Optics: eye examining – vision testing and correcting – Eye examining or testing instrument – Methods of use
Patent
1997-07-17
1999-08-03
Mai, Huy
Optics: eye examining, vision testing and correcting
Eye examining or testing instrument
Methods of use
351243, A61B 300
Patent
active
059332107
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to a method and apparatus for presenting dichoptic stimuli, particularly for the determination of dark vergence of an observer.
BACKGROUND OF THE INVENTION
U.S. patent application Ser. No. 034,717 filed on Mar. 22, 1993 by the present Applicant and entitled "Ophthalmological Instrument For Producing Dichoptic Stimuli On A Visual Display Terminal" describes a method and system for presenting two successive images on a video display unit as a dichoptic stimulus of substantially equal intensity. A first image is displayed at a predetermined intensity so as to be seen by a first eye of an observer only, an optical shutter being used to prevent the displayed image from reaching the second eye of the observer. The first image is then extinguished and the state of the optical shutter reversed so that the first eye can no longer see the video display unit, whilst a second image is presented to the second eye. In order to prevent the second eye from seeing an after image of the first image, a neutral density filter is disposed within the light path, and the second image is illuminated at a correspondingly higher intensity so as to compensate for the attenuation factor of the neutral density filter.
Such a method may be employed to measure the dark vergence of the observer which has been found to be a major factor in determining fatigue of computer operators, in order that corrective action may be taken so that the angle subtended by an image on the computer screen at the eyes of the observer may be adjusted to the angle of dark vergence. Such corrective action may be provided by individually tailored prismatic spectacles.
U.S. application Ser. No. 034,717 thus addresses the specific need of computer operators and permits the dark vergence of such operators to be determined when looking in a substantially straight-ahead position towards a computer screen. Since images are presented on computer screens and the like as a raster scan, there is at any time only a single pixel which is illuminated, the effect of a complete image being obtained owing to the eye's persistence of vision. Further, since both the left and right eye images are presented on the same screen, any effect of after image must be suppressed so that each eye will see only its own image as a dichoptic stimulus.
Prior art relating to determination of dark vergence is fully described in the above-referenced U.S. application, the complete contents of which are incorporated herein by reference.
The above-referenced U.S. application and the prior art references discussed therein are, as indicated above, principally directed to the determination of dark vergence in a straight-ahead direction. This is fine for computer operators who do indeed work with their gaze directed substantially straight ahead. However, most text is, in fact, read with the eyes directed not straight ahead but inclined downwards. Fixating at text whilst looking down requires the cooperation of all twelve extraocular muscles attached to both eyes (i.e. medial rectus, lateral rectus, superior and inferior recti and superior and inferior oblique recti).
The prior art does not define the dark vergence when the eyes are not directed straight ahead in the horizontal direction. However, it may be expected that in any plane formed by the visual axis of the two eyes, a different dark vergence exists. Specifically, when looking down at an angle commensurate with reading, the dark vergence will be different to that associated with straight-ahead vision. To converge on text whilst looking down requires that some of the extraocular muscles contract and are thus under tension. This tension, when applied for several hours, can produce visual strain similar to the strain which results when an observer is looking at a visual display unit such as a computer screen.
From the above-referenced U.S. patent application three considerations are apparent: first, dark vergence varies between limits from one subject to another; secondly, dark vergence depends on t
REFERENCES:
patent: 4561723 (1985-12-01), Hamano et al.
patent: 5233458 (1993-08-01), Moffitt et al.
W. Jaschinski-Kruza; "Stimulus Distance on Measurements of Dark Convergence"; Ophthalmic and Physiological Optics, vol. 10; pp. 243-251; Jul. 1990.
D. Alfred Owens; "The Resting State of the Eyes"; American Scientist, vol. 72; pp. 378-387.
W. Jaschinski-Kruza; "Eyestrain in VDT Users: Viewing Distance and Resting Position of Ocular Muscles"; pp. 69-83; 1991.
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