures appear evenly black as their bright strips are being masked by the dark ones of the front screen.
Both are available in two sizes corresponding to acuities of 0.2 and 0.7. The luminous density of the transluminated parts of the discs is so high that the test marks appear in distinct contrast, but without the luminous field causing any dazzling effect.
After switching on the lamp the patient, sitting 5 meters away, is to be instructed to align his sideway position in such a way that when covering one eye he can perceive the full figure 6 or 9 in black. Minor lateral movements of the head do not matter, so that it is not necessary to fx the hitherto known separating method. To produce the refraction balance the correcting lenses are to be changed until the patient perceives the two groups of figures at the same density. To obtain the muscular balance, correction is carried out in the known manner prismatic lenses until a stationary position of the two groups of figures is obtained at normal distance.
Besides the direct observation of the test marks at a distance of 5 m, also indirect observation is possible via a mirror (2x2.5 m distance). For this purpose the instrument must be provided with mirror-image test marks. To prevent the mirror from stimulating fusion it should have a diameter of at least 200 m.
[Figure - 1], [Figure - 2], [Figure - 3]
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