Patent Description:
Determining the refractive error of an eye is an important part of eye examinations, the result of which is for example used to produce lenses for eyeglasses with appropriate optical properties for the respective eye or to prescribe appropriate contact lenses. In the field of ophthalmology, the refractive error of an eye is usually given in terms of sphere, also referred to as spherical power, as defined in <NUM> of DIN EN ISO <NUM>:<NUM>-<NUM>, cylinder, also referred to as cylindrical power, as defined under <NUM> of DIN EN ISO <NUM>:<NUM>-<NUM> and axis, more precisely cylinder axis, as defined under <NUM> of DIN EN ISO <NUM>:<NUM>-<NUM>. These values give essentially the deviations from an emmetropic eye (zero refractive error). Values of <NUM> for sphere and cylinder indicate that the eye is emmetropic, while values different from <NUM> indicate various kinds of ametropia like hyperopia or myopia.

A determination of the refractive error may be a subjective refractive error determination or objective refractive error determination. Methods for subjective refractive error determination are based on a (subjective) feedback of an examined person regarding his or her visual perception. An example is a measurement on the basis of eye charts with small and smaller symbols, letters, numbers or the like, where the person gives feedback which numbers, letters or symbols he or she can recognize. Lenses may then be placed in front of the eye of the person, and the person may give feedback with which lenses the best visual perception is obtained. An example for apparatuses allowing subjective refractive error determinations are phoropters. Methods and apparatuses for objective refractive error determination do not need such a feedback of the examined person regarding his or her visual perception.

Examples of apparatuses for objective refractive error determination are Hartmann-Shack-based aberrometers, as for example described in <NPL>. Such aberrometers are comparatively complex and therefore are costly apparatuses which require a camera and lenslet array for sensing wavefronts. The measurement range of such aberrometers is limited by the parameters of the lenslet array and is inversely proportional to the accuracy, i.e. a higher measurement range typically will result in lower accuracy.

Another class of apparatuses used for objective refractive error determination are referred to as autorefractors are based on principles described in Spanish patent application <CIT> and are available commercially in many forms, for example as the Zeiss Visuref <NUM> or <NUM>. Such autorefractors often exhibit a comparatively low accuracy compared to aberrometers, in particular in case of a positive refractive error, i.e. hyperopic subjects. Therefore, the result of such autorefractors is often only used as a starting point to then perform a subjective refractive error determination as a more exact measurement.

Commercial autorefractors and aberrometers are comparatively costly and have a certain size, which makes them difficult to move around. Therefore, starting from such conventional apparatuses for objective refractive error determination and corresponding methods, it is an object of the present invention to provide apparatuses and methods which are capable of remedying at least some of the drawbacks above, in particular providing a possibility for objective refractive error determination with low costs and/or with a compact design.

<CIT> discloses an apparatus for determining the refractive error of an eye using a smartphone. An image of a retina of the eye is captured and convolved with point spread functions corresponding to different defocus values. Point spread functions are determined based on images using complex mathematical operations like a neural network or using convolutions and deconvolutions. The refractive error is the determined based on the defocus of the best fitted point spread function.

<CIT> uses wavefront sensing and laser illumination to determine a refractive error of an eye.

<CIT> discloses a subjective refraction error determination method.

An apparatus for determining the refractive error of an eye as defined in claim <NUM> and a method for determining the refractive error of an eye as defined in claim <NUM> are provided. The dependent claims furthermore define a corresponding computer program, a kit for a mobile computer device and a method for producing a lens for spectacle glasses.

An apparatus for determining a refractive error of an eye is provided, comprising:.

According to the claimed invention, the apparatus is characterized in that the computing device is configured to calculate the refractive error based on a series of images of light from the eye captured by the camera with varying optical power using the series of images as at least approximate point spread functions. In this second aspect, to calculate the refractive error, for at least three different meridians corresponding to three different directions in the images captured, an optical power setting where the point spread function is sharpest (narrowest) may be determined. This is equivalent to determining the maximum of an area (integral) of the modulation transfer function (MTF) for the three meridians.

The point spread function of the optical path from the eye to the camera is an intensity distribution resulting from the eye being illuminated at a single spot. As will be explained further below, in some embodiments no point illumination device, but a more extended illumination device may be used, in which case the intensity distributions of the images represent approximate point spread function.

In this way, in contrast to for example autorefractors which require several series of images with different illuminations for different meridians, the apparatus may calculate the refractive error, for example in terms of sphere, cylinder and axis, based on a single measurement series with varying optical power. As any measurement, the measurement may be repeated to increase accuracy, but a single measurement is sufficient.

In addition, in contrast to <CIT>, the series of images of light from the eye captured is used as a point spread function, so no image of the retina needs to be captured. The term "directly" emphasizes this, i.e. point spread functions are not calculated based on retinal images using convolutions or neural networks, but the images themselves are used as at least approximate point spread functions. The term "image of light from the eye" indicates this, i.e. the images do not have the purpose of capturing structures of the eye itself, but have the purpose of capturing a light intensity distribution of light coming from the eye, e.g. in response to illumination with a light source internal or external to the apparatus as explained further below.

The optical power of the light path determines the focus with which the light from the eye is imaged to the camera and/or the size with which the light from the eye appears in the image. In some embodiments, the optical power changing device may include a variable focus optic. A variable focus optic generally is an optical device the focal length of which may varied. Such a variable focus optic may for example comprise one or more lenses, a focal length of which may be varied. Such lenses may for example include a lens with electrically variable focus as discussed in<NPL>, liquid lenses, or an optic with two or more lenses, some of which are movable with respect to other lenses. Other examples include lens- or mirror-based Badal systems (<NPL>)), deformable mirrors, spatial light modulators (SLMs) or a set of polarization-dependent lenses.

Another example for an optical power changing device causes axial movement of fixed focal length lenses within the system, or of the detector device. A range through which the optical power of the optical power changing device can be varied corresponds to a measurement range of the apparatus. By providing on optical power changing device with a wide range of optical power settings, a correspondingly large measurement range may be easily obtained. In other embodiments, the optical power setting device may be implemented in software on the computing device to computationally vary the optical power.

For determining the refractive error of an eye, the computing device may control the optical power changing device to vary the optical power, while the controlling the camera to capture images of light from the eye for each optical power setting to obtain the series of images.

In some implementations, the computing device is a mobile computer device like a smartphone and a tablet computer, and the camera may be a camera of the smartphone or tablet computer. In this way, the apparatus may be implemented at comparatively low costs by using an existing smartphone or tablet computer. Furthermore, the optical power changing device may be an autofocus device of the camera, for example camera of the mobile computer device. This may further decrease implementation costs. However, also in case of using a mobile computer device like a smartphone, a separate optical power changing device may be provided to increase a measurement range compared to an autofocus. In yet other embodiments, the optical power changing device may be implemented in software on the computing device to computationally change the optical power. A computational implementation of an optical power changing device may for example be implemented similar to the above mentioned <CIT>, with the difference that no retinal image is convolved with point spread functions representing different optical power (defocus values), but the image used as an at least approximate point spread function is convolved with point spread functions representing different optical powers.

The apparatus may further include an illumination device for creating a focal spot on the retina of the eye for the measurement, which, when back-propagated, produces point spread functions (of the point corresponding to the focal spot) for the varying optical powers on the camera. In case a non-point illumination device like a light emitting diode, LED, is used, the detector device measures essentially a superposition of all point spread functions of points of the light on the eye. This, may be seen as an approximation of the point spread function and may also be used for measurements as described herein. Therefore, a point illumination device like a focused laser is not required, but may be used for more precise measurements. Preferably, the illumination device uses infrared light, in particular an infrared laser. In this way, the person examined does not notice the light used for the measurement. Such an illumination device may be placed at various parts of the apparatus, and light may be guided to the eye using elements like mirrors, including for example a semitransparent mirror, and/or lenses.

However, due to chromatic aberration of the eye, the values of sphere, cylinder and axis thus calculated using infrared light differ from the values for visible light, for which lenses ultimately have to be designed. Therefore, a correction of this deviation due to chromatic aberration of the optical power of the eye as a function of wavelength may be performed in some embodiments, for which previously reported values of longitudinal chromatic aberrations may be used, such as in<NPL>).

In other embodiments, an illumination device external to the apparatus may be used, which may be a point illumination device like a laser or an approximation of a point source like a light-emitting diode. For example, a beam diameter of the illumination device may be below <NUM>, for example below <NUM>, preferably below <NUM>.

In the following, for simplicity's sake, reference will be made only to the point spread function, understanding that depending on the illumination used this may also be an approximate point spread function.

Several approaches may be made for determining the refractive error in terms of sphere, axis and cylinder. According to the claimed invention, for at least three different meridians corresponding to three different directions in the images captured, an optical power setting where the point spread function is sharpest (narrowest) may be determined. This is equivalent to determining the maximum of an area (integral) of the modulation transfer function (MTF) for the three meridians. The modulation transfer function essentially corresponds to the absolute value of the optical transfer function (OTF), which is defined as the Fourier transfer of form of the point spread function.

For example, for the three meridians, <NUM>°, <NUM>° and <NUM>°, which may correspond to a horizontal direction, <NUM>° direction and vertical direction in the images captured, may be used. With these angular values, the calculation is particularly simple, but other angular values may be used as well. Details regarding such calculations may be found in<NPL>).

With P<NUM>, P<NUM>, and P<NUM> being the respective optical power settings in Diopters (D) of the optical power provided by the optical power changing device or computationally varied optical power in the pupil plane of the eye in terms of a deviation from a <NUM> setting for three meridians of <NUM>°, <NUM>° and <NUM>°, respectively, where the point spread function is sharpest (or the MTF area is at its maximum), a value M can be calculated according to <MAT> and power vectors J<NUM>, J<NUM> can be calculated according to <MAT> <MAT>.

From that, the cylinder J may be calculated according to <MAT> and the axis α may be calculated according to <MAT>.

Thus, sphere, cylinder and axis may be calculated.

The typical optometric notation of negative cylinder can be calculated as follows: <MAT> <MAT>.

As briefly mentioned above, in case an infrared illumination device is used, the above values have to be corrected taking the chromatic aberration of the human eye into account. For instance, chromatic shift from <NUM> to <NUM> would be around <NUM> D. Then, for calculating the refraction values, <NUM> D should be subtracted from M, P<NUM>, and P<NUM> as used in the equations above.

In a second approach which is not part of the claimed invention to determining sphere, axis and cylinder, the maximum area of the modulation transfer function depending on optical power setting is calculated for a plurality of meridians over at least <NUM>° with a step size smaller than <NUM>°, preferably smaller than <NUM>°, more preferably <NUM>° or smaller, for example <NUM>°. Then, the refractive error is calculated based on the area of the modulation transfer function as a function of optical power setting and angle.

For example, in case of the eye having no cylinder (i.e. no spherical aberration), the optical power setting is setting at which the modulation transfer area has its maximum corresponds to the sphere. In case of an astigmatism (cylinder ≠ <NUM>), there are two maximums in the modulation transfer function area depending on angle and optical power setting. The sphere is then the optical power setting value of one of the maximums, the cylinder is the difference of optical power settings between the maximums, and the axis is the angle difference between the two maximums. The number of maximums and their angular position may be calculated first by averaging the modulation transfer function are over <NUM>°.

This evaluation may for example be performed by image analysis efficiently.

A corresponding method is also provided, which includes capturing a series of images of an eye with varying optical power, and calculating the refractive error based on the series of images, as explained above for the first and second aspect. The method may further include illuminating the eye with an illumination device as discussed above. The above explanations made for the apparatus also apply to the method, for example the different ways of calculating the refractive error based on the images.

The method may be implemented in form of a computer program, for example in form of an application (app) for a mobile computer device like a smartphone or a tablet. The computer program may be provided on a tangible storage medium. In other words, the computer program may, when executed on a computing device, cause the computing device to cause execution of the method as discussed above, for example by controlling an apparatus as discussed above and performing calculations for refractive error determination.

In this case, the computer program may be included in a kit for a mobile computer device, where the kit, in addition to the computer program, may include a hardware module to be connected to the smart phone. The hardware module may include one or more hardware elements from the group consisting of an illumination device as mentioned above and an optical power changing device as mentioned above, and may also include further optical elements like lenses or mirrors. With such a kit, a conventional mobile computer device like a smartphone may be converted to an apparatus as mentioned above.

The refractive error determined by the above apparatuses, methods, computer programs and kits, may then be used for producing lenses based on the refractive error, i.e. lenses to correctly refractive error of the eye.

Various embodiments will be described referring to the attached drawings, wherein:.

<FIG> is a diagram illustrating an apparatus for determining a refractive error of an eye <NUM> having optical properties represented by a lens 10A according to an embodiment.

It should be noted that while single lenses are depicted in several instances in the embodiment of <FIG>, these may be replaced by combinations of two or more lenses or by other optical elements like diffractive elements having the same or similar optical function.

The apparatus of <FIG> includes a mobile computer device <NUM>, in the example of <FIG> a smartphone, and a hardware unit <NUM> coupled to computer device <NUM>. In other embodiments, instead of a smartphone other mobile computer devices like a tablet PC, or a laptop computer may be used.

Hardware unit <NUM> may be coupled to mobile computer device <NUM> using for example adhesive, brackets or other fastening elements, which align hardware unit <NUM> to a camera <NUM> of mobile computer device <NUM>.

In other embodiments, instead of using a pre-existing mobile computer device <NUM> like a smartphone, a dedicated apparatus may be provided including for example elements of hardware unit <NUM>, a camera corresponding to camera <NUM> and a computing device having processing capabilities using processors, memories and the like, which in case of <FIG> is performed by mobile computer device <NUM>.

Hardware unit <NUM> includes an infrared laser <NUM> as an illumination device. Light from infrared laser <NUM> is reflected by a semitransparent mirror <NUM> through lens 10A into eye <NUM>.

By using an infrared laser, or infrared light in general, the person examined (i.e. the person to whom eye <NUM> belongs) does not notice the examination due to light coming into the eye. Infrared laser <NUM> may be regarded as approximately being a point illumination device. As mentioned above, also other illumination devices, for example light-emitting diodes, may be used.

Light reflected from the retina of eye <NUM> through lens 10A passes through semitransparent mirror <NUM>, a variable focus optic <NUM> symbolized by a focus-adjustable lens in <FIG> and an optics arrangement including lenses 14A and 14B. Variable focus optic <NUM> is a simple example for an optical power changing device and may be implemented as explained above, for example using an adjustable liquid lens or any of the other possibilities mentioned above, including an arrangement of several lenses or other optical elements as well as moving lenses or moving a camera <NUM>. The placement of variable focus optic <NUM> is also only an example, and other placements may be used, for example a placement between lens 14B and camera <NUM>, or a placement between lenses 14A and 14B.

Lenses 14A, 14B form a telescope arrangement imaging light from eye <NUM> on camera <NUM>.

A setting of variable focus optic <NUM> will also be referred to as defocus in the following. For an emmetropic eye (no astigmatism, sphere = <NUM>), a defocus of <NUM> means that the retina of eye <NUM> is imaged sharply on camera <NUM>.

In other embodiments, variable focus optic <NUM> may be omitted, and an autofocus of camera <NUM> may be controlled to perform the function of an optical power changing device, as explained above and further below. However, by using a separate variable focus optic, a measurement range (a range from maximum negative defocus to maximum positive defocus) may be extended. In yet other embodiments, as explained above, other optical power changing devices that a variable focus optic may be used.

<FIG> is a flowchart illustrating a method according to an embodiment. The method of <FIG> is an example for operation of the apparatus of <FIG> and will be explained referring to <FIG>.

In step <NUM>, the method includes illuminating an eye. In case of <FIG>, this illumination is performed by infrared laser <NUM> via semitransparent mirror <NUM>.

In step <NUM>, the method comprises capturing a series of images with varying optical power. In the embodiment of <FIG>, computing device <NUM> controls variable focus lens <NUM> to provide different defocus settings, corresponding to varying optical power, and controls camera <NUM> to capture an image for each defocus setting, thus resulting in a series of images. In step <NUM>, the method comprises calculating the refractive error, for example in terms of sphere, cylinder and axis, based on the series of images. Each image essentially corresponds to the point spread function or at least approximate point spread function for the respective defocus setting, and the refractive error is calculated essentially based on said point spread functions.

Different possibilities for calculating the refractive error will now be explained using examples referring to <FIG>.

<FIG> shows example images representing the point spread function captured with camera <NUM> of <FIG> for defocus settings from -<NUM> diopters (D) to <NUM> D in steps of <NUM> D. Of course, the step size of <NUM> D is only an example, and to increase accuracy also a smaller step size, for example <NUM> D, may be used. The intensity in the image (e.g. greyscale value, mean RGB value) correspond to the amplitude of the point spread function.

In a first evaluation approach corresponding to the claimed invention, the "best focus", i.e. a defocus setting where the point spread function is sharpest, is determined for three different meridians, i.e. three different directions in the images of <FIG>. In an example discussed herein below, meridians of <NUM>°, <NUM>° and <NUM>° are used, where the directions are shown at <NUM> in <FIG>. In other embodiments, other angles may be used, with corresponding modifications to the calculations set forth below, in some embodiments, the best focus may be found by evaluating the images directly, i.e. by evaluating the shape of point spread function curves in three directions. To illustrate this, <FIG> illustrates intensities for the images of <FIG> along lines having a <NUM>° angle (top row), <NUM>° angle (middle row) and <NUM>° (bottom row). As can be seen, for different meridians the peaks are sharpest at different defocus settings, for example at -<NUM>. 5D for <NUM>° or at <NUM>. 0D for <NUM>° and <NUM>°.

In other embodiments, the modulation transfer function (MTF) may be calculated for the three meridians, and an area (integral) of the modulation transfer function may be determined.

An example for the images of <FIG> is shown in <FIG>, where the modulation transfer function area (MTFa) is shown over the defocus for the three meridians of <NUM>° (MTFa<NUM>), <NUM>° (MTFa<NUM>) and <NUM>° (MTFa<NUM>). The best focus then corresponds to the respective maximum of the area. In the example shown, for <NUM>° the maximum is at -<NUM> D, for <NUM>° the maximum is at <NUM> D, and for <NUM>° the maximum is at <NUM> D.

Furthermore, the measurement may be repeated, to increase accuracy through a plurality of measurements by statistics, as essentially possible for all measurements. For such multiple measurements, also the angle may be varied slightly. For example, for finding the best focus for <NUM>°, meridians in a range around <NUM>°, for example from <NUM>° to <NUM>°, may be evaluated, and the average best focus settings then may be used as for all these angles then may be used as the best focus setting for <NUM>°. This was done for the MTFa functions shown in <FIG>.

With P<NUM> being the defocus value of the best focus setting for <NUM>°, P<NUM> being the best focus setting for <NUM>° and P<NUM> being the best focus setting for <NUM>°, sphere, cylinder and axis may then be calculated according to the following equations. In the example of <FIG> and <FIG>, P<NUM> would be -<NUM> D, P<NUM> would be <NUM> D, and P<NUM> would be <NUM> D. Of course, these values are very depending on the eye being examined.

In particular, sphere M may be calculated as <MAT>.

Cylinder J and axis α may be calculated according to <MAT> <MAT> <MAT> and <MAT>.

These values may then be corrected to compensate chromatic aberration as discussed above.

For instance, chromatic shift from <NUM> to <NUM> would be around <NUM> D. Then, for calculating the refraction values, <NUM> D should be subtracted from M, P<NUM>, and P<NUM> as used in the equations above.

An alternative evaluation method which is not part of the claimed invention of the image captured will be explained referring to <FIG> (including subfigures 5A and 5B) and <NUM> (including subfigures 6A and 6B). In the alternative approach, as explained for <FIG>, the modulation transfer area MTFa is calculated for each optical power setting, for example defocus settings as shown in <FIG>. However, in contrast to the example of <FIG>, the modulation transfer focus area is not calculated only for three meridians, that over <NUM>° in a certain step size, for example <NUM>°, <NUM>° or <NUM>°, even if other step sizes or varying step sizes may also be used. Generally, using a smaller step size increases the accuracy, but also increases the computation time required. In this way, the modulation transfer area MTFa is determined as a function of defocus setting and angle.

This function may be represented as an image, where the modulation transfer function area corresponds to the intensity (e.g. greyscale value, brightness value or the like) and the x- and y- coordinates in the image represent defocus setting and angle or as a 3D Plot. <FIG> and <FIG> show corresponding examples for two sets of measurements, where in <FIG> no astigmatism is present in the eye (cylinder = <NUM>), whereas in <FIG> an astigmatism is present. In <FIG> and <FIG>, an image representation is given, whereas in <FIG> and <FIG>, a representation as a 3D Plot is used. In <FIG> and <FIG>, a more dense hatching represents a higher intensity. In <FIG> and <FIG>, the defocus is on the x-axis, the angle of the Meridian measured is on the y-axis, and the intensity of the image represents the area of the modulation transfer function.

The values of sphere, cylinder and axis may then be found using simple image processing. In case of no astigmatism present (<FIG>), independent of the direction, the maximum area is always at the same defocus, indicated by a line <NUM> in <FIG>. In the example of <FIG>, for example the sphere value would be <NUM>. This value may for example also be found by calculating a mean area across all meridians (angles). In case of a significant value of astigmatism, for example more than <NUM> D, two peaks will be found in the mean modulation transfer function overall angles depending on the focus. This corresponds to two peaks found in <FIG>. The sphere value then is at the defocus value corresponding to the peak at a higher defocus, in case of <FIG> at <NUM> D, as indicated by a line <NUM>. The cylinder value corresponds to the difference in defocus between the two peaks, i.e. the difference on the defocus, x, axis between line <NUM> and <NUM> of <FIG>. The axis value corresponds to the position of the second peak (with lower defocus) on the y, angle, axis as indicated by a line <NUM>. In this way, by evaluating the modulation transfer area as a function of angle and defocus, sphere, cylinder and axis may be found.

Claim 1:
An apparatus for determining a refractive error of an eye (<NUM>), comprising:
a camera (<NUM>) configured to capture an image of light from the eye, and
an optical power changing device (<NUM>) configured to vary an optical power of a light path from the eye (<NUM>) to the camera (<NUM>),
wherein the apparatus comprises a computing device which is configured to calculate the refractive error of the eye (<NUM>) based on a series of images of light from the eye (<NUM>) captured by the camera (<NUM>) with varying optical power using the series of images as at least approximate point spread functions, by determining an optical power where the point spread function is sharpest for at least three meridians, and calculate the refractive error based on the optical powers where the point spread function is sharpest determined for the at least three meridians.