Improving Visibility of the Retinal Image
Into this category come a wide range of very simple strategies which are designed to aid vision, but do not affect the vergence or direction of rays of light entering the eye. They do not increase the size or improve the focus of the retinal image, but rather improve the visibility of the image in other ways. In addition to making an object easier to see, it is often possible to add a tactile back-up to give extra help in recognition—for example, one may put a coloured mark on a light switch to make it easier to see, but also make the mark raised above the surface so that it can be easily felt. ‘Hi-marks’ is a fluorescent orange substance like toothpaste which is squeezed from a tube to make coloured, tactile markings which set to a hard lump; ‘Bump-ons’ are self-adhesive plastic dots which are clearly visible and easily felt when used as markers ( Figs. 10.1 and 10.2 ), and there are many other examples. Improving object visibility often involves increasing the contrast of the retinal image, and these strategies will be equally effective regardless of the cause of the visual impairment. For those patients with media opacities, however, the ability to detect low-contrast objects may be particularly impaired in the presence of high ambient illumination when there may be scattering of light within the eye, reducing the contrast of the retinal image. This can be tackled by removing any possible sources of scattered light, thus maximising the contrast of the retinal image: these methods will be considered later in the section ‘Possible Approaches to Glare Reduction’.
Luminance contrast
Michelson contrast is equal to
Lmax−LminLmax+Lmin
where L max is the luminance of the brightest and L min the luminance of the dimmest areas within the image. In the case of a black object on a white background, or a white object on a black background, contrast can approach 100%. Many low-vision patients have very poor sensitivity to low-contrast targets, and if the contrast is insufficient, then improving the lighting or magnifying the image may not help. If, for example, the patient has difficulty in seeing the edge of a sheet of white paper against a pale desk-top when writing, then there is insufficient contrast between it and the background because the two surfaces have almost equal luminance. The simple solution of providing a darker surface or edge (a sheet of black paper covering the desktop or surrounding a light switch) will make the border of the sheet or the switch visible, even if vision is poor ( Figs. 10.3A and B ).
It has been suggested that electronic enhancement of video images could be performed in order to improve the contrast (and hence visibility) of images presented on a TV screen. This might apply to television programmes, films or videos, or might extend to image enhancement for text and images presented on an electronic vision enhancement system (EVES). Peli and coworkers found improved performance of visually impaired patients for recognition of faces, expressions, and other details on still photographs ( ) and video films ( ). This group did not find any improvement in reading performance for text which had been enhanced in this way: this is in marked contrast to the results of who reported dramatic increases in reading speed. This technique is incorporated on several new wearable EVESs although data have not been published on how many people use these modes.
Chromatic (colour) contrastIt is possible to have chromatic contrast within an image in addition to, or instead of, luminance contrast. In fact, it is possible for someone with good colour vision to be able to distinguish an object from a background of equal luminance, if the two are of different colours—such as a green object against a red background. There are some ocular pathologies in which colour perception would be impaired to the extent that such an object would be invisible, whereas luminance contrast is always a potent source of visual information. This might suggest that black-and-white should be the only colour combination used, but in many cases extra information can be provided by maximising both luminance and chromatic contrast ( ).
There have been several studies looking at preferred colour combinations in a variety of contexts. looked at reading performance with various colour combinations for text displayed on a screen and found that whilst there were strong subjective preferences, there was no combination which led to improved performance. measured reading rate for different coloured letters against a black background and found very little variation in performance. In a study by of the preferred colours for the screen display of a bank self-service cash dispenser, there was a marked preference for white-on-black in most older subjects: in the group of cataract patients, white-on-blue was slightly more popular but this is likely to offer almost as much contrast when one considers the attenuation of the blue background luminance by the yellowed crystalline lens. For reading text, then, it appears that maximising luminance contrast is the only way to optimise performance, although patients may subjectively prefer to introduce colour contrast as well: this may be beneficial providing that high luminance contrast is maintained.
investigated the hypothesis that colour would improve object recognition for the blurred images seen by low-vision patients even more than it would for those with good vision (as with good vision, additional shape and texture information can be seen, which might render colour redundant). After carefully controlling for luminance contrast in the images, they found that colour did improve the speed and accuracy of naming familiar objects (food items): this led them to suggest that colour contrast is a useful practical strategy to use when the patient has to perform a recognition task.
Optimising both luminance and colour contrast would appear to offer the best practical approach. In considering which colours to select, it is worth remembering that the normal visual system is much more sensitive to wavelengths from mid-spectrum—the peak of photopic sensitivity being at 555 nm (yellow)—with less sensitivity to the spectral extremes. Even if there is no specific colour vision defect associated with the visual impairment, one would expect the patient to lose the ability to detect the red and blue wavelengths as sensitivity overall diminishes ( ). It is also likely that the ability to discriminate between similar hues of equal luminance will become impaired: patients often report a difficulty in distinguishing between black and blue, or between white and yellow ( Fig. 10.4 ). To maximise luminance contrast needs a bright object and a dark background, or vice versa, and chromatic contrast requires selection of colours widely separated in the spectrum. If choosing a colour from mid-spectrum and one from a spectral extreme, it makes sense to have that from the extreme at low-luminance, with the brighter one chosen from mid-spectrum. If the choice is reversed, it is possible that the patient’s loss of sensitivity will tend to equalise the apparent brightness of each. Thus, a ‘good’ combination would be bright yellow and dark blue, but bright red and dark green would be a poor choice. Other poor choices would be those hues close together in the spectrum (such as green and turquoise) or pastel shades where hue is indistinct (such as yellow and grey): in these cases, detection would be especially difficult if the two were of equal brightness.
There are many suggestions for putting such strategies (along with many other helpful adaptations) into practice for everyday tasks:
https://www.rnib.org.uk/sight-loss-advice/reading-home-and-leisure/your-home/practical-adaptations
https://www.henshaws.org.uk/knowledge-village/henshaws-life-hacks/
Using Luminance Contrast
- 1.
Writing with a fine felt-tip or fibre-tip pen produces higher-contrast letters, which are more visible than those written with ball-point pen, even when the two are the same size. It is also possible to have lined paper with lines which are thicker and darker than ‘normal’. Various writing frames can also be used which provide the patient with a dark marker along which to write, thus keeping the words in straight lines: this might be done, for example, by moving a dark elastic line marker progressively down the page ( Fig. 10.5 ), or having a cover over the page which is folded down section-by-section to reveal each new line in turn. There are also envelope and cheque guides and signature guides ( Fig. 10.6 ): each is in essence a dark card with sections cut out to reveal the spaces on the page beneath into which the patient must write.
Fig. 10.5
Writing frames.
Fig. 10.6
Signature and cheque guide.
- 2.
A staircase should be evenly lit to avoid shadows from the vertical risers.
- 3.
A white napkin or handkerchief on a dark table-top helps the patient to locate dark objects placed there, such as a purse or spectacle case.
- 4.
Tools can be carried around the house or garden in a large white bucket. This stops them from being lost, or forming a hazard when left lying on the ground, and the bucket will be noticed easily against the background.
- 5.
Decor should include pale carpets and walls (matt) to reflect the available light, and to contrast with dark furniture. If the furnishing fabric is striped, then the stripes will orient in different directions on the horizontal and vertical planes and will define the direction of each.
- 6.
Dark contrasting door knobs will be more easily seen on a light surface and may stop the patient from bumping into the door. Shiny door knobs are most effective on a dark door.
- 7.
Vegetables should be peeled or chopped against a light work surface, whereas pastry should be rolled out on a dark board. Sponge cake mixture should be made in a dark-coloured bowl, but chocolate cake mixed in a light container!
- 8.
Pale crockery will be seen best against a dark tablecloth or, if using a white tablecloth, choose crockery with a dark edge band.
- 9.
Kitchens are often uniformly pale in colour, providing no contrast against which to view light objects. A sheet of black paper against the wall forms a useful background for pouring pale liquids.
- 10.
If making tea/coffee in white cups, the tea/coffee should be poured in before the milk because milk will not be seen ( Fig. 10.7 ).
Fig. 10.7
Coffee grains in a dark and a light cup.
Using Chromatic Contrast
The selection of appropriate coloured backgrounds is often a very individualised choice. If there is a particular task to be performed, this should be tried against different backgrounds provided by coloured sheets of paper to see which is best. A ‘good’ combination should also provide significant luminance contrast. In some of these examples, colour is also used for ‘ coding’ and identifying particular objects. This will only be possible if the patient retains useful colour perception.
- 1.
Glass tumblers are available which have coloured plastic holders to make them easy to see. A clear glass container is almost invisible against a white tablecloth.
- 2.
Food should be arranged on a plate of contrasting colour, such as carrots on a white plate, or fish on a blue plate. Green vegetables placed between fish and mashed potatoes on the plate will aid in the location of each.
- 3.
The handles of garden tools could be painted bright yellow to make them easy to locate.
- 4.
Brightly coloured straps around suitcases can be used for identification.
- 5.
A standard lamp with a pale shade will not contrast with the wall, and may be knocked into, but a brightly coloured wall hanging behind it will make it clearly visible.
- 6.
Coloured electrical sockets and/or plugs can be used. The colour contrast makes each easier to see, and can be used for identification (e.g. red for kettle, blue for microwave).
- 7.
Old felt-tip pens can be used as markers for seedlings in the garden.
- 8.
Toothpaste and shaving cream should be bought in different coloured tubes, and dangerous substances like bleach or white spirit in a distinctively coloured bottle so that these are not mistaken.
- 9.
Grab rails on the sides of the bath can be wound with fluorescent tape.
- 10.
Crockery could be selected from nonmatching sets: the shape of cups, milk jugs and sugar bowls is often similar, so choose the latter two from sets of different colour, preferably with a different shape.
- 11.
Kitchen utensils should not be from a matched set, but deliberately chosen to differ in design: the can opener with a red handle, potato peeler with a yellow handle, etc. If utensils are all the same colour, different coloured tape can be wound around the handles.
- 12.
Plastic freezer bags are available with different coloured stripes, and these can also be used for coding—green for vegetables, red for meat, blue for fish, etc.
- 13.
Tablecloths should be selected which contrast with the crockery, placemats and serving bowls.
- 14.
Low vision aids (LVAs) should have different colour tape attached so they stand out against a table or shelf: something which manufacturers often overlook.
Glare
Illumination is often extremely valuable for vision enhancement, but there are occasions when it must be carefully controlled, due to the symptoms of glare described by the patient. ‘Glare’ is a word which is used very informally by many clinicians and patients; however, it covers a range of difference experiences, and it is not clear that everyone is applying the term in the same way. Glare phenomena are experienced by everyone, but patients with visual impairment face some specific/additional challenges. Careful investigation and questioning are required to determine the exact nature of the problem.
Definitions
Photophobia is acquired as a result of pathology affecting the trigeminal (ophthalmic division) axon reflex ( ). Stimulation of the trigeminal sensory nerve endings of the cornea by keratitis, for example, causes reflex vasodilation in the iris. This leads to miosis and a painful response to further light-induced miosis. It is this pain, accompanied by blepharospasm and tearing, which is characteristic of true photophobia: symptoms are alleviated by mydriasis, pending treatment of the underlying condition. In contrast, dazzling or glare is a sense of excessive brightness within the visual field which can create discomfort (discomfort glare) or impair visual performance (disability glare) ( ). It must be emphasised that glare and photophobia are completely different phenomena: mydriasis might reduce the photophobia experienced by the patient with anterior segment disease, and yet the dilated pupil may increase the glare experienced ( ).
Discomfort glare occurs physiologically (and transiently) in normal vision when a person is suddenly subjected to a much higher level of luminance than that to which they have adapted. The discomfort can be long-lasting if the visual environment requires a difference in adaptation level between adjacent areas of the visual field. For example, the recommendations for workplaces (BS EN 12464: 2011) ( ) suggest that for a task illuminance of 750 lx, the immediate surround (<50 cm) of the task should be at least 500 lx, and the background (3 m) at least 160 lx. The amount of discomfort glare created by a discrete light source within the visual field can be predicted to be proportional to the brightness of the source and its angular subtence at the eye, and inversely proportional to its distance from the visual axis and the brightness of the background. However, such a formula tells us nothing about the subjective perception of the glare phenomenon. This glare does not affect vision and may just be annoying, or it may cause the individual to screw up their eyes or avert their gaze. In pathological conditions, discomfort glare occurs when the eye is constantly subjected to levels of illumination which are higher than those to which it can adapt. Sometimes it is easy to see why this would be the case, with conditions such as albinism or aniridia (where the light reaching the retina will be considerably greater than normal) or rod monochromatism (where only the low-light scotopic rod photoreceptor system is operational). Many other conditions (such as refractive error, glaucoma, optic atrophy and retinitis pigmentosa) have been associated with discomfort glare, although in these cases the mechanism of the visual symptoms is not so obvious. There are various subjective scales with which a patient could be asked to rate the amount of glare they experience ( ): the simplest is a four-point scale of imperceptible, noticeable, disturbing, intolerable . The LUMIZ 100 ( ) consists of a headset which the patient holds over their eyes, and this allows the amount of light required to reach the individual’s discomfort threshold to be accurately measured. It is not yet clear how such a device will be used in clinical practice.
Disability glare may occur at the same time as discomfort glare, but is distinguished by the change in retinal image contrast, and hence reduction in vision, which it creates.
By definition,
contrast=Lmax−LminLmax+Lmin
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