WO2004089200A1

WO2004089200A1 - Arrangement for measuring the optical quality of the anterior segment of the eye and grating element therefor

Info

Publication number: WO2004089200A1
Application number: PCT/GB2004/001587
Authority: WO
Inventors: Robert Alexander Weale; Nlgel Philip Davies
Original assignee: University College London Hospitals Nhs Trust
Priority date: 2003-04-12
Filing date: 2004-04-13
Publication date: 2004-10-21
Also published as: GB0308537D0

Abstract

An arrangement for measuring the optical quality of the anterior segment of the human eye has a source of illumination, a grating element marked with a novel pattern, a camera and a processing means. The grating element pattern consists of a series of parallel lines which are made up of respective numbers of alternate light and dark dashes, the respective numbers becoming greater going from one end of the series to the other. In use, the source of illumination projects a light through the grating element towards the eye to be examined, the camera forms an image corresponding to the image of the grating element pattern reflected from the posterior surface of the eye's lens and the processing means carries out processing on the camera image such as to provide a measure of the anterior-segment optical quality. Preferably the processing stage measures quality using a modulation transfer function of the system.

Description

ARRANGEMENT FORMEASURING THE OPTICAL QUALITY OF THE ANTERIORSEGMENT OF THE EYEAND GRATING ELEMENT THEREFOR

Technical Field of the Invention

The invention relates to an arrangement for measuring the optical quality of the anterior segment of the eye, in particular the human eye, and to a grating element for use with such an arrangement. The device is adaptable for an independent assessment of the cornea and anterior chamber combined.

Background of the Invention

There has long been a need for an arrangement which would enable medical practitioners to assess the quality of a patient's optics, especially in the anterior portion, in a way which did not depend on a subjective evaluation of the patient's visual acuity. It is also desirable to be able to differentiate a poor optical performance which is due to a defective lens from one which is due to a defective retina. The visual acuity of a patient can easily be established using, for example, a word-reading test from a distance, but it is not always clear what the cause of a poor visual -acuity result is: whether it is purely lens- related or due to some retinal dysfunction. It may even be a combination of the two. One known way of achieving these aims is to use a piece of equipment known as the Oqual™ "Mark 1", developed by the present inventor. This equipment is a simple addon to a standard slit-lamp and consists of a cylindrical ring which fits onto the illuminating tube of the slit-lamp. Held in the ring in an internal flange thereof is a circular grating element having a pattern shown in Figure 1. This pattern consists of a series of pairs of dots (in this case, square dots) of a fixed width, but separated by respectively different amounts: the spacing between each pair of dots is half that between the preceding pair. In use, the slit-lamp projects an image of this pattern onto the eye and the observer then looks out for a particular reflection of the pattern originating from a specific part of the anterior segment. There are four such reflections in total (see Figure 2), these being: a reflection 10 from the convex anterior surface of the cornea, a reflection 11 from the convex posterior surface of the cornea, a reflection 12 from the convex anterior surface of the lens and a reflection 13 from the concave posterior surface of the lens. The reflection used in the Oqual™ "Mark 1" is the fourth of these, i.e. reflection 13. Since the posterior lens-surface is concave, the fourth image is inverted (the lens surface acts here somewhat in the manner of a shaving mirror).

The more dots in the fourth image that the observer can see, the better the quality of the patient's anterior segment. In a healthy eye, all six pairs of dots shown in Figure 1 can be clearly seen. If the eye has a mild haze, perhaps only four or five will be observed, whereas for a complete cataract none may be visible. Where the eye (particularly the lens) has areas which are clear, as in a healthy eye, but also areas which are opaque, it may be that a closely spaced pair of dots will be resolved, whereas a more widely spaced pair will not be, or will be distorted.

While this particular known system works well, it still relies to some extent on the accurate observation of the observer and results may indeed vary from observer to observer. It would be useful to have a system which obviated human observation altogether and it is with such a system that the present invention is concerned. Summary of the Invention

In accordance with a first aspect of the invention, there is provided a grating element for use in an arrangement for measuring the optical quality of the anterior segment of a patient's eye, the grating element comprising a series of parallel lines, the lines being made up of respective pluralities of alternate light and dark dashes, the respective pluralities increasing from one end of the series to the other.

This configuration of the grating element allows an automatic, computer-based assessment of the fourth image as picked up by a (digital) camera pointing at the eye.

In order to facilitate computer processing the grating element may in any one line have light and dark dashes which are of substantially equal length.

As a suitable threshold for the highest frequency of dots and dashes, the length of the dashes in the line with the greatest number of dashes may be such as to correspond to a limit of resolution of a normal anterior segment. It is then convenient if the length of the dashes in the line at the other end of the series is approximately 10 times that length, with the dash lengths of the intervening lines being approximately linearly distributed.

In a second aspect of the invention, an arrangement for measuring the optical quality of the anterior segment of the eye comprises: a source of illumination; a grating element as just described; a camera, and a processing means, wherein, in use, the source of illumination projects a light through the grating element towards the eye to be examined, the camera forms an image corresponding to the image of the grating element pattern reflected from the posterior surface of the eye's lens and the processing means carries out processing on the camera image such as to provide a measure of said anterior-segment optical quality. The processing means may determine the modulation transfer function of the eye's lens for each line of the grating element. This method of processing the camera's image is relatively simple, but yields reliable results.

The arrangement advantageously further comprises a graticule for the fixation of the patient's other eye, the graticule comprising a white cross on a black background. This has the benefit that white on black minimises glare and the configuration of the graticule as a cross makes it is easier for the user of the equipment to communicate what he wants the patient to do in order to fixate the other eye. (The patient may only speak a foreign language and therefore any measure which facilitates communication is a good thing). Earlier patterns have included a black disk in a bull's eye, but the disk tended to be too large to lead to good fixation and also caused subjective glare to the patient.

As an alternative to the use of a thin-line cross, a cross having arms of varying width (eg. a Maltese-type cross) may be employed and a suitable anti-glare filter (preferably yellow- green) used in conjunction with it. This is useful where the patient's other eye cannot resolve a thin line.

Brief Description of the Drawings

An embodiment of the invention will now be described, by way of non-limiting example only, with the aid of the drawings, of which:

Figure 1 shows a known grating element for helping to establish a measure of the optical quality of the anterior segment of the human eye;

Figure 2 is a view (not to scale) of the human eye showing reflections of an incident ray of light from different parts of the eye;

Figure 3 depicts a, grating element according to the invention; Figure 4 is a characteristic curve of the response of the human eye to a point source of light;

Figures 5 and 6(a), 6(b), 6(c) are characteristic curves showing different responses to different frequencies of source image;

Figure 7 is a diagram illustrating an eye-scanning operation, and

Figures 8, 9 and 10 are measurement results for actual patients using the measuring arrangement according to the invention.

Detailed Description of an Embodiment of the Invention

As with the lαiown system, the present arrangement for measuring the optical quality of the eye's anterior segment makes use of a conventional slit-lamp with an add-on cylindrical ring, but uses instead of the grating element having six pairs of variously spaced dots a grating element having a series of broken lines. Figure 3 illustrates this pattern, in which in this particular version a series of ten parallel lines is provided, each composed of a series of light dashes 15 separated by a dark space. (The diagram, like Figure 1, shows a negative version of the pattern, hence the light dashes are shown as dark dashes and the dark spaces as white areas). The number of dashes varies for each line and increases monotonically and also approximately linearly from one end of the series to the other. In addition the mark-space-ratio for each line is advantageously 1:1, since this greatly simplifies computer processing when the element is in use.

In contrast with the known arrangement, in which the fourth image from the back of the lens is observed by a human observer, in this arrangement a camera is used, which is directed towards the eye and registers the fourth image in any way convenient for processing. The ideal kind of camera in this regard is a digital camera, since this yields an output relating to light intensity across an image plane in the form of a digital (binary) representation which can then be easily manipulated by a processing means such as a computer.

The digital camera output is fed to a processor, where it undergoes processing suitable for providing an indication of optical quality, The preferred form of processing is the formation of the modulation transfer function of the lens being examined. This process is now described with the aid of Figures 4, 5 and 6(a), (b) and (c).

Figure 4 shows a "point spread function" which is a characteristic of the response of the human eye. In Figure 4 it is assumed that a very small point of light is directed towards the eye. Ideally, if the eye's optics were perfect, the image of this point on the retina would be identical to the point of light and therefore the relative intensity of this point as a function of distance across the retina would appear as line 18 in Figure 4. Instead, however, the relative intensity is actually spread out across the retina, as shown by curve 19. It is this curve which describes the "point spread function". (The actual curve includes additional maxima at either end, but these are of significantly lower amplitude and are therefore ignored in this explanation).

When a regular series of light and dark areas is presented to the eye, the point spread function causes the eye's response to vary according to the "frequency" and contrast of these areas. This is illustrated in Figures 5 and 6. In Figure 5, when a step change in intensity 20 is presented to the eye, the response of the eye is modified by the point spread function 21, so that the perceived change is no longer a step change, but is "blurred" (curve 22). The same blurring occurs also on the "falling edge" of the step change and when a series of step changes is generated, the result is a periodic intensity or luminance curve as shown by, for example, curve 23 in Figure 6(a). When the frequency of the grating line (i.e. number of light and dark dashes per unit distance) increases, the blurring due to the point spread function plays an even more significant role and results in an ever decreasing amplitude 24 and 25, respectively (see Figures 6(b) and (c)). Eventually the frequency will be so high as to render resolution impossible. The present invention makes use of this change in amplitude to measure the performance of the anterior segment and does so by forming, as already mentioned, the modulation transfer function in a way which will now be explained.

A parameter can be derived which is called the "modulation" of the incoming light signal from the grating element. The modulation is defined as:

-M. ^— L_maχ — Lminj ' (-Lmax ^"■^" -L/min)

where L_max is the luminance of the light dashes of the grating element pattern as presented to the eye and L_mi_n the luminance of the intervening dark spaces. The modulation is thus a measure of the "contrast" of the broken lines of the pattern. A similar definition applies to the maximum and minimum luminances of the image coming from the posterior surface of the lens, which the camera picks up.

The modulation transfer function (MTF) is now defined as the modulation of the image, Mj, divided by the modulation of the source (the grating element), M_s, i.e.:

This parameter, MTF, is a function of the frequency of the lines in the pattern, the frequency being - as already mentioned - dependent on the length of the dashes, or the number of dashes (and spaces) per unit distance. Referring again to Figure 6, the MTF is simply the ratio of the amplitude of the image (e.g. amplitude 24 in Figure 6(b)) to that of the stimulus or source (amplitude 20 in Figure 6(b)) for any one frequency.

The modulation of the source signal (i.e. of the square- wave stimulus springing from the grating element) is already known and is fed to the processing means (digital computer). The modulation of the fourth image from the lens is calculated from the image supplied by the camera, so that the processing means is then able to form the modulation transfer function (MTF) for each of the lines (frequencies) of the pattern, the result then being a measure of the quality of performance of the lens.

One problem with making measurements such as those described is the tendency of the human eye to wander, which would tend to smear the images taken from the eye. In order to reduce this tendency, the known system makes use of a "bull's eye" target, on which the patient is asked to focus his other eye (the eye not under examination). This "bull's eye" target is conventionally a fairly large black spot on a white background. Unfortunately, the effect of this is to introduce a sensation of glare to the patient and in addition its ability to enable fixation by the patient is limited. In an attempt to overcome this drawback, the present invention envisages the alternative use of a thin white cross on a black background, the patient then being asked to fixate on the point of intersection of the arms of the cross. The white on black reduces the glare and the thin cross pattern with its very precise point of intersection enables effective fixation of the eye in question.

While the use of a thin- line cross would be ideal, it does presuppose that the patient's other eye is good enough to resolve it. Where this is in doubt, an alternative measure is to use a cross of variable size, eg. a Maltese cross. However, since this now represents a broader target than the thin-line cross, some measure is needed to reduce the attendant glare. One solution is to allow the patient to view the cross through a coloured filter (a yellowish-green filter would be suitable).

In spite of the superior fixation properties of the white cross on the black background, some residual eye movement still takes place, so-called "saccadic" movement, which takes the form of low-amplitude jerks. The present invention takes these into account to create a cleaner digital interpretation of the camera image and does so by suitable routines in the programming employed. A preferred routine is to take a substantial number of recordings of the MTF and then identify which recording gives the highest reading, since readings which have been affected by saccadic movement will generally be lower. The same procedure can be employed to optimise the focusing of the slit-lamp. When the lamp is defocused, MTF readings will likewise be lower than if the lamp is properly focused, so that, by monitoring the readings at the same time as changing the focus of the slit-lamp, the optimum focus level can be achieved. This is explained further below.

It was stated at the beginning that it is necessary to be able to distinguish eye problems which are due to neural causes from those due to a flaw in the lens. In order to be able to distinguish between these two causes, it is important that the reading taken by the arrangement according to the invention be the best possible reading. As already brought up in connection with the lαiown system, patients often make use of "windows of opportunities", i.e. clearer areas of an otherwise opaque lens, when looking at objects and an attempt should be made to discover this "window", if it exists.

In order to achieve this, the eye is preferably scanned across its surface, the angle of incidence of the source light from the grating element varying accordingly. This is illustrated in Figure 7 for one of two planes of scanning, it being appreciated that, in practice, scanning will take place in the other orthogonal plane also. In Figure 7 a representative point of the source grating pattern is located at Psi and the fourth-image reflection is located at PR_I in one particular plane, while a representative point of the source pattern in a second position is located at Ps₂ and of the fourth-image reflection is located at PR₂. Clearly, the two rays involve different areas of the lens and in this case, if one position of the source pattern only were used, e.g. that corresponding to point Psi, and the relevant part of the lens (part A) for that position were defective, the fact that a less defective part of the lens (part B) existed would not be picked up unless the target were moved to correspond to position Ps₂. Having found the more optimum angle of illumination, this can be used to provide a reliable basis for later comparison with standard visual-acuity readings. When the slit-lamp is adjusted to the above optimum angle, it is then a preferred step to move the slit-lamp light source toward and away from the eye, the computer processing then signalling when optimal contrast has been obtained. This then produces a diagnostic object having a refined focus. Movement of the light source takes, place by moving the slit-lamp's joystick.

Actual results of using the described equipment with patients are illustrated in Figures 8 to 10, which are graphs of relative amplitude of modulation (i.e. modulation transfer function) versus the number of the grating-pattern line. It is assumed that the results with a normal eye will be monotonically decreasing very approximately linear with frequency of grating from a value of 1.0 at the lowest frequency to zero at the cut-off frequency. As can be seen, the first patient (Figure 8) appears to have a fairly normal lens, since there is only a slight departure from approximate linearity and monotonicity. The second patient (Figure 9) has a marked departure from normal at the start of the second half of the frequency range (between grating lines 5 and 8), while the third patient (Figure 10) clearly has a badly deteriorated lens.

While it has so far been assumed that the graticule will have a total of ten lines, it is possible to have more or less than ten. A greater number would enable a finer assessment of the quality of the interior segment, but would occupy generally more space; fewer lines would make for a more compact grating, but would conversely provide a coarser resolution of quality. Also, while a linear distribution of frequency across the series of lines is preferred, it is conceivable to employ a different distribution.

Claims

1. A grating element for use in an arrangement for measuring the optical quality of the anterior segment of a patient's eye, the grating element having a pattern comprising a series of parallel lines, the lines being made up of respective pluralities of alternate light and dark dashes, the respective pluralities increasing from one end of the series to the other.

2. A grating element as claimed in Claim 1, wherein the light and dark dashes of any one line are of substantially equal length.

3. A grating element as claimed in Claim 2, wherein the length of the dashes in the line with the greatest number of dashes is such as to correspond to a limit of resolution of a normal anterior segment.

4. A grating element as claimed in Claim 3 wherein the length of the dashes in the line at the opposite end of the series is approximately ten times the length of the dashes in the line with the greatest number of dashes.

5. An arrangement for measuring the optical quality of the anterior segment of the eye, comprising:

- a source of illumination;

- a grating element as claimed in any one of the preceding claims;

- a camera, and

- a processing means, wherein, in use, the source of illumination projects a light through the grating element towards the eye to be examined, the camera forms an image corresponding to the image of the grating element pattern reflected from the posterior surface of the eye's lens and the processing means carries out processing on the camera image such as to provide a measure of said anterior- segment optical quality.

6. Arrangement as claimed in Claim 5, wherein the processing means determines the modulation transfer function of the eye's lens for each line of the grating element pattern.

7. Arrangement as claimed in Claim 5 or Claim 6, further comprising a graticule for the fixation of the patient's other eye, the graticule comprising a white cross on a black background.

8. Arrangement as claimed in Claim 5 or Claim 6, further comprising a graticule for the fixation of patient's other eye and a filter for the viewing of the graticule, the graticule comprising a Maltese-type cross and the filter being a yellow-green filter.

9. Grating element substantially as shown in, or as hereinbefore described with reference to, Figure 3 of the drawings.

10. Arrangement for measuring the optical quality of the anterior segment of the eye substantially as hereinbefore described with reference to Figure 3 or Figures 3 and 7 of the drawings.