Abstract: A visual fatigue level measuring device includes: a video presenting unit which presents 3D video; a test period determining unit which determines two or more time periods each of which is for a test for measuring a level of visual fatigue; an eye movement obtaining unit which obtains data indicating eye movements of left and right eyes of a player with respect to the presented video; an eye movement extracting unit which extracts, from the obtained eye movement data, eye movement data in each of the two or more time periods determined by the test period determining unit; an eye movement comparing unit which compares eye movements in the two or more time periods based on the extracted data; and a fatigue level determining unit which determines the level of visual fatigue of the player based on the comparison result.

Abstract: The invention relates to an instrument for simulation of multifocal ophthalmic corrections, comprising two optical channels with different optical power values in the beams coming from the object observed, wherein at least one channel comprises a Badal system. This instrument simultaneously provides images of objects near and far focused. The system provides the same optical magnifications for each channel, regardless of the optical power thereof, and produces superimposed retinal images with different degrees of focus which, unlike other devices, are all of the same size. The instrument allows simulating different optical powers for near vision and different refractive corrections for distant vision.

Abstract: Method and system for treating of visual impairment associated with misalignment between eyes of a patient is described. The invention allows to obtain electronically of an image of an object at which one eye of the patient is staring, calculating angular deviation between staring direction of this eye and the second eye and displaying before the second eye the image after it has been electronically processed so as to correct the angular deviation and to obtain the image, which would be perceptible by the brain as a single three dimensional stereoscopic image.

Abstract: An eyesight testing apparatus examines associated heterophoria of the eyes of a subject. The apparatus includes an image generation device having a display unit for generating test patterns for display to the eyes. An optical assembly is arranged on that side of the display unit facing the eyes. This assembly separates light supplied by a first group of selected zones of the display unit from light supplied to a beam path by a second group of selected zones of the display unit. The left eye only receives the light from the first group. The light from the second group only passes to the right eye. The optical assembly includes a prism matrix having a multiplicity of prism portions extending in the vertical direction and each having a lens-shaped region with a convex surface facing the display unit.

Abstract: The invention relates to a stereovision optometry apparatus comprising: a 3D display unit for displaying 3D image; a reference 2D display unit for displaying reference 2D image; an eyepiece in opposite side of the 2D display unit configured to view the images; a reflective minor in opposite side of the 3D display unit configured to move toward and away from the 3D display unit; and a half minor arranged between the 3D display and the reflective mirror and disposed at a point where propagation paths of the 2D image and the 3D image intersect; wherein the 2D image penetrates through the half mirror to reach the eyepiece; wherein the 3D image penetrates the half mirror to be reflected on the reflective mirror and then the reflected image is reflected on the half mirror to reach the eyepiece.

Abstract: An apparatus and method are disclosed that provide for establishing and/or improving binocular vision by adaptation of perceptual motor learning for the visual system of a patient suffering of a binocular vision disorder. The apparatus has a set of two units, one for respectively eye, for manipulation of a vision parameter of the visual presentation, e.g. picture, movie, image. To control the visual presentation on respectively unit a processing unit is connected. The processing unit is adapted to determine a boundary value of a first and/or second vision parameter for the first and/or second manipulation of the first and second unit, where binocular vision disappears for the patient. Furthermore, the processing unit controls at least one vision parameter by manipulating, including oscillating or fluctuating, it within a first range, the first range having a maximum value that is less than the boundary value, for the perceptual motor learning.

Abstract: A vision testing device includes a partition defining a first side and a second side and an electronic device operable to project a first image to the first side and a second image to the second side. The first image and the second image comprise computer-generated images, and the first image is identical to but offset from the second image. A mirror extends from the first side to the second side and is positioned to reflect the first image and the second image. A first viewing lens is in optical communication with the first side and configured to receive the first image reflected from the mirror. A second viewing lens is in optical communication with the second side and configured to receive the second image reflected from the mirror.

Abstract: A visual function testing device includes: a visual target image presentation unit that presents a visual target image; a visual target image rendering unit that renders the visual target image to be presented on the visual target image presentation unit; a visual function test item selection unit that selects a visual function test item; a visual target image generation unit that generates a visual target image corresponding to the test item selected by the visual function test item selection unit; a viewpoint distance input unit that inputs a distance between the visual target image presentation unit and a viewpoint of an observer; and a visual angle input unit that inputs an angle to be made by the visual target image and the viewpoint of the observer. Based on the viewpoint distance and the visual angle, a display size and display position of a visual target image are calculated.

Abstract: A device for measuring depth of field of an eye optical system that includes a lens Badal having positive optical power, a positioning apparatus adapted to maintain the eye optical system on the image side of the lens at a first focal plane of the lens and a multi-vergency target that provides a plurality of objects that are simultaneously viewable, through the lens, by the eye optical system.

Abstract: An imaging apparatus for obtaining a tomographic image of an object based on light obtained by combining returning light from the object, which is irradiated with measurement light, and reference light corresponding to that measurement light, the imaging apparatus comprising: a reference light splitting unit adapted to split the reference light into a plurality of reference light beams of different wavelength ranges, and a plurality of dispersion compensation units adapted to compensate dispersion in accordance with wavelength ranges of the plurality of reference light beams, the dispersion compensation units being provided in respective light paths of the plurality of reference light beams.

Abstract: A binocular vision apparatus allows a patient to view objects through the apparatus with polychromatic light and monochromatic aberration correction, such that the chromatic aberration of the eye can be combined with the monochromatic aberration correction, so as to provide a more accurate determination of vision quality. The binocular vision apparatus provides left and right viewing optics that can substantially maintain the line of sight of each eye, such that objects can be viewed in a room with 3D depth perception corresponding to the distance of the object from the patient. As both near and far objects can be viewed with binocular aberration correction, the patient can alternate binocular viewing between near and far vision with chromatic aberration so as to evaluate a proposed treatment such as a presbyopia correction.

Abstract: Method for determining the acceptance of progressive addition lenses of a wearer, the method comprising: a fusional vergence parameter providing step during which at least one fusional vergence parameter representing the fusional vergence of the wearer is provided, an acceptance determining step during which the value of the at least one fusional vergence parameter is compared to a predetermined threshold value so as to determine the probability of acceptance of progressive addition lens of the wearer.

Abstract: The invention relates to an ophthalmic instrument and method for measuring, controlling, and handling aberrations (1) of the eyes (2, 3), which further simultaneously provide visual stimulants when operating same. The instrument consists of a single device for correcting aberrations (4) as well as a single aberration sensor (5), optically connected by an optical system (14). A lighting system (9) introduces beams of light into both eyes. The measurement, control, and handling of the aberrations (1), as well as the providing of visual stimulants (6), are simultaneously and binocularly (7, 8) carried out.

Abstract: A method and apparatus for vision testing and for simulation of eyesight correcting modalities is disclosed, the method including generating one or more images to be viewed by a patient, modulating the wavefront of each image by a differing amount and/or changing other optical attributes of one or more images by differing amounts, and selecting the preferred image based upon patient response. The apparatus includes devices for generating one or more images to be viewed by a patient, modulating the wavefront of each image by a differing amount and/or changing other optical attributes of one or more images by differing amounts, and devices for selecting the preferred image based upon patient response.

Abstract: A substrate includes a diffracting structure providing a hologram which encodes one or more holographic images. Every image of the one or more holographic images has a holographic location at either a finite or substantially infinite effective optical distance. The hologram is to be used as an ophthalmic eye chart or an eye exercise device or to provide a scale for measuring distances.

Abstract: An anaglyphic image presentation system is provided to evaluate and train a user's depth perception abilities. In embodiments, anaglyphic image target components are presented to a user on a display device. The image target components are then viewed by the user through a set of transmission filter lenses. The transmission filter lenses present and block one or more target components based on the peak wavelength transmission associated with each lens. As a result, a user perceives an anaglyphic target image that is resultant from the perceived image target components when viewed through the transmission filter lenses.

Abstract: Systems, methods, and apparatuses for amblyopia and ocular deviation correction, as well as visual interfaces, are disclosed. In one aspect, embodiments of the present disclosure include a system for amblyopia correction, the system includes, an image processing unit to identify a set of image parameters and, when, in operation, the image processing unit modifies a source visual content based on one or more of the set of image parameters to generate visual content and a visualization unit coupled to the image processing unit operable to receive visual content from the image processing unit, the visualization unit having a screen, when, in operation, the screen displays the visual content.

Abstract: A machine and method for balancing across the brain's fusion bridge rivalrous stimuli of image pairs having diametrically opposed brightness and means of adjusting the rivalrous balance to measure the depth of a vision defect.

Abstract: A vision testing device includes a partition defining a first side and a second side and an electronic device operable to project a first image to the first side and a second image to the second side. The first image and the second image comprise computer-generated images, and the first image is identical to but offset from the second image. A mirror extends from the first side to the second side and is positioned to reflect the first image and the second image. A first viewing lens is in optical communication with the first side and configured to receive the first image reflected from the mirror. A second viewing lens is in optical communication with the second side and configured to receive the second image reflected from the mirror.

Abstract: A visual function testing device includes: a visual target image presentation unit that presents a visual target image; a visual target image rendering unit that renders the visual target image to be presented on the visual target image presentation unit; a visual function test item selection unit that selects a visual function test item; a visual target image generation unit that generates a visual target image corresponding to the test item selected by the visual function test item selection unit; a viewpoint distance input unit that inputs a distance between the visual target image presentation unit and a viewpoint of an observer; and a visual angle input unit that inputs an angle to be made by the visual target image and the viewpoint of the observer. Based on the viewpoint distance and the visual angle, a display size and display position of a visual target image are calculated.

Abstract: A device for at least one of diagnosis and treatment of binocular vision disorders is disclosed. The device comprises a portable, wearable viewing apparatus that includes a left eye electronic display and a right eye electronic display, a dual data channel input, wherein each data channel corresponds to a respective one of the left eye and right eye electronic displays, a left eye adjustable optic structure adjusting a view of the left eye electronic display, and a right eye adjustable optic structure adjusting a view of the right eye electronic display.

Abstract: An apparatus and method are disclosed that provide for establishing and/or improving binocular vision by adaptation of perceptual motor learning for the visual system of a patient suffering of a binocular vision disorder. The apparatus has a set of two units, one for respectively eye, for manipulation of a vision parameter of the visual presentation, e.g. picture, movie, image. To control the visual presentation on respectively unit a processing unit is connected. The processing unit is adapted to determine a boundary value of a first and/or second vision parameter for the first and/or second manipulation of the first and second unit, where binocular vision disappears for the patient. Furthermore, the processing unit controls at least one vision parameter by manipulating, including oscillating or fluctuating, it within a first range, the first range having a maximum value that is less than the boundary value, for the perceptual motor learning.

Abstract: A binocular vision analyzer apparatus is used for determining a prescription of visual aids for the eyes of an individual. The apparatus includes first and second targets respectively corresponding to the first and second eyes of the individual and at least one illuminator unit for illuminating the targets. An optical system is configured to present respective virtual images of the first and second targets so as to be viewable by the respective first and second eyes. At least one beam splitter disposed in front of the first and second eyes directs the virtual images toward the respective eyes. The apparatus also includes first and second spherical correction devices and first and second cylindrical correction devices that respectively correspond to the first and second eyes.

Abstract: A testing device and method are presented for detecting binocular vision problems in patients. The testing device includes a body with a flat portion disposed along one side. Extending outwardly from the top of the body is an actuating means. The device further includes a first set of lights disposed on the surface of the device a first distance away from a center point and a second set of lights disposed a second distance away from the center point. Each of the lights in the first set are smaller than each of the lights in the second set. Further, the second distance is shorter than the first distance.

Abstract: An optometric apparatus for a subjective binocular visual performance test includes right and left test units including right and left housings, a plurality of optical elements disposed in the right and left housings, right and left test windows provided to the housings, in which desired optical elements selected from the optical elements are to be placed and through the selected optical elements, an optotype is presented to examinee's right and left eyes by an optotype presenting apparatus, right and left polarization plates having polarization axes perpendicular to each other and arranged to be placed in the test windows, and right and left protective covers arranged to be placed in the test windows at a side facing the optotype presenting apparatus and having polarization optical characteristics of changing circularly polarized light that passes through the right and left protective covers into linearly polarized light having polarization axes perpendicular to each other.

Abstract: An optotype presenting apparatus presents optotypes for testing an examinee's binocular visual function. The apparatus includes: an optotype presenting part to display an optotype at a predetermined presenting region; a pupillary distance input part that inputs a pupillary distance between an examinee's left and right eyes; and a control unit that makes the optotype presenting part display left- and right-eye optotypes for a stereoscopic vision test. The control unit determines a lateral inter-optotype spacing between the left- and the right-eye optotypes based on the input pupillary distance so that a fusion optotype seen with the examinee's both eyes is perceived as an optotype that is seen floating or sinking by a fusion distance between a reference position and a predetermined fusion position, and makes the optotype presenting part display the left-eye optotype and the right-eye optotype based on the determined inter-optotype spacing.

Abstract: A pair of eyewear has transparent waveguides in each eyewear lens. An illumination subsystem directs a beam of light through the waveguides onto a wearer's eyes. A First Purkinje point imaging subsystem for each lens uses light reflected from the eyes through the waveguides. A controller calculates from the positions of the First Purkinje points the wearer's gaze parallax angle and/or the distance to an object of interest on which the gaze of the wearer's eyes converges. The output of the controller may drive variable-focus eyeglass lenses to enable a presbyopic wearer to focus on the object. Modified Alvarez variable-focus lenses are described.

Abstract: An ophthalmic instrument for measurement of ocular refraction and visual simulation and a method of measurement obtaining the ocular refraction in a binocular manner, which incorporates a digitally controlled phase modulator for generating best ophthalmic correction in each subject. The instrument enables measuring refraction, not only associated with blurring and astigmatism, but also optical aberration of any order. The phoropter can simulate vision by any phase profile, including those of a diffractive or discontinuous type. The instrument also incorporates a subsystem for presentation of stimuli that produces stereoscopic vision of the same, enabling the subject to enjoy a three dimensional perception during the process, and two output pupils. The instrument, by its electro-optical features, enables simulation of vision as it would be modified after submitting an eye to different surgical techniques, such as refractive surgery or interocular lens implantation.

Abstract: A device for at least one of diagnosis and treatment of binocular vision disorders is disclosed. The device comprises a portable, wearable viewing apparatus that includes a left eye electronic display and a right eye electronic display, a dual data channel input, wherein each data channel corresponds to a respective one of the left eye and right eye electronic displays, a left eye adjustable optic structure adjusting a view of the left eye electronic display, and a right eye adjustable optic structure adjusting a view of the right eye electronic display.

Abstract: The invention relates to an ophthalmic instrument and method for measuring, controlling, and handling aberrations (1) of the eyes (2, 3), which further simultaneously provide visual stimulants when operating same. The instrument consists of a single device for correcting aberrations (4) as well as a single aberration sensor (5), optically connected by an optical system (14). A lighting system (9) introduces beams of light into both eyes. The measurement, control, and handling of the aberrations (1), as well as the providing of visual stimulants (6), are simultaneously and binocularly (7, 8) carried out.

Abstract: A binocular vision apparatus allows a patient to view objects through the apparatus with polychromatic light and monochromatic aberration correction, such that the chromatic aberration of the eye can be combined with the monochromatic aberration correction, so as to provide a more accurate determination of vision quality. The binocular vision apparatus provides left and right viewing optics that can substantially maintain the line of sight of each eye, such that objects can be viewed in a room with 3D depth perception corresponding to the distance of the object from the patient. As both near and far objects can be viewed with binocular aberration correction, the patient can alternate binocular viewing between near and far vision with chromatic aberration so as to evaluate a proposed treatment such as a presbyopia correction.

Abstract: An anaglyphic image presentation system is provided to evaluate and train a user's depth perception abilities. In embodiments, anaglyphic image target components are presented to a user on a display device. The image target components are then viewed by the user through a set of transmission filter lenses. The transmission filter lenses present and block one or more target components based on the peak wavelength transmission associated with each lens. As a result, a user perceives an anaglyphic target image that is resultant from the perceived image target components when viewed through the transmission filter lenses.

Abstract: The following is a method to represent images for dissociative stereoscopic vision, which allows, by means of some simple operations on stereoscopic images, to develop a device comprising a processing card and LCD shuttering goggles, which together with a computer equipped with a graphic card with LVDS output and any image display device, enable to perform visual acuity tests on patients.

Abstract: This invention is a game platform for the assessment and/or treatment of disorders of binocular vision, such as amblyopia. The game content is devised to maximize the possible therapeutic effects by leveraging advanced research in ophthalmology, as well as advanced display technology to render images independently to each eye. In particular, the game content engages both eyes at different levels of difficulty, forcing an amblyopic eye to work harder to regain its performance in the visual system. The invention herein described provides a mobile device, capable of interaction with an eye care specialist, for the assessment/treatment of binocular vision using innovative mechanisms for ensuring proper use thereof.

Abstract: An information difference between a left eye image and a right eye image is adjustable to achieve binocular vision in a patient having a deficiency of binocular vision. A source of image pairs is used along with a dichoptic display system to present a selected one of the image pairs as a right eye image to a patient's right eye and a left eye image to a patient's left eye. The difference at which a patients achieves binocular vision is a measure of a level binocular vision health or function, and continued exposure to the image pairs is therapeutic. The difference can be adjusted during therapy, and restoration of regular binocular vision is possible.

Abstract: The present invention relates to an ophthalmic diagnostic apparatus. The apparatus comprises: a display means (12) arranged to be worn in proximity to a user's eyes and arranged for displaying images relating to at least one visual function test to a user; a first user interface (14) for providing instructions to a user relating to said at least one visual function test and a second user interface (16) for receiving responses from said user in relation to said at least one visual function test; storage means (22) for storing data relating to said at least one visual function test; control means (18) for transferring said data relating to said at least one visual function test to said display means and said user interface; and output means for outputting a signal responsive to the user responses so as to assist with ophthalmic diagnosis.

Abstract: An anaglyphic image presentation system is provided to evaluate and train a user's depth perception abilities. In embodiments, anaglyphic image target components are presented to a user on a display device. The image target components are then viewed by the user through a set of transmission filter lenses. The transmission filter lenses present and block one or more target components based on the peak wavelength transmission associated with each lens. As a result, a user perceives an anaglyphic target image that is resultant from the perceived image target components when viewed through the transmission filter lenses.

Abstract: An anaglyphic image presentation system is provided to evaluate and train a user's depth perception abilities. In embodiments, anaglyphic image target components are presented to a user on a display device. The image target components are then viewed by the user through a set of transmission filter lenses. The transmission filter lenses present and block one or more target components based on the peak wavelength transmission associated with each lens. As a result, a user perceives an anaglyphic target image that is resultant from the perceived image target components when viewed through the transmission filter lenses.

Abstract: Generally, the present invention relates to medical devices and a method of vision screening, and more particularly to a pediatric vision screening system and method thereof that identifies a risk factor for amblyopia or diagnoses amblyopia by measurement of microstrabismus. An embodiment of the invention is directed to a method of patient screening for risk factors for amblyopia which includes the steps of illuminating the eye with polarized light, scanning the polarized light about the eye, capturing the retro-reflected light emanating back from the eye, analyzing the retro-reflected light to determine ocular misalignment; and calculating a metric to determine if the patient passes or fails the screening test thereby providing an indication that the patient may have a risk of amblyopia based on either strabismus or anisometropia. The method is effective at detecting amblyopia related to focusing problems without the measuring the focus of the eye directly.

Abstract: A binocular wavefront measurement system for performing wavefront analysis on the eyes of a patient, the system comprising an optics system for providing an image to a first eye along a first optical path and an image to a second eye along a second optical path and a sensor system, said sensor system configurable in a first mode for performing a wavefront measurement of a first eye through a portion of the first optical path and configurable in a second mode for performing a wavefront measurement of a second eye through a portion of the second optical path.

Abstract: An anaglyphic image presentation system is provided to evaluate and train a user's depth perception abilities. In embodiments, anaglyphic image target components are presented to a user on a display device. The image target components are then viewed by the user through a set of transmission filter lenses. The transmission filter lenses present and block one or more target components based on the peak wavelength transmission associated with each lens. As a result, a user perceives an anaglyphic target image that is resultant from the perceived image target components when viewed through the transmission filter lenses.

Abstract: An anaglyphic image presentation system is provided to evaluate and train a user's depth perception abilities. In embodiments, anaglyphic image target components are presented to a user on a display device. The image target components are then viewed by the user through a set of transmission filter lenses. The transmission filter lenses present and block one or more target components based on the peak wavelength transmission associated with each lens. As a result, a user perceives an anaglyphic target image that is resultant from the perceived image target components when viewed through the transmission filter lenses.

Abstract: This invention is a game platform for the assessment and/or treatment of disorders of binocular vision, such as amblyopia. The game content is devised to maximize the possible therapeutic effects by leveraging advanced research in opthalmology, as well as advanced display technology to render images independently to each eye. In particular, the game content engages both eyes at different levels of difficulty, forcing an amblyopic eye to work harder to regain its performance in the visual system. The invention herein described provides a mobile device, capable of interaction with an eye care specialist, for the assessment/treatment of binocular vision using innovative mechanisms for ensuring proper use thereof.

Abstract: Apparatuses and methods are provided for measuring the critical flicker fusion frequency of a subject. The apparatus in one aspect comprises two or more light sources, each flickering at a different predetermined frequency. The subject selects one light source from an array of light sources defining a first range of frequencies, the selection representing the light source of lowest frequency that appears as fused to the subject. The frequencies of the light sources are adjusted to second predetermined frequencies defining a second range. The subject selects the light source of lowest frequency that appears as fused to the subject. The frequency value of the second selection can be assigned as the subject's critical flicker fusion frequency.

Abstract: An optotype presenting apparatus for presenting an optotype to be used for testing a visual function of an eye of an examinee, the apparatus comprises: a display unit including a memory in which a plurality of optotypes is stored and a display on which at least one of the stored optotypes is displayed; an operation unit by which at lest one of the stored optotypes to be displayed on the display is selected; a pilot lamp placed to be visible by the examinee who is in front of the display; and a control unit which turns on the pilot lamp when the display unit is powered on and the display is in an off-state, and turns off the pilot lamp when the display unit is powered on and the display is in an on-state.

Abstract: An information content difference between a left eye image and a right eye image is adjustable to achieve binocular vision in a patient having a deficiency of binocular vision. A source of image pairs is used along with a dichoptic display system to present a selected one of the images pairs as a right eye image to a patient's right eye and a left eye image to a patient's left eye. The difference at which a patient achieves binocular vision is a measure of a level binocular vision health or function, and continued exposure to the image pairs is therapeutic. The difference can be adjusted during therapy, and restoration of regular binocular vision is possible.

Abstract: A physiologic test, and a system for implementing this test, including, from a methodologic point of view, (a) illuminating the central field in a test subject's eye with a relative-motion test image produced on an image display structure by a motion-image-creation structure, (b) by such illuminating, creating a related, subject-perception image, (c) requesting a subject report describing the observed presence and nature, if any, of a distortion, relative to the test image, in the perception image, and (d) thereafter utilizing such a report to assess a test subject's physiologic condition involving macular, paramacular, and neural-pathway physiologic degeneration. A preferred test image, which is fully adjustable by a test administrator with respect to substantially all of its image parameters, takes the form of an image field of elongate, spaced, parallel lines, having edges which distinctly contrast with a background field, and which move smoothly and linearly across the image field.

Abstract: The following is a method to represent images for dissociative stereoscopic vision, which allows, by means of some simple operations on stereoscopic images, to develop a device comprising a processing card and LCD shuttering goggles, which together with a computer equipped with a graphic card with LVDS output and any image display device, enable to perform visual acuity tests on patients.

Abstract: Generally, the present invention relates to medical devices and a method of vision screening, and more particularly to a pediatric vision screening system and method thereof that identifies a risk factor for amblyopia or diagnoses amblyopia by measurement of microstrabismus. An embodiment of the invention is directed to a method of patient screening for risk factors for amblyopia which includes the steps of illuminating the eye with polarized light, scanning the polarized light about the eye, capturing the retro-reflected light emanating back from the eye, analyzing the retro-reflected light to determine ocular misalignment; and calculating a metric to determine if the patient passes or fails the screening test thereby providing an indication that the patient may have a risk of amblyopia based on either strabismus or anisometropia. The method is effective at detecting amblyopia related to focusing problems without the measuring the focus of the eye directly.

Abstract: A subjective vision testing system is used for visual function tests for examinees. The subjective vision testing system is provided with an optometric apparatus which selectively sets optical elements in front of both eyes of an examinee, an optotype presenting apparatus for displaying optotypes to the examinee, and a controller. The optotype presenting apparatus, based on a command signal for implementation of a binocular balance test, displays optotypes used for binocular balance tests. The optometric apparatus, based on a command signal for implementation of a binocular balance test, sets a first prism in front of the examinee's right eye and a second prism in front of the examinee's left eye. The second prism has a base direction vertically opposite to the first prism and the same prism power as the first prism.