WO2015107373A1 - Appareil ophtalmique - Google Patents

Appareil ophtalmique Download PDF

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Publication number
WO2015107373A1
WO2015107373A1 PCT/GB2015/050122 GB2015050122W WO2015107373A1 WO 2015107373 A1 WO2015107373 A1 WO 2015107373A1 GB 2015050122 W GB2015050122 W GB 2015050122W WO 2015107373 A1 WO2015107373 A1 WO 2015107373A1
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Prior art keywords
pupil
light
patch
eye
boundary
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PCT/GB2015/050122
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English (en)
Inventor
Peter West
Alan Robinson
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Keeler Limited
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Publication of WO2015107373A1 publication Critical patent/WO2015107373A1/fr

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    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B3/00—Apparatus for testing the eyes; Instruments for examining the eyes
    • A61B3/10—Objective types, i.e. instruments for examining the eyes independent of the patients' perceptions or reactions
    • A61B3/103—Objective types, i.e. instruments for examining the eyes independent of the patients' perceptions or reactions for determining refraction, e.g. refractometers, skiascopes
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B3/00—Apparatus for testing the eyes; Instruments for examining the eyes
    • A61B3/10—Objective types, i.e. instruments for examining the eyes independent of the patients' perceptions or reactions
    • A61B3/1015—Objective types, i.e. instruments for examining the eyes independent of the patients' perceptions or reactions for wavefront analysis

Definitions

  • This invention relates to ophthalmic apparatus for use in determining refractive errors in an eye.
  • the invention is particularly applicable to apparatus that can provide an automated measurement of occular refractive power.
  • retinoscopy One method of objective assessment of refractive error is retinoscopy. This involves sweeping a narrow collimated beam of light over a pupil and assessing the movement of the retinal reflex in comparison with movement of the incident beam. The method is typically used by clinicians in conjunction with trial lenses to determine which lens would adequately compensate for the patient's refractive error. Although retinoscopy does not require feedback from the patient, it does call for considerable skill on the part of the clinician.
  • US7,001,020 shows an auto-refractor system in which an illumination system projects a spot of light onto the retina of an eye to be examined, and light reflected from the retina is relayed to a common sensor along two paths of different lengths.
  • the sensor defines, therefore, two measurement plane conjugates (each corresponding to a respective path), which are respectively situated in front of and behind the retina.
  • the data from the sensor is then processed so as to determine refractive error.
  • the various embodiments shown in Yancey have a limited dynamic range (i.e. are only linear over a limited range of errors), and require that only a small retinal beacon (i.e. spot projected onto the retina) be used.
  • Curvature sensing is another technique for measuring wavefront distortion (from which refractive error can be determined). It was originally developed by Roddier (Applied Optics, vol. 27 no. 7 ppl223-1225, April 1998) to apply adaptive optics to astronomical telescopes.
  • An example of ophthalmic apparatus using this sort of approach is shown in US 6,439,720 (Graves et al), in which light reflected from the retina of an eye is relayed to a common sensor along two different optical paths, so that the sensor defines a measurement plane having two conjugates, each corresponding to a respective path, situated on either side of the pupil.
  • the difference in intensity between the two measurement planes is related to the curvature of the wavefront of the pupil, and differential displacement of the pupil boundary in the two images are proportional to the normal component of wavefront slope at the boundary.
  • the wavefront distortion at the pupil is computed by combining the interior curvature and boundary slope information. As with Shack-Hartman measurements, the computational requirements for this analysis are relatively high.
  • ophthalmic apparatus for use in determining refractive errors in an eye, the apparatus comprising illumination means for projecting light on to the retina of the eye to be reflected or scattered from the retina and transmitted back through the pupil of the eye; optical relay means for relaying said light from the eye to sensor means for detecting characteristics of the light incident thereon to enable refractive errors to be determined, wherein the optical relay means and sensor means are so arranged that the sensor means is operable to provide an output for use in determining the transverse extent of the transmitted light projected onto two measurement positions, intersecting a direction, or respective direction, of propagation of the transmitted light at different locations therealong, the positions being situated in the near field of the pupil or a pupil conjugate and distant from a retinal conjugate.
  • a retinal conjugate is distant from a measuring position, then features of the retina which would be in focus at that conjugate are not imaged at the measurement position in question.
  • the measurement positions can be so arranged that features of the retina do not have a detrimental effect on the analysis of the effect of the eye lens on the light reflected or scattered from the retina.
  • the measurement positions may be on a common plane, defined for example by a single sensor, the light from the eye under examination being split into two branches which are then directed to the common plane along paths of differing lengths.
  • each measurement position may comprise a respective measurement plane.
  • the sensor means may comprise two sensors, each defining a respective one of the measurement planes.
  • each measurement plane is situated closer to the pupil or a pupil conjugate than it is to a retinal conjugate.
  • ophthalmic apparatus for use in determining refractive errors in an eye, the apparatus comprising illumination means for projecting light onto the retina of the eye to be reflected from the retina and transmitted through the pupil of the eye and back to the apparatus; optical relay means for relaying said light from the eye to sensor means for detecting characteristics of the light incident thereon in at least one measurement plane to enable refractive errors to be determined, wherein the relay means defines two conjugates for said at least one measurement plane, each of which is situated in the path of the light from the retina to the apparatus and is closer to the pupil than it is to the retina.
  • the sensor means may define two measurement planes, each corresponding to a respective conjugate, the optical relay means being arranged so that, in use, each measurement plane is closer to a conjugate of the pupil than it is to the closest retinal conjugate.
  • Measurements in planes which are close to a pupil conjugate permit a substantially linear response over a wide range of refractive errors.
  • Such measurements can also tolerate relatively large diameter retinal beacons, either arising directly from the use of an extended illumination source such as a surface-emitting LED, or due to broadening of the retinal spot when the refractive error of the subject's eye is large. In the latter case, the need for tracking optics can be avoided.
  • the measurement plane conjugates of the apparatus when in use, are situated in the near field of the pupil of the eye.
  • the near field may be constituted by the range of distances from the pupil for which the lateral extent of the defocussed image at each conjugate is determined predominently by beam size and vergence of the light transmitted by the eye, and not by diffraction or the size of the patch of light being projected onto the retina.
  • the near field distance of each measurement plane conjugate from the pupil is less than 60mm, is in the range of l-20mm or is less than 15mm.
  • the conjugates of the measurement planes of the apparatus are, when in use, situated on either side of the pupil.
  • potential systematic errors in the measurement are minimised when the measurement planes are each located at substantially equal distances to either side of a pupil conjugate.
  • the sensor means is connected to processor means for analysing the output of the sensor means to determine said characteristics, the processor means being programmed or arranged to analyse said output in such a way that said characteristics are, or are indicative of, the lateral extent, in at least one direction, of the defocussed patches of light defined by the light incident on the sensor means, each said patch corresponding to the light passing through a respective measurement plane conjugate.
  • the processor meams is programmed or arranged to determine said characterstics by a process comprising analysing substantially all points in a patch defined by said incident light.
  • the analysis comprises identifying the points at the periphery of each patch, to enable the radial extent of each patch in at least two directions to be determined.
  • the processor means may to advantage be so programmed or arranged that said analysis comprises the steps of:
  • the average radius may be the RMS radius of the points within the patch.
  • the processor means is programmed or arranged to refine an estimate of the co-ordinates of the centre of the patch, using co-ordinates of the patch boundary.
  • the processor means is also programmed or arranged to determine astigmatic errors in the eye, using data relating to the difference in shape of the patches defined by light incident on the sensor means, and to determine spherical refractive error by comparing the lateral extent of the patches defined by the light incident on the sensing means.
  • the sensor means comprises a two-dimensional sensor array of pixels, each pixel corresponding to a respective point in a patch of light incident on the array.
  • the sensor means may conveniently comprise two such arrays, each of which defines a respective measurement plane.
  • the illumination means is adjustable so as to adjust the vergence of light projected into the eye.
  • a collimated illumination beam is suitable for eyes having low refractive errors, but altering the vergence of the illuminating beam enables eyes with larger refractive errors to be tested using the apparatus.
  • the optical relay means preferably comprises a telecentric imaging system for forming at least one real image of the pupil.
  • the relay optics can be configured to increase the working distance between the instrument and subject, and facilitate both retinal illumination and provision of a fixation target on which the subject can focus.
  • the telecentric imaging system also greatly reduces the sensitivity of the apparatus to longitudinal alignment variations (at least for eyes having low refractive errors).
  • the measurements of pupil size and shape are, in effect, only made at the pupil boundary, they are highly tolerant of intensity perturbations within the pupil, and in particular of Fresnel reflections from the front surface of the cornea.
  • the illumination means is operable to provide pulsed or modulated illumination of the eye whereby the retina is illuminated for an exposure time which is sufficiently short for inaccuracies in refractive error measurements due to fluctuation in gaze, saccades or changes in accommodation of the eye to be mitigated or avoided.
  • each exposure time is preferably less than 100 ms, more preferably less than 20 ms.
  • the invention also lies in ophthalmic apparatus for use in determining refractive errors in an eye by comparing two patches each defined by light incident on at least one measurement plane, said at least one measurement plane having two conjugates spaced apart along the path of light from the eye to the apparatus, the apparatus comprising illumination means for projecting light onto the retina of the eye, sensor means, located at said measurement plane or planes for detecting patches defined by incident light reflected or scattered from the retina, and processor means connected to the sensor means and arranged or programmed to determine the lateral extent, in at least one direction, of each patch by a process comprising analysing substantially all points in each patch and using this information to identify an effective boundary for each patch.
  • the effective boundary may for example be defined by the points of the patch the intensity of which is a given fraction of the average intensity of the points within the patch.
  • the boundary may be defined by the points of the patch, the intensity of which is a given fraction of the local intensity near to the boundary and within the patch. In this case, there is no need to analyse substantially all points within the patch provided the location and approximate extent of each patch on the sensor means is known with sufficient accuracy.
  • each point in the patch may correspond to a respective pixel of the array, but the points defining the boundary may be such pixels or positions intermediate to adjacent pixels, determined by a process of interpolation.
  • Figure 1 is a ray diagram illustrating the operation of a first embodiment of apparatus in accordance with the invention
  • Figures 2-4 show how an eye the retina of which has been illuminated by the apparatus transmits light rays respectively in the case of an emmetropic eye, a hyperopic eye and a myopic eye;
  • Figure 5 is a more detailed ray diagram of an eye illuminated by the apparatus, showing how some of the illuminating light undergoes specular reflection from the cornea of the eye;
  • Figure 6 is a side view of a second embodiment of apparatus in accordance with the invention.
  • Figure 7 is a corresponding view of a third embodiment of apparatus in accordance with the invention, showing an alternative configuration of light source and optical elements for constituting the illumination means of the apparatus;
  • Figure 8 shows simulated intensity profiles for images of a 4mm diameter pupil illuminated by a cone of converging rays at the pupil conjugate (at which the image is focussed) and at various defocussed planes either side of the conjugate;
  • Figure 9 is a more detailed view of a portion of the profiles shown in Figure 8.
  • Figure 10 illustrates a patch of light as detected at a measurement plane, showing a central portion of substantially constant intensity and a boundary region (at which blurring of the patch would be perceived by the viewer) at which the intensity progressively diminishes in a direction away from the patch centre, the diagram also illustrating the location of a boundary point between neighbouring pixels in the outer region of the patch;
  • Figure 11 corresponds to Figure 10, and illustrates a refinement to the approach of determining the location of the boundary
  • Figure 12 is a ray diagram showing converging rays which are being transmitted from an illuminated eye (which is myopic) and also shows measurement plane conjugates.
  • the apparatus shown in Figure 1 is for use in measuring refractive error in an eye 1 by a process which involves projecting a beam 2 of collimated light through the pupil 4 and thence onto the retina 6 of the eye 1. This forms a retinal beacon from which light is scattered in all directions, illuminating the back of the pupil 4, through which some of that light then passes back to the apparatus.
  • Optical relay means 8, 10 and 12 supplies this light to two sensors 14 and 16 which may be CMOS or CCD arrays, for example, and which define two measurement planes which are situated close to respective pupil conjugates 18 and 20 respectively situated in front of and behind the associated measurement plane. It will be appreciated that the relay means also define a respective measurement plane conjugate for each of the measurement planes 14 and 16, the conjugates being situated close to, and on either side of, the pupil 4.
  • processor means comprising a programmable microprocessor which will analyse those signals in a way described below.
  • the illumination means comprises a light source in the form of an LED 22 which emits light at near infra-red wavelengths (between 780nm and lOOOnm). Near infrared light at wavelengths between 780 nm and 1000 nm is reflected from the retina more strongly than shorter visible wavelengths, and poses a negligible photochemical hazard. Moderately high powers may be used which are well within accepted safety limits (specifically ISO 15004-2: 2007), with no risk or discomfort to the subject, and which do not perturb the subject's response to a fixation target or fogging means employed to control accommodation.
  • the illumination means further comprises a collimating lens 24 which collimates light emitted by the LED 22 (situated at the focus of the lens 24), and a beam splitter 26 which may be in the form of a half-silvered mirror but is preferably a polarising beam splitter, which reflects the collimated illuminating light into the eye 1.
  • the optical relay means comprises two lenses 8 and 10 which capture the exit beam of light (depicted by rays 28 and 30 in Figure 1), after those rays have passed through the splitter 26, and bring light from the pupil 4 to a focus at the pupil conjugate 20.
  • a further beam splitter 12 is interposed between the lens 10 and measurement plane 16, so as to reflect a portion of that light up to the measurement plane 14 via the second pupil conjugate 18. It will be appreciated that a focussed image of the pupil is formed at a conjugate 18, but not the conjugate 20 since in the latter case, the conjugate is behind the sensor array 16.
  • the relay means is preferably telecentric, so that when low vergence rays from the pupil of an emetropic eye are focused at the conjugates 18, 20, the rays passing through the pupil conjugates also have low vergence. For the configuration in figure 1, this is achieved when the respective principal planes of lenses 8 and 10 are separated by a distance equal to the sum of the focal lengths of the two lenses.
  • An aperture 32 is placed between the lenses 8 and 10, near to a retinal conjugate, and together with the polarising beam splitter helps to block specular reflections from the cornea of the eye 1.
  • the incident light 38 is reflected from the polarising beam splitter 26, it remains in its polarised state on reflection from the cornea 40, so that the specular reflections are predominantly reflected rather than transmitted by the beam splitter 26.
  • the aperture 32 is chosen so that the rays (for example 46 transmitted from the retinal beacon through the pupil 4) are not obstructed, but that more divergent rays from the corneal reflex (that may have managed to pass through the beam splitter 26) are blocked.
  • the aperture does not prevent all the light from the corneal reflex from reaching the measurement planes defined by the sensor arrays 14 and 16, but will generally restrict the resultant bright spot to a restricted disc near the centre of each defocussed pupil image so that measurements near the pupil boundary are not affected.
  • light from the source 22 is collimated by the lens 24 and projected through the pupil of the eye which focuses the light onto a small region of the retina 6 to define a retinal beacon from which light is scattered in multiple directions.
  • Light from the beacon which passes out of the eye will be affected by the refractive properties of the eye, and Figures 2-4 show the effect of the refractive properties on the exiting beam of light for different eyes.
  • the exit beam (depicted by rays 28) is collimated, as shown in Figure 2.
  • the diameter of the beam is equal to the diameter of the virtual image of the pupil 4 viewed through the cornea 48.
  • the exit beam 28 diverges, the diameter of the beam increasing with distance from the eye 1.
  • the vergence of the beam can be calculated by measuring the beam diameter at the measurement planes defined by the arrays 14 and 16, and which in this example have conjugates at 50 and 52. It will be appreciated that the apparatus of Figure 1 can be set up so that the plane 50 is positioned behind the pupil 4, whilst the plane 52 is positioned in front of the pupil 4, i.e. between the eye 1 and the apparatus.
  • Figure 4 shows a myopic (short-sighted) eye with the opposite sign of refractive error from the eye shown in Figure 3.
  • the exit beam 28 converges so that the diameter measured in the plane having the conjugate 52 will be smaller than that measured in the plane having the conjugate 50 closer to the eye 1. Since eye accommodation and pupil diameter can change within less than a second, the apparatus in Figure 1 is set up to measure the beam dimensions in the two different measurement planes simultaneously.
  • the conjugates 50 and 52 are shown at the same side of the pupil 4, it is advantageous to measure the de-focussed pupil image in planes situated symmetrically to either side of the exact focus position (i.e. with one measurement plane conjugate behind the pupil 4 and the other in front of the pupil 4).
  • the embodiment shown in Figure 6 has many features which correspond to those of the embodiment shown in Figure 1, and these are indicated by the reference numerals of Figure 1, raised by 100, as are the structures of the eye 101 under examination.
  • the beam splitter 112 is positioned in front of the lens 110, so that a further lens 115 is provided (between the beam splitter 112 and the array 114) in order to project the de-focussed patch of light on to the array 114.
  • the telecentric lens pairs 108, 110 and 108, 115 have a smaller magnification than the corresponding components of the Figure 1 apparatus, allowing the use of inexpensive COMS or CCD photodiode sensor arrays such as those developed for digital cameras.
  • An imaging system with transverse magnification which is less than unity requires that the numerical aperture of the optical relay means increases in inverse proportion to the decreased in magnification.
  • the design of the lenses 108, 115, 110 and 124 is chosen from known designs that keep optical aberrations at acceptably low levels. This typically results in a decreased working distance between each pupil conjugate 118 and 120 and the respective lens 115 and 110.
  • the embodiment shown in Figure 6 provides a fixation target on which the subject attempts to focus.
  • the target may, for example, be an illuminated slide or screen.
  • the fixation target is denoted by reference numeral 158, and takes the form of a slide or screen and a suitable source of visible light for illuminating the slide or screen.
  • Light from the target 158 is directed towards the subject by a further beam splitter 160 positioned behind the lens 108.
  • the beam splitter 160 is preferably a dichroic filter, polarising beam splitter or equivalent device which transmits wavelengths scattered from the source 122 via the retinal beacon, but reflects wavelengths from the fixation target 158.
  • the target 158 is adjustable by being displaced along the direction of propagation of light therefrom (i.e. in either direction indicated by the double-headed arrow 160). This enables the fixation target to be located beyond the ideal focus for distance vision (which will vary from one eye to the other), which is helpful in minimising fluctuations in accommodation, a procedure known as fogging. Variations including a progressive change of the refractive error of the fixation image may also be used to counter accommodation.
  • the source 158 may be moved in the directions indicated by arrow 160 by any suitable control means, for example by means of a manual, mechanical adjustment mechanism or an electronic system using a stepper motor. A similar arrangement may be used to move the lens 124 in either direction indicated by the arrow 162 and the source 122 in either direction indicated by the arrow 164.
  • the illumination means of the apparatus shown in Figure 7 includes a further lens 268 interposed between a pair of apertures 270 and 272.
  • the beam splitter 226 is positioned on the other side of the lens 208 from the eye 201 so that the beam of illuminating light also passes through the lens 208 on its way to the eye 201.
  • the aperture 272 is close to a retinal conjugate and its diameter and the focal length of the lens 208 determine the diameter of the beacon spot on the retina 206.
  • Aperture 270 is also close to a pupil conjugate and its diameter and focal length and positions of lenses 208 and 268 determine the diameter of the incident beam 202 as it enters the pupil 204.
  • the incident beam 202 it is advantageous for the incident beam 202 to overfill the pupil so that the illumination of the retinal beacon is insensitive to misalignment between the subject's eye and the apparatus.
  • the scattered light which exits the pupil 204 may suffer vignetting or excessive aberrations within the apparatus, problems which are avoided or at least mitigated by restricting the width of the incident beam.
  • An alternative means to control the divergence of light from the pupil at high refractive errors is to adjust the focus of the collimation of the illumination beam, for example by changing the separation between light source 40 and lens 42 in Figure 6, so that the light source is focused onto the retina.
  • the described embodiments do not require a very small diameter retinal beacon, so sufficient power for an acceptable signal to noise ratio can be coupled from a conventional surface-emitting light emitting diode (LED) source.
  • a moderately large retinal beacon area is beneficial in permitting operation at higher power levels without exceeding retinal exposure limits such as those specified in ISO 15004-2: 2007.
  • Retinal beacon diameters between 0.1 mm and 2 mm are expected to provide good performance, although successful operation outside this range is feasible. The optimum beacon diameter will depend on the range of refractive errors likely to be encountered and the detailed design of the instrument. Factors include whether focus tracking is implemented for retinal beacon or relay optics, operational parameters including the subject's pupil diameter, and the tolerance to longitudinal and transverse alignment errors.
  • the light source can be switched or modulated with a response time of a few milliseconds or less.
  • the source is pulsed to illuminate the retina for an exposure time which is preferably less than 100 ms, and more preferably less than 20 ms, in order to reduce image degradation due to fluctuation in gaze, to saccades, and to changes in accommodation.
  • the cameras are operated with their shutters open for at least the period in which the source is on. Alternatively, the camera shutters can be synchronized to open simultaneously.
  • Figure 8 shows simulated intensity profiles of images of a 4 mm diameter pupil, illuminated by a cone of converging rays.
  • the vergence is 12 dioptre at the pupil, corresponding to -12 D refractive error according to the usual ophthalmic convention.
  • the intensity profile 274 at the pupil conjugate has a sharp outer boundary between illuminated and dark regions.
  • the intensity profiles 276, 278, 280 and 282 are for distances -10, -5, +5 and +10 mm from the pupil conjugate, and show a more gradual transition between inner and outer zones.
  • Figure 9 is an expanded view of the same profiles, showing that the width of the transition zone increases with the magnitude of the defocus distance.
  • Profiles 278, 280 at -5 and +5 mm exhibit a slight over-shoot inside the outer perimeter. This is caused by Fresnel diffraction which also broadens the boundary width in proportion to the square root of the product of distance from pupil conjugate and wavelength of the light. At larger defocus distances, geometric blur proportional to the product of retinal beacon diameter and refractive error dominates. In this simulation the nominal diameter of the retinal beacon was 0.5 mm, with the retinal spot blurred by an additional 0.4 mm due to the 12 D refractive error and an incident collimated beam width of 2 mm at the pupil.
  • the relationship between the width of the translation zone and the defocus distance can be used to estimate the position of the pupil conjugate. This, in turn, enables longitudinal alignment errors of the instrument relative to the eye under examination to be calculated.
  • Many alternative means to measure the transverse extent or diameter of the defocused pupil images will be apparent to those skilled in the art. These include, but are not limited to: linear arrays of photodetectors, one or more scanned detector, means to project a portion of each image onto a stationary detector via a scanning mirror, or slits or edges scanned across the image with one or more detectors to measure the transmitted light.
  • the intensity distribution of the light transmitted from the pupil is recorded in each measurement plane by a two-dimensional array of photo detectors, including those manufactured using CMOS or CCD technology and incorporated in digital cameras.
  • the sensors comprise multiple sensor elements arranged on a regular rectangular array, each of which defines a respective measurement plane.
  • the mean square radius of all points within the image, or alternatively the second moment of the intensity distribution, is:
  • the intensity within the pupil image boundary is constant, apart from a slight blurring of the boundary due to diffraction, as shown for intensity profile 274 in figure 9.
  • the intensity is zero outside the boundary.
  • This approximation can be used to estimate the vergence of the light transmitted from the pupil, but is subject to systematic errors if the intensity distribution within the pupil boundary is not uniform.
  • Figure 10 illustrates a more robust method of measuring the transverse extent, applied to a defocused pupil image comprising an inner zone 290 in which intensity is substantially uniform, an outer zone 292 in which the intensity of illumination is much lower, and a boundary zone 294 in which the intensity falls with increasing distance from the inner zone.
  • the recorded intensities at successive points along a radius 295 from the estimated centre of the image 296 are compared until a location is found where the intensity falls below a selected threshold value, typically chosen to be a fraction, / (between zero and unity) of the mean intensity.
  • the coordinates of point 302 at which the intensity is equal to the selected boundary threshold are estimated by interpolation, for example using:
  • the boundary location procedure is repeated for directions oriented at different azimuthal angles, A*.
  • A* azimuthal angles
  • an improved estimate of the pupil image boundary radius is given by the average of the radial distances over all K directions:
  • K k yoB yoM +— ⁇ ( r k - R B )si n A k (I 4 )
  • Figure 12 shows rays converging to a focus at distance zv along the axis.
  • the vergence at distance z 1 is equal to the reciprocal of the distance to the focus, and may be calculated from the image diameter and the rate of change of diameter between the measurement planes at zi, z 2 .
  • the spherical vergence mid-way between the two measurement planes is calculated from the boundary radius measurements derived above using:
  • the astigmatic component of vergence is similarly calculated from the differential elongation of the pupil in azimu hal directi ns 0 and 45°.
  • V s - V c Minimum vergence (V s - V c ) is at azimuthal angle A c +90°.
  • the radius estimate is sensitive to the boundary threshold selected.
  • the threshold intensity is calculated using equation (7), results are sensitive to variations in intensity within the pupil image. These could arise from random noise, or from speckle in the reflected light if the source is partially coherent. An incompletely suppressed corneal reflex will increase the intensity near the centre of the pupil image.
  • there are systematic variations of scattered intensity such as that associated with the Stiles-Crawford effect, which increases the relative intensity of light scattered back towards the direction of incidence, i.e. towards the centre of the pupil.
  • Tolerance to systematic variations of intensity across the pupil is improved if the threshold is related to a local measurement of intensity near to each boundary measurement, rather than to a single sample, or to an average over all or a selected part of the illuminated region.
  • Figure 11 shows a pupil image again containing an inner illuminated zone 290, an outer dark zone 292, and a boundary transition zone 294.
  • a local reference intensity P k,re f is measured, preferably by averaging the intensity over a small local area 304 near to, but inwards from the edge of the illuminated region.
  • a local threshold intensity is calculated which is a fraction, /, of the reference.
  • the diameter of the reference area 304 is typically comparable with, but smaller than the transition zone width.
  • An RMS blur radius equal to half the Fresnel distance may be suitable, as described by equation (38) below. If the entire image is low-pass filtered before boundary tracing, additional averaging of the reference regions is acceptable, but may not be necessary.
  • Equations (20-21) apply directly for measurements close to the subject's pupil, and for telecentric imaging at unit magnification. More commonly, the transverse magnification of the subject's pupil, m, will be less than unity. In this case, the magnitude of the vergence at the physical pupil will be smaller than that at the pupil image by a factor m .
  • Vs.M - — ⁇ (36)
  • the cylinder component Figures 8 and 9 show a gradual transition between the illuminated inner zone of the pupil image 290, and the dark outer zone 292.
  • the width of the transition zone 294 depends on Fraunhofer diffraction limited by the numerical aperture of the relay optics, Fresnel diffraction proportional to the square root of the distance from the pupil conjugate and the wavelength, and the geometric blur due to the finite size of the illuminated spot on the retina 6.
  • the boundary threshold intensity in equations (9, 10) should correspond to the intensity at the 'ideal' boundary radius.
  • the ideal threshold scaling factor, / is 0.25.
  • the ideal threshold scaling factor, / is 0.50.
  • the factor/ should be adjusted according to the diameter of the retinal beacon, the estimated refractive error and the width of the transition zone in each image (which in turn depends on the longitudinal offset, z). It depends on the ratio of the magnitude of blur from Fresnel diffraction to the blur from other mechanisms. Suitable values can be found by measurement or by computer simulation. In the course of ocular measurements, values for /may be estimated from polynomial approximations to pre-computed values or interpolated from values stored in a look-up table. In many circumstances, particularly when the diameter of the retinal beacon is relatively large (for example greater than 0.1 mm), a fixed value for/ between 0.45 and 0.50 may give acceptable accuracy and reproducibility.
  • the recorded images will contain random noise in addition to the desired intensity information. Potentially deleterious effects of such noise are mitigated by low pass filtering or smoothing the images. Excessive filtering should be avoided, as this will broaden the edge transition, and will also change the optimum boundary threshold/.
  • a blur radius equal to half the Fresnel blur distance is a reasonable value which should not broaden the edge transition substantially.
  • z is the distance of the sensor from the pupil conjugate
  • ⁇ is the wavelength of the illumination
  • the intensity outside the pupil boundary 292 should be close to zero.
  • ambient light can raise the background level at the sensor, and distort the boundary measurement.
  • the average background intensity For a uniform background it is straightforward to estimate the average background intensity and subtract this from the image. A varying background intensity is more likely to cause problems.
  • One option is to modify the procedure above for each step around the pupil boundary to include a measurement of the intensity Pu , b ⁇ at a point 308 outside the transition zone, 294, but close to the interpolation points 298, 302 shown in figure 11.
  • the modified threshold value in place of equation (23) is:
  • the location of the reference measurements 304, 308, can be set at fixed radial offsets from the most recently estimated boundary location, 302. This risks use of unrepresentative values if the offset is too large, or systematic distortion if the offset is too small.
  • a better solution is to make a first analysis using conservative (i.e. large) offsets, in which the normalised boundary slope is calculated for each boundary point k: p (Xa > y a ) - p ( x c , y c -cos tan -i (40)
  • a second analysis of each image uses radial offsets for the reference intensities which are equal to, or a small multiple of the average boundary width.
  • the width of the pupil image boundary is proportional to the axial distance from the pupil conjugate.
  • the negative sign reflects the ophthalmic convention that refractive error has the opposite sign to vergence.
  • the estimated position of the pupil conjugate may also be used to calculate the relative contributions of Fresnel diffraction and geometric blurring of the pupil boundary for each image, so that a more accurate boundary threshold scaling factor can be calculated for each image.
  • a feature of this invention is that it relies on 'near field' measurements of intensity close to a pupil image.
  • the measurement plane is 'near' in the sense that the diameter of the defocused image is determined predominantly by the pupil size and beam vergence.
  • the boundary is blurred by the finite size of the retinal beacon and by diffraction, but these have only a second order effect on the measured diameter.
  • the retinal conjugate is very distant.
  • the geometric blur at distance 50 mm from the pupil is 0.29 mm.
  • Fresnel diffraction blur is around 0.21 mm for 850 nm illumination. This blurring is only a fraction of the pupil diameter which might be in the range 2 to 7 mm, depending on the ambient illumination and the subject.
  • the image diameter at 50 mm is reduced to half the diameter at the pupil conjugate, and may result in significant measurement distortion for smaller pupil diameters.
  • Instruments of this type are often expected to measure refractive errors of 15 D or larger. Linearity over the full measurement range is improved by using smaller defocus distances in the range 1-20 mm at unit magnification.
  • the corresponding sensor displacement range when imaging at magnification 0.5 would be 0.25 to 5 mm. Smaller defocus distances are preferred when a large diameter retinal beacon diameter is employed.
  • One option to improve dynamic range while preserving high sensitivity is to track the refractive errors in the subject's eye by adjusting the collimation of the retinal beacon by adjusting the distance between source 122, 222 and lens 124 or 262 by translation means in Figures 6, 7, and by varying the separation of lenses to maintain low vergence at the pupil conjugates.
  • the measured refractive error now depends on the distance between lenses 108 and 110, or 208 and 210, which are measured and calibrated appropriately, either by simulation or by direct measurement of suitable calibration artefacts such at those specified in appendix A of ISO 10342:2010.
  • Some ophthalmic instruments use a laser source or super-luminescent (SLD) edge- emitting diode source (sometimes referred to as ELED sources). These are high brightness devices, capable of projecting substantial optical powers into a small area of the retina. When used directly with a simple lens-based collimator, as shown in Figures 6 and 7, their high spatial coherence leads to significant interference between light scattered from adjacent parts of the retinal beacon. This manifests as strong variations of intensity in the pupil images, so that robust high accuracy measurement of the pupil image boundary location is more difficult. For SLD or ELED sources, spatial coherence is reduced by coupling light from the diode source to the collimating optics using a multi-mode optical waveguide with high modal dispersion.
  • SLD super-luminescent
  • ELED sources super-luminescent
  • the same principle can be applied to laser sources, but these typically have much higher temporal coherence than light emitting diode sources, and so require unfeasibly long waveguides to de-cohere the illumination. Exceptions include lasers which support multiple longitudinal modes and exhibit a relatively broad spectral width.
  • Another option to improve dynamic range and sensitivity is to provide multiple sensors at different defocus distances. Those at short distances will provide accurate measurements at large refractive errors, with larger defocus distances capable of more sensitive measurement when refractive errors are less extreme.
  • An additional advantage of collecting data at multiple defocus distances is that the planes closest to the pupil conjugate can be used when the longitudinal alignment varies appreciably, as may be the case in a hand-held instrument.
  • An apparatus to measure the refractive error of an eye comprising means to project a spot of light onto the eye's retina so that light reflected from said retinal spot is transmitted through the eye's pupil and comprising means to measure the transverse extent of the transmitted light projected onto at least two planes each intersecting the direction of propagation at different locations along the direction of propagation, and located in the near field of the pupil or a pupil conjugate and distant from a retinal conjugate.
  • An apparatus according to paragraph 1 comprising a means to form an image of the subject's pupil.
  • An apparatus as described in paragraph 1 in which the distance of the measurement planes from the pupil is less than 60 mm.
  • An apparatus as described in paragraph 1 in which the distance of the measurement planes from the pupil is less than 15 mm.
  • An apparatus as described in paragraphs 2 or 3 in which the distance of the measurement planes from the pupil conjugate divided by the square of the transverse magnification of the apparatus is less than 60 mm.
  • An apparatus as described in paragraphs 2 or 3 in which the distance of the measurement planes from the pupil conjugate divided by the square of the transverse magnification is less than 15 mm.
  • An apparatus as described in paragraphs 2 or 3 in which at least one measurement plane is located before the pupil image and at least one other measurement plane is after the pupil image along the direction of propagation of light from the pupil.
  • An apparatus in which the extent of the transmitted light transverse to the mean direction of propagation is measured in at least two planes and from said measurements the vergence of the light transmitted out of the pupil is calculated from the rate of change of transverse extent with displacement in the mean direction of propagation.
  • An apparatus in which the transverse extent of the transmitted light is measured using five or more locations around the perimeter of the illuminated region in each plane, from which the vergence is determined for multiple meridional planes, each containing the mean direction of propagation but oriented at different azimuthal angles.
  • An apparatus according to paragraphs 9 or 10 in which the transverse extent of each pupil images is measured by identifying the location of the pupil boundary at multiple points around said boundary.
  • An apparatus according to paragraphs 10 or 11 in which sphere and cylinder components of refractive error are calculated.
  • An apparatus according to any of paragraphs 2 to 12 in which the rate of change of intensity at the edge of the defocused pupil image is estimated in at least two measurement planes, from which the location of the pupil conjugate is estimated.
  • An apparatus according to any of paragraphs 2 to 12 in which the width of the transition zone at the edge of the defocused pupil image is measured in at least two measurement planes, from which the location of the pupil conjugate is estimated.
  • An apparatus according to paragraphs 13 or 14 in which an estimate of the longitudinal alignment error is calculated and an indication of the direction and magnitude of the error displayed for use by the operator.

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Abstract

La présente invention concerne un appareil ophtalmique destiné à être utilisé dans la détermination d'erreurs réfractives dans un œil (1; 101; 201), comprenant des moyens d'illumination (par exemple, 22, 24 et 26) pour projeter de la lumière sur la rétine de l'œil. La lumière réfléchie par la rétine et transmise à travers la pupille de l'œil est reçue par l'appareil et relayée par des moyens de relais optique (8, 32, 10, 12; 108, 112, 110 et 115; 208, 232, 212, 210 et 215) vers des capteurs (14, 16; 154, 156; 254, 256) pour détecter des caractéristiques de la lumière incidente sur ceux-ci dans au moins un plan de mesure afin de permettre que des erreurs réfractives soient déterminées. Les moyens de relais définissent deux positions de mesure dont chacune est située dans le trajet de la lumière depuis la rétine vers l'appareil et est dans le champ proche du conjugué de pupille (4; 104; 204) et distante d'un conjugué rétinien. De plus ou en variante, les zones de lumière peuvent être analysées selon un procédé comprenant l'analyse de sensiblement tous les points dans chaque zone et l'utilisation de ces informations pour identifier une limite effective pour chaque zone.
PCT/GB2015/050122 2014-01-20 2015-01-20 Appareil ophtalmique WO2015107373A1 (fr)

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CN110192840A (zh) * 2018-02-27 2019-09-03 皇家飞利浦有限公司 获得用于确定对象的皮肤的一个或多个特性的图像

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US7001020B2 (en) * 2001-08-02 2006-02-21 Daphne Instruments, Inc. Complete autorefractor system in an ultra-compact package
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WO2013165689A1 (fr) * 2012-04-30 2013-11-07 Clarity Medical Systems, Inc. Capteur de front d'onde ophtalmique fonctionnant en mode échantillonnage parallèle et en mode détection synchrone

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US6439720B1 (en) * 2000-01-27 2002-08-27 Aoptics, Inc. Method and apparatus for measuring optical aberrations of the human eye
US7001020B2 (en) * 2001-08-02 2006-02-21 Daphne Instruments, Inc. Complete autorefractor system in an ultra-compact package
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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN109640788A (zh) * 2016-08-31 2019-04-16 株式会社尼康 基于手机的眼底相机的广角光瞳中继器
JP2019526346A (ja) * 2016-08-31 2019-09-19 株式会社ニコン 携帯電話ベースの眼底カメラのための広角瞳孔リレー
US11717161B2 (en) 2016-08-31 2023-08-08 Nikon Corporation Wide-angle pupil relay for cellphone-based fundus camera
CN110192840A (zh) * 2018-02-27 2019-09-03 皇家飞利浦有限公司 获得用于确定对象的皮肤的一个或多个特性的图像
CN110192840B (zh) * 2018-02-27 2023-12-22 皇家飞利浦有限公司 获得用于确定对象的皮肤的一个或多个特性的图像

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