Perimeter with adjustable lenses

Abstract

The perimeter comprises a screen (2), a light source assembly (12), a projection optics (14), and an adjustable beam deflector (16). The projection optics (14) comprises an adjustable lens (26) having a focal length that can be changed by the device's control unit (18). The projection optics (14) further comprises a projection lens element (28). The focal length of the adjustable lens (26) is changed as a function of the position (L, L') of the stimulation light spot, thereby compensating for the position-dependent optical path length from the projection optics (14) to the screen (2). The distance between the projection lens element (28) and the adjustable lens (26) is equal to the focal length of the projection lens element (28), which makes the spot size independent of the position (L, L').

Description

[Technical field]

[0001] The present invention relates to a perimeter and a method for operating same.

[0002] In the context of this application, a perimeter is used to test a patient's subjective visual field. [Background technology]

[0003] A perimeter, such as that described in US Pat. No. 5,399,633 or 6,399,645, comprises a light source, an adjustable deflector, and a screen. The adjustable deflector is used to generate a stimulating light spot at a desired position on the screen for evaluating the sensitivity of the patient's visual field for a particular spatial direction.

[0004] Typically, this type of perimeter device faces the problem that the optical path length of the light depends on the coordinates at which the stimulus light spot appears on the screen.

[0005] For spherical screens, such as those shown in US Patent No. 5,399,633, the patient's eye is typically located at the center of the screen's sphere, and therefore the deflector, such as a set of movable mirrors, is positioned off-axis. Thus, the path length that light must travel from the source through the deflector to the screen varies depending on the screen position selected.

[0006] On the other hand, the projected light spot should have substantially the same brightness and size for all screen positions. Therefore, adaptive optics is required. For example, US Pat. No. 5,399,433 discloses the use of path length correction implemented with a displaceable mirror. However, this is mechanically complex, and furthermore, the actuation of the mirror generates noise that may alert the patient to new stimuli, which in turn may affect the accuracy of the test. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] U.S. Pat. No. 4,561,738 [Patent Document 2] U.S. Pat. No. 7,475,987 [Patent Document 3] European Patent Application Publication No. 1380251

[0008] The present invention therefore aims to solve the problem of providing a perimeter and a method of this type, in which the size of the stimulating light spot can be corrected in a simple manner.

[0009] This problem is solved by a perimeter and a method according to the independent claims.

[0010] Thus, the perimeter contains at least the following elements: - A screen. This screen is used to receive the stimulating light spots for the patient. a light source assembly adapted to generate the visible light used to provide the stimulation; - A projection optics system, which is adapted to process the light from the light source and to project the stimulating light spot onto a screen. an adjustable deflector, the adjustable deflector adapted to deflect light to generate stimulating light spots at a plurality of selectable coordinates on the screen;

[0011] According to the invention, the projection optics comprises an adjustable lens whose focal length is adjustable, which can be used to quickly and easily correct the focus of the stimulation light spot, which can further be controlled as a function of the light path length from the light source to the screen.

[0012] Advantageously, the projection optical system comprises a projection lens element arranged coaxially with and at a distance from the adjustable lens, which lens element is capable of supporting the imaging movement of the adjustable lens, which advantageously has a fixed focal length.

[0013] In particular, the projection lens element may be a positive lens element, which may enable, for example, an adjustable lens to operate at a large focal length (whether positive or negative).

[0014] In such an example, the focal length of the projection lens element is advantageously equal to within ±20% of the axial distance between the adjustable lens and the projection lens element. As shown below, this ensures that the size of the stimulation light spot on the screen is substantially independent of the focal length used for the adjustable lens.

[0015] In another advantageous embodiment, the aperture of the projection optics and deflector combination is selected such that any light beam from the light source that traverses the adjustable lens reaches the screen, ensuring that the total amount of light at the stimulation point does not change when changing the focal length of the adjustable lens.

[0016] The light source assembly typically forms an aperture that defines the illumination area projected by the projection optics onto the screen. This aperture is advantageously uniformly illuminated (e.g., with a uniformity of greater than 25%). The diameter of this aperture, together with the magnification of the projection optics, therefore defines the diameter of the stimulation light spot.

[0017] This stop may be formed, for example, by the diaphragm of a Köhler illumination, or it may be formed, for example, by the exit aperture of a multimode light guide which homogenizes the light from the light source.

[0018] Advantageously, the diameter of the restriction may be varied, for example by using an adjustable diaphragm to define the restriction.

[0019] The perimeter may further comprise a control unit connected to the adjustable lens and the adjustable deflector for controlling them.

[0020] This control unit is advantageously adapted to carry out at least the following steps: - positioning the stimulation light spot at a plurality of positions on the screen by adjusting the adjustable deflector, such that the optical path length from the light source assembly to the position of the stimulation light spot varies among at least some of those positions, as described. - Adjusting the focal length of an adjustable lens as a function of the path length.

[0021] This scheme provides automatic compensation for changing path lengths.

[0022] Advantageously, the control unit adjusts the focal length of the adjustable lens such that the projection optics projects the field stop onto the screen for all of said positions, in other words the field stop and the screen are kept in a conjugate plane of the projection optics for the currently selected position.

[0023] In this example, the projection optics are advantageously designed to have the same magnification for all of the above positions, accurate to within 20%.

[0024] The adjustable deflector may advantageously include a mirror positioned radially offset from the central axis of the screen to provide a wide field of view for the patient.

[0025] In a simple design, the adjustable deflector comprises a mirror that is tiltable about a first axis and a second axis that are transverse to each other, which makes it possible to use only one mirror to adjust the position of the stimulating light spot on the screen in two dimensions.

[0026] Advantageously, for simple computational control, the first axis and the second axis are mutually orthogonal and / or they intersect.

[0027] In a simple design, the adjustable deflector includes: - a first rotary actuator, such as a stepper motor, having a first shaft rotatable about a first axis. - a second rotary actuator, such as a stepper motor, having a second shaft rotatable about a second axis. - a first coupler that couples the mirror to the first shaft. It rotationally locks the mirror to the first shaft for rotation about the first axis, but allows the mirror to tilt about a third axis transverse to the first axis. In other words, when the first shaft rotates, the mirror rotates with it, but the mirror can tilt about the third axis. - a second coupler that couples the mirror to the second shaft. It rotationally fixes the mirror to the second shaft for rotation about the second axis, but allows the mirror to tilt about a fourth axis transverse to the second axis. In other words, when the second shaft rotates, the mirror rotates with it, but the mirror can tilt about the fourth axis.

[0028] In particular, the third shaft rotates together with the first shaft about the first axis, and the fourth shaft rotates together with the second shaft about the second axis.

[0029] In particular, the third axis is orthogonal to the first axis, and the fourth axis is orthogonal to the second axis.

[0030] The screen of the perimeter is typically rotationally symmetrical with respect to the screen axis. In order to provide the patient with a wide field of view, the first and second rotary actuators are advantageously arranged radially outward (with respect to the screen axis) than the mirror, i.e. the mirror is closer to the screen axis than the first and second rotary actuators.

[0031] The invention also relates to a method of operating such a perimeter, which comprises the step of positioning a stimulating light spot at a plurality of positions on a screen by adjusting an adjustable deflector.

[0032] Advantageously, as described, the optical path length from the light source assembly to said locations varies between at least some of said locations, in this example the method advantageously includes adjusting a focal length of the adjustable lens as a function of the path length.

[0033] In particular, for the reasons stated above, the method may include the step of adjusting the focal length of the adjustable lens such that the projection optics projects the field stop of the light source assembly onto the screen for all of the above positions. [Brief description of the drawings]

[0034] The invention will be better understood, and objects other than those set forth above will become apparent, when the following detailed description is considered, such description making reference to the accompanying drawings.

[0035] [Figure 1] 1 shows a schematic diagram of one embodiment of a perimeter and a patient using it. [Diagram 2] 1 shows a cross-section of the perimeter along a vertical plane passing through the central axis of the screen. [Diagram 3] 1 shows the optical elements that generate a stimulating light spot on the screen. [Figure 4] 1 illustrates an embodiment of a light deflector. [Diagram 5] 1 illustrates one embodiment of a light source assembly, an adjustable lens, and a projection lens element. [Figure 6] The distances between the field stop, the adjustable lens, the projection lens element, and the screen along the optical axis of the system are shown. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] (definition) A "lens" as defined herein is either a simple lens or a complex lens.

[0037] A "compound lens" includes a number of single lenses arranged coaxially, the distance between the lenses being significantly less than the focal length of the lens. Advantageously, the single lenses of a compound lens are arranged axially adjacent to one another, i.e. they may be in contact. As known to those skilled in the art, the single lenses of a compound lens may have different dispersion characteristics, for example to produce the effect of an achromatic lens as a whole.

[0038] The term "lens element" may be a lens and / or a curved mirror with transformation properties that can be approximated using a ray matrix of the following type: where f is the focal length of the lens element.

[0039]

number

[0040] A controllable liquid lens is a lens whose focal length depends on the curvature of at least one liquid interface. This curvature can be altered by an electric field and / or by mechanical means. Advantageously, this curvature is altered by an electric field applied to the liquid.

[0041] The term "axial" refers to the centerline of the light beam from the light source assembly as it travels through the projection optics.

[0042] The term "axial distance" describes the distance between two components along the axis of the light beam.

[0043] (Overview of the perimeter) 1 and 2 show a perimeter comprising a screen 2 and a projection system 4 for projecting stimulating light spots at positions L, L′ on the screen 2.

[0044] The perimeter further comprises a chin rest or head rest 6 for positioning the head of the patient 8, and in particular the patient's eyes, in a predetermined position in front of the screen.

[0045] As known to those skilled in the art, the screen 2 of a modern perimeter is typically curved to have a central axis 10 of the screen, and the patient's eyes are advantageously positioned close to or at the intersection with the screen axis 10.

[0046] Advantageously, the screen 2 is spherical, the screen axis 10 extending through the centre point of the sphere defined by the screen 2 .

[0047] The projection system 4 includes a light source assembly 12, projection optics 14, and an adjustable deflector 16. A control unit 18 controls the light source assembly 12, projection optics 14, and beam deflector 16 to generate stimulating light spots at multiple positions L, L' on the screen 2.

[0048] To perform a perimeter test, the control unit 18 sets the beam deflector 16 to a predetermined position L and adjusts the projection optics 14 accordingly, as described in more detail below. It then operates the light source assembly 12 to generate a stimulus light spot of desired brightness and size at that position L. The brightness of the stimulus light is varied to determine the visibility threshold. This operation is repeated for multiple positions.

[0049] (Projection System) The projection system 4 is shown in more detail in figures 3-5.

[0050] As mentioned, it includes a light source assembly 12 for generating a predetermined light spot. In this embodiment, the light source assembly 12 comprises a light source 20 for generating white light. The light source 20 may be a white LED. The light from the light source 20 is projected by a lens element 22 onto a field stop 24. These elements advantageously form a Kohler illumination, as known to those skilled in the art.

[0051] The light intensity at the field stop 24 is advantageously uniform in the sense that it varies by less than 20% over the transparent area of ​​the field stop 24 .

[0052] The field stop 24 is advantageously an adjustable diaphragm or a revolver-type mechanism having a number of diaphragms with different diameters on a revolver carrier, the control unit 18 being adapted to vary its diameter as required by the current examination, thereby making it possible to present stimulating light spots of various sizes to the patient.

[0053] The projection optics 14 is adapted to project a field stop 24 onto the screen 2. That is, the field stop 24 and the stimulation light spot on the screen 2 are in a conjugate plane with respect to the projection optics 14, such that the stimulation light spot on the screen 2 is an image of the field stop 24 and therefore has a well-defined diameter.

[0054] In this embodiment, the projection optics 14 includes an adjustable lens 26 and a projection lens element 28 , with the adjustable lens 26 disposed between the light source assembly 12 and the projection lens element 28 .

[0055] The adjustable lens 26 has an adjustable focal length, which for example is controllable by a control voltage U or a control current I generated by the control unit 18 .

[0056] The adjustable lens 26 may be a controllable liquid lens. In such a case, the curvature of the interface of the liquids may be changed by an applied electric field or by mechanical means. An example of a field-controllable liquid lens is described in EP 167892 A1, where the curvature of the interface between two immiscible liquids may be changed by an electric field.

[0057] In other embodiments, a controllable liquid lens, for example as described in EP 2 860 555 A1, may be used.

[0058] In yet other embodiments, lenses using electroactive polymers may be used, such as those described in US Pat. No. 1,000,7034.

[0059] For example, adjustable lens 26 can be a Corning A-series variable focus liquid lens.

[0060] Advantageously, adjustable lens 26 contains no mechanical motors for quiet operation.

[0061] The projection lens element 28 is a positive lens element, that is, it is a converging lens element.

[0062] The projection system 4 may further comprise an auxiliary mirror 30 that projects light from the projection optics onto the adjustable deflector 16 if the adjustable deflector 16 is not positioned coaxially with the adjustable lens 26 and the projection lens element 28.

[0063] (Adjustable Deflector) The adjustable deflector 16 is best seen in FIGS.

[0064] In the current embodiment, it comprises a tiltable mirror 36 that can be tilted about a first axis A1 and a second axis A2. As mentioned above, the axes A1 and A2 are transverse and intersect. Advantageously, they are perpendicular to each other.

[0065] A first rotary actuator 38-1 and a second rotary actuator 38-2 are provided for rotating the mirror 36. They may be, for example, stepper motors and their rotational positions may be controlled by the control unit 18.

[0066] The first rotary actuator 38-1 has a first shaft 40-1 rotatable about a first axis A1. The first shaft 40-1 is connected to the mirror 36 via a first coupler 42-1. The first coupler 42-1 rotationally fixes the mirror relative to the first shaft 40-1 for rotation about the first axis A1, but allows the mirror 36 to tilt about a third axis A3 transverse to the first axis A1. The third axis A3 can advantageously be pivoted parallel to the second axis A2 by rotation about the first axis A1.

[0067] In the current embodiment, the first coupler 42-1 includes a first arm 44-1a rigidly connected to the first shaft 40-1 and a first arcuate section 44-1b rigidly connected to the first arm 44-1a. The first arcuate section 44-1b is connected to the mirror 36 by one or two rotation bearings 46-1.

[0068] The second rotary actuator 38-2 has a second shaft 40-2 rotatable about a second axis A2. The second shaft 40-2 is connected to the mirror 36 via a second coupler 42-2. The second coupler 42-2 rotationally fixes the mirror relative to the second shaft 40-2 for rotation about the second axis A2, but allows the mirror 36 to tilt about a fourth axis A4 transverse to the second axis A2. The fourth axis A4 can advantageously be pivoted parallel to the first axis A1 by rotation about the second axis A2.

[0069] In the current embodiment, the second coupler 42-2 includes a second arcuate section 44-2 rigidly connected to the second shaft 40-2. The second arcuate section 44-2 is connected to the mirror 36 by one or two rotation bearings 46-2.

[0070] The first axis A1 and the second axis A2 are advantageously perpendicular to each other, thereby obtaining the best angular resolution of the mirror 36.

[0071] As shown in FIG. 4, they advantageously extend symmetrically relative to the plane 48 of the screen axis 10, ie relative to the plane in which the screen axis 10 lies.

[0072] As shown in FIG. 2, the mirror 36 can be tilted by rotary actuators 38-1, 38-2 to guide light from the projection optical system 14 to various positions L, L' on the screen 2.

[0073] The mirror 36 is radially offset from the central axis 10 of the screen 2. Therefore, since it is not located at the center of the spherical screen surface, the path length of the light from the projection optics 14 to the screen 2 is not constant, but is a function of the positions L, L'. For this reason, the focal length of the adjustable lens 26 is adapted, as will be described in more detail in the next section.

[0074] As can also be seen in FIG. 2, the first actuator 38-1 and the second actuator 38-2 are positioned radially outward from the mirror 36 with respect to the central axis 10 of the screen 2 to avoid interference with the patient's field of view (i.e., they are positioned farther away from the central axis 10 than the mirror 36).

[0075] (Adjustment for path length) As mentioned in the previous section, the path length of the light to the screen 2 is not constant but is a function of the positions L, L'.

[0076] For this reason, the control unit 18 controls the focal length of the adjustable lens 26 as a function of the position L, L' in order to generate a well-focused stimulation light spot for every position L, L' on the screen 2. In addition, the projection optics 14 is adapted to have a magnification and focal length setting of the adjustable lens 26 that is independent of the position L, L'. This will be explained in more detail below.

[0077] This section refers to Figure 6, in which the projection of the field stop 24 onto the screen 2 by the adjustable lens 26 and the projection lens element 28 is shown diagrammatically, with the optical axis 50 of the system shown as a straight line (in reality the optical axis 50 is bent by the two mirrors 30, 36). The following parameters are used to describe the system: f1: focal length of the adjustable lens 26. This distance is varied by the control unit 18 as a function of the position L of the stimulating light spot on the screen 2. f2: the focal length of the projection lens element 28. This distance is advantageously fixed. d1: the axial distance between the field stop 24 and the adjustable lens 26. This distance is advantageously fixed. d2: The axial distance between the adjustable lens 26 and the projection lens element 28. This distance is advantageously fixed. d3: the axial distance between the projection lens element 28 and the screen 2. This distance is a function of the position L.

[0078] In the following, the projection system is approximated by ray matrix analysis, where the individual elements are described by their ray matrices.

[0079] As known to those skilled in the art, a ray matrix (or ABCD matrix) describes how an element transforms the radial position and the beam angle between its input and its output, see for example https: / / en.wikipedia.org / wiki / Ray_transfer_matrix_analysis.

[0080] The projection system of FIG. 6 converts a beam angle α1 and radial position r1 at its input (in the plane of the field stop 24) to a beam angle α2 and radial position r2 at its output (on the screen 2) as follows:

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[0081] This right hand side matrix, when multiplied, gives:

[0082]

number

[0083] Here, B in Equation 3 must be 0 if it is desired that the output plane be conjugate with the input plane, that is, the following equation must hold:

[0084]

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[0085] This equation gives, for the focal length f1 of the adjustable lens 26,

[0086]

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[0087] The values ​​of f2, d1, and d2 are constant and known. The distance d3 along the optical axis between the projection lens element 28 and the screen 2 can be calculated from the position of the lens element 28, the positions of the mirrors 30, 36, the position of the screen 2, and the current spot position L, L'. Thus, the control unit 18 can calculate the focal length f1 of the adjustable lens 26 from Equation 5 or an appropriate look-up table, and set f1 accordingly. Thereby, it is ensured that the field stop 24 is focused and projected onto the screen 2.

[0088] A in Equation 3 is the magnification factor between the input plane and the output plane, i.e., between the field stop and the screen.

[0089]

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[0090] As can be seen, the magnification factor A generally depends on the distance d3. Therefore, for a large number of f2, d1 and d2, the control unit 18 needs to adapt the diameter of the field stop 24 as a function of the current spot position L, L', so as to keep the spot size on the screen 2 constant. Furthermore, it needs to adapt the brightness of the light source 20 so as to keep the brightness of the spot constant. Although these measurements are available, they are advantageously avoided.

[0091] Therefore, in an advantageous embodiment, the scaling factor A is selected such that it is independent of d3, ie, the derivative of A with respect to d3 should be zero.

[0092]

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[0093] That is, the following formula should hold:

[0094]

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[0095] Therefore, if the focal length f2 of the projection lens element 28 is chosen to be equal to the distance d2 between the projection lens element 28 and the adjustable lens 26, then the magnification A of the projection optics becomes independent of the distance d3.

[0096] Therefore, advantageously, f2 is selected to be equal to d2. In such a case, the control unit 18 only needs to maintain the focal length f1 given by Equation 5 in order to generate a well-focused stimulation light spot with constant size and brightness at all positions L, L'.

[0097] The condition f2=d2 does not have to be strictly met. Small deviations will result in changes in luminance and stimulus spot size. According to the perimetry ISO standard EN ISO 12886:1999 (Ophthalmic instruments-Perimeters), luminance is allowed to vary within the range of -20% to +25% and spot size is allowed to vary within the range of -15% to +20%. Advantageously, therefore, the focal length f2 of the projection lens element is equal to the axial distance d2 within the range of ±20% of f2.

[0098] In operation, the distance d3 has a minimum value d 3,min and the maximum value d 3,max The time varies between 0 and 100 ms.

[0099] d 3,min , d 3,max The value of depends on the geometry of the device. Perimeters often use a Goldmann type screen with a radius of R=300 mm (see FIG. 2). As can be calculated for a geometry of the type of FIG. 2, where the position of the mirror 36 is offset from the central axis 10 by about 100 mm and the incident light has an angle α of about 60° with respect to the central axis 10, the distance d3 typically varies between about 550 mm and 750 mm.

[0100] For example, adjustable lenses of the type described above are usually best operated with low curvature, ie, with a large or infinitely long focal length, ie, f1=∞.

[0101] In other words, the method advantageously includes the step of setting the focal length of the adjustable lens 26 to be infinitely long, at least for some of the positions L, L'.

[0102] f1=∞ is true when the denominator of Equation 5 is zero. The corresponding value of d3 is as follows:

[0103]

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[0104] Therefore, advantageously, the components of the perimeter are 3,inf But, d 3,min and d 3,max That is, the following is true:

[0105]

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[0106] In the case of the embodiment of Equation 8, i.e., when d2=f2, we obtain:

[0107]

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[0108] In other words, the above components are advantageously: d1, d2, d 3,min , d 3,max are arranged so as to satisfy the following:

[0109]

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[0110] For d1=50mm and d2=160mm, assume that d2=f2, and then d3=d 3,min = 550mm and d3 = d 3,max = 750 mm, the corresponding ranges of focal length f1 are f1 = 200 mm (+5 diopters) and f1 = -328 mm (-3 diopters), respectively, i.e. in this case an accommodation range of about 8 diopters is required for the adjustable lens 26.

[0111] (Note) In the above embodiment, the deflector has only one mirror that is tiltable about two axes, alternatively, two independently tiltable mirrors may be used.

[0112] In still other embodiments, there may be no tiltable mirror, or the mirror may be tiltable about only a single axis. In such instances, the beam deflector may be implemented by a pivot mechanism that tilts the light source assembly and / or the projection optics along at least one direction or both directions.

[0113] At least one mirror of the adjustable deflector is advantageously a flat mirror, but it may also be a curved mirror or the mirror 30 may be curved, in which case it is considered to form part of the projection optics, in particular part of the projection lens element.

[0114] In the above example, the projection lens element 28 is a lens. Alternatively, it may be formed exclusively by a curved mirror, in particular by a curved mirror of an adjustable deflector.

[0115] While the presently preferred embodiments of the invention have been shown and described, it is to be clearly understood that the invention is not limited thereto and can be variously embodied and practiced within the scope of the following claims.

Claims

1. A perimeter, comprising: A screen (2); A light source assembly (12); A projection optical system (14); an adjustable deflector (16); The projection optical system (14) comprises an adjustable lens (26) whose focal length is adjustable.

2. 2. The perimeter of claim 1, wherein the projection optical system (14) comprises a projection lens element (28), the projection lens element (28) being arranged coaxially with the adjustable lens (26) and at a distance from the adjustable lens (26).

3. 3. The perimeter of claim 2, wherein the projection lens element (28) is a positive lens element.

4. The focal length (f ２ ) is the axial distance (d) between the adjustable lens (26) and the projection lens element (28). ２ 4. The perimeter of claim 3, wherein the perimeter is equal to within ±20% of the perimeter.

5. 5. The perimeter of claim 2, wherein the adjustable lens (26) is disposed between the light source assembly (12) and the projection lens element (28).

6. The following formula is established: d １ is the axial distance between the field stop (24) of the light source assembly (12) and the adjustable lens (26), d ２ is the axial distance between the adjustable lens (26) and the projection lens element (28), f ２ is the focal length of the projection lens element (28), and d ３，ｍｉｎ and d ３，ｍａｘ 6. The perimeter according to claim 2, wherein x is the minimum and maximum axial distance possible between the projection lens element (28) and the screen (2). [0010]

7. 7. The perimeter according to claims 4 and 6, wherein the following formula is satisfied: [0025]

8. 8. The perimeter of claim 1, wherein the aperture of the projection optical system (14) and the deflector (16) are selected such that any light beam from the light source assembly (12) that traverses the adjustable lens (26) is large enough to reach the screen (2).

9. 9. The perimeter of claim 1, wherein the light source assembly (12) comprises a field stop (24), and the projection optical system (14) projects the field stop (24) onto the screen (2).

10. The perimeter comprises a control unit (18) connected to the adjustable lens (26) and the adjustable deflector (16); and The perimeter is adapted to position a stimulating light spot at a plurality of positions (L, L') on the screen (2) by adjusting the adjustable deflector (16), wherein an optical path length from the light source assembly (12) to the positions (L, L') varies among at least some of the positions (L, L'); and 10. The perimeter of claim 1, further adapted to adjust the focal length of the adjustable lens (26) as a function of the path length.

11. The perimeter according to claims 9 and 10, wherein the control unit (18) is adapted to adjust the focal length of the adjustable lens (26) such that the projection optical system (14) projects the field stop (24) onto the screen (2) for all of the positions (L, L').

12. 12. The perimeter according to claim 11, wherein the projection optics (14) has the same magnification for all of the positions (L, L').

13. 13. The perimeter according to any one of the preceding claims, wherein the adjustable deflector (16) comprises a mirror (36) radially offset from the central axis (10) of the screen (2).

14. 14. The perimeter according to claim 1, wherein the adjustable deflector (16) comprises a mirror (36) tiltable about a first axis (A1) and a second axis (A2), the first axis (A1) and the second axis (A2) being transverse to each other.

15. The adjustable deflector (16) a first rotary actuator (38-1) having a first shaft (40-1) rotatable about a first axis (A1); a second rotary actuator (38-2) having a second shaft (40-2) rotatable about the second axis (A2); a first coupler (42-1) coupling the mirror (36) and the first shaft (40-1), the first coupler (42-1) rotationally fixing the mirror (36) relative to the first shaft (40-1) for rotation about the first axis (A1) but allowing the mirror (35) to tilt about a third axis (A3) transverse to the first axis (A1); a second coupler (42-2) coupling the mirror (36) and the second shaft (40-2), the second coupler (42-2) rotationally fixing the mirror (36) relative to the second shaft (40-2) for rotation about the second axis (A2) but allowing the mirror (36) to tilt about a fourth axis (A4) transverse to the second axis (A2); The perimeter of claim 14 further comprising:

16. The perimeter according to claim 15, wherein the first actuator (38-1) and the second actuator (38-2) are arranged radially outward from the mirror (36) with respect to the central axis (10) of the screen (2).

17. 17. The perimeter according to any one of claims 14 to 16, wherein the first axis and the second axis are symmetrical with respect to the plane of the screen axis (10).

18. 18. The perimeter according to any one of the preceding claims, wherein the adjustable lens (26) is a controllable liquid lens, in particular an electric field-controllable liquid lens.

19. A method of operating a perimeter as described in any one of claims 1 to 18, comprising the step of positioning a stimulating light spot at a plurality of positions (L, L') on the screen (2) by adjusting the adjustable deflector (16).

20. an optical path length from the light source assembly (12) to the locations (L, L') varies between at least some of the locations (L, L'), and the method includes:

20. The method of claim 19, comprising adjusting the focal length of the adjustable lens (26) as a function of the path length.

21. 21. The method of claim 20, further comprising the step of adjusting the focal length of the adjustable lens (26) such that the projection optical system (14) projects a field stop (24) of the light source assembly (12) onto the screen (2) for all of the positions (L, L').

22. 22. The method according to any one of claims 19 to 21, comprising setting the focal length of the adjustable lens (26) to infinity for at least some of the positions (L, L').

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