Method and device for visual field testing

Abstract

Visual field tests for a given location of a patient's eye are performed by means of probing cycles. In each probing cycle, a visual stimulus with a defined stimulus luminance (L) and stimulus size (D) is displayed on a perimeter screen (12), and it is tested if the patient detects the stimulus. The method comprises a first and a second series of such probing cycles. In the first series, the stimulus luminance (L) extends up to a first threshold. In the second series, the stimulus size (D) is larger than in the first series. Then, the stimulus luminance (L) and / or the stimulus size (D) may be increased again. A transfer function is provided for scaling the luminances measured for different stimulus sizes to a reference stimulus size.

Description

[0001] Method and device for visual field testing

[0002] Technical Field

[0003] The invention relates to a method and a device for visual field testing.

[0004] Background Art

[0005] Visual field testing (perimetry) is a psychophysical testing method used to establish a baseline for visual function, to compare it with healthy subjects (normal) and to assess change over time (progression). Various sets of parameters for testing, some more suitable for certain pathologies, others more suitable for different stages of a disease, have been established. For example, SWAP, or short wavelength automated perimetry, uses specific parameters for testing while SAP, standard automated perimetry uses other parameters. Even for the same type of testing, standard white / white perimetry or SAP, may use different parameters on different devices.

[0006] A typical perimetry measurement is carried out by means of repetitive probing cycles, with each probing cycle comprising displaying, on a perimeter screen, a visual stimulus at a desired location, with a given stimulus luminance and a stimulus size, and testing for a patient response indicative of the patient detecting the stimulus. These cycles are repeated while increasing the stimulus luminance until a patient response is received.

[0007] One important parameter in this type of measurement is the size of the stimulus. Smaller stimuli provide better resolution, but they are harder to see. Hence, for poor-sighted patients, the measurements are often carried out for larger stimuli, which requires additional steps and, also, makes it hard to compare measurements, e.g., as a patient's sight is degraded over time.

[0008] Further, patients with poor eye sight tend to generate poorly reproducible results, especially for large stimulus luminance at small stimulus size.

[0009] Disclosure of the Invention

[0010] Hence, the problem to be solved is to find a method and a device for visual field testing that provides a simple measurement with good accuracy for a large range of eyesights.

[0011] This problem is solved by the method and device of the independent claims. Accordingly, the method for performing a visual field test for a given location of a patient's retina is carried out by means of several probing cycles Ci, with i = 1 ... N. Each probing cycle Ci comprises the step of displaying, on a perimeter screen, a visual stimulus, with a stimulus luminance Li and a stimulus size Di, and testing for a patient response indicative that the patient detected the stimulus.

[0012] The method comprises at least the following:

[0013] - performing a first series of N1 > 1 probing cycles, and

[0014] - performing a second series of N2 > 1 probing cycles.

[0015] The stimulus cycles Ci in the first series have luminances Li up to a first threshold Tl.

[0016] The second series comprises at least one stimulus cycle Ci having a stimulus size Di larger than any stimulus size in the first series.

[0017] This is based on the understanding that the reproducibility of measurements become poor for small stimuli with large luminance. By using a probing cycle with larger stimulus size for higher luminance, the process adapts to the patient's needs and produces more reliable results.

[0018] As mentioned above, the method comprises "at least" performing the first and second series. It may include further cycles, such as small stimuli Di not larger than at least some stimulus size of the first series and having a luminance larger than the first threshold Tl, that are not part of the first or second series.

[0019] For a broader scope of measurement, N1 and / or N2 may be larger one. In some embodiments, N1 and N2 are larger than one in order to probe with several smaller and larger stimuli.

[0020] For a more flexible assessment of the visual field, the measurement at a given location of the patient's retina comprises at least two probing cycles Ci at different stimulus luminances. If N1 > 1, the first series may comprise several probing cycles Ci at different stimulus luminances up to said first threshold.

[0021] Based on the understanding that lager stimuli can more reproduci- bly be combined with larger luminance, the second series may comprise at least one stimulus cycle Ci having a luminance Li larger than the first threshold Tl and a stimulus size Di larger than any stimulus size in the first series.

[0022] In order to better compare the stimulus luminance recorded for stimuli having different stimulus size, a transfer function L(D0) = S(L(D), D) may be used. For a given stimulus having a size D and a luminance L(D), the transfer function returns the luminance L(D0) of a statistically equally well visible stimulus having a reference stimulus size DO. For example, if, in a number of measurements, patients have, for a given field-of-view location, their detection threshold at a luminance L(D0) for stimuli with a stimulus size DO, a median or average of these patients has, for the same location, their detection threshold at stimulus luminance L(D) for stimuli with a stimulus size D, with L(D0) = S(L(D), D), or vice versa.

[0023] This transfer function can be used to translate the stimulus luminance Li of at least some of the probing cycles Ci to the scaled luminance L(D0) = S(Li(Di), Di) for the reference stimulus size DO.

[0024] The method may further comprise at least the following:

[0025] - Calculating, for several probing cycles having different stimulus luminances Li and equal or different stimulus sizes Di, scaled luminances L(D0) = S(Li(Di), Di) using a transfer function L(D0) = S(Li(Di), Di) that, for a given stimulus having the stimulus size Di and the stimulus luminance Li(Di), returns a scaled luminance L(D0) of a statistically equally well visible stimulus having a reference stimulus size DO Di.

[0026] Hence, the transfer function is able to translate different luminances Li for a stimulus sizes Di DO to a corresponding scaled luminance at the reference size DO. Again, this allows to carry out a series of probing cycles at the size Di at different stimulus luminances and still compare them to measurements at a reference luminance size.

[0027] If, for these "several probing cycles having different stimulus luminances Li and equal or different stimulus sizes Di", the stimulus size(s) Di is / are larger than the reference stimulus size DO, the transfer function allows to perform measurements at larger stimulus sizes while still comparing them to measurements at smaller stimulus sizes, thereby allowing more accurate results if the patient has poor eyesight.

[0028] In one embodiment, the method comprises calculating, for several probing cycles having different stimulus luminances Li but equal stimulus size Di DO, the scaled luminances L(D0) = S(Li(Di), Di).

[0029] The method may comprise calculating the scaled luminance for several points of the second series.

[0030] If, in the case of the previous paragraph, at least some of the probing cycles in the first series have a stimulus size Di = DO, the respective probing cycles can be easily compared to those probing cycles of the second series to which the transfer function has been applied.

[0031] For a large range of measurement in the respective series, the first and / or the second series may comprise different scaled luminances L(D0) = S(Li(Di), Di) In order to progress quickly through the probing cycles, the scaled luminance L(D0) may be increased continuously between at least a majority of, in particular all of, the subsequent probing cycles Ci and Ci+i over both the first and the second series. In other words, the scaled luminance S(Li+i(Di+i), Di+i) is higher (i.e. brighter) than the scaled luminance S(Li(Di), Di).

[0032] For an equal density of measurements, the scaled luminances S(Li(Di), Di) of at least a majority of the probing cycles Ci of the first and the second series may be equally spaced. In this context, if a logarithmic scale is used for the luminance, "equally spaced" is to be understood as a constant difference while, if a linear scale is used for the luminance, "equally spaced" is to be understood as a constant ratio. In view of the limited accuracy of psychophysical measurements, such "equal spacing" is considered to be fulfilled if it is fulfilled within an accuracy of 33%.

[0033] In the first series, the stimulus size Di may be the same for all the probing cycles, taking into account that, even for small stimulus size, reliable measurements are possible over a whole range of low luminances, e.g., up to 24 dB. (For a definition of the dB-based luminance, see section "Definitions" below.)

[0034] Alternatively, though, the first series may comprise cycles having stimulus sizes Di.

[0035] The probing cycles of the first and / or second series may be executed in an order of increasing stimulus luminance Li, i.e., the luminance Li of a cycle in the series is at least as large or larger than the luminance Li of any earlier cycle in the same series. This can expedite the process because the procedure can be stopped once the patient is able to see the stimulus.

[0036] For the same reason, the first series may be carried out before the second series.

[0037] However, as explained in the section "Order of Measurements" below, the order of cycles and the order of the series can also be selected differently. Also, the algorithm may "hop" between the series, i.e., it may, e.g., start with one or more cycles of one series, then move to one or more cycles in the other series, then again go to one or more cycles of the former series, etc., or vice versa. This applies to all aspects of the present method. Further, the cycles for one given location of the eye (i.e., of the field of view), may be interspersed with one or more cycles for other locations of the eye.

[0038] The second series may comprise cycles having several different luminances Li with these cycles having stimulus size Di larger than any stimulus size in the first series. In particular, there may be several such probing cycles with Li > Tl. By varying the luminance for a large stimulus size, the range of measurement at this stimulus size is increased.

[0039] The methods may be implemented by means of a perimetry device that comprises at least the following elements:

[0040] - A perimeter screen: This screen may e.g., be a spherical screen or a screen built into a headset or desktop device.

[0041] - A stimulus generator for generating the stimulus on the screen. It may, e.g., be implemented as a projector sending the stimulus to a desired point on the screen, or it may be software adapted to light up certain pixels of the screen.

[0042] - A control unit: The control unit is adapted to carry out the present method, e.g., by being suitably programmed to control the stimulus generator. The control unit may be integrated into a device, like a headset, or may be separate, or may be remote.

[0043] Brief Description of the Drawings

[0044] The invention will be better understood and objects other than those set forth above will become apparent when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawings, wherein:

[0045] Fig. 1 shows a schematic perimetry device,

[0046] Fig. 2 illustrates a series of probing cycles,

[0047] Fig. 3 illustrates the relationship between a patient's sensitivity to stimuli at size 3 and to stimuli at size 5,

[0048] Fig. 4 illustrates transfer functions between various stimuli sizes and a reference stimulus size 3,

[0049] Fig. 5 shows a series of probing cycles in a diagram that shows the scaled stimulus luminance for a stimulus size 3 along the horizontal axis and the stimulus size along the vertical axis, with the continuous curves showing the locations along which the transfer function has constant value,

[0050] Fig. 6 shows another series of probing cycles in the same diagram as Fig. 5,

[0051] Fig. 7 shows another series of probing cycles in the same diagram as Fig. 5,

[0052] Fig. 8 shows another series of probing cycles in the same diagram as Fig. 5, Fig. 9 shows another series of probing cycles in the same diagram as Fig. 5,

[0053] Fig. 10 illustrates an embodiment for deriving the transfer function, and

[0054] Fig. 11 compares several types of transfer functions to measured data.

[0055] Modes for Carrying Out the Invention

[0056] Definitions

[0057] "Stimulus luminance" L, as used herein, is a parameter indicative of the luminous flux of the stimulus per source area, e.g., expressed in cd / m2.

[0058] In perimetry, sensitivity and stimulus luminance are often expressed on a logarithmic attenuation scale in decibels. Given a luminance L indicative of the luminous flux per source area, a luminance L' in this decibel-scaled "attenuation luminance" L' is given by

[0059] L' = 10 • log(Lmax / L) (1) with Lmax being a maximum luminance L of the stimulus of the perimetry device, and L the current stimulus luminance.

[0060] L' = 0 dB corresponds to a stimulus at the maximum luminance of the device, L' = 40 dB to a stimulus having 1 / 10000 of the maximum luminance of the device.

[0061] In the present context, terms such as "higher" or "lower" luminance or "increasing" or "decreasing" luminance refer to the stimulus luminance L. For example, a higher luminance is expressed by a larger value of L and a lower value of L', and "increasing luminance" expresses making the stimulus brighter (thereby decreasing L').

[0062] In the graphs of the figures (with the exception of Fig. 2), luminance is expressed by L', i.e., lower numerical values correspond to a brighter stimulus.

[0063] The values of L' given in the examples are based on the Octopus series of perimeters, where a 0 dB value represents 10000 apostilbs (with one apostilb being 1 / cd / m2) and a 40 dB value represents 1 apostilb. As known to the skilled person, other perimeters may use a different value of Lmaxand, therefore, specific dB values correspond to other values the stimulus luminance. "Stimulus size" is a parameter indicative of the size of the stimulus. In perimetry, stimulus size is often expressed as the "Goldmann size", with a Gold- mann size III corresponding to an angular size (as seen from the patient) of 0.43° and Goldmann size V corresponds to an angular size of 1.72°, i.e., a Goldmann size of D corresponds to, approximately, an angular size of 0.054°-2D

[0064] In the graphs of the figures, stimulus size is expressed in Goldmann size and interpolations thereof, with 3, e.g., corresponding to Goldmann size III and 5, e.g., corresponding to Goldmann size V.

[0065] Two probing cycles Ci+i and Ci are considered to be "subsequent" if both probing cycles are carried out for the same location of the eye in the same patient session and probing cycle Ci+i is carried out later than probing cycle Ci. However, there may be other probing cycles carried out between Ci+i and Ci.

[0066] Perimetry Device

[0067] Fig. 1 shows an example of a perimetry device 8 (also called a perimeter) comprising a perimetry measurement unit 10 (schematically shown in sectional view). It may comprise a spherical screen 12 and a stimulus generator 15. Another, similar example of such a device is shown in US9084540.

[0068] The stimulus generator 15 includes an image projector 16 to generate visual stimuli, which are projected, via a mirror 14, onto screen 12. A subject, resting her / his head on a headrest 18, observes screen 12 and indicates, by means of an input means, such as a button 20, when a stimulus appears, i.e., button 20 is a tool to generate a patient response indicative of the patient detecting the stimulus.

[0069] The device further comprises a control unit 22, which may, e.g., be equipped with a microprocessor 24. Control unit 22 may be physically integrated in measurement unit 10, or it may be a device separate from measurement unit 10.

[0070] Control unit 22 operates measurement unit 10 to provide stimuli to the subject at various locations in the visual field. For a given location, control unit 22 operates the device in a series of probing cycles as described in more detail below. Thus, it records a visual field map.

[0071] In addition or alternatively to a spherical screen 12, the device may, e.g., comprise a head-mounted display 26 where the stimulus is shown on a built-in perimeter screen.

[0072] To ease working with small stimuli, the device may further comprise corrective optics (not shown) for correcting the patient's eyesight as it is known to the skilled person. General Measurement Procedure

[0073] The general measurement procedure is illustrated in Fig. 2, where the first signal trait shows the pulsing of the stimulus (on / off), the second curve shows the stimulus luminance L, and the third curve shows the stimulus size D.

[0074] As can be seen, a measurement procedure for detecting the patient's sensitivity to a stimulus at a certain location on the screen (i.e., on the patient's retina) comprises a series of probing cycles ci, C2, C3, etc. Each probing cycle Ci, with i = 1 ... N comprises displaying a visual stimulus on the perimeter screen for a stimulus duration ti.

[0075] Control unit 22 executes the probing cycles Ci. In probing cycle Ci, it sets the stimulus luminance to the value Li and the stimulus size to the value Di and displays the stimulus for the stimulus duration ti.

[0076] After each stimulus, control unit 22 tests for a patient response, e.g., by monitoring button 20.

[0077] Once the patient provides a response, the measurement procedure for the given location is ended, and the stimulus luminance Li, stimulus diameter Di, and (optionally) the stimulus duration ti are used to indicate the eye sensitivity for the given location. Details how these parameters may be processed can be found in the section Result Processing below.

[0078] Adapting Stimulus Size

[0079] As mentioned above, control unit 22 varies the stimulus luminance Li and stimulus size Di over the probing cycles Ci. It may start with a low value of the stimulus luminance Li and the stimulus size Di, e.g., with a luminance of 40 dB and a size of 3 (Goldmann size III). (As mentioned in the section "Definitions" above, a low luminance corresponds to a high dB value.)

[0080] In some embodiments, in a first series of probing cycles, the luminance is then increased in steps of, e.g., 1 to 4 dB until the patient indicates that they have detected the stimulus.

[0081] This proceeds until the stimulus luminance Li exceeds a first threshold TI. This first threshold may, e.g., be between 20 dB and 28 dB, for example at 24 dB.

[0082] In absolute terms, for an Octopus perimeter where 0 dB corresponds to 10000 apostilbs and using Eq. (1) above, this corresponds to TI being between, for example, between 16 and 100 apostilbs (corresponding to 28 dB and 20 dB), such as 40 apostilbs (corresponding to 24 dB), i.e., to TI being between 3 cd / m2and 30 cd / m2, such as 13 cd / m2. Setting T1 to such a low level means that the spot size is increased long before approaching the maximum luminance that the perimeter can generate, but it has the advantage to generate more repeatable results, in particular with poor-sighted patients. In particular, T1 is not larger than 15 cd / m2.

[0083] In more general terms, to improve the reliability of the measurements, if the perimeter can generate spots with a maximum luminance Lx on its perimeter screen, threshold T1 may be chosen to be no larger than Lx / 10, in particular no larger than Lx / 100, (with T1 and Lx being expressed in a scale linear to light luminance, such as in cd / m2). In other words, the threshold T1 is at least one order of magnitude, in particular at least two orders of magnitude, lower than Lx.

[0084] In that case, to still exploit the luminance range of the perimeter device, the second series may comprise stimulus cycles Ci having a luminance Li larger (i.e., brighter) than the first threshold Tl. In other words, after switching to larger spot sizes, the luminance will also, at some point, be increased again, i.e., the luminance is increased in several steps.

[0085] The first series of probing cycles consists of N1 probing cycles, typically with N1 > 1. In the example above, with Tl = 24 dB, Li = 40 dB, and a step size between 1 and 4 dB, N1 is, e.g., between 5 and 17. For Tl being 20 dB, it may, e.g., be as high as 21.

[0086] As shown in Fig. 2, in the first series of probing cycles, the stimulus Di may be kept constant, e.g., at 3. Goldmann size III, which is a well-established parameter that provides reliable results up to about threshold T 1.

[0087] Once threshold Tl would be exceeded if the stimulus luminance Li were increased by yet another step, a second series of probing cycles starts.

[0088] In the example of Fig. 2, the first series or probing cycles ends with probing cycle C7.

[0089] In the first probing cycle of the second series (cs in the example of Fig. 2), the stimulus size Di is increased. Hence, in the example of Fig. 2, we have Ds > D7. The stimulus luminance Li may be kept constant (i.e., Ls = L7). This results in a stimulus that is larger and therefore easier to recognize.

[0090] If the patient does not recognize the stimulus in probing cycle cs, the stimulus size and / or the stimulus luminance may be increased for the next probing cycle.

[0091] In the example of Fig. 2, the stimulus luminance is kept constant (i.e. L9 = Ls) while the stimulus size is increased once more (i.e., D9 > Ds).

[0092] If the patient does not recognize the stimulus in cycle C9, the stimulus size and / or the stimulus luminance may be increased for the next probing cycle. In the example of Fig. 2, the stimulus luminance is now increased again (i.e., Lio > L9) while the stimulus size is now kept constant (i.e., Dw = D9). Transitioning to a larger luminance at C10 is possible because Dw is larger than Gold- mann size III and less prone to inaccurate measurements even at a luminance larger than first threshold Tl.

[0093] Result Processing

[0094] As mentioned, the series of probing cycles may end when the user confirms that they have recognized the stimulus, or it may end when the algorithm finds for other reasons that the given probing cycle is at the limit of what the patient can see (see section "Order of Measurements" below). If this occurs in probing cycle Ck, the sample luminance will be Lk and the sample size will be Dk. These two values can be used to characterize the patient's sensitivity to stimuli at the given location.

[0095] However, comparing the result of the measurement to other measurements, e.g., performed in other parts of the eye or at other times or on other patients, would difficult because, in such other measurements, the stimulus may have had a different stimulus size when it was detected.

[0096] For better comparison between measurements carried out at different stimulus sizes, a transfer function S may be provided that allows to rescale a stimulus luminance L(D) at a given stimulus size D to a scaled luminance L(D0) at a reference stimulus size DO, i.e.

[0097] L(D0) = S(L(D), D). (2)

[0098] For example, DO may be the popular Goldmann size 3, and the transfer function S can be used to calculate, given a luminance L(D) at a size D 3, a luminance L(D0=3) having, statistically, the same visibility at size 3.

[0099] Such a transfer function S can be established using a series of test measurements where one or more patients are exposed to, e.g., measurements at the reference stimulus size DO and at a stimulus size D.

[0100] For a given location in the field of view, and for each stimulus size, the stimulus luminance may be increased until the patient sees stimulus. This allows to compare the visibility threshold luminance at D and DO and to establish a relationship between L(D) and L(D0). Using enough measurements and, e.g., linear or nonlinear curve fitting techniques, a numerical model for the transfer function S then be derived. For example, Fig. 10 shows the results of a number of different measurements where the patient first saw the stimulus at 33 dB at stimulus size 5. Each bar represents one patient and at which luminance the patient detected the stimulus at the same location but for a spot size of 3. The bars are ordered by length.

[0101] As can be seen, the median of the patients saw the stimulus for spot size 3 at approximately 27 dB. Therefore, it can be concluded that a stimulus luminance of 33 dB at a stimulus size of 5 corresponds to a stimulus luminance of 27 dB at a spot size of 3.

[0102] Performing the same analysis for other size-5 -luminance values allows to derive a relationship as shown in Fig. 3, where the luminance L at spot size 5 is shown in relation to the corresponding luminance L at spot size 3.

[0103] As mentioned, the points in Fig. 3 (apart from those that were not visible at spot size 3, i.e., which sit on the vertical axis at the left) can be used for curve fitting the transfer function S(L(5), 5).

[0104] For D = 3, the transfer function S(L(3), 3) is equal to L(3).

[0105] If S(L(D), D) needs to be known for other values of size D (other than 3 and 5), further test measurements may be carried out for other values of D as described above.

[0106] In addition or alternatively thereto, the transfer function may be modeled by a function with parameters al, a2, a3..., i.e.,

[0107] L(D0) = S(L(D), D, al, a2, a3...) (3)

[0108] The parameters al, a2, a3... may then be fitted to the result of a number of test measurements.

[0109] An example of this is illustrated in Fig. 4, which shows a transfer function for values of D of 3, 3.5, 4, 4.5, 5, and 6, the parameters of which have been obtained by fitting the function to the experimental values.

[0110] In an example, the transfer function S may be modeled as

[0111] L'(DO) = x + (Dn- 3n) (a + b x + e x2+ d x3), (4) with x = L'(D), the parameters a, b, c, d being fitted parameters, and n being either a fixed value (e.g., 1 or 2) or being a fitted parameter. The third-order term d x3may be omitted if a second-order polynomial is found to provide an approximation of sufficient accuracy. In another example, a model of the inverse S'1of the transfer function S may be modeled as

[0112] L'(D) = x + (Dn- 3n) (a + b y + c y2+ dy3), (5) with y = L'(DO), the parameters a, b, c, d being fitted parameters, and n being either a fixed value (e.g., 1 or 2) or being a fitted parameter. The third-order term dy3may again be omitted if a second-order polynomial is found to provide an approximation of sufficient accuracy. Once the fitted parameters of S'1are known, S may, e.g., be calculated using a root finding algorithm to determine the value of y for a given L(D).

[0113] Eq. (5) can be rewritten as

[0114] L'(D) = po + pi L'(DO) + p2L'(DO)2+ P3 L'(DO)3(6) with po, pi, p2, P3 being parameters depending on D.

[0115] In contrast to this, conventional approaches for the transfer function are based on the laws of spatial summation, such as, e.g., described by Adrian Dart Cheong Chan, "Head-Mounted Perimetry", master thesis submitted at the University of Toronto, 1999, Chapter 3.1. These approaches are based on the assumption that detectability of a stimulus increases linearly with the luminance L (see Chan, p. 44) in linear luminance scale, i.e.,:

[0116] L Ak= C, (7) with A being the area of the stimulus, k being a coefficient of spatial summation, and C being a constant. According to Chen, different values have been suggested for coefficient k, e.g., k = 0.3, k = 0.5, k = 0.8, and k = 1.0.

[0117] From Eq. (7), the relation between a luminance L(D0) and L(D) for two difference spot sizes DO, D can be calculated as

[0118] L(D0) A(D0)k= L(D) A(D)k, (8) with A(D0) and A(D) corresponding to the areas at spot sizes DO and D, respectively.

[0119] For converting this transfer functions into logarithmic luminances L', Eq. (8) can be combined with Eq. (1), which results in a transfer function as follows: L'(D) = L'(DO) + 10 klog(A(D) / A(0)) (9)

[0120] Comparing Eqs. (6) and (9), one can see that Eq. (9) corresponds to Eq. (6) with po = 10 klog(A(D) - A(0)), pi = 1, and p2 = p3 = 0.

[0121] Fig. 11 compares transfer functions of the type of Eq. (6) and Eq. (9) to measured data (shown as dots in the graph). It plots Eq. (9) for k = 0.3 and k = 1 and linear and quadratic fits of Eq. (6) for po = 15.1, pi = 0.66, and p2 = p3 = 0 as well as for po = 14.7, pi = 0.77, p2 = -0.0032, and p3 = 0.

[0122] As can be seen, the conventional transfer functions of Eq. (9) provide a poor description of the measured data while those of Eq. (6) provide much better results.

[0123] In particular, the slopes of the transfer functions of Eq. (9), which are always equal to 1, are much higher than what the measured data suggests. In contrast to this, the fitted transfer function of Eq. (6) as shown in the figure has slopes smaller than 1 for D > DO. For the example of D = 5 and DO = 3 in the range of 0.5 and 0.8 over the measured range of luminances. In Fig. 11, for example, the linear version of Eq. (6) has a first-order derivative of 0.66 and the quadratic version has a first-order derivative between 0.77 (at L'(D0) = 0) and 0.55 (at L'(D0) = 35).

[0124] Hence, for an improved match with real-world data, the transfer function may be such that, for logarithmic luminances L', the first-order derivative of L'(D) vs. L'(D0) for at least some of the stimulus sizes D and DO, with D > DO, is smaller than 1. In particular, the derivative may be between 0.5 and 0.8 for D = 5 and DO = 3 (Goldmann size) over a range of stimulus luminances at DO between 0 and 35 dB.

[0125] Examples

[0126] Fig. 5 shows an example of how the probing cycles ci, C2, etc. may be selected. The diagram shows the scaled luminance L(D0) for DO = 3 along its horizontal axis and the actually used stimulus size (in Goldmann size) along its vertical axis. Each circle symbol illustrates the scaled luminance L(D0) as well as the stimulus size D for one probing cycle Ci. The curves in the graph illustrate trajectories along which the probe luminance L(D) has a constant value.

[0127] The measurement starts with probing cycle ci, e.g., at a stimulus size D = DO with a low stimulus luminance of Li = 40 dB. Since D = DO, the stimulus luminance L corresponds to the scaled luminance.

[0128] As long as the patient does not detect the stimulus, the probing cycles proceed as follows: - In a first series of probing cycles, which contains, in this example, the cycles ci to C5, the stimulus size D is kept at 3 while the stimulus luminance Li is increased with fixed steps of, e.g., a value between 1 dB and 4 dB. The graph shows an example with a step of 4 dB. The first series ends when the stimulus luminance Li would exceed the threshold Tl. In the shown example, this threshold is 24 dB.

[0129] - In a second series of probing cycles, which contains, in this example, the cycles C6 ... C13, a stimulus size Di larger than 3 is used. a) For example, the stimulus size for cycle C6 is increased to slightly more than 4 while keeping the luminance constant at 24 dB, with stimulus size De being such that the scaled luminance L(D0 = 3) = S(24 dB, De) = 20 dB in order to keep the step of increase of the scaled luminance L(D0 = 3) constant at around 4 dB. b) For the following two cycles C7, Cs, the stimulus size D? =Ds is kept at the same value as De while the stimulus luminance L? and Ls are increased by such amounts that S(L?, D?) = 16 dB and S(Ls, Ds) = 12 dB c) For cycle C9, the stimulus luminance is kept constant (i.e., L9 = Ls at about 17 dB) while the stimulus size is increased once more (i.e., D9 > Ds) such that S(L9, D9) = 8 dB. d) In the shown example, three cycles cio, cn, C12 at the same size as D9 follow, then the luminance is increased again for cycle C13.

[0130] As can be seen, the cycles move to larger stimulus sizes as the luminance increases, thereby avoiding the problems observed for small stimuli at high luminance. Also, the scheme expands the range of measurement to values of the scaled luminance L(D0) even smaller than zero.

[0131] Fig. 6 illustrates another example where, in the second series starting with cycle C6, the luminance Li is kept constant at 24 dB while increasing the stimulus size in several steps up to a fairly large stimulus size of 5.5 at cycle C9, and only then is the luminance Li increased again.

[0132] Fig. 7 shows yet another example where, in the second series starting with cycle C6, the stimulus size Di is increased by constant steps of 0.5 (i.e. Di+i = Di + 0.5), and the stimulus luminance Li is adjusted such that S(Li, Di) changes in constant steps of about 4 dB.

[0133] In the example of Fig. 8, the first and second series of cycles apply substantially the same algorithm for determining stimulus size and luminance Di, Li in that both values are increased, e.g., with a constant increase between Di+i and Di and with Li being chosen such that S(Li, Di) changes in constant steps of about 4 dB. This continues until a maximum size, such as 6, is reached, whereupon the size Di is kept constant (if necessary, further steps at this maximum size with increasing luminance Li may follow, but this is not shown in the example).

[0134] In the example of Fig. 9, the step for the scaled luminescence is 1 dB, and the maximum luminescence size is 5. Here, there are about 17 cycles in the first series. In the second series, the luminance Li is kept constant at 24 dB while increasing the stimulus size in 11 steps to a size of 5. Then, the stimulus size Di is kept constant while the luminance is increased again, up to 0 dB, which corresponds to a scaled luminescence of about -9 dB.

[0135] The examples above illustrate various aspects of the method, such as:

[0136] - In the examples of Figs. 5 - 7 and 9, the stimulus size Di in the first series (e.g., ci ... cs) remains unchanged. In this way, the first series uses the same scheme as conventional methods at least for the first series, thereby making the results more straightforward to compare with those of the conventional methods.

[0137] - In the example of Fig. 8, the stimulus size Di is increased already in the first series, which allows to use smaller increase steps in stimulus size over the whole measurement.

[0138] - In all the shown examples, the stimulus size is increased between at least some of the cycles of the second series. (Alternatively, though, the second series may, e.g., directly skip to a large stimulus size (e.g., at cycle ce), which would, however, require a decrease of the stimulus luminance to, e.g., a value around 32 dB.)

[0139] - In the example of Fig. 5, in the second series, the stimulus luminance Li is increased, and the stimulus size Di is kept constant (at slightly above 4) between at least some of the probing cycles (in cycles ce, C7, cs) until the stimulus luminance Li exceeds a second threshold T2 (12 dB). At this point, the stimulus size Di may again increased again (to approximately 4.6). Alternatively, though, the stimulus luminance Li of all cycles in the second series may be the same.

[0140] - In all of the shown embodiments, the stimulus luminance Li in the second series is at least as large as a largest stimulus luminance Li in the first series.

[0141] - In all the examples, at least some of the probing cycles of the first series are performed with a stimulus size Di = DO, i.e., at the reference size of, e.g., 3. As mentioned, this makes these measurements easier to compare to conventionally obtained results. In the examples of Figs. 5 - 7, all of the cycles of the first series are performed with a stimulus size Di = DO.

[0142] - In all of the shown examples, the scaled luminance S(Li+i(Di+i), Di+i) of probing cycle Ci+i is higher (in the sense of "brighter") than the scaled luminance S(Li(Di), Di) of probing cycle Ci, and the change is even constant. It may, however, also be possible to "backtrace" in the sense that the luminance Li is, between some of the cycles, decreased to such a degree that cycle Ci+i has a scaled luminance dimmer than cycle Ci, e.g., for checking the consistency between measurements at different sizes.

[0143] Order of Measurements

[0144] The probing cycles for a given location of a patient's retina, i.e., for a given location in the patient's field of view, may be subsequent with increasing index, i.e., for all i, probing cycle Ci+i is carried out after probing cycle Ci. There may be other probing cycles, e.g., for other locations, between Ci+i and Ci.

[0145] Alternatively, though, any other ordering may be used. In particular, the strength of the stimuli for a given location does not necessarily have to increase over time. For example, the measurement may start with a stimulus at a middle range of the scaled luminance. Depending on if the patient sees that or not, the probing cycles may then concentrate on a scaled luminance subrange above or below the first stimulus, e.g., again starting with a stimulus having a scaled luminance at a middle range in the given subrange.

[0146] Also, depending on a patient's history, the measurement may already start with probing cycles in the second series, omitting the first series entirely.

[0147] In some embodiments, the first and / or second series of probing cycles may be skipped completely, e.g., depending on patient history and / or patient reaction.

[0148] Varying other parameters

[0149] In the examples above, the parameters changed over the cycles Ci are the stimulus size Di and the stimulus luminance Li. Other parameters may, however, also be varied. In some embodiments, stimulus duration ti may be varied, too. For example, the Octopus perimeters often use a stimulus duration ti of 100 ms for smaller stimulus sizes Di and a stimulus duration of 200 ms for larger stimulus sizes. If the duration ti is changed independently of both other parameters (Li, Di) and has a significant influence on the visibility of the stimulus, it may be added as a further input value to the transfer function S, i.e., S(L(Di), Di, ti). This allows to compare measurements recorded while varying all three of these parameters. For stimulus duration ti, the influence of visibility is small when ti is at least 100 ms, but it may become more important for shorter stimuli. The transfer function S(L(Di), Di, ti) may again be obtained from test measurements in the same manner as described in the section "Result Processing" above.

[0150] Other parameters that may be changed between the probing cycles Ci may, e.g., include the stimulus shape and / or the pattern of the stimulus (in particular for the larger stimulus sizes) and / or the spectral composition of the stimulus and / or a temporal modulation of the stimulus. Again, one or more of these parameters can be included in the transfer function S in order to compare the measurement for given settings with a measurement at reference settings.

[0151] Trend Analysis Over Larger Times

[0152] When monitoring a patient's visual fields over longer periods of time in different sessions with the patient, e.g., over weeks, months, or even years, in order to recognize trends, the present methodology is particularly advantageous because the transfer function allows to compare measurements at different spot sizes. This is advantageous, e.g., because earlier measurements on a given patient may have been carried out at a small size (such as Goldmann size 3) while later measurements carried out as the patient's sight becomes poorer, may have to be performed at larger sizes (such as Goldmann size 3 or even larger).

[0153] Hence, the method may comprise at least the following:

[0154] - Performing a first set of probing cycles Ci.

[0155] - Performing a second set of probing cycles Ci at least one week after performing the first set of probing cycles. The second set of probing cycles comprises at least some probing cycles Ci performed at a stimulus size Di different from the first set of probing cycles Ci. The second set of probing cycles may even be performed at least one month or even at least one year after the first set of probing cycles.

[0156] - Using the transfer function to compare the stimulus luminances Li of the first and the second sets of probing cycles Ci.

[0157] The use of the transfer function allows to compare probing cycles even if the patient's visual resolution has suffered substantially. The comparison, may, e.g., include a trend analysis performed over the first and the second set of probing cycles Ci.

[0158] The first and second sets of proving cycles may even be carried out on different devices. Notes

[0159] While there are shown and described presently preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.

Claims

Claims1. A method for performing a visual field test for a given location of a patient's retina by means of probing cycles Ci, with i = 1 ... N, and with each probing cycle Ci comprising displaying, on a perimeter screen, a visual stimulus, with a stimulus luminance Li and a stimulus size Di, and testing for a patient response indicative of the patient detecting the stimulus, the method comprising- performing a first series of N1 > 1 of the probing cycles Ci, and- performing a second series of N2 > 1 of the probing cycles Ci, wherein the stimulus cycles Ci in the first series have a stimulus luminance Li up to a first threshold Tl, and wherein the second series comprises at least one stimulus cycle Ci having a stimulus size Di larger than any stimulus size in the first series.

2. The method of claim 1 wherein at least one of N1 and N2 is larger than 1.

3. The method of any of the preceding claims wherein there are at least two probing cycles Ci with different stimulus luminances, and in particular wherein N1 > 1 and wherein the first series comprises probing cycles Ci with different stimulus luminances up to the first threshold TL4. The method of any of the preceding claims wherein the second series comprises at least one stimulus cycle Ci having a luminance Li larger than the first threshold TL5. The method of any of the preceding claims comprising providing a transfer function L(D0) = S(L(D), D) that, for a given stimulus having size D and luminance L(D), returns a scaled luminance L(D0) of a statistically equally well visible stimulus having a reference stimulus size DO, using the transfer function to translate the stimulus luminance Li of at least some of the probing cycles Ci to a scaled stimulus luminance L(D0) = S(Li(Di), Di) for the reference stimulus size DO.

6. The method of claim 5 wherein at least some of the probing cycles of the first series are performed with a stimulus size Di = DO.

7. The method of any of the claims 5 or 6 comprising calculating, for several probing cycles having different stimulus luminances Li and equal or different stimulus sizes Di, scaled luminances L(D0) = S(Li(Di), Di) using the transfer function L(D0) = S(Li(Di), Di).

8. The method of claim 7 comprising calculating, for several probing cycles having different stimulus luminances Li but equal stimulus size Di DO, the scaled luminances L(D0) = S(Li(Di), Di).

9. The method of any of the claims 7 or 8 wherein DO < Di for the several probing cycles having different stimulus luminances Li but equal stimulus sizes Di.

10. The method of any of the claims 5 to 9 comprising calculating the scaled luminance for several points of the second series.

11. The method of any of the claims 5 to 10 wherein at least one of the first and the second series comprises several probing cycles having different scaled luminances L(D0) = S(Li(Di), Di).

12. The method of claim 11 wherein, between at least a majority, in particular all of, the subsequent probing cycles Ci and Ci+i, over both the first and the second series, the luminance S(Li+i(Di+i), Di+i) is higher than the luminance S(Li(Di), Di)13. The method of any of the claims 11 or 12 wherein the scaled luminances S(Li(Di), Di) of at least a majority of the probing cycles Ci of the first and the second series are equally spaced.

14. The method of any of the claims 5 to 13 wherein the transfer function is such that, for logarithmic luminances L' and at least some of the stimulus sizes D and DO, with D > DO, a first-order derivative of L'(D) vs. L'(D0) is smaller than 1,in particular wherein the first-order derivative is between 0.5 and 0.8 for Goldmann sizes D = 5 and DO = 3 over a range of stimulus luminances between 0 and 35 dB at DO.

15. The method of any of the claims 5 to 14 comprising performing a first set of probing cycles Ci, performing a second set of probing cycles Ci at least one week after performing the first set of probing cycles, wherein the second set of probing cycles comprises at least some probing cycles Ci performed at a stimulus size Di different from the first set of probing cycles Ci, and using the transfer function to compare the stimulus luminances Li of the first and the second sets of probing cycles Ci, and in particular wherein the first set of probing cycles Ci and the second set of probing cycles Ci are carried out on different devices.

16. The method of any of the preceding claims wherein, in the first series, the stimulus size Di is the same for all probing cycles.

17. The method of any of the preceding claims wherein the first series comprises probing cycles have different stimulus sizes Di.

18. The method of any of the preceding claims wherein the probing cycles of at least one of the first and second series are executed in an order of increasing stimulus luminance Li.

19. The method of any of the preceding claims wherein the first series is carried out before the second series.

20. The method of any of the preceding claims wherein the second series comprises cycles having different luminances Li with a stimulus size Di larger than any stimulus size in the first series.

21. The method of claim 20 wherein the second series comprises cycles having different luminances Li > T1 with a stimulus size Di larger than any stimulus size in the first series.

22. The method of any of the claims 1 to 19 wherein all the cycles of the second series have the same stimulus luminance Li.

23. The method of any of the claims 1 to 21 wherein, in the second series, the stimulus luminance Li is increased, and the stimulus size Di is kept constant between at least some of the probing cycles until the stimulus luminance Li exceeds a second threshold T2 whereupon the stimulus size Di is increased.

24. The method of any of the preceding claims wherein the second series comprises cycles having different stimulus sizes Di.

25. The method of any of the preceding claims wherein the stimulus luminance Li of all the cycles in the second series is at least as large as a largest stimulus luminance Li of the cycles in the first series.

26. The method of any of the preceding claims wherein the stimulus size Di in the first series of probing cycles Ci is Goldmann size III.

27. The method of any of the preceding claims wherein the first threshold T1 is between 3 cd / m2and 30 cd / m2.

28. The method of any of the preceding claims wherein the first threshold T1 is not larger than 15 cd / m2.

29. The method of any of the preceding claims wherein the threshold T1 is no larger than Lx / 10, in particular no larger than Lx / 100, with Lx being a maximum luminance of spots generatable on the perimeter screen.

30. The method of claim 29 wherein the second series comprises measurement cycles Ci with a stimulus luminance Li lager than the threshold Tl.

31. The method of any of the preceding claims wherein, in each probing cycle Ci, the stimulus is shown over a stimulus duration (ti), wherein the pulse stimulus duration is varied between at least some of the pulse cycles Ci.

32. The method of any of the preceding claims wherein N1 > 1 and N2 > 2,wherein the first series comprises probing cycles Ci with different stimulus luminances up to the first threshold Tl, and wherein the second series comprises at least one stimulus cycle Ci having a luminance Li larger than the first threshold Tl .

33. A perimetry device comprising a perimeter screen (12), a stimulus generator (15), and a control unit (22), wherein the control unit (22) is adapted to carry out the method of any of the preceding claims.

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