heterotropia or heterophoria

The present invention relates to an apparatus for assisting in establishing a correction for correcting heterotropia or heterophoria. In addition, the invention relates to a method of operating a computer for assisting in establishing a correction for correcting heterotropia or heterophoria and a computer program product.

Binocular vision, or the use of two eyes working in conjunction, can be achieved only with a well-developed coordinated oculomotor and neural system and with the optical functioning of each eye in reasonable adjustment. When some of the mentioned mechanisms fail, binocular vision can be impaired, as it is described by Bennett & Rabbetts 1998, Clinical Visual Optics, 3 ^rd edition.

Heterophoria and heterotropia are anomalies of a binocular vision and are conditions that prevent a person from directing both eyes simultaneously towards to a fixation target. These anomalies can be diagnosed by dissociating the eyes. In the case of heterophoria, for example, if a patient fixates a stationary fixation target and one of the eyes is covered (or dissociated, as it is called) the covered eye will turn by an angle a so that the visual axis no longer passes through the fixation target, as it is shown in panel A of figure 1 . When the cover is removed, bifoveal fixation is rapidly regained, as shown in panel B of figure 1 . This behaviour is described in Bennett & Rabbetts 1998, Clinical Visual Optics, 3 ^rd edition.

In the case of heterotropia, patients do not achieve bifoveal fixation of any object and one of the eyes shows a manifested deviation, even without dissociation, as shown in panel A of figure 2. This is also called strabismus. In this case, if the strabismic eye is covered while the patient looks at the fixation point, neither the covered nor the uncovered eye will move, however if the originally fixating eye is covered, the strabismic eye will turn through the angle of misalignment and foveate the fixation point, as it is shown in panel B of figure 2. This behaviour is also described in Bennett & Rabbetts 1998, Clinical Visual Optics, 3 ^rd edition.

A prevalence of strabismus ranging from 2 % to 5 % among preschool and school-aged European children has been found in some population-based studies (Frandsen AD. Occurrence of Squint", Acta Ophthalmol Suppl. 1960, 62, pages 1 - 158 and "Prevalence and risk factors for common vision problems in children: Data from the ALSPAC study", Williams C, Northstone K, Howard M, Harvey I, Harrad RA, Sparrow JM, Br J Ophthalmol., July 2008, 92(7) pages 959-964) and similar values have been found in African American populations (Baltimore Vision Screening Project, Preslan MW, Novak A, Ophthalmology", January 1996, 103(1 ), pages 105 - 109 and Giordano L, Friedman DS, Repka MX, et al. "Prevalence of strabismus and amblyopia in preschool-aged children: The Baltimore Pediatric Eye Disease Study", Invest Ophthalmol Vis Sci., 2008, 49, E-abstract 1552). In a study of Japanese school children a prevalence of 1 % was found ("The prevalence of strabismus and amblyopia in Japanese elementary school children", Matsuo T, Matsuo C, Ophthalmic Epidemiol., February 2005 12(1 ), pages 31 - 36 and "Comparison of prevalence rates of strabismus and amblyopia in Japanese elementary school children between the years 2003 and 2005", Matsuo T, Matsuo C, Acta Med Okayama, December 2007, 61 (6), pages 329 - 334) and same prevalence of strabismus was found in a population-based sample of native American children in the Kindergarten and first-grade (Garvey 2010 "Prevalence of strabismus among preschool, kindergarten and first-grade Tohono O'Odham children"). Usually, cover tests are performed to determine the presence, classification and magnitude of an ocular deviation. The test classifies the heterophoria based on the movement of the eye under cover. The classification can be esophoria or exophoria for horizontal movements and hyperphoria or hypophoria for vertical movements. The classification of heterotropia is based on the relative position of the deviating eye and can be esotropia or exotropia for horizontal eye movements and hypertropia or hypotropia for vertical movements. Another method is the so-called MKH ("Mess- und Korrektur nach Hase") measure and correction method which is critically discussed in the community of opticians and ophthalmologists.

Once a heterophoria or heterotropia is diagnosed, it is treated by refractive, prismatic or orthoptic means. Also surgery is an option, typically the last one.

The current methods of manually diagnosing and treating heterophoria or heterotropia often have the following issues: - Diagnosis, classification and the determination of magnitude of heterophoria or heterotropia lead to a prescription which is verified with the standard optometric methods. This prescription, e.g. prismatic glasses, is never tested in a realistic life environment before it is actually applied.

Diagnosis is subjective and based on the experience of the optometrist or examiner.

Determination of the magnitude of correction is estimated by subjective observation of the optometrist and, therefore, the accuracy of the correction also depends on the experience of the examiner.

Determination of oblique deviations is hard to be achieved and, thus, often uncorrected.

Strabismus testing apparatus as they are described in US 5,094,521 and in CN101 147670 (A) can overcome some of these issues. In particular, US 5,094,521 describes a measuring device which allows diagnosing strabismus and suggesting a treatment. However, even when using such testing apparatus there is no possibility for testing suggested prisms in a realistic life environment.

WO 201 6/139662 A1 discloses a method and a device for measuring strabismus or heterophoria. EP 3 109 694 A1 discloses a spectacle lens supply system in which a size and an area of clear vision in a progressive power lens is determined according to a level of prescribed fixation disparity amount.

EP 2 856 931 A1 discloses a simulation device for simulating the effect of a spectacle lens where, i.a., prismatic power is considered. In the context of the simulation, a stereoscopic image acquired by imaging cameras is processed to create a simulation image which is supposed to be viewed by the spectacle wearer through the spectacle lens.

With respect to EP 2 856 931 A1 , it is a first objective of the present invention to provide an advantageous apparatus for assisting in establishing a correction for correcting heterotropia or heterophoria by testing a prescription for correcting the heterotropia or heterophoria before the prescription is actually realized. In particular, the invention aims to increase the accuracy of a simulation used in the testing.

It is a second objective of the present invention to provide an advantageous method of operating a computer for assisting in establishing a correction for correcting heterotropia or heterophoria by testing a prescription for correcting the heterotropia or heterophoria before the prescription is actually realized. In particular, the invention aims to increase the accuracy of a simulation used in the testing. The mentioned objectives are achieved by an apparatus as claimed in claim 1 and a method of operating a computer as claimed in claim 1 1 . The depending claims contain further developments of the invention.

An inventive apparatus for assisting in establishing a correction for correcting heterotropia or heterophoria by testing a prescription for correcting the heterotropia or heterophoria before the prescription is actually realized comprises a simulation device with:

An input interface for receiving diagnostic data indicating whether heterotropia or heterophoria is present, classification data indicating which kind of heterotropia or heterophoria is present, magnitude data indicating the magnitude of the heterotropia or heterophoria and prescription data indicating a prism or prisms suitable for correcting the heterotropia or heterophoria. The simulation device may also comprise a user interface allowing a user to input manually or verbally, or by other suitable means, the diagnosis data, the classification data, the magnitude data and the prescription data. Moreover, as an addition or as an alternative, the simulation device may comprise a reader for reading a storage medium containing stored diagnosis data, stored classification data, stored magnitude data and stored prescription data.

A generator unit for generating a right simulation image for the right eye and a left simulation image for the left eye where the right simulation image and the left simulation image each represent the same object or scene looked at by the right eye under a right viewing angle and the left eye under left viewing angle, respectively. The right viewing angle and the left viewing angle are calculated based on the diagnostic data, the classification data, the magnitude data and the prescription data such that the right viewing angle and/or the left viewing angle correspond to viewing angles which would be achieved by applying the prism or prisms indicated by the prescription data.

A viewing device with separate displays or display sections for the right eye and the left eye of the patient for displaying the simulation image for the right eye and the simulation image for the left eye, respectively.

Using the simulation device offers the possibility to test the prescription in a simulated real life environment before the prescription is actually realized. This allows for correcting the prescription if the patient feels uncomfortable with the prescription before the prisms are actually manufactured. In particular, calculating the right viewing angle and the left viewing angle based on the diagnostic data, the classification data, the magnitude data and the prescription data such that the right viewing angle and/or the left viewing angle correspond to viewing angles which would be achieved by applying the prism or prisms indicated by the prescription data allows for a individually adapting the simulation image to wearers of spectacle lenses and thus allows for high accuracy in the simulation as compared to simulation devices in which the simulation relies on the view angle of an imaging result obtained by cameras as it is, e.g., the case in the simulation device of EP 2 856 931 A1 .

The right simulation image and the left simulation image used in the simulation device may be generated from an image or a video of an object or a scene stored in a memory. As an alternative, the right simulation image and the left simulation image may be generated from a real time video of the environment, e.g. of the examination room. In both cases a test of the prescription under realistic conditions can be achieved. This is particularly true when a real time image of the examination room is used as basis for the simulation images. However, for example if the examination room is not big enough for simulating an object at a certain distance the generator unit of the simulation device may use a stored object or scene instead of the real time video of the examination room.

If heterotropia or heterophoria is accompanied by myopia or hyperopia it is advantageous that the simulation device further comprises a means that allows for correcting the refraction of the patient's eye during the simulation. Such a means may, for example, include a trial frame or at least one wave front manipulator such as a liquid lens, an Alvarez-element, or the like.

In a further development of the inventive apparatus it further comprises a measuring device and an analysing unit. The measuring device is equipped with a means for displaying a fixation target for providing a patient with a target to gaze at, a switchable blocking means which allows selectively blocking the sight of the right eye and the left eye when the patient gazes at the fixation target, a control unit for switching the switching means according to a blocking sequence describing a sequence of blocking and unblocking the right eye and/or the left eye, an eye tracker for tracking the line of sight of at least one of the patient's eyes during the blocking sequence, and an output interface for outputting data representing the blocking sequence and data representing the line of the sight of the patient's eyes during the blocking sequence.

The blocking sequence resembles at least one event of blocking and unblocking at least one of the eyes according to a cover test.

As switchable blocking means of the measuring device various implementations are conceivable. In a first implementation, the switchable blocking means comprises at least one cover that can be moved to selectively cover and uncover the right eye or the left eye. In particular, there may be two covers, one for the right eye and one for the left eye which can be controlled to cover and uncover the right eye and the left eye, independently of each other. However, it would also be possible to provide only a single cover which can be switched to cover either the left eye or the right eye and which may also be brought into a neutral position, in which none of the eyes is covered.

In a second implementation of the switchable blocking means the means comprises at least one switchable transmission display that is located in front of the patient's eyes. The display can be switched back and forth between a transmissive state and an opaque state. There may be individual displays for each eye, or there may be a single display with left and right sections that can be switched independently.

In a third implementation of the switchable blocking means the fixation target emits polarized light and the switchable blocking means comprises at least one switchable polarizer located in front of the patient's eyes. The polarizer can be switched back and forth between a first polarizing state and a second polarizing state where the first polarizing state is transmissive for the polarized light of the fixation target while the second polarizing state is blocking the polarized light of the fixation target. Like in the first two implementations there may be individual polarizers for each eye, or there may be a single polarizer with switchable sections that can be switched independently where the switchable sections are associated to the left eye and the right eye, respectively.

In order to allow the eye tracker to accurately track the gazing direction of the patient's eyes the inventive apparatus may include a fixation unit for fixing the patient's head so that eye movement is prohibited or at least restricted during the measurement. The need for prohibiting or restricting the movement of the patient's eye may be overcome if the measuring device comprises a head tracker for tracking the movement and/or orientation of the patient's head in addition to the eye tracker. In this case, the head tracker establishes the movement and/or orientation while the eye tracker measures the gazing direction so that the gazing direction established by means of the eye tracker can be corrected for changes in the head position and/or the head orientation. However, neither a fixation for the head nor a head tracker is necessary if the measuring device is integrated into a head mounted device (HMD). By integrating it into a head mounted device, the eye tracker is always fixed relative to the head so that a head movement is not reflected in the gazing direction determined by use of the eye tracker. If the measuring device is integrated into a head mounted device it is advantageous if the simulation device is also integrated into the head mounted device. However, it is generally also possible that only the simulation device is integrated into a head mounted device while the measuring device is a stationary device which relies on a fixation of the head or a head tracker. The head mounted device may, for example, be based on binocular disparity, holographic projection, projection on the retina, virtual retinal display, etc.

The analysing unit is equipped with - an input interface that is connected or connectable to the output interface of the measuring device for receiving the data representing the blocking sequence and the data representing the line of the sight of the patient's eyes during the blocking sequence, an evaluation unit which determines based on the line of sight of the patient's eyes during the blocking sequence deviation data which represents deviations of the line of sight of the patient's eyes from a line of sight necessary for gazing the fixation target, - a diagnostic unit which is coupled to the evaluation unit for receiving the deviation data and which determines from the deviation data the diagnostic data, the classification data, the magnitude data and the prescription data, and an output interface for outputting the diagnostic data, the classification data, the magnitude data and the prescription data.

The output interface of the analysing unit is connected or connectable to the input interface of the simulation device.

With the described further development of the inventive apparatus a means is provided for objectively measuring eye movement in a test for diagnosing heterophoria or heterotropia since the gazing directing of the eye is not estimated by the examiner but directly measured by use of the eye tracker. In addition, the accuracy of the measurement does not depend on the experience of the examiner but only on the measurement accuracy of the eye tracker. Thus, a highly reliable and accurate determination of the line of sight of the patient's eye or eyes during the cover test by means the switchable blocking means can be achieved. In particular, by the use of the eye tracker the measurement device also allows for determining oblique gazing directions which in turn allows for determining oblique heterotropia or heterophoria. Furthermore, by use of the analysing unit diagnostics of heterophoria and heterotropia as well as the prescription for a treatment can be fully automated. Hence, diagnosing and treating heterotropia and heterophoria is based on objective measurements and does not depend on the examiner's experience. Furthermore, due to the fact that the measuring device is able to also detect oblique ocular deviations diagnosis and prescription is not restricted to diagnosing and treating horizontal and/or vertical heterotropia or heterophoria.

The invention also provides a method of operating a computer for assisting in establishing a correction for correcting heterotropia or heterophoria by testing a prescription for correcting the heterotropia or heterophoria before the prescription is actually realized. The method includes using diagnostic data indicating whether heterotropia or heterophoria is present, classification data indicating which kind of heterotropia or heterophoria is present, magnitude data (m) indicating the magnitude of the heterotropia or heterophoria and prescription data (p) indicating a prism or prisms suitable for correcting the heterotropia or heterophoria for generating a right simulation image for the right eye and a left simulation image for the left eye where the right simulation image and the left simulation image each represent the same object or scene looked at by the right eye under a right viewing angle and the left eye under left viewing angle, respectively. In the method, the right viewing angle and the left viewing angle are calculated based on the diagnostic data (d), the classification data (c), the magnitude data (m) and the prescription data (p) such that the right viewing angle and/or the left viewing angle correspond to viewing angles which would be achieved by applying the prism or prisms indicated by the prescription data (p). The right simulation image and the left simulation image may be generated from a stored image or video of an object or may be generated from a real time video of the environment. The method also includes outputting the right simulation image and the left simulation image, e.g. to a viewing device. The inventive method allows using a computer together with a viewing device with separate displays or display sections for the right eye and the left eye of the patient as an apparatus for assisting in establishing a correction for correcting heterotropia or heterophoria.

The method may further include determining, based on data representing a blocking sequence describing a sequence of blocking and unblocking the right eye and/or the left eye and date representing the line of sight of the patient's eyes during the blocking sequence, deviation data which represent deviations of the line of sight of the patient's eyes from a line of sight necessary for gazing at the fixation target during the blocking sequence, and determining the diagnostic data, the classification data, the magnitude data, and the prescription data, from said deviation data. By this development, the computer can also be used for determining the diagnostic data, the classification data, the magnitude data, and the prescription data, from deviation data.

In addition, the invention provides a computer program product including computer readable instructions for performing the method of operating a computer for assisting in establishing a correction for correcting heterotropia or heterophoria. A computer program product may be a computer readable storage medium such as, for example, a USB mass storage device, a flash card, a DVD. As an alternative, the computer program product may be implemented in form of one or more data packages which is/are made available for downloading form the internet or any other computer network. Further features, advantages and properties of the present invention will become clear from the following description of embodiments in conjunction with the accompanying drawings.

Figure 1 shows an example of dissociation and regain of bifoveal fixation for individuals with heterophoria. Figure 2 shows an example of a strabismic eye and of the effect on axis location during dissociation.

Figure 3 shows an apparatus for assisting in establishing a correction for correcting heterotropia or heterophoria.

Figure 4 shows a detail of the apparatus shown in Figure 3. Figure 5 shows an alternative implementation of the detail shown in

Figure 4.

An embodiment of the present invention will be described with respect to Figure 3 which shows an apparatus for assisting in establishing a correction for correcting heterotropia or heterophoria. The apparatus comprises a measuring device 1 , an analysing unit 21 and a simulation device 41 . The measuring device 1 and the analysing unit 21 are connected to each other via an output interface 3 of the measuring device and an input interface 23 of the analysing unit 21 . In addition, the analysing unit 21 includes an output interface 25 by which it is connected to an input interface 43 of the simulation device 41 . The interfaces can be standard interfaces or dedicated interfaces especially designed for connecting the measuring device 1 with the analysing unit 21 and the analysing unit 21 with the simulation device 41 , respectively. Moreover, the interfaces may be connected to each other by cable or by wireless means. The measuring device 1 includes a fixation target 5 to which a patient to be examined directs its gaze. For stopping the head from moving when the patient gazes at the fixation target 5, the measuring device may provide a fixation unit 14 with a support 15 for the patient's chin and a rest 16 for the forehead. The measuring device 1 further comprises a blocking means 7 which is, in the present embodiment, a device with moveable covers 9A, 9B, where one of the covers can be moved in front of the patient's right eye while the other one of the covers can be moved in front of the patient's left eye. Movement of the covers 9A, 9B is controlled by control unit 1 1 which controls moving the covers 9A, 9B in front of the eyes according to a blocking sequence describing a sequence of blocking an unblocking the right eye and/or the left eye of the patient. This blocking sequence includes the covering und uncovering actions of an eye that are necessary for determining heterophoria or heterotropia. While the blocking sequence is performed an eye tracker 13 which is also part of the measuring device 1 tracks the gazing directions of at least one of the patient's right eye and the patient's left eye. The control unit 1 1 outputs data b presenting the blocking sequence through the output interface 3 and causes the eye tracker 13 to output data I representing the line of sight of the at least one of the patient's eyes during the blocking sequence via the output interface 3,

Although the switchable blocking means 7 which allows selectively blocking the sight of the right eye and the left eye when the patient gazes at the fixation target 5 comprises two covers 9A, 9B in the present embodiment other implementations of the blocking device are possible. For example, instead of moveable covers 9A, 9B liquid crystal displays 19 could be present before the patient's eyes which can be switched by the control unit 1 1 into a transmissive state and an opaque state (Figure 4). When an eye shall be covered (dissociate) the respective display 19 becomes opaque. In a still further implementation the switchable blocking means can be realized by the use of polarizing filters 20 in front of the patient's eyes if the light of the fixation target 5 is polarized, e.g. by a further polarizing filter 20 in front of the light source of the fixation target 5 (Fig. 5). If, for example, the polarizing filter 20 in front of the light source of the fixation target 5 polarizes the light linearly in a first direction the polarizing filter 20 in front of the patient's eye can be switched between polarizing states perpendicular and parallel to the polarizing direction of the polarizer in front of the fixation target 5 so that the polarized light from the fixation target 5 can pass the polarizing filter 20 in front of the patient's eye (polarising filters are in front of the light source and in front of the eye are oriented parallel to each other) or is blocked the polarizing filter 20 in front of the patient's eye (polarising filters are in front of the light source and in front of the eye oriented perpendicular to each other).

As has already been mentioned, during the measurement the head of the patient is kept fixed by the chin support 15 and the rest 1 6 for the forehead. However, if the measuring device 1 is equipped with a head tracker 17 which tracks the position and/or orientation of the head a fixation unit 14 for fixing the patient's head is not necessary because the gazing direction measured by the eye tracker 13 can be corrected for taking into account the head position and/or head orientation given by the result of the measurement performed by the head tracker 17.

A further possibility is to integrate the measuring device into 1 a head mounted 18 device like for example a helmet or goggles like structure which includes the switchable blocking means 7 and the eye tracker 13. Since the orientation of the head mounted device 18 is fixed relative to the head a fixation unit 14 or a head tracker 17 are not necessary during the measurement of the gazing direction of the patient's eyes. The analysing unit 21 receives from measuring device 1 through the input interface 23 the data b representing the blocking sequence and the data I representing the line of sight of at least one of the patient's eyes during the blocking sequence. In the analysing unit 21 , an evaluation unit 27 evaluates the data b representing the blocking sequence and the data I representing the line of sight to determine whether the line of sight of the patient's eyes measured during the blocking sequence deviates from the line of sight necessary for gazing at the fixation target. The determined deviation is then output from the evaluation unit 27 in form of a deviation data Δ.

A diagnostic unit 29 of the analysing unit 21 receives the deviation data Δ and determines from the deviation data Δ diagnostic data d, classification data c, magnitude data m, and prescription data p. The diagnostic data d represents whether or not heterotropia or heterophoria is present, the classification data c indicates which kind of heterotropia or heterophoria is present, the magnitude data m indicates the magnitude of the heterotropia or heterophoria, and the prescription data p indicates a prism or prisms suitable for correcting the heterotropia or heterophoria which has been found to be present with a certain direction and magnitude. The diagnostic data d, the classification data c, the magnitude data m and the prescription data p is then output from the analysing unit 21 through the output interface 25.

The data d, c, m, and p output through the output interface 25 of the analysing unit 21 is received by the input interface 43 of the simulation device 41 . The simulation device 41 includes a generator unit 45 which generates a stereoscopic simulation image, i.e. a right simulation image for the right eye and a left simulation image for the left eye, where both simulation images represent the same object or scene under a right eye viewing angle and a left eye viewing angle, respectively. The right eye viewing angle and the left eye viewing angle are calculated based on the diagnostic data d, the classification data c, the magnitude data m and the prescription data p such that the right viewing angle and/or the left viewing angle correspond to the viewing angles achieved with the prism or prisms indicated by the prescription data. In other words, to simulate the effect of the prism or prisms without using an actual prism the viewing angle of the simulation image presented to the affected eye or eyes is/are calculated such as to simulate the line of sight which would be achieved with the prism or prisms indicated in the prescription data p installed in front of the affected eye. For example, in the case shown in Figure 2 the diagnostic data d would indicate heterotropia, the classification data c would indicated a deviation of the strabismic eye which directs away from the other eye, the magnitude data m would give the angle β and the prescription data p would describe a prism suitable for correcting the line of sight of the affected eye so that, with the prism or prisms installed, both eyes would gaze at the fixation target. By use of the simulation device 41 , the effect of the prism or prisms can be simulated before the prism or prisms is/are manufactured and, in case the patient feels uncomfortable with the simulation, the diagnostics can be reviewed and the prism or prisms can be amended if the review reveals that a different kind of prism or prisms would lead to a result more comfortable for the patient.

For presenting the stereoscopic simulation image to the patient the simulation device 41 comprises a viewing device 47, which, in the present embodiment, includes two displays 49A, 49B, where one of the displays is provided for the right eye and the other one is provided for the left eye. The viewing device 47 can be located in a stationary optical instrument or in a head mounted device. In particular, it may be integrated in the same optical instrument or the same head mounted device as the measuring device 1 .

The right and left simulation images may be generated based on images or videos of objects or scenes stored in a memory 53 of the simulation device 41 . When using images or videos stored in a memory 53 a large number of objects or scenes can be presented to the patient. Hence, the right and left simulation images can closely meet the needs and preferences of a patient. On the other hand, instead of using videos or images stored in the memory 53 a live video of the examination room could be used for generating the stereoscopic simulation image. This way of generating the simulation image allows presenting a very realistic image impression to the patient. If the patient does not only show heterophoria or heterotropia but is also nearsighted (myopic) or farsighted (hyperopic) the lack of refraction of the patient's eyes needs also to be corrected. To this end, the viewing device preferably also includes a refraction correcting device, which may in the simplest case be a trial frame that allows putting lenses 51 A, 51 B in front the patient's eyes. However, a kind of wave front manipulator which allows introducing a defocus could be used instead of a trial frame. In this case, a wave front manipulator would be present in front of each eye. As wave front manipulators liquid lenses, Alvarez-elements or the like could be used. By varying the liquid lens or the Alvarez-element the necessary refraction correction could be provided.

Although the simulation device 41 has been described to receive the diagnostic data d, the classification data c, the magnitude data m and the prescription data p from the analyser unit 21 it is, in the present embodiment also equipped with a user interface 44 allowing for manually or verbally inputting data relating to a diagnosis indicating whether heterotropia or heterophoria is present, a classification indicating which kind of heterotropia or heterophoria is present, a magnitude indicating the magnitude of the heterotropia or heterophoria and a prescription indicating a prism or prisms suitable for correcting the heterotropia or heterophoria. The user interface 44 may include a keyboard, a touchpad or touchscreen, a speech recognition unit, or any other unit that allows a user to input the mentioned data. In addition, in the present embodiment the simulation device 41 also comprises a reader 48 for reading a storage medium containing stored data relating to a diagnosis indicating whether heterotropia or heterophoria is present, a classification indicating which kind of heterotropia or heterophoria is present, a magnitude indicating the magnitude of the heterotropia or heterophoria and a prescription indicating a prism or prisms suitable for correcting the heterotropia or heterophoria. The storage medium may be any medium suitable for storing the mentioned data, for example a memory stick, a memory card, a DVD, etc.

In another embodiment of the invention, the tasks of the generator unit 45, the input interface 43 and the user interface 48 are realized by a computer running a software implementing a method of using the computer for assisting in establishing a correction for correcting heterotropia or heterophoria. By running of the software, the computer uses the diagnostic data (d), the classification data (c), the magnitude data (m) and the prescription data (p) for generating a right simulation image for the right eye and a left simulation image for the left eye. The right simulation image and the left simulation image each represent the same object or scene looked at by the right eye under a right viewing angle and the left eye under left viewing angle, respectively, where the right viewing angle and the left viewing angle are calculated based on the diagnostic data (d), the classification data (c), the magnitude data (m) and the prescription data (p) such that the right viewing angle and/or the left viewing angle correspond to viewing angles which would be achieved by applying the prism or prisms indicated by the prescription data (p). The computer then outputs the right simulation image and the left simulation image to a viewing device 47 which may be a viewing device as used in the first embodiment.

In a further embodiment of the invention, the computer also realizes the tasks of the analysing unit 21 . In this embodiment, the computer running the software also determines, based on the data representing the line of sight of the patient's eyes during a blocking sequence and data representing the blocking sequence, deviation data (Δ) which represent deviations of the line of sight of the patient's eyes from a line of sight necessary for gazing at the fixation target (5) during the blocking sequence. It then determines the diagnostic data (d), the classification data (c), the magnitude data (m), and the prescription data (p), from deviation data (Δ).

The present invention has been illustrated by describing embodiments of the invention. However, a person skilled in the art realizes that deviations from the described embodiments are possible since such deviations have already been described throughout the embodiment. For example neither of the user interface, the reader, the analyser device and the measuring device is mandatory as long as there is a device which allows inputting the diagnostic data, the classification data, the magnitude data and the prescription data. The present invention shall, therefore, not be restricted by the described embodiment but only by the accompanying drawings.

Reference numerals

I measuring device

3 output interface

5 fixation target

7 blocking means

9A, B cover

I I control unit

13 eye tracker

21 analysing unit

23 input interface

25 output interface

27 evaluation unit

29 diagnostic unit

41 simulation device

43 input interface

44 user interface

45 generator unit

47 viewing device

48 reader

49A, B display

51 A, B refraction correcting device

53 memory

b data representing blocking sequence I data representing line of sight

Δ deviation data

d diagnostic data

c classification data

m magnitude data

p prescription data