The present invention relates to glasses (or spectacles) for use in visual training, for example of sportsmen who have to observe a moving items such as a ball, or for aircrew or emergency personnel who may have to operate in a low-contrast environment such as cloud, fog or smoke.

Background

In ball-sports in which players need to make contact with a moving object, for example with a bat, stick or racquet, the player's vision is clearly a vital component. Close observation of the movement of the ball or object is essential to pick up information such as speed, direction, trajectory and spin to maximise the chance of correct contact. An enhanced ability to observe the ball is likely to yield improved sporting skill, and a means of increasing that ability would therefore be of benefit to those participating in such sports.

Training effects in sport rely on practice, repetition and intense effort at the limits of ability to stimulate higher or enhanced aptitude. Whilst the effects of this on physical ability are well established the impact of such training techniques on the visual system is less clear. One reason for this may be the highly complex nature of the visual system. As much as 40% of the brain's activity may be devoted to the visual system and there are multiple factors that affect the information received by the brain such as an object's luminance, contrast, colour or movement as well as the optics of the eye and the health and integrity of the retina and subsequent visual pathways. Simply asking a player to 'watch the ball closely' is rarely effective and may be counterproductive as it requires an active effort of thought that may distract from or prevent efforts in other aspects of play that a player also needs to consider, such as movement or preparation. If it were possible to require a player to make an enhanced effort at ball watching, while not requiring additional thought to ensure it, this might be advantageous in stimulating close ball-watching ability. This would be also applicable in training personnel who are required to carry out tasks that necessitate seeing items in situations of poor visibility, for example in cloud, fog or smoke.

Making a task more difficult, to stimulate additional ability, is frequently at the heart of sports training; and attempts to develop visual abilities in this way have been made previously. For example, the Nike Vapor SPARQ Strobe eye training glasses had lenses that would rapidly alternate between being clear and opaque, producing a stroboscopic effect resulting in vision being alternately normal or absent. The consequent proportional reduction in visual information made extra effort necessary to track a moving object.

Subsequent research indicated that there was indeed evidence for an eye training effect (Mitroff et al. Athletic Training and Sports Health Care 2013 5(6) 261 -264). Nevertheless the glasses were withdrawn, possibly due to risks of inducing epilepsy. Indeed, glasses with adjustable slots in the lenses, which thereby blank out the peripheral parts of the visual field, have been used for eye training in sport. The benefit is in attempting to reduce distraction from the peripheral parts of the vision; however, in themselves, they do not stimulate the additional effort required to train the eyes in greater ball watching ability.

Summary of the Invention According to the present invention there are provided glasses for visual training, wherein the glasses comprise optical elements arranged to reduce the contrast between an item that is to be tracked and a background to that item. The glasses may be used for training of a sportsman who has to observe and track a moving item such as a ball or a puck.

The glasses may incorporate achromatic contrast reduction filters as the optical elements. Such contrast reduction filters are available for use in photography, and looking through them is similar to looking through an optically clear film that has a grey colour. Their effect is that lighter features of the view appear darker but dark features do not, or indeed appear lighter, so the contrast between light features and dark features is reduced without necessarily leading to a significant reduction in overall light transmission.

Alternatively, or additionally, the glasses may incorporate coloured filters as the optical elements, selected in accordance with the colour of the item to be tracked, so as to reduce colour contrast between the item and its background.

In another aspect the invention provides a method of visual training of a trainee, wherein the trainee wears glasses that are arranged to reduce the contrast between an item that is to be tracked, and a background to that item; and the trainee then repeatedly performs a task that requires the trainee to spot, track and possibly also hit the item. For example, for training a tennis player, the training method may use yellow tennis balls, and the glasses may selectively inhibit transmission of yellow light as scattered by the tennis balls, so as to reduce contrast between the tennis balls and their background; and the tennis player would then practice playing tennis with the yellow tennis balls while wearing the glasses. If the light scattered by the yellow tennis balls is in a narrow wavelength band in the yellow part of the spectrum, then the glasses would selectively reduce transmission of that band of yellow light; while if the light scattered by the yellow tennis balls is a

combination of narrow bands of red and green light, then the glasses would selectively reduce transmission of those narrow bands of red and green light. In both cases the filter would be a blue filter. It will be appreciated that the glasses may therefore be arranged to selectively reduce the light scattered by the balls without having a major impact on the colour of any other features of the view: the ball will appear darker and the background appear bluer. Alternatively, the filter may be selected in accordance with the colour of the

background, to minimise the reduction of light that corresponds to the background against which the item is viewed.

Reducing the amount of information available on the object being tracked in a sport appears counter-intuitive in trying to improve ball-watching, where optimising clarity, brightness etc may seem the more natural approach. Nevertheless it can be expected that forcing a player to make an extra effort to track a moving object can indeed lead to subsequent enhancements in the player's visual ability, when the training is completed.

The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings in which:

Figure 1 a is a graph showing the variation of contrast sensitivity with spatial frequency, with a linear scale for the spatial frequency;

Figure 1 b is a graph showing the variation of contrast sensitivity with spatial frequency, with a logarithmic scale for the spatial frequency;

Figure 2 shows a perspective view of a pair of glasses of the invention;

Figure 3 shows a perspective view of a modification to the pair of glasses of figure 2; and Figure 4 shows a sectional view on the line 4-4 of figure 3. Technical Aspects Associated with the Invention

With healthy eyes and visual pathways whether an object can be seen depends on a number of factors. Visual acuity refers to the spatial resolution of visual processing. It is the level at which a subject can just distinguish two adjacent light points with 100% contrast and the level is presented as an angular resolution using scales such as Snellen or LOGMAR. Optimum visual acuity is only achieved with a focussed retinal image and therefore requires correction of any refractive error for example with glasses or contact lenses. In a healthy eye the limit of resolution is determined by the separation of the cone light-sensitive receptors in the retina.

To be seen an object also needs adequate luminance. Luminance is the light intensity coming from the object that determines how bright it appears. At low luminance (scotopic conditions) only the rod retinal photoreceptors are active. At higher luminance

(photopic conditions) the cone photoreceptors are active. The eye has the ability to adjust to orders of magnitude changes in luminance through changes in the diameter of the pupil and through variation in the sensitivity of the retina. However the angular resolution of the rod photoreceptors is substantially less than that of the cones and optimum visual acuity is therefore only achieved under photopic conditions, i.e. at higher luminance.

For an object to be perceived not only must there be adequate luminance but also the object's luminance relative to its background is important. Contrast refers to the difference in luminance or colour between an object and its background. The human visual system is more sensitive to contrast than to absolute values of luminance, so the

environment can be perceived similarly despite wide variations in illumination. There are a number of different definitions of contrast, some of which take colour into account. One definition is the Weber contrast, which is the difference in luminance between the object and the background, divided by the luminance of the background; another definition, the

Michelson contrast, is commonly used for patterns such as sine wave gratings, and this is defined as the ratio of the difference between, and the sum of, the maximum and minimum values of luminance. The greater the contrast the easier it is to see an object, and there is a threshold below which the eye is no longer able to see the object. This threshold can be determined with charts (such as the Pelli-Robson chart) that present an object in progressively lower contrast, from black against white through increasingly pale shades of grey. The reciprocal of the contrast threshold is known as the contrast sensitivity.

However the level of contrast needed for a person to detect an object varies according to the spatial frequency of the object viewed (assessed in terms of the number of sine wave cycles in a sinusoidal grating per degree of visual angle). Referring now to figures 1 a and 1 b these show graphically how the contrast sensitivity, CS, varies with spatial frequency, f, figure 1 a showing the spatial frequency, f, on a linear scale and figure 1 b showing the spatial frequency, f, on a logarithmic scale. It will be seen that sensitivity is low at low spatial frequencies, increases to a maximum at around 8 cycles/degree and diminishes at higher frequencies reaching zero at around 60 cycles/degree, which corresponds to the subject's generally measured visual acuity i.e. maximum distinguishable spatial frequency at maximum contrast. The relationship between contrast sensitivity and spatial frequency may be referred to as the contrast sensitivity function. Thus objects with combinations of spatial frequency and contrast that fall below the curve are visible, those above are not.

Although the results above are shown for contrast from black to white, similar considerations apply to chromatic contrast. Coloured objects to be visible also need to have adequate contrast in their colour from their background when the background and object's luminance is the same. Similar contrast sensitivity function graphs can be generated for colour vision. There are three types of cones in the retina which respond maximally to light of wavelengths that correspond to red (570 nm), green (530 nm) and blue (420 nm). Colour contrast is typically assessed using axes defined by two contrasting colour responses: either red v green or blue v yellow (yellow being the response to combined stimulation of the 'red' and 'green' cones). Using sinusoidal gratings of alternating red and green or blue and yellow, each of these responses can be plotted against the spatial frequency of the grating in a similar way to that used to generate the black/white contrast sensitivity function. To be effective, glasses (or spectacles) to stimulate the effort needed to see an object must reduce the visual information presented to the eye whilst still allowing the object to be visible. Thus any reduction in focus of the object will be counter-productive because the image of the object generated on the retina will not be clear. Recognition of the object would then only take place when it is much closer to the eye which in a sporting situation would be highly disadvantageous. Such a situation would occur for example if glasses or contact lenses normally used to correct refractive errors were removed. It is therefore important that the glasses do not create a loss of focus or blur, as it is necessary that the visual acuity is not reduced. (Glasses that mimic the effects of defects of the eye or visual system, such as the defects caused by cataract, glaucoma, macular degeneration or stroke, are also known, but have no place in eye training because they impair the wearer's visual acuity.)

Similarly, cutting down the amount of light reaching the eye is also likely to be counter-productive not only because the eye can adjust to substantial reductions in luminance, but also because significant enlargement of the pupil can induce aberration and because visual acuity diminishes once the luminance drops to scotopic levels.

Reducing contrast of the object however does provide the possibility of reducing the information available to the eye whilst not affecting the focus of the image. Moreover it is possible to do this without reducing the visual acuity as long as the object being viewed, such as a ball, is not at either the higher or lower ends of spatial frequency on the contrast sensitivity function graph. For example objects with a size that equates to a spatial frequency of around 8 cycles/degree could have a very substantial reduction in their contrast relative to their surrounding yet still be clearly seen, albeit with significantly less ease than at maximum contrast. Objects such as balls that need to be tracked in sports are usually of a size that is substantially larger than the limit of visual acuity (about 60 cycles/degree). For example, at the full extent of the playing space or court, the size of the playing object subtends an angle at the eye equivalent to a spatial frequency (cycles/degree) of approximately 12.5 in ice hockey, 6.1 in tennis, 4.3 in squash, 4.2 in baseball and 1 .2 in table tennis. There is therefore the potential to substantially reduce the contrast of these objects to ensure they are harder to see yet for them to still be visible and in focus. Greater effort is therefore required to retain fixation on these objects as they move. With time the extra effort required stimulates the mechanisms used to fixate moving objects and provides the training effect to produce enhanced ball-watching abilities that remain beneficial once the contrast of the object is returned to normal.

Details of the Invention

Referring now to Figure 2 there is shown a pair of glasses 10 of the invention. The glasses are sports glasses (spectacles), with a conventional frame 12 and hinged side arms 13, and with lenses 14 of uniform thickness that are filters to make it more difficult to see the ball or object that must be tracked in a sport. The "lenses" 14 are optical elements that do not focus the light; the uniform thickness of the lenses 14 ensures they do not affect the wearer's ability to focus. The filter does not reduce the contrast to a level that prevents the object being seen at the typical spatial frequency of the object in that sport. Thus the lenses 14 may be contrast reducing filters arranged to reduce contrast in the mid-ranges of object spatial frequency to a level that remains below the contrast sensitivity threshold, so the object remains visible. Such filters could be independent of colour (achromatic), affecting the luminance of all colours equally. Alternatively the filters may be chromatic with peak reductions at wavelengths selected to be appropriate for the colour of the ball or object in a particular sport, to reduce the luminance of light scattered by the ball or object, and so to reduce colour contrast for that sport. As another alternative the lenses 14 may combine both achromatic and chromatic filters. The lenses 14 must not create any significant blur, dazzle or glare effect, and preferably do not lead to a substantial reduction in overall light reaching the eye.

Filters that reduce contrast are known to those working in, for example, photography and film to try to improve the visibility of components of the image that are poorly illuminated relative to the brighter parts of the scene. The filters for use in the present invention would typically provide a greater reduction in contrast than is normally required in photography, in order to reduce the level of contrast of the ball or the object of a sport to a level that stimulates extra effort in fixation. Filters made of glass, resin, polyester or other materials with selected transmission characteristics are well known to those skilled in the art. It is important that the glasses 10 do not impair visual acuity.

When considering the requirements of glasses to develop sporting ball-watching abilities it is necessary to select a level of contrast reduction appropriate to the sport. This is based upon the angle subtended at the eye by the playing object at the typical sighting distance. This determines the effective spatial frequency of the object and therefore the level to which the contrast can be reduced without preventing the object from being seen. Where used, appropriate chromatic filter transmission characteristics need to be selected to produce the required reduction in colour contrast in a given sport based on the colour of the object or ball used. For example yellow filters to match the colour of a tennis ball could be used or red filters for a cricket ball, whilst for a white ball such as a baseball the filter would be achromatic. A number of additional features could be applied depending on the requirements of the sport and on the level of training desired. For example the glasses 10 may, as shown, include side baffles 16 attached to the side arms 13, to obstruct peripheral vision, to avoid distraction. The glasses 10 may be provided with replaceable and interchangeable pairs of lenses 14 that produce different degrees of contrast reduction.

Referring now to figure 3 there is shown a perspective view of a pair of glasses 20 which are similar to the glasses 10 of figure 2, equivalent items being referred to by the same reference numerals. The glasses 20 are sports glasses (spectacles), with a

conventional frame 22 and hinged side arms 13, and with lenses 14 of uniform thickness that are filters to make it more difficult to see the ball or object that must be tracked in a sport. The frame 22 surrounds each of the lenses 14, rather than being attached only along the top of each lens 14. In this example the peripheral portions 24 of each lens 14 are opaque; this ensures the wearer's eyes are in the neutral (primary) position.

Alternatively, the peripheral portions 24 may provide greater contrast reduction than the central part of the lens 14. The peripheral portions 24 may be graduated in contrast reduction, and may be graduated in opacity rather than providing an abrupt transition to being opaque, by comprising a graduated dark filter or graduated and increasing contrast reduction, or a combination.

The purpose of the peripheral portions 24 is to keep the eyes in the primary position, as the process of moving the eyes in the eye socket can itself set up postural reflexes, that is to say corresponding movements of the head, neck, or body, that maybe disadvantageous to hitting the ball (or the other item with which contact is to be made). Keeping the eyes stationary in the socket may ensure that the head and body have to move directly, which may avoid such reflex movements. As another option the frame 12 or 22 may be arranged so that a second or third lens may be provided in combination with each lens 14, for example to provide increased contrast reduction and so higher levels of eye training, or to add in chromatic contrast reduction where desired. Such lenses could be supplied as a set. For example, referring now to figure 4, this shows a sectional view on the line 4-4 of figure 3, showing only the portions of the frame 22 above and below the lens 14. The frame 22 defines a groove 26 into which the lens 14 is fixed, for example by gluing. The groove 26 is wider than the thickness of the lens 14, so a second such uniform-thickness lens (not shown) can be clipped into position fitting up against the back of the lens 14, with the periphery of the second lens locating in the groove 26; part 28 of the frame 22 is notched so that part of the periphery of the second lens is accessible to allow the second lens to be removed. By way of example, where the lens 14 is an achromatic contrast reduction filter, the second filter may be a coloured filter, or may be a second achromatic contrast reduction filter.

Where the glasses 10 or 20 of the invention are to be worn by a person who requires prescription glasses to see clearly, the glasses 10 or 20 of the invention may be arranged to be fitted over the person's normal glasses. For example the glasses 10 or 20 may be modified to omit the side arms 13 and to omit the side baffles 16 (where provided); the frame 12 or 22 holding the lenses 14 may be arranged so it can be clipped over the prescription glasses.

As indicated above, the glasses 10 or 20 may be worn by a sportsman training to play a game such as cricket or tennis, each of which requires a player to hit a moving ball. The sportsman would wear glasses 10 or 20 that reduce the contrast between the ball and the background for the game in question. For example, assuming that cricket is played with a red ball that scatters red light in a narrow part of the spectrum, a batsman might wear glasses 10 or 20 that reduce the intensity of red light of that part of the spectrum, so as to reduce the contrast between the ball and the background. While wearing these glasses the batsman would practice, for example being bowled at in cricket practice nets. Because of the reduced contrast, the batsman will have to make a greater effort to track the ball, and may thereby improve his visual abilities in that respect.

Similarly, assuming that tennis is being played with yellow tennis balls that scatter light in a narrow band in the yellow part of the spectrum, a person training as a tennis player would wear the glasses 10 or 20 with lenses 14 that act as filters to selectively reduce transmission of yellow light in that band of the spectrum, thereby reducing contrast between the tennis balls and their background. The tennis player would then play tennis with the yellow tennis balls while wearing the glasses 10 or 20. Because of the reduced contrast, the person wearing the glasses 10 or 20 would have to make a greater effort to track the ball, and may therefore improve and train his visual abilities at tracking a ball. In each case, or indeed in other sports training situations, the glasses 10 or 20 might reduce contrast in every colour, rather than selectively reducing the intensity of a particular part of the spectrum, that is to say using an achromatic contrast-reduction filter.

The glasses 10 or 20 might also provide additional features. For example the filters 14 might include neutral density darkening filters for use in bright light, or a photochromatic element which becomes darker in bright light, so that the same pair of glasses 10 or 20 can be used for both indoor sports and outdoor sports, and in both dim and bright light, without having to add any additional filters.

Other variations and modifications will be apparent to the skilled person. Such variations and modifications may involve equivalent and other features that are already known and which may be used instead of, or in addition to, features described herein.

Features that are described in the context of separate embodiments may be provided in another embodiment, so for example the lenses 14 with the peripheral portions 24, as shown in figure 3, might instead be provided in frames 12 as shown in figure 2; and side baffles 16, as shown in figure 2 might be provided on frames 22 as shown in figure 3.

It should be noted that the term "comprising" does not exclude other elements or steps, the term "a" or "an" does not exclude a plurality, a single feature may fulfil the functions of several features recited in the claims and reference signs in the claims shall not be construed as limiting the scope of the claims. It should also be noted that the Figures are not necessarily to scale; emphasis instead generally being placed upon illustrating the principles of the present invention.