Lang Stereotest Technical Article
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A New Stereotest
Joseph
Lang, MD Zurich, Switzerland
Recently
a number of stereo tests have become available, among them in particular random
dot tests.
The
advantage of random dots in not giving monocular clues is well known. However,
the disadvantage of most random-dot tests is that they required red-green or
Polaroid glasses Small children may refuse glasses, or time maybe necessary to
convince them to accept glasses. Furthermore, observation of the eyes and of eye
movement is impossible o more difficult with glasses. The Frisby test which does
not require glasses is somewhat cumbersome and not very attractive to small
children.
To avoid
the difficulties described above, we have combined Jules' random dot with the
panography technique of Hess. Panography or lenticular screen was invented in
1912 by the Swiss ophthalmologist and physiologist, WR Wess. The same Hess had
described the Hess coordimetry method for examining eye muscle paresis which is
still in use today. in 1949 he was awarded the Nobel prize for his research on
the diencephalon. He called this technique "free eye" stereoscopy.
Cylindrical
screens provide a separate image to each eye. The two pictures consist of fine
strips which are alternatively seen by each eye when focused through the
cylindrical lens elements. This is shown in the patent specification of Hess
(Figure 1). Since there are 23 cylinders per centimeter, 46 fine strips per
centimeter provide 23 strips to be seen by each eye. The two pictures are fused,
and if disparity in some parts exists, this disparity provides a stereoscopic
impression (Figure 2).
The
great advantage of panography and of our new test is that no glasses are
necessary. The stereoscopic picture is seen spontaneously when the test card is
held in a front parallel position in front of the examined person. The test
should be held by the examiner and not by the patient. Otherwise, the examinee
may try to get monocular clues by moving the stereo test in a flip-flop manner.
But the patient is permitted by small movements of his head to adjust to the
best viewing position.
The
correctness of answers can be checked by holding the test vertically stereo
clues then disappear or by holding the test upside down the star is then seen at
the bottom. By holding one test horizontally and the other one vertically,
preferential looking can be tested. When the test is tilted horizontally ,
binocular clues disappear. exact front parallel positioning therefore is
mandatory.
In
the test card measuring 9.5 x 14.5 cm, the examinee sees a car, a star and
nearer to him a cat. when held at a distance of 40 cm, the disparity of the car
and star is 600 seconds and of the cat 1200 seconds. (Figure 3).
Due
to technical reasons this test is to suited for assessing the stereo threshold,
but it provides a very quick screening for binocular vision even in young
children by a very simple procedure. If children are not willing, or to able to
pronounce the name of the object, the ophthalmologist still can judge whether
the child sees the objects from the eye movements or the child may try to grasp
them. Our procedure is therefore especially suited for screening in kindergarten
and also for pediatricians. It is also extremely useful in everyday ophthalmologial routine examination and in orthoptics. Curiously enough,
children mostly respond quicker than adults when shown the test. when a patient
fails the test, we immediately proceed to a vertical two-pencil test under
monocular and binocular conditions. This shows whether he has all the same gross
binocularity. (1).
We worked with great success several months with a
prototype which showed only one object, namely a car. This was published in our monography
on Microtropia in May 1982. (4). Presenting three objects proved to be
more satisfactory. More work will have to be done in this interesting field and
comparative studies with other stereo tests are in preparation.
UPDATE:
As of 11/03 we have found some quality problems with the LANG 2 test. It has
become visible monocularly which make the test void. When new items have been
received we will resume shipment of the LANG 2 tests.
REFERENCES
-
Jules B: Foundations of Cyclopean Perception. Chicago, The University of
Chicago Press, 1971.
-
Hess WR: Direkt wirkende Steroskopbilder. Zeitschrift fur wissenschaftliche
Photographie. Photophysik and Photochemie Leipzig, Bank, Heft, 1914, Vol. 14, PP 34-38.
-
Lag J: Deer Treffversuch zur Prufung des stereoskopischen Sehens. Klin
Monstabl Augenheilk 1974; 165:895-897
-
Lang J:Mikrostrabismus. Heft 62.
Bucherei des Augenarztes. 2. Stuttgart, Auflage, Mai 1982, PP 39-41.
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