Methods and apparatus for providing a first refractive correction and a second refractive correction to be compared to each other for at least one eye of a subject

Abstract

In the method according to the application, the optical system (2) switches from a first dioptric power state to a second dioptric power state to provide a first dioptric correction and a second dioptric correction to the eye (4) of the subject, said switching being performed without interrupting the light beam (6) coming from the target object (7) and transmitted by the optical system to the eye of the subject. Said switching is performed so as to bypass an intermediate dioptric correction between said first and second dioptric corrections, or at a speed higher than a given limit. The application also relates to an associated instrument (1).

Description

TECHNICAL FIELD

[0001] The present invention relates to a method for providing a subject with a first refractive correction and a second refractive correction to be compared with each other by the subject.

[0002] The present invention also relates to an optometric instrument configured to implement such a method. BACKGROUND

[0003] During a subjective refractometry examination protocol, in order to determine the refractive characteristics of the eye of a subject, such as the spherical power of the refractive error of this eye, different refractive corrections are generally offered to the eye of the subject. The subject then indicates which of these refractive corrections enables him to see the target object most clearly and with the least blurring.

[0004] In order to gradually approach the optimal refractive correction that best compensates for the refractive error of the eye of the subject, a first refractive correction is generally offered to the eye of the subject, followed by a second refractive correction that is close to the first refractive correction. The subject is then asked to indicate which of these two corrections enables him to see the target object most clearly.

[0005] Depending on the result of the comparison of these two corrections, the mean refractive power of this set of first and second refractive corrections can be increased or decreased until the subject indicates that the two corrections produce similar and small amounts of blurring. In this case, the mean refractive power described above, for which the first and second corrections produce similar amounts of blurring, is close to the refractive power that best compensates for the refractive error of the eye of the subject.

[0006] In some optometric instruments, in order to switch from a first refractive correction to a second refractive correction, a given set of lenses initially placed in front of the eye of the user is replaced by another set of lenses. During this lens replacement, the subject is temporarily masked from the target object. Cutting off the field of view of the eye of the subject in this way prevents the subject from being able to finely compare the first and second refractive corrections. In other words, as a result of this cut-off, only when the first and second refractive corrections are sufficiently different from each other can the subject compare the first and second refractive corrections, i.e. indicate which is the best, since their perception is reset to some extent as a result of said cut-off.

[0007] Other optometric instruments have an optical system through which the subject views the target object, this optical system being able to change the refractive correction offered to the eye of the user without cutting off the field of view of the user. Such an optical system can be implemented by means of, for example, continuously deformable lenses with adjustable refractive power as described in document WO 2017 / 013343. Such an optometric instrument should enable the subject to compare said corrections, even when these corrections are close to each other, thus enabling an estimate of the refractive characteristics of the eye with greater precision.

[0008] However, even with such optometric instruments in which the refractive correction varies continuously without cut-off, it proves difficult for the subject to compare between the first and second refractive corrections, and this comparison is not well repeated when these refractive errors corrections are close to one another, in particular around the optimal refractive correction. SUMMARY

[0009] It is therefore an object of the present disclosure to provide a method for providing a subject with a first refractive correction and a second refractive correction to be compared with one another for at least one eye of the subject, in which the switch from the first refractive correction to the second refractive correction is performed without interrupting the field of view of the eye of the subject, and enables the subject to finely compare these first and second refractive corrections even when they are close to one another. This is achieved by suitably controlling the blur variation during the switch from the first refractive correction to the second refractive correction.

[0010] This method of optically transmitting a light beam from a target object to the eye of the subject comprises the steps of:

[0011] - providing, from the optical system to the eye of the subject, a first dioptric power state of the first refractive correction to

[0012] - the optical system providing, to the eye of the subject, a second dioptric power state of the second refractive correction, the switch being performed without interrupting the light beam transmitted by the optical system,

[0013] Each dioptric power state of the optical system is represented by a corresponding point in a coordinate system, the coordinates of the point representing the values of different dioptric power characteristics of the optical system for the dioptric power state of the optical system,

[0014] the first dioptric power state being represented by a first point in the coordinate system and the second dioptric power state being represented by a second point, a line segment starting at the first point and ending at the second point, the optimal refractive correction for the eye of the subject being represented by an optimal dioptric point in the coordinate system.

[0015] Notably, in this method:

[0016] when an intermediate point located in an intermediate portion of the line segment is closer to the optimal dioptric point or further away from the optimal dioptric point than each of the first and second points,

[0017] The switch is then performed so that:

[0018] - for each point of the trajectory representing the switch, the distance between the point and the intermediate point in the coordinate system is higher than or equal to a quarter of the smallest of the first distance between the first point and the intermediate point and the second distance between the second point and the intermediate point, or such that

[0019] - at the point of the trajectory closest to the intermediate point, the speed of variation of the optical power of the optical system is higher than the variation speed limit at which the subject cannot perceive a variation of optical power.

[0020] In the case of a gradual or even continuous switch between the first and second refractive corrections, one explanation for the difficulty of comparing the first and second refractive corrections is that, during the switch, when in the vicinity of the best refractive correction of the eye, the intermediate refractive correction corresponding to the intermediate point provided to the eye of the subject during the switch generally corresponds to a blur level perceived by the subject that is significantly lower than the blur levels corresponding to the first and second refractive corrections to be compared.

[0021] It turns out that perceiving such a reduced blur level during the switch from the first and second refractive corrections makes it more difficult for the subject to compare the first and second refractive corrections.

[0022] Similarly, when the intermediate point is further from the best correction point than the first and second points (that is to say, when the distance between the intermediate point and the best refractive point in the coordinate system is higher than the distance between the first point and the best refractive point, and higher than the distance between the second point and the best refractive point), passing through the intermediate point during the switch leads to a significant increase in the blur level. In other words, in this case, the blur level perceived by the subject when the optical system is statically in the intermediate refractive state is higher than the blur levels corresponding to the first and second refractive states. It turns out that this also disturbs the subject and makes it more difficult for the subject to compare the first and second refractive corrections.

[0023] Switching the optical system from its first refractive power state to its second refractive power state while remaining far from the intermediate point, or at a speed higher than the variation speed limit at least during part of the switch, prevents the subject from perceiving a blur level corresponding to the intermediate refractive correction that is significantly different (lower or higher) from the blur levels corresponding to the first and second refractive corrections in the above-mentioned cases.

[0024] Thus, compared to a switch performed at low or medium speed and along a straight trajectory in said coordinate system, this feature enables to reduce unnecessary and potentially annoying blurring variations during said switch (and thus will pass slowly through said intermediate point). And it turns out that reducing these blurring variations improves the accuracy and repeatability of the comparison of these first and second refractive corrections by the subject.

[0025] Moreover, the method can also comprise the following steps performed before providing said first and second refractive corrections to the eye of the subject:

[0026] - collecting personal data related to the subject; and

[0027] - determining, based on said personal data, the coordinates of the first and second points representing said first and second states of refractive power.

[0028] According to the application, the above-mentioned objects are also achieved by providing an apparatus. Optional features of the above-mentioned method can also be applied to this apparatus. DETAILED DESCRIPTION

[0029] The following description with reference to the accompanying drawings will make the content included in the application and the way of realizing the application clear. The application is not limited to the embodiment(s) shown in the drawings. Accordingly, it should be understood that, in the case where reference signs are followed by reference signs in the claims, the inclusion of these reference signs is merely for the purpose of enhancing the intelligibility of the claims, and by no means limits the scope of the claims.

[0030] In the drawings:

[0031] - Figure 1 schematically represents some elements of an apparatus for providing to the eye of a subject a refractive correction to be tested by the subject from above;

[0032] - Figure 2 、 Figure 3 and Figure 4 schematically represents different trajectories for switching the optical system of the apparatus from a first state of refractive power to a second state of refractive power of Figure 1 ;

[0033] - Figure 5 schematically represents the evolution over time of the cylindrical power feature of the optical system for a switch corresponding to Figure 4 ;

[0034] - Figure 6 schematically represents some steps of a subjective refraction protocol;

[0035] - Figure 7Some steps for determining the speed limit of variation of the optical power of the optical system are schematically represented; and

[0036] - Figure 8 is a schematic representation of a cost function depending on the distance between two refractive corrections.

[0037] Apparatus for providing a subject's eye with a refractive correction to be tested by the subject

[0038] Figure 1 A subject 5 is schematically represented, the subject looking at a target object 7 through an optical system 2 providing him / her with an adjustable refractive correction for his / her eye 4, in order to test the vision of the subject and determine at least one refractive feature of his / her eye 4.

[0039] This optical system 2 is part of an instrument 1 which also comprises a control unit 3 for controlling the optical system 2 in order to vary the refractive correction provided to the subject during the eye examination. The instrument can comprise another optical system (not represented in the figure) similar to the above-described optical system, for determining the refractive error of the other eye 4' of the subject.

[0040] The target object 7 can be a screen or panel displaying one or more optotypes, or any image suitable for testing the vision of the subject.

[0041] The optical system 2 is configured to provide an adjustable cylindrical refractive correction for the eye 4 of the subject, in order to determine the astigmatism feature of this eye. Thus, the optical system 2 is configured so that its cylindrical power feature (like its "cylinder") as well as the axis position of this "cylinder" can be adjusted.

[0042] In the embodiments described here, the optical system 2 is further configured to provide an adjustable spherical refractive correction for this eye 4. In other words, the spherical power of the optical system 2 is also adjustable.

[0043] The optical system 2 is configured so that its above-mentioned refractive power features can be adjusted gradually, in small steps or even continuously.

[0044] The optical system 2 is also configured so that its dioptric power characteristics can be changed without interrupting the light beam 6 coming from the target object 7 and passing through the optical system 2 to the subject’s eye 4. This light beam 6 is constituted by the portion of light coming from the target object 7 and collected by the optical system 2 then transmitted to the eye 4 of the subject 5 (this light being initially emitted, diffused or reflected by the target object). Thus, the optical system 2 is configured so that its dioptric power characteristics can be changed without cutting off the field of view of the eye 4 of the subject. In other words, during such a dioptric power change, the target object 7 remains unmasked, that is to say unobstructed, for the eye 4 of the subject. In particular, the optical system 2 is configured so that such a dioptric power characteristic adjustment can be achieved without having to replace a given lens by another lens, which would temporarily mask the target object for the eye of the subject.

[0045] The optical system 2 comprises one or more lenses and / or mirrors. At least one of these lenses and / or mirrors:

[0046] - is movable, with a position that can be controlled by the control unit 3 of the instrument 1, relative to the target object, or

[0047] - has an adjustable shape, the shape being controllable by the control unit 3.

[0048] Thus, the configuration of the optical system 2 can be changed in a controlled manner by the control unit 3 in order to change at least some dioptric power characteristics of the optical system 2.

[0049] For example, the optical system 2 can comprise a single deformable liquid lens with said adjustable shape such as disclosed in EP 3096677. The optical system can also comprise such a deformable lens and additional optical components, and / or a deformable mirror, an Alvarez lens or a light field display. The optical system can also comprise a Badal-like system comprising at least one lens and a displacement system to change the length of the optical path connecting the target object to this lens in order to form an image of the target object 7 at an adjustable distance from the eye 4 of the subject.

[0050] In the embodiments described here, the optical system 2 forms an image of the target object 7 at an adjustable distance from the eye 4 of the subject, which makes it possible to test whether this eye is myopic or hyperopic, or otherwise. The spherical dioptric power of the optical system 2, that is to say the spherical dioptric correction provided by the optical system 2 to this eye 4 (which is an effective spherical dioptric power in the case of a Badal-like system), is directly related to the distance at which the optical system 2 forms the image of the target object 7. The spherical dioptric power of the optical system 2 can be defined as being equal or approximately equal to the reciprocal of the algebraic distance between the eye 4 of the subject and the image of the target object 7 formed by the optical system 2.

[0051] More generally, the optical system 2 is optically equivalent to a single lens that would be placed in front of the eye 4 of the subject, close to the eye 4 (not more than three centimeters) and would provide for this eye 4 an image of an object located at a fixed distance in front of the subject (several meters, even infinity), this effective lens having an adjustable refractive power feature. The cylindrical refractive power feature of the optical system 2 is the cylindrical refractive power feature of this equivalent lens. This applies to the spherical refractive power of the optical system. For example, in the particular case where the optical system 2 comprises the single deformable liquid lens described above and the target object 7 is located several meters away from the subject, the equivalent lens described above is this single liquid lens itself.

[0052] The expression "refractive power state" of the optical system 2 refers to the set of refractive power features that the optical system 2 has when it is in a given configuration, providing a given refractive correction for the eye 4 of the subject. Each refractive power state of the optical system 2 can be defined by a set of values of at least three different refractive power features of the optical system 2 (as is the case here), the optical system being in this given configuration. One of these three refractive power features is related to or even represents the spherical refractive power of the optical system 2. The other two refractive power features described above represent a first cylindrical refractive power feature of the optical system 2 and a second different cylindrical refractive power feature of the optical system 2.

[0053] Each refractive power state of the optical system 2 can therefore be represented by a corresponding point in a coordinate system. The coordinates of this point represent the values of the three different refractive power features described above for the refractive power state of the optical system.

[0054] These three coordinates can be, for example:

[0055] - the spherical refractive power of the optical system 2, also called the spherical S of the optical system;

[0056] - the cylindrical C of the optical system 2;

[0057] - the angle a representing the orientation of the cylindrical axis of the optical system 2.

[0058] The spherical S can be defined as the refractive power (sometimes called optical power) of the equivalent lens described above, given by the spherical component of the front and back shapes of this equivalent lens.

[0059] The cylindrical C can be defined as the refractive power of this equivalent lens, given by the cylindrical component of the front and back shapes of this equivalent lens.

[0060] The angle a is formed between a fixed reference direction and the central axis of the cylindrical component described above.

[0061] Another vectorial decomposition of the cylindrical component of the optical system 2 can be used to characterize the cylindrical power characteristics of the optical system 2 instead of the amplitude and orientation decomposition described above (i.e. instead of specifying the cylinder and the orientation of the cylinder).

[0062] For example, the power state of the optical system 2 can be represented by three orthogonal components (M, Jo, J45) instead of three spherical-cylindrical components (spherical S, cylinder C, orientation a), where Jo and J45 are the powers of two Jackson crossed-cylindrical lenses representing the cylindrical power characteristics of the optical system 2, and where M is the equivalent sphere equal to the spherical S plus half of the cylinder C: M = S + C / 2. The cylinder of the first Jackson lens among the two "Jackson lenses" is oriented at a 45-degree angle with the cylinder of the other Jackson lens among the "Jackson lenses". When the first "Jackson lens" is aligned with the above-mentioned reference direction, that is to say when one of its crossed cylinders is aligned with this direction, the first crossed cylinder power Jo is equal to (-C / 2)*cos(2* a) and the second crossed cylinder power J45 is equal to (-C / 2)*sin(2* a).

[0063] Another coordinate system that can be used to represent the power state of the optical system 2 is based on a Zernike polynomial decomposition of the characteristic wavefront output by the optical system 2 when the optical system is illuminated by a point source instead of the target object 7, this point source being located at the same position as the target object 7. The three components described above are then the three second-order coefficients c o 2, c 2 2and c -2 2of this Zernike polynomial decomposition. It is to be noted that this coordinate system is equivalent to the coordinate system based on the three orthogonal components M, Jo and J45, since the three second-order coefficients c o 2, c 2 2and c -2 2are proportional to M, Jo and J45, respectively, according to the formula 1 of the article by Thibos et al. cited further below.

[0064] Other coordinate systems than the ones described above can be used to represent the power state of the optical system 2.

[0065] The control unit 3 is programmed to control the optical system 2 so as to provide various refractive corrections to the eye 4 of the subject according to a refraction protocol, for example according to the refraction protocol of the Figure 6 application, in order to determine the refractive error of the eye 4 of the subject.

[0066] In particular, the control unit 3 is programmed so as to implement a method for providing to the eye 4 of the subject a first refractive correction and a second refractive correction to be compared with each other, which is described below. According to this method, the optical system 2 switches from:

[0067] - the optical system 2 provides to the subject's eye 4 a first dioptric power state of this first dioptric correction to

[0068] - the optical system 2 provides to the subject's eye 4 a second dioptric power state of this second dioptric correction.

[0069] Then, the subject 5 designates which one of these first and second corrections enables him / her to see the target object 7 most clearly (see the target object more clearly, darker and / or with less distortion), for example by entering this information by means of a user interface of the instrument 1.

[0070] This comparison step plays an important role in several optometric protocols. For example, it enables to gradually approach the best dioptric correction that best compensates for the refractive error of the subject's eye 4, as explained below with reference to Figure 6 which are further illustrated.

[0071] In the above-mentioned coordinate system Figures 2 to 4 ), the first dioptric power state and the second dioptric power state between which the optical system switches are respectively represented by a first point P1 and a second point P2.

[0072] In prior art instruments, when this switching is performed continuously, there is no interruption of the subject's visual field, meaning that the trajectory of the switching is generally a straight line, constituted by a segment [P1P2] starting at the first point P1 and ending at the second point P2.

[0073] But when the first correction and the second correction are in the vicinity of the best dioptric correction of the subject's eye 4, the intermediate point P I located in the middle of this segment, for example the middle of this segment, is generally closer to the best dioptric point P O representing the best dioptric correction than the first point P1 and the second point P2. In other words, in this case, the intermediate dioptric correction represented by the intermediate point P I corrects the refractive error of the subject better than the first dioptric correction and the second dioptric correction. Thus, for the above-mentioned prior art instruments, switching from the first dioptric correction to the second dioptric correction temporarily reduces the level of blur perceived by the subject during the switching. Indeed, the level of blur corresponding to the intermediate dioptric correction is lower than the level of blur corresponding to the first dioptric correction and than the level of blur corresponding to the second dioptric correction. This is annoying for the subject 5 and prevents him from making a reliable, repeatable comparison of the first dioptric correction and the second dioptric correction. The expression "best dioptric correction" means the dioptric correction that will be determined as a conclusion of the subjective optometric protocol as the one that best corrects the refractive error of the subject's eye 4.

[0074] Conversely, in the opposite case to optimal refractive correction, switching from the first refractive correction to the second refractive correction while following a straight path may cause the subject's perceived level of blurring to temporarily increase (rather than temporarily decrease) during the switch, which is also distressing for the subject.

[0075] In the method according to the invention, in order to avoid these undesirable effects, the control unit 3 of the instrument 1 controls the optical system 2, such that...

[0076] When P is located at the midpoint of the middle part of the above line segment I When the first point P1 and the second point P2 are closer to the optimal refractive point P0 than each of the aforementioned points, or alternatively, when the first point P1 and the second point P2 are farther from the optimal refractive point P0 than each of the first point P1 and the second point P2, the two points are considered to be closer.

[0077] During the switching period:

[0078] i) For each point P representing the trajectory of the switch in the coordinate system, the point P is relative to the midpoint P in the coordinate system. I The distance d between them is greater than or equal to one-quarter of the smallest of the following: the distance between the first point P1 and the intermediate point P I The first distance d1 between them, and the second point P2 and the midpoint P I The second distance d2 between them, or

[0079] ii) The point P closest to the midpoint on the trajectory I At point , the rate of change s of the refractive power of optical system 2 V Above the rate of change limit s L When the rate of change exceeds the limit, subject 5 cannot perceive the change in refractive power.

[0080] The optical system 2 is switched from its first refractive power state to its second refractive power state while remaining away from the midpoint P. I Or, during at least a portion of the switching, at a rate exceeding the rate of change limit s. L The speed switching prevents subject 5 from perceiving the blur level corresponding to the intermediate refractive correction, which under the above conditions is significantly different (lower or higher) from the blur level corresponding to the first and second refractive corrections.

[0081] Therefore, this feature can reduce unnecessary and potentially annoying blurring during the switching process, which improves the accuracy and repeatability of these comparisons of first and second refractive corrections performed by subject 5.

[0082] As an alternative or supplement, control unit 3 can control optical system 2 such that when the midpoint P...I between the best point of refraction P0 or the intermediate point P I Switching according to feature i) or ii) when the distance between the estimate of the best point of refraction is less than one quarter or alternatively half of the smallest of the first and second distances.

[0083] In the embodiments described here, the distances between points in the coordinate system are each equal to the Euclidean distance between the considered points, that is to say to the "ordinary" straight-line distance. However, in other embodiments, the distances between points in the coordinate system can be determined according to other metrics, for example equal to the sum of the absolute values of:

[0084] - the difference between the first coordinate of the considered first point and the first coordinate of the second point;

[0085] - the difference between the second coordinate of this first point and the second coordinate of this second point; and

[0086] - the difference between the third coordinate of this first point and the third coordinate of this second point,

[0087] Such a distance is sometimes referred to as "city-block distance" or "Manhattan distance".

[0088] The specific ways in which the optical system 2 described above switches from the first dioptric power state to the second dioptric power state are described in detail below in the section "Switching from a first dioptric correction to a second dioptric correction" and are illustrated by six different switches corresponding to the same first and second dioptric power states. Then, in the section "Subjective refraction scheme", the subjective refraction scheme represented in Figure 1 is described from a more general point of view Figure 6 The subjective refraction scheme represented in Figure 1 is based on a method for providing the eye 4 of a subject with the first and second dioptric corrections described above.

[0089] Switching from a first refractive correction to a second refractive correction

[0090] In practice, the distance d12 between the first point P1 and the second P2 of these two corrections in the coordinate system is typically in the range 0.1 to 2 dioptres.

[0091] This distance d12 can be set to a constant value, independently of the subject. It can also be set in dependence on the subject's sensitivity to dioptric variations. This sensitivity can be characterised by a value of a sensitivity parameter such as the sensitivity parameter described in European patent application n° 18305996.3 owned by the applicant (and not yet published at the filing date of the present patent application). This sensitivity parameter represents, for example, the minimum variation of one or several optical characteristics of a lens placed in front of the eye 4 of the subject that can be perceived by the subject 5. Since this sensitivity parameter is small, the above-mentioned distance d12 can be smaller.

[0092] It is further noted that, in the embodiments described here, the control unit 3 is programmed so as to switch the optical system 2 from the first dioptric power state to the second dioptric power state according to the above-mentioned feature i) or ii) when the middle of the line segment [P1P2] is closer to the best fitting point P0 than each of the first point P1 and the second point P2, or, alternatively, is further away from the best fitting point P0 than each of the first point P1 and the second point P2. In this case, the middle point P I (i.e. the point whose representation the trajectory of the switch bypasses, or in the vicinity of which the trajectory of the switch will quickly pass) is the middle of this line segment.

[0093] This situation corresponds in particular to Figures 2 to 4 the situation represented in the middle of the figure, in which the middle of the line segment [P1P2] is closer to the best fitting point P0 than each of the first point P1 and the second point P2 (i.e. the distance between the middle of the line segment and the best fitting point P0 is smaller than the distance between the first point P1 and the best fitting point P0, and smaller than the distance between the second point P2 and the best fitting point P0, in the said coordinate system).

[0094] Since the middle point P I then lies in the middle of the line segment [P1P2], the first distance d1 (the distance between the points P1 and P I ) is equal to the second distance d2, and the base distance d m (i.e. the smallest of the first and second distances d1, d2) is indifferently equal to d1 or d2: d m = d1 = d2.

[0095] It should however be noted that, in other cases, the middle of the line segment [P1P2] can not be closer to or further away from the best fitting point P0 than each of the first point P1 and the second point P2, and another point of the middle of this line segment will be closer to or further away from the best fitting point P0 than each of the first point P1 and the second point P2. In this case, the middle point (whose representation the trajectory of the switch will bypass, or in the vicinity of which the trajectory of the switch will quickly pass) will be this other point.

[0096] Furthermore, the control unit 3 can be further programmed to control the optical system 2 so that:

[0097] when the middle point P I is closer to the best fitting point P0 than each of the first point P1 and the second point P2, or, alternatively, is further away from the best fitting point P0 than each of the first point P1 and the second point P2,

[0098] then, during the switch, for each point P of the representation of the trajectory for which the switch, the distance d between the point P and the middle point P I is higher than the proximity distance threshold d apthis approach distance threshold d ap higher than 0.05 dioptres, or even higher up to 0.125 dioptres.

[0099] The control unit 3 can also be programmed to control the optical system 2 so that

[0100] when the intermediate point P I closer to the optimum point P0 than each of said first and second points P1 and P2, or, alternatively, further from the optimum point P0 than each of said first and second points P1 and P2,

[0101] then during said switching:

[0102] i') for each point P of the trajectory representing said switching, the distance between said point P and each point of the intermediate portion of the segment [P1 P2] is higher than or equal to a base distance d m which is a quarter of the base distance d (this is the case for the trajectories T2 to T5 described below), or

[0103] ii') the speed s V of variation of the optical power of the optical system 2 is higher than the limit of speed of variation s L for each point of said trajectory, which is located at a distance from the intermediate point P I which is less than a quarter, or even half, of the base distance d m (corresponding to the switching of the trajectory T6).

[0104] Thus, according to this last feature, not only is the intermediate point P I skipped, or passed near quickly by the system, but also the entire intermediate portion of the segment [P1 P2] is skipped.

[0105] The length of the intermediate portion of the segment [P1 P2] (which itself is a smaller segment) is equal to half the length of this segment. The middle of this intermediate portion coincides with the middle of the segment. Alternatively, the length of the intermediate portion of the segment can be smaller, for example equal to a quarter of the length of the segment [P1 P2].

[0106] According to the application, Figures 2 to 4 the six Trajectories T1 to T6 trajectories represented in the diagram of Figure 1 illustrate more particularly the different switchings of the optical system 2 that can be implemented by the instrument 1.

[0107] The three coordinates of the coordinate system representing these trajectories are the equivalent sphere M of the optical system 2, and the first and second cross cylinders J0 and J45. However, as mentioned above, other coordinate systems can be employed to represent the state of the optical power of the optical system 2.

[0108] The first point P1 (from which each of these trajectories T1, T2, T3, T4, T5, and T6 begins) and the second point P2 (where it ends) are identical for all six trajectories. Whether the eye is in the first refractive power state represented by the first point P1 or the second refractive power state represented by the second point P2, the optical system 2 has the same equivalent spherical power M (in other words, the same total spherical power) for all six transitions. In effect, here, a comparison of the subject's first and second refractive corrections is made to determine the astigmatic characteristics of his / her eye 4.

[0109] In the specific cases illustrated in these diagrams, the first point P1 and the second point P2 correspond to the value of the equivalent spherical mirror M, which is essentially zero, and also to the value of the coordinate J45, which is also essentially zero. The value of the equivalent spherical mirror M of the optimal correction point P0 is slightly negative, and this optimal correction point is also close to the middle of the line segment [P1P2].

[0110] Of course, the switching technique described here is also applicable to the first and second refractive power states of other groups. For example, instead of the same orientation corresponding to the cylinder axis of optical system 2 and different values ​​of this cylinder (as shown in the figure), the two refractive power states to be compared by the subject can correspond to the same value of cylinder C of optical system 2, but to two different orientations of this cylinder (corresponding to two different values ​​of angle α).

[0111] The first five Trajectories T1 to T5 It is to stay away from the midpoint P I trajectory ( Figure 2 and Figure 3 ).

[0112] for First trajectory T1 Each point P( Figure 2 This point and the midpoint P I The distance d between them is included in the base distance d m Between half of this base distance: d m / 2≤d≤d m As a variant, distance d can be included in the base distance d. m One-quarter of the distance between this base distance and the base distance.

[0113] for Second, fourth and fifth trajectories T2, T4 and T5 For each point on the considered trajectory, this point is related to the intermediate point P. I The distance d between them is equal to the base distance d m d = d m Therefore, each of these trajectories T2, T4, and T5 is centered at the intermediate point P. I The semicircle centered on ( Figure 2 and Figure 3). This semi-circular trajectory allows to minimize the temporary blur level variation that occurs during the switch, making the comparison of the first and second dioptric power states very comfortable and reliable. More generally, the trajectory can obtain this advantageous effect such that the distance d remains comprised between 0.8 times the base distance d m and 1.2 times the base distance d m .

[0114] and Third trajectory T3 such that for each point of this trajectory, the distance d between this point and the intermediate point P I is higher or equal to the base distance d m : d > d m . More specifically, except for the first point P1 and the second point P2, the distance d is higher than d m : d > d m .

[0115] Along the first trajectory T1, the equivalent sphere M of the optical system 2 remains constant. It applies to the second trajectory T2 and to the third trajectory T3. Thus, in the coordinate system considered here, these trajectories are planar.

[0116] The fourth trajectory T4 and the fifth trajectory T5 are also planar, but along them the equivalent sphere M of the optical system 2 varies while one of the cylindrical power characteristics of the optical system 2 (J45 as represented) remains constant.

[0117] Fourth trajectory T4 By increasing then decreasing the equivalent sphere M while changing one of the cylindrical power characteristics of the optical system 2 (J0 as represented) to reach the second point P2, the intermediate point P I is bypassed.

[0118] Thus, along this trajectory, the equivalent sphere M of the optical system 2 varies while remaining higher than the lowest of: the first value M1 of the equivalent sphere M, corresponding to the first point P1, and the second value M2 of the equivalent sphere M, corresponding to the second point P2. This feature advantageously prevents accommodation of the eye 4 of the subject. In other words, thanks to this feature, along the whole of said switch, the eye 4 of the subject focuses on a point as far as possible from this eye.

[0119] Fifth trajectory T5 Similar to the fourth trajectory, but by decreasing then increasing the equivalent sphere M while changing the above-mentioned cylindrical power characteristic (J0) to reach the second point P2, the intermediate point P Figure 4 is bypassed. In this case, the fifth trajectory is represented by a dashed line while the fourth trajectory is represented by a solid line.

[0120] Different trajectories than T1 to T5 can be followed to bypass the intermediate point P Iother trajectories to switch from the first dioptric power state to the second dioptric power state, in particular non-planar trajectories.

[0121] Furthermore, instead of avoiding the intermediate point P I , the trajectory representing the switch from the first dioptric correction to the second dioptric correction can pass through this point, but at high speed. Sixth trajectory T6

[0122] This sixth trajectory T6 is a straight line Figure 4 , and thus coincides with the segment [P1P2].

[0123] During this sixth switch, the dioptric power of the optical system varies at a speed s I at the intermediate point P V , that is to say when the trajectory passes through said intermediate point, which is higher than the speed limit s L .

[0124] More particularly, during this switch, the speed s I is higher as the distance d between the intermediate point P V and the point P representing the dioptric power state of the optical system 2 is smaller.

[0125] The speed s V of variation of the dioptric power of the optical system 2 is the speed at which the point P representing the current dioptric power state of the optical system 2 moves in the coordinate system on the trajectory representing the switch. It can be expressed in dioptres per second, for example.

[0126] Thus, for example, when the value of the cylinder C of the optical system 2 remains constant while its orientation varies, the speed of variation is equal to C.(dα / dt) (a being expressed in radians).

[0127] And in the particular case illustrated in Figure 4 , since the equivalent sphere M and the coordinate J45 remain constant during the switch, the speed s V of variation is equal to d(J0) / dt. Thus, in this case, for example at the intermediate point P I , the slope of the line L tangent to the curve C representing the variation of the coordinate J0 over time t is higher than the speed limit s L . Figure 5 This curve is represented in

[0128] The speed limit s L of variation is the highest dioptric power variation speed that the subject 5 is able to perceive.

[0129] In other words, when the dioptric power of the dioptric correction provided to the eye of the subject varies over time at a speed higher than the speed limit s L of variation, the subject is not able to perceive this variation. ​

[0130] And when the dioptric power of the refractive correction provided to the eye of the subject varies over time at a speed lower than the change speed limit s L , the subject is able to perceive at least partially this variation.

[0131] To determine the change speed limit s L of the subject, a skilled vision scientist, optometrist can submit to the subject the image seen through the optical system 2 set to a first dioptric power and vary the dioptric power to a second dioptric power at an initial speed variation s I . The subject has only 2 alternative forced choices: 2 response options: yes, the variation was seen; no, it was not seen. If the subject says yes, the first and second dioptric powers remain identical but the speed of variation is increased. When the subject answers no, the speed is decreased. Using the classical psychophysical staircase method, the speed variation limit s L of each person can be defined.

[0132] For almost any subject, it is proven that the subject is unable to perceive a corresponding dioptric power variation when the speed of variation of the dioptric power is higher than 20 diopters per second. In practice, 20 diopters / second is therefore a suitable value for the change speed limit s L . Indeed, when switching from a first dioptric power state to a second dioptric power state at a speed higher than this value, even if the trajectory followed by the optical system 2 passes through the intermediate point P I , the subject 5 is unable to perceive the corresponding reduced blur level, or alternatively the increased blur level corresponding to the intermediate point.

[0133] 10 diopters / second is also a suitable value for the change speed limit s L , because when the trajectory passes through this point at this speed, this value is still high enough for most subjects to be unable to perceive the reduced / increased blur level corresponding to the intermediate point P I .

[0134] In an embodiment of the method described here, the change speed limit s L ( s Figure 7 ) is determined during a step S200, before switching from a first dioptric power state to a second dioptric power state. This step can be performed by the control unit 3 of the optical instrument 1. The step S200 comprises the following steps:

[0135] - collecting data relating to the response speed at which the subject 5 reacts to a dioptric power variation (step S201); and

[0136] - determining the change speed limit s L based on said data (step S202).

[0137] Data relating to the speed of reaction of the subject, called RSD, can include:

[0138] - the age of the subject 5; ocular conditions / properties such as the transparency of the intraocular environment, the presence of pathologies, visual performances: high and low contrast visual acuity, the type and level of refractive errors, data relating to the installation of the subject: such as the magnification of the system, which can affect the apparent size of the stimuli, and therefore the distance of the vertex from the subject, the type of target and its distance with respect to the subject, which can affect the RSD data relating to the stimuli...

[0139] - a previous value of the speed of reaction, determined during other tests implemented before the method, this previous value being for example loaded from a remote server or read into the electronic health card of the subject 5;

[0140] - a set of reaction times that the subject 5 has made in response during tests previously carried out during the course of the same refraction protocol, for example during the course of the refraction protocol of Figure 6 .

[0141] When the data relating to the speed of reaction RSD of the subject include a previous value of the speed of reaction as described above, then the variation speed limit s L can be set equal to this previous value.

[0142] When this data includes a set of reaction times as described above, the variation speed limit s L can be determined to be higher, since these reaction times are short.

[0143] When this data includes the age of the subject, the variation speed limit s L can then be determined as a function of the nominal maximum variation speed s max of the optical system 2 and the age of the subject.

[0144] The nominal maximum variation speed s max of the optical system 2 is the highest variation speed of the optical power of the optical system 2 for which the optical system 2 is designed. In other words, this is the highest variation speed that can be repeatedly implemented by the optical system 2 without damaging the optical system or excessive heating. Depending on the optical system 2 considered, the nominal maximum variation speed s max may for example be comprised between 15 and 50 dioptres / second.

[0145] The variation speed limit s L is then determined equal to a fraction of this nominal maximum variation speed s max , this fraction being higher as the subject is younger.

[0146] For example, when the subject is less than 20 years old, then the variation speed limit sL may be set to 70% of the nominal maximum variation speed s max , preferably higher than 20 diopters / second.

[0147] When the age of the subject is comprised between 20 and 50 years, then the variation speed limit s L may be set to 70% of the nominal maximum variation speed s max , preferably higher than 10 diopters / second.

[0148] While the age of the subject is over 50 years, then the variation speed limit s L may be set to 50% of the nominal maximum variation speed s max , preferably higher than 5 diopters / second.

[0149] As the variation speed limit s L is adapted to the age of the subject, the optical power of the optical system 2 is only varied at its nominal maximum variation speed s max when necessary (only when the subject is young), thus preventing unnecessary heating or wear of the optical system.

[0150] The value of the variation speed limit s L may also be determined in step S202 by combining a value determined based on the age of the subject, a previous value of the response speed, and / or a value derived from the set of response times described above. This combination may, for example, be achieved by averaging the three values.

[0151] The value of the variation speed limit s L may also take into account a sensitivity parameter of the subject. In particular, as the minimum diopter variation that can be detected by the subject is small, the value of the variation speed limit s L may be set higher. Indeed, when the subject is very sensitive to diopter variations, the variation speed limit s L is preferably set to a high value, as the subject will additionally clearly notice the temporary blur level variation caused by the proximity to the optimal point P0 (as the subject is very sensitive to the diopter values provided to him / her).

[0152] In case of failure of step S201 (for example because the data related to the response speed RSD of the subject are not available), or if the control unit 3 determines that these data are not sufficient / reliable enough, the variation speed limit s L may be set to a default value in step S202 (for example equal to 20 diopters / second), 10 diopters / second or the nominal maximum variation speed s max of the optical system 2.

[0153] Alternatively, step S200 may include the reaction speed test described above, instead of collecting the subject's reaction speed data and then deriving an appropriate rate of change limit s from that data. L The steps to determine the value. In other words, in the method described here, the limit of the subject's rate of change s can be directly assessed by performing this reaction speed test. L Then, the aforementioned switching is performed depending on the rate of change limit measured in this way.

[0154] As described above, for trajectories T1 to T5, by keeping the coordinate system away from the midpoint P... I This is to avoid a temporary decrease in the level of blur perceived by subject 5 during the switching process. Furthermore, for the switching corresponding to the sixth trajectory T6, the trajectory is at the midpoint P. I It passes through or even crosses the midpoint, but passes through or even crosses it at high speed.

[0155] By combining the aforementioned geometric and temporal features, the aforementioned temporary ambiguity level changes can also be avoided or at least reduced; that is, by keeping the distance from the intermediate point P... I And simultaneously, switching is achieved at high speed, at least near the midpoint. Specifically, for each point P on the trajectory followed during the switching, since this point P is close to the midpoint P... I Refractive power V The rate of change can be higher to avoid [the situation] at the intermediate point P. I Spending an extended period of time in the vicinity.

[0156] More specifically, in the method according to the invention, in order to avoid [the situation at the intermediate point P]... I If you stay nearby for an extended period, you can switch modes to:

[0157] - Given the integral Int of the cost function f over time t from the start to the end of the switch along the trajectory representing the switch, with its independent variables being the current variable point P and the intermediate point P of the trajectory. I The distance d between them

[0158] Less than

[0159] -Nearest time span limit Δ T When the independent variable of the cost function is equal to one-quarter of the minimum of the first distance d1 and the second distance d2, that is, when the independent variable of the cost function is equal to the base distance d... m The product of the values ​​of the cost function f when it is one-quarter of the value.

[0160] Therefore, according to this standard:

[0161]

[0162] The integral Int of formula F1 corresponds to an average switching time, where the time the system spends near a given point P on the trajectory is weighted by the cost function f(d[P]).

[0163] Because the independent variable d of the cost function is small, the cost function f is higher. In other words, as point P gets closer to the midpoint P... I The cost function increases. This allows for a significant penalty in the integral Int for points close to the midpoint PI, due to their high corresponding weights. Therefore, during the switching period satisfying Formula F1, approaching the midpoint at a slow or moderate speed is avoided, thus preventing temporary changes in the level of ambiguity that would otherwise occur as perceived by the subject.

[0164] In fact, the limit of the nearest time span Δ T The value is typically between 1 millisecond and 1 second, preferably between 10 milliseconds and 0.3 seconds, or even, as here, between 25 milliseconds and 250 milliseconds. This value is proven short enough that, for almost any subject, it corresponds to the midpoint P. I A temporary decrease in the level of ambiguity (or conversely, a temporary increase in the level of ambiguity) will not be perceived by the subject.

[0165] The switching begins at time t1, which is the time when the refractive power characteristic of optical system 2 becomes different from the refractive power characteristic corresponding to the first refractive correction. Similarly, the switching ends at time t2 when the refractive power characteristic of optical system 2 becomes equal to the refractive power characteristic corresponding to the second refractive correction. In the case of the sixth switching described above, these start and end times t1 and t2 are... Figure 5 The Chinese side indicated that...

[0166] For example, the value of the cost function f could be:

[0167] -When the distance d exceeds the nearest distance d p When the time is equal to zero, the nearest neighbor distance d p For example, equal to the base distance d m Half of, and

[0168] -When the distance d is lower than this nearest distance d p The time is right.

[0169] More specifically, such as Figure 8 As shown, when d exceeds d p When the time condition is met, the cost function f can be zero; otherwise, it is equal to the positive constant Co.

[0170] In formula F1, other cost functions can be used, which are more suitable when point P is more expensive than point P. Figure 8 The point in the middle is closer to the midpoint P. I The frequency will gradually increase.

[0171] It should be noted, for example, if we consider Figure 8 If the cost function f is such that for the five first trajectories T1 to T5 mentioned above, the criterion of formula F1 is satisfied because the integral Int equals zero, and therefore is less than f(d). m / 4).Δ T .

[0172] Furthermore, for the switching corresponding to the sixth trajectory T6, when the refractive power s V If the rate of change is constant, for example, on the locus corresponding to the middle part of the line segment [P1P2], and equals vo = 20 diopter / second, then the integral Int is found to be equal to Co.d. m / vo, which is less than the typical 0.5 diopters of d. m d m / vo equals 25 milliseconds while Δ T Including f(d) between 25 milliseconds and 250 milliseconds m / 4).Δ T =Co.ΔT. Therefore, for this sixth trajectory, and for this value of various parameters, the criteria for formula F1 are also satisfied.

[0173] It is further noted that, according to an optional feature of the method according to the invention, the switching of the optical system 2 from its first refractive power state to its second refractive power state can be performed, such that for each point P representing the trajectory of the switching, the blur level BL (which is determined based on a theoretical model of the eye 4 and the vision of the subject 5, and taking into account that the subject's eye 4 is provided with a correction corresponding to the refractive power state associated with said point P):

[0174] -The product of the margin factor k and the minimum of the first blur level BL1 associated with the first refractive power state of the optical system and the second blur level BL2 associated with its second refractive power state, and

[0175] - Less than the ratio of the largest of the first fuzzy level BL1 and the second fuzzy level BL2 to the margin coefficient k.

[0176] In other words, for each point P representing the trajectory of the switch, the corresponding fuzziness level BL(P) conforms to the following formula F2:

[0177] BL(P)>k×Min(BL1,BL2)and BL(P)<[Max(BL1,BL2)] / k(F2).

[0178] The margin factor k is between 0.5 and 1, or even more preferably between 0.8 and 1.

[0179] The first blur level BL1 and the second blur level BL2 are determined as blur levels based on the theoretical model of the eye 4 and the vision of the subject 5 and taking into account that the subject's eye 4 is provided with a first refractive correction or a second refractive correction, respectively.

[0180] To meet the criteria of the formula F2, a trajectory meeting this last criterion can be calculated before the switch, and then the optical system 2 is controlled by the control unit 3 so that it follows this pre-calculated trajectory.

[0181] The blur level determined based on this model represents in a realistic and precise manner the blur level that the subject should perceive at the beginning, during and at the end of the switch.

[0182] Implementing the switch so that the above criteria are met ensures that the variation of blur perceived by the subject during the switch is strictly limited, the blur level perceived by the subject remaining approximately bounded by the first blur level BL1 and the second blur level BL2, the first blur level and the second blur level corresponding to the beginning and the end of the switch.

[0183] The above theoretical model can assume that the subject's eye 4 is perfectly corrected optically, when provided with an intermediate refractive correction corresponding to the intermediate point P I , or alternatively when provided with another refractive correction considered optimal for the subject's eye. This other refractive correction can correspond to the optimal refractive point P0, or to an estimate of the optimal refractive correction. Thus, according to this theoretical model, for each correction considered, the optical resolution of the eye 4, such as the width of its point spread function, is calculated based on the difference between this correction and the intermediate refractive correction (considered as a reference, perfect correction), or alternatively based on the difference between the refractive correction to be evaluated and the above-mentioned refractive correction considered optimal.

[0184] Under this assumption, the blur level BL can then be determined according to any one of the following:

[0185] - the width of the point spread function,

[0186] - the spatial frequency cut-off of the optical modulation transfer function,

[0187] - the Strehl ratio of the subject's eye 4 provided with the refractive correction considered and modelled as described above.

[0188] Thus, when the blur level BL is determined as the width of the point spread function of this theoretical eye provided with the refractive correction considered, it is assumed that, for example, if the subject's eye were to be provided with a refractive correction corresponding to the intermediate point, this point spread function would be limited by diffraction (the subject's eye would then be perfectly corrected).

[0189] The theoretical model of the eye 4 and the vision of the subject 5 can take into account the neural characteristics of the visual response of the subject. For example, it is known that the best spatial frequency perceived by the subject is not necessarily the lowest spatial frequency. Thus, the blur level BL can be determined for example as the spatial frequency cut-off of the visual modulation transfer function, which takes into account the optical and neural characteristics of the eye 4 and the vision of the subject, as described in the appendix of the article: "Accuracy and precision of objective refraction from wavefront aberrations", Journal of Vision (2004) 4, 329-351, L. N. Thibos et al.

[0190] Similarly, the blur level BL can be determined as the visual Strehl ratio, which takes into account the optical and neural characteristics of the eye 4 and the vision of the subject (as described in the article mentioned above), his / her eye being provided with the considered refractive correction.

[0191] Subjective refraction protocol

[0192] Figure 6 Some steps of the subjective refraction protocol are schematically represented, which are implemented with the aid of the instrument 1 and on the basis of the method described above, for providing the first and second refractive corrections for the eye 4 of the subject.

[0193] This protocol begins with a preliminary data collection step So, during which personal data relating to the subject 5 are collected by the instrument 1. These personal data include, for example, data relating to the refractive errors of the eye 4 of the subject, such as:

[0194] - the previous refractive prescription of the subject 5, which is entered for example with the aid of the user interface of the instrument 1, loaded from a remote server, read into an electronic health card of the subject 5, or determined from the ophthalmic lenses normally worn by the subject 5;

[0195] - the preliminary refractive prescription, obtained by implementing a subjective refraction protocol prior to the subjective refraction protocol described here.

[0196] These data can also include the data relating to the response speed RSD of the subject described above.

[0197] These data can also include data relating to the sensitivity parameter of the subject, which represents his / her sensitivity to refractive changes.

[0198] The protocol can also include the reaction speed test described above, during which the change speed limit s of the subject 5 being tested for vision is determinedL individual values.

[0199] After this data acquisition step So, the protocol comprises a spherometer phorometry sub-protocol, denoted by step S1. During this spherometer phorometry sub-protocol, the best spherometer refraction Mop is determined, which best corrects the spherometer refractive error of the eye 4 of the subject. To this end, the eye of the subject can be provided with a refraction correction adapted to blur its vision (by means of a spherometer correction higher than the best spherometer refraction Mop), then its vision is progressively made clear until the subject indicates that he sees the target object 7 as clearly as possible. The spherometer refraction thus obtained can then be refined or confirmed by means of a dichromatic test.

[0200] The way in which this spherometer phorometry sub-protocol is carried out can take into account at least some of the personal data acquired in step So, in order to converge quickly and / or with an appropriate precision taking into account the sensitivity parameters of the subject, to the best spherometer refraction Mop.

[0201] Steps So and S1 are both optional. Indeed, the eye care professional supervising the protocol can choose to carry out only step So, only step S1 or both (not necessarily in the order presented above).

[0202] In any case, before the following step S2, the best spherometer refraction Mop, or at least an estimate thereof, is acquired or determined.

[0203] During step S2, a cylinder phorometry sub-protocol is carried out. During this cylinder phorometry sub-protocol, a cylinder refraction correction is determined which best corrects the spherometer refractive error of the eye 4 of the subject. To this end, the spherometer refraction correction provided to the subject can be slightly increased compared to the best spherometer refraction Mop (for example, +0.5 diopters can be added to Mop) to slightly blur the vision of the subject. Then, various cylinder refractions to be evaluated are provided to the subject.

[0204] More particularly, this cylinder phorometry sub-protocol comprises a step S23 of providing the eye 4 of the subject with a first refraction and a second refraction to be compared with each other. At step S24, the subject 5 then designates which of these first and second corrections enables him / her to see the target object 7 most clearly.

[0205] Before providing the subject with the first refraction and then switching to the second refraction (during step S23), the control unit 3:

[0206] - determines, at step S21, the coordinates of these first and second refractions, and then

[0207] - determines, at step S22, the characteristic of the switch from this first refraction to this second refraction.

[0208] In the embodiments described here, the set of steps S21 to S24 is executed several times in succession until the control unit 3 determines that the optimal refractive correction of the eye 4 of the subject has been reached, or until the control unit has determined this optimal refractive correction based on the answers provided by the subject during the successive executions of step S24.

[0209] More specifically, the control unit 3 can be programmed to control the instrument 1 so that the set of steps S21 to S24 is repeated for different values of the mean refractive correction corresponding to the mean of the first and second refractive corrections until the subject indicates in step S24 that the blur level he perceives is the same for the first and second refractive corrections and that these blur levels are low.

[0210] The optimal refractive correction can then be determined by the control unit 3 as equal to the mean refractive correction in this last case for which the blur levels corresponding to the first and second refractive corrections are the same. The control unit can determine the optimal refractive correction based on this last mean refractive, but also taking into account the previous answers provided by the subject 5 during the course of the protocol.

[0211] In the repeated sequence of the set of steps S21 to S24, when step S21 is executed again, the coordinates of the first and second refractive corrections, that is to say the coordinates of the first point P1 and of the second point P2, are determined according to the answer or answers provided by the subject 5 during one or more previous executions of step S24.

[0212] For example, if the answer provided by the subject during a previous execution of step S24 indicates that the first and second refractive corrections are close to the optimal refractive correction, the control unit 3 can determine new coordinates of the first and second refractive corrections so that these new refractive corrections are closer to the previous refractive corrections than during the previous repetitions of the set of steps S21 to S24. In other words, the gap between the previous first refractive correction provided during a previous execution of step S23 and the next first refractive correction provided during the next execution of step 23 decreases as this refractive correction approaches the optimal refractive correction.

[0213] When the answer provided by the subject at step S24 expresses a high uncertainty of the subject as to which of the first and second refractive corrections best corrects his vision, the control unit 3 can determine that the first and second refractive corrections are already close to the optimal refractive correction.

[0214] When a "reversal" of the refractive correction that best corrects the vision of the subject is detected between the first and second refractive corrections, the control unit 3 can also determine that the first and second refractive corrections have approached the optimal refractive correction. This reversal can occur during a series of successive executions of this set of steps S21 to S24, during which the average refractive correction monotonously increases (or monotonously decreases). This reversal corresponds to the fact that the subject 5 signals, during a new execution of the step S24, that it is no longer the second refractive correction that best corrects his vision, but now it is the first refractive correction, and vice versa. This reversal occurs when the above-mentioned average refractive correction passes from one side to the other of the optimal refractive correction in the coordinate system. This reversal thus reveals that the first and second corrective refractive approaches the optimal refractive correction.

[0215] The control unit 3 can also determine that the first and second refractive corrections approach the optimal refractive correction on the basis of the former refractive prescription or on the basis of the preliminary refractive prescription on the subject 5 collected at the step So.

[0216] Now regarding the gap between the first and second refractive corrections to be compared to each other, the control unit 3 can be programmed to determine the distance d12 between the first point P1 and the second point P2 at the step S21 as a function of the sensitivity parameter of the subject as explained at the beginning of the section "Switching from the first refractive correction to the second refractive correction".

[0217] The control unit 3 can also be programmed to determine the distance d12 at the step S21 so that its value is smaller when the control unit 3 has determined that the first and second refractive corrections approach or are approaching the optimal refractive correction than when the first and second refractive corrections are considered to be far from the optimal refractive correction. For example, the distance d12 can be set to an initial value of 1 diopter, then this distance can be set to 0.5 diopter once the control unit 3 has determined that the first and second refractive corrections approach or are approaching the optimal refractive correction.

[0218] Now regarding the step S22, in the embodiment described here, the control unit 3 is programmed so that:

[0219] - for example according to one of the criteria mentioned above, if the control unit 3 has previously determined that the first and second refractive corrections approach or are approaching the optimal refractive correction,

[0220] - then the control unit infers that the middle point of the middle portion of the segment [P1 P2] is closer to the optimal point P0 than each of said first and second points P1 and P2.

[0221] And when the control unit 3 has determined that this intermediate point of the intermediate portion of the line segment [P1P2] is closer to the optimum point P0 than each of said first point P1 and second point P2, the control unit 3 then determines the characteristics of the switch from the first dioptric correction to the second dioptric correction according to feature i) or feature ii), that is to say such that the trajectory representing said switch avoids the intermediate point P I , or passes near it at high speed.

[0222] And as long as the control unit 3 has not determined that the intermediate point of the intermediate portion of the line segment [P1P2] is closer to the optimum point P0 than each of said first point P1 and second point P2, the control unit 3 determines the characteristics of said switch without particular constraint, the trajectory representing the switch being for example a straight line and being carried out at any speed ((not necessarily high speed)).

[0223] Alternatively, the control unit 3 can be programmed to determine, on the basis of the coordinates of the first point P1, the coordinates of the second point P2, and an estimate P 0E of the optimum point, whether the intermediate point of the intermediate portion of the line segment [P1P2] is closer or alternatively further away from the optimum point P0 than each of said first point P1 and second point P2. Such an estimate P 0E of the optimum point represents for example the dioptric power state corresponding to the former or preliminary dioptric prescription of the subject 5. On the basis of this estimate, the control unit 3 can then calculate the following values:

[0224] - the distance d I between the intermediate point P 0E and the estimate P PI of the optimum point;

[0225] - the distance d 0E between the first point P1 and the estimate P P1 of the optimum point; and

[0226] - the distance d 0E between the second point P2 and the estimate P P2 of the optimum point.

[0227] If the distance d PI is less (alternatively higher) than d P1 and d P2 , the control unit 3 determines that the intermediate point under consideration is closer (or alternatively further away) to the optimum point P0 than each of said first point P1 and second point P2.

[0228] In the above exemplary solutions, before controlling the optical system 2 so that the criterion of feature i) or ii) is met, the control unit is programmed to test, during the switch, whether the intermediate point of the intermediate portion of the line segment [P1P2] is closer to or, optionally, further away from the best point of refraction P0 than each of said first and second points P1, P2.

[0229] However, in other embodiments, the control unit 3 can be programmed to systematically determine the feature of switching from the first dioptric correction to the second dioptric correction according to the above feature i) or feature ii) regardless of any condition, that is, all the time.

[0230] In this case, the condition corresponding to feature i) or feature ii) is met each time the optical system 2 switches from the first dioptric correction to the second dioptric correction (and therefore, in particular, in the case where the intermediate point located in the intermediate portion of the line segment is closer to or further away from the best point of refraction P0 than each of the first point P1 and the second point P2). Thus, in these last embodiments, the condition corresponding to feature i) or feature ii) is met each time step S23 is performed regardless of any condition.

[0231] Moreover, some of the operations performed at steps S21 or S22, such as the determination of the variation speed limit s L , or the determination of the distance d12, can be performed only once instead of each time these steps are performed.

Claims

1. A method for providing a first and a second refractive correction to be compared with each other for at least one eye (4) of a subject (5), wherein - an optical system (2) transmitting a light beam (6) coming from a target object (7) to the eye (4) of the subject, the method comprising the steps of: - providing, from the optical system (2) to the eye (4) of the subject, a first dioptric power state of the first refractive correction to - providing, by the optical system (2) to the eye (4) of the subject, a second dioptric power state of the second refractive correction, the switching being performed without interrupting the light beam (6) transmitted by the optical system (2), Each power state of the optical system (2) is represented by a corresponding point (P, P1, P2, P0, P I ) in a coordinate system, the coordinates (M, J0, J45) of the point (P, P1, P2, P0, P I ) representing values of different power characteristics (M, J0, J45) of the optical system for the power state of the optical system (2), the coordinates of the point (P, P1, P2, P0, P I ) representing the power state in the coordinate system comprising, for any power state of the optical system (2): - at least two of the following: a spherical power (S, M) of the optical system (2) in the dioptric power states, a first cylinder power feature (Jo, C) and a second cylinder power feature (J45, a), or - the second order coefficient (c2) of the Zernike polynomial decomposition of the wavefront output by the optical system (2) in the refractive power state o 2, c 2 2, c -2 2) of at least two among: the first dioptric power state being represented by a first point (PI) in the coordinate system and the second dioptric power state being represented by a second point (P2), a segment starting at the first point (PI) and ending at the second point (P2), the best refractive correction to the eye (4) of the subject being represented by a best refraction point (PO) in the coordinate system, the best refractive correction being a refractive correction compensating for a refractive error of the eye (4) of the subject, wherein, when an intermediate point (P I ) located in an intermediate portion of the line segment is closer to the optimal point of refraction (P0) or further away from the optimal point of refraction (P0) than each of the first point (P1) and the second point (P2), then the switching is performed so that: - for each point (P) of the trajectory (T1, T2, T3, T4, T5) representative of said switch, said point (P) is such that the distance (d) in said coordinate system between said point (P) and said intermediate point (P I ) is higher than or equal to one quarter of the smallest of the following distances (d m ): a first distance (dl) between said first point (P1) and said intermediate point (P I ), and a second distance (d2) between said second point (P2) and said intermediate point (P I ), or such that - the power of the optical system (2) varies at a speed (s V ) higher than a variation speed limit (s L ) at a point (P I ) of the trajectory (T6) closest to the intermediate point (P I ), the variation speed limit (s L ) being the highest power variation speed at which the subject (5) is not able to perceive a power variation.

2. The method of claim 1, wherein, the switching is performed so that: - an integral along the trajectory (T1, T2, T3, T4, T5, T6) representing the switch, from the start (t1) until the end (t2) of said switch, of a given function (f) of the distance (d) between the current variable point (P) of the trajectory and the intermediate point (P I ) of the trajectory, less than - the product of the proximity time span limit (Δ T ) and the value of the function (f) when the argument of the function is equal to one quarter of the minimum (d m ) of the first distance (d1) and the second distance (d2).

3. The method of claim 2, wherein, The proximity time span limit (Δ T ) is comprised between 1 millisecond and 1 second.

4. The method of claim 1, wherein, The switching is carried out so that for each point (P) of the trajectory (T2, T3, T4, T5) representing the switching in the coordinate system, the distance (d) between the point (P) and the intermediate point (P I ) is higher than or equal to the smallest of the first distance (dl) and the second distance (d2).

5. The method of claim 1, wherein, The intermediate point (P I ) is the middle of the line segment, the trajectory (T2, T4, T5) representing the switching in the coordinate system is a semicircle centered on the intermediate point (P I ).

6. The method of claim 1, wherein, the spherical power (M) of the optical system (2) varies during the switching while remaining higher than the smallest of a first spherical power (Ml) that the optical system has when it is in its first dioptric power state and a second spherical power (M2) that the optical system has when it is in its second dioptric power state along the switching.

7. The method according to claim 1, comprising the following steps performed before switching from the first dioptric power state to the second dioptric power state: - acquiring data relating to a response speed (RSD) of the subject (5) in reaction to a dioptric power change; and - determining said variable speed limit (s L ) based on said data (RSD).

8. The method of claim 1, wherein, said speed limit of change (s L ) is higher than or equal to 20 diopters per second.

9. The method of claim 1, wherein, The trajectory (T6) representing the switch in the coordinate system is the line segment, and wherein the speed of change (s V ) of the optical power of the optical system (2) is higher than the change speed limit (s L ) when the point (P) representing the state of the optical power of the optical system passes through the intermediate point (P I ).

10. The method of claim 1, wherein, the optical system (2) in its second dioptric power state has the same spherical power (M2) as in its first dioptric power state, and wherein at least one cylinder power feature (Jo, J45) of the optical system (2) is different when in its second dioptric power state than when in its first dioptric power state.

11. The method of claim 1, wherein, an intermediate portion of the segment has a length equal to half the length of the segment, and wherein a middle of the intermediate portion coincides with a middle of the segment.

12. The method of claim 1, wherein, along the switching, for each point (P) of the trajectory (Tl, T2, T3, T4, T5, T6), a blur level (BL) determined on the basis of a theoretical model of the eye (4) and of the vision of the subject (5) and taking into account that the eye (4) of the subject is provided with a refractive correction corresponding to the dioptric power state associated with the point (P): - higher than the product of a margin coefficient (k) and the minimum of a first blur level (BL1) associated with a first dioptric power state of the optical system (2) and a second blur level (BL2) associated with a second dioptric power state of the optical system (2), and - lower than the ratio of the maximum of the first blur level (BL1) and the second blur level (BL2) to the margin coefficient k, the first blur level (BL1) and the second blur level (BL2) being determined on the basis of the theoretical model of the eye (4) and the vision of the subject (5) and taking into account that the eye (4) is respectively provided with the first dioptric correction or the second dioptric correction, the margin coefficient (k) being comprised between 0.5 and 1.

13. The method of claim 1, wherein, The intermediate point (P I ) is the middle of the line segment.

14. An instrument (1) for providing a first dioptric correction and a second dioptric correction to be compared with each other for at least one eye (4) of a subject, comprising: - an optical system (2) arranged to transmit a light beam (6) coming from a target object (7) to the eye (4) of the subject; and - a control unit (3) for controlling the optical system (2), the control unit (3) being programmed for controlling the optical system (2) to: - switch from the optical system (2) to the eye (4) of the subject from a first dioptric power state of the first dioptric correction to - the optical system (2) providing a second dioptric power state of the second dioptric correction to the eye (4) of the subject, the optical system (2) being configured so that the light beam (6) transmitted by the optical system (2) is not interrupted during the switch, - at least two of the following: the spherical power (S, M) of the optical system (2) in the dioptric power state, a first cylinder power feature (J0, C) and a second cylinder power feature (J45, a), or Each power state of the optical system (2) is represented by a corresponding point (P, P1, P2, P0, P I ) in a coordinate system, the coordinates (M, J0, J45) of the point (P, P1, P2, P0, P I ) representing values of different power characteristics (M, J0, J45) of the optical system for the power state of the optical system (2), the coordinates of the point (P, P1, P2, P0, P I ) representing a power state of the optical system (2) in the coordinate system comprising: the first dioptric power state being represented by a first point (PI) in the coordinate system and the second dioptric power state being represented by a second point (P2), a line segment starting at the first point (PI) and ending at the second point (P2), the best dioptric correction for the eye (4) of the subject being represented by a best dioptric point (PO) in the coordinate system, the best dioptric correction being a dioptric correction that compensates for the refractive error of the eye (4) of the subject, - the second order coefficient (c2) of the Zernike polynomial decomposition of the wavefront output by the optical system (2) in the dioptric power state o 2, c 2 2, c -2 2) of at least two among: wherein the control unit (3) is programmed to control the optical system (2) so that then, during the switch: when an intermediate point (P I ) located in an intermediate portion of the line segment is closer to the optimal point of refraction (P0) or further away from the optimal point of refraction (P0) than each of the first point (PI) and the second point (P2), - the light beam (6) transmitted by the optical system (2) is not interrupted, - for each point (P) of the trajectory (T1, T2, T3, T4, T5) representing the switch in the coordinate system, the distance (d) in the coordinate system between the point (P) and the intermediate point (P I ) is higher than or equal to one quarter of the smallest of the first distance (d1 ) between the first point (P1 ) and the intermediate point (P I ), and the second distance (d2) between the second point (P2) and the intermediate point (P I ); or -The refractive power of the optical system (2) is closest to the midpoint (P) I The point (P) of the trajectory (T6) of the ) I The rate of change at point (s) V ) higher than the rate of change limit (s) L When the rate of change exceeds the limit, the subject (5) cannot perceive the change in refractive power. The rate of change limit (s) L ) is the highest rate of change in refractive power that the subject (5) can perceive.

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