TNOA Journal of Ophthalmic Science and Research

: 2020  |  Volume : 58  |  Issue : 2  |  Page : 101--105

Integrating low vision service in clinical practice

Krishnamurthy Ilango 
 Department of Ophthalmology, Velammal Medical College Hospital and Research Institute, Madurai, Tamil Nadu, India

Correspondence Address:
Prof. Krishnamurthy Ilango
Department of Ophthalmology, Velammal Medical College Hospital and Research Institute, Ring Road, Anupanadi, Madurai, Tamil Nadu


Low vision service has been least prioritised for decades in eye care. Vision 2020[1] the Global initiative for avoidable blindness has identified Refractive errors and low vision as a priority. In spite of low vision has been a focus area in vision 2020, the service has not been widely accepted in eye care for the benefit of patients, except for a few tertiary eye care institutions. This article discusses in detail, about clinical low vision, basic optics, types of low vision aids, and inventory, required at an eye care practitioners' level to integrate lowvision as a part of refraction service in their eye care services.

How to cite this article:
Ilango K. Integrating low vision service in clinical practice.TNOA J Ophthalmic Sci Res 2020;58:101-105

How to cite this URL:
Ilango K. Integrating low vision service in clinical practice. TNOA J Ophthalmic Sci Res [serial online] 2020 [cited 2023 Jan 27 ];58:101-105
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Low vision patients are suspended between the blind and the sighted. They neither enjoy the benefits for the blind nor enjoy this world as sighted. There is no clarity on how best their remaining visual potential can be utilized to make them see better. With advancing technology, there are many options available for these patients. They have to be given proper professional guidance what best aids will help them. Low vision service is multidimensional and multidisciplinary. Eye care professionals should provide the basic low vision service and refer them to appropriate caretakers to provide a comprehensive service.

 Global Blindness on Low Vision

Currently used blindness and low vision data reports estimate 180 million visually impaired persons globally. Of these, 45 million are blind, of whom approximately 20% (i.e. 9 million) have no perception of light. These later persons require mobility training and other forms of rehabilitation. Out of the total 171 million which comprise 135 million persons with low vision and 36 million blind with residual vision (i.e. with light perception or better but < 3/60), an estimated 60% can be improved with surgical treatment, mainly for cataract and some for corneal opacity. This accounts for an estimated total of 103 million persons who would benefit from treatment (NOTE, those requiring standard refractive correction would have been already excluded from this group by virtue of the definition international classification of diseases-10 (ICD-10)). Remaining estimated 68 million persons require low vision care and are likely to benefit from such a care.[2]

The functional definition of low vision defines as a person with low vision one who has impairment of visual functioning even after treatment, surgery, or standard refractive correction and has a visual acuity of <6/18 to light perception or a visual field of <10° from the point of fixation in the better eye but who is potentially able to use vision for the planning and/or execution of task.[3]

The impact of low vision has multiple effects in a patient's life such as occupational, emotional, social, financial, and educational aspects.

 Optics of Low Vision

The concept of providing low vision aid is based on the following three different types of magnifications. (a) Relative distance magnification is achieved by reducing the reading distance. the retinal image subtended increases so the patient is able to see the print. (b) angular magnification is achieved using tow or more lenses in a telescopic system. (c) Electronic magnification also known as projection magnification which is achieved by enlarging an object by projecting in a screen, for example, the portable video magnifiers falls in this category.

 Clinical Aspects of Low Vision Care

In general, stationary low vision conditions such as albinism are benefitted by low vision aids than progressive clinical diseases such as retinitis pigmentosa or glaucoma. For example, albinism patients apart from low vision also experience dazzling glare with nystagmus. These patients will be benefitted with refractive correction that gives the best possible vision. Second, they have to be prescribed a tinted glass which is available as clip on and wear over. In general, yellow-tinted glasses are prescribed which improve the contrast, but it is better to have a set of tinted glasses in various shades and ask them to try which suits them best and then prescribe them. It is better to avoid fixed colour tints in low vision patients in their refractive correction.the colour tints blocks the light entering the eyes, when they are at indoors. Hence, they should use their tinted glasses only when they go outdoors in sunlight to avoid glare. The recently introduced transition lenses in the market avoid this problem but are expensive. Albinos can be suggested to use a wide-brimmed hat for protection against sunlight.

Macular degeneration patients have interesting observations. These patients experience central scotoma and difficulty in face recognition. At the same time, when we examine their distant vision, they will be able to read most of the lines with some head tilt and eccentric fixation. The same patient when they are given a near vision print, they will struggle to read a large N36 print. This is because the scotoma which appears small at a distance now appears large when looking at a near vision print. Reading will be a big challenge for them. These patients should be trained to use their eccentric fixation with preferred retinal locus. They will require a higher magnification and more illumination and will be benefitted with portable electronic handheld magnifiers where the text can be enlarged. If they are using a smartphone, they can increase the font size using the settings.

On the other hand patients with post viral subepithelial keratitis has crenated edges. This crenated edges scatter more light causing Glare. These patients will require least possible lighting and least magnification that is just enough to make them recognise a print.

RP patients need a compassionate and a comprehensive low vision care. When they have a maculopathy along with RP, they will have both peripheral and central field defects. Understanding their problems with a detailed history is important while giving counseling and guidance. Majority will not be benefitted by magnifiers, and even when they are prescribed as the disease advances, they will not be able to use it. Magnifiers to some extent will help them if their visual axis is clear and provided with a support of good lighting.

The reverse Galilean telescopes that are recommended to enlarge the field have no practical use except to compress the objects in the field. This will help them to know what is in the room as a spotting device. With their increasing dark adaptation time and night blindness, patients have to be explained about the disease and counseled for adaptations. They can be advised to use a torchlight in dim illuminations. However, if the vision is tubular and vision falls <3/60, they have to take up a mobility training with mobility instructors using a cane. Many patients at the first instance will not be willing for this idea, as they may not be able to accept due to the stigma in the society of using a cane. The other alternative will be to take a sighted guide. In developed countries, guide dogs are popular and most of the blind people have a guide dog trained for mobility.

 Vision Assessment and Prescription of Low Vision Aids

Distant vision

A detailed history and examination with standard log mar charts at 4-m distance to be used is now available in regional languages. A standard logMAR chart comes in a mounted stand with backlit illumination and a wheelbase, so that it can be moved closer to patient till they identify the optotypes. For near vision assessment, it is better to test the reading ability (number of words read per minute) than the reading acuity (size of the letter print). Depending on the patient requirement, a suitable low vision aid has to be given trial with appropriate lighting and prescribed.

At the end of the examination, there should be an understanding of what low vision devices and other low vision services the person needs.[4]

Distant vision aids

As far as distant vision is concerned, the only low vision that can be recommended is a telescope which is available as handheld or spectacle-mounted device. Low vision telescopes are Galilean telescopes, and the image is erect in the eyepiece. [Figure 1] shows Galilean handheld telescope. Whereas, in Keplerian telescope, the image is inverted, so a reinverting prism is incorporated and used for astronomical purpose. Conventional telescopes have shallow depth of field and narrow field of view and the need to change focus for different distance. These characteristics make low vision telescopes difficult to use, especially at important midrange distances. Hence, the Ocutech company has come out with an innovative Keplerian optical design called vision-enhancing system (VES).[5] The VES eliminates these constraints and is cosmetically acceptable. It can be mounted on the frame of a spectacle. The VES combines the principle of telescope and periscope.{Figure 1}

Low vision aids for near vision

The following formulae can be used as a starting point to determine the near vision addition.

Kestenbaum's formulae[6]

If the distant vision is 6/60, the near vision will be the reciprocal of the distant vision which is 60/6 = 10 Diopters. This is just a rule of thumb, but the patient will require more addition also depending on their contrast sensitivity and the nature of disease. It is also important to instruct the patient to hold the print at correct focal distance. For example, a +10 Diopter spectacle has to be held at a 10-cm distance from the eyes which will be it focal length.

Spectacle low vision aids[7] are used for patients with binocular visual potential, and the prismospheres are most commonly recommended. These are high addition spherical lenses with base-in prisms. Base-in prisms are available as half eyeglasses from the range +4 to +10 Diopters. [Figure 2] shows a prismosphere.{Figure 2}

The advantage of prism glasses is that the prism is placed with base in and avoids convergence strain.

They can be recommended for reading for prolonged use provided the patient has binocular potential.

More than +10 diopters a patient cannot use the glasses for binocular use. In that case for monocular use, the dominant eye with a better vision is given a high addition lens. When we go to higher additions, aspheric lenses are preferred as they avoid the distortions present in spherical lenses.

 Hand Magnifiers and Stand Magnifiers

Hand magnifiers are high convex lens mounted in a handle. Hand magnifiers are available with built-in light-emitting diode (LED) lights for illumination over the print. Hand magnifiers are available from +6 Diopters to +24 Diopters.

The magnification in hand magnifiers are mentioned in terms of ×, and each × corresponds to 4 Diopters. Hence, a × 4 hand magnifier has a 16D convex lens mounted in it. [Figure 3] shows illuminated hand magnifiers.{Figure 3}{Figure 4}

 Stand Magnifiers

Stand magnifiers have a high-power convex lens mounted on a stand with fixed focal distance. The patient has to hold the stand magnifiers flat to the print material to fix his dominant eye in the magnifier and move the stand over the print line by line and read the words. To do this, he needs a reading stand and a light that is kept behind the stand so that the illumination falls over the print. A stand magnifier is more commonly prescribed for this purpose which has a 24D lens mounted on the stand.

Instead of a stand magnifier, a dome-shaped magnifier, which looks like a paperweight, can also be used. This type of magnifier is usually preferred by children.

Recent advances in low vision aids

Closed-circuit television systems

The closed-circuit television cameras are used for security purpose. The same camera is used for low vision purpose also. The camera is mounted inside a computer mouse and kept over a print material and connected to a television monitor. We see a projected enlarged image of the print material.

Those patients who do not improve with conventional optical magnifiers will require a projected magnification to see enlarged text. This will be used in a stationary setup. The magnification can be increased or decreased depending on the patient's visual requirement. It has been shown that for low vision patients who read more slowly, reading speed may improve at higher magnifications despite reduced field size.[8]

 Portable Video Magnifiers

Currently portable video magnifiers are popular globally with a number of choices from various companies and with various options. Video magnifiers use the same camera lens used in mobile cameras, and the image captured is projected to a liquid crystal display screen. These video magnifiers have additional LED lights at back surface to illuminate the text so that the patients can see the print clearly. The background color can be changed with contrasting text so that the patient can make a choice of the color background that best suits their visibility. The text page can be taken screenshots to read later and can be stored. There are video magnifiers with ergonomic handles so that the patient can hold the video magnifier over a sheet of paper and write with the other hand and use it for signing cheques etc., [Figure 3] shows the different types of portable video magnifiers.

 Smartphones and Applications (Apps) as Low Vision Aids

Smartphones have come as a big boon for low vision patients.[9] The apple and android phones have many accessibility features that are inbuilt for a low vision patient that comes handy.

If we look into the accessibility feature of an android phone, it has various categories as vision, hearing, dexterity, and interaction. In the vision category, the following options are available. The font size can be increased. There are magnification gestures such as zoom in and out with tapping by fingers or pan dragging two or more fingers across the screen. The zoom level can be adjusted by pinching two or more fingers together or spread them apart. Android has a feature called talkback which functions as a voice-over so that a low vision patient can use it as a supplementary to read E-mails along with increasing the font size. I phone has the Siri voice-over and Windows has the Cortana and Google with the Alexa. All these make the life of low vision patient much easier.

There are several Apps available in Android and Apple platform that helps low vision patients.

 Fresnel Prisms

A Fresnel prism is nothing but a sheet of multiple prims stacked together, so it moves the objects from the nonseeing temporal hemifield to the seeing nasal hemifield.

Preferably young patients with stable vertical hemianopia with clear visual axis are suitable candidates for trying Fresnel prisms. A 40-prism press on 3M Fresnel is fixed to spectacle lens with base out as a small strip on the inside of the temporal aspect of the spectacle. [Figure 5] shows a Fresnel prism sticked to the back surface of spectacle on the temporal side.

The concept is that a prism moves the objects from base to apex. Hence, this is an awareness aid for the objects in the temporal field.[10]

Apart from optical aids, there are nonoptical aids such as reading light, large print books, writing sheets with black slits, bold tip pens, talking clocks, and reading stands which can be supplemented to optical aids and also helpful for the Blind. [Figure 4] shows nonoptical aids.

 Dispensing Low Vision Aids

Low vision aids have to be stocked in the optical so that the accessibility to patients is made easier.

Low vision devicesInventory for setting up a low vision clinicEquipments required are. Streak retinoscope, ophthalmoscope, lensometer, trial lens set (full aperture) adult trial frame (2) pediatric trial frames (3 pairs of different sizes) trial lens holder, Halberg trial lens clip, occluder with pinholes, cross cylinders (0.5, 1) Pen torch.

 Requirements for Vision Assessment

Distant vision tests

LogMAR test chart with backlit illuminated drum – with letter, number, symbol – E Chart (one for each type), and contrast sensitivity test chart.

Near vision tests

LEA symbol pediatric test chart, Near vision test – numbers and continuous reading text, preferential looking charts with LEA paddles and cards of differential contrasts, color vision test charts, Amsler grid, and perimeter test with Tangent screen.

 Optical Low Vision Devices

Spectacle magnifiers (half eyeglasses) from 6D to 10D in 2D steps with base-in prisms12–40D in 4D steps as half eyeglassesHandheld magnifiers with and without built-in light source, from 5D to 42DStand magnifiers with and without built-in light source, from 12D to 56DDome and bar magnifiersHandheld monocular telescopes from 2.5 ×, 3 ×, and 4 × one in each for trialColored filter glasses of five different shades with UV protection and luminous transmission of 40%, 18%, 10%, 2%, and 1%,CCTV device tabletop and portable handheld magnifierComputer devices with preloaded computer software having text enlargement and voice outputFresnel prisms 40D single piece for trial. The suppliers of low vision aids are given in ANNEXURE I.

 Non Optical Aids

The non optical aids includes the following

Table lamps.large print books.Bold tip pens.large print calculatorsletter writing guides.

[Figure 4] shows Non optical aids.


This article attempts to provide a comprehensive overview of low vision services which can be provided at a practitioner's level of eye care. As the legend Helen Keller has quoted, “here is an opportunity” to help the visually impaired. Low vision aids improve the patient quality of life. What is needed is the willingness to use the opportunity and help the low vision.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

 Annexure I: Suppliers of low vision devices

Baliwalla and Homi Pvt Ltd, MumbaiLensel Enterprises, PuneLow Vision Resource Centre, Hong KongMadhu Instruments, New DelhiOm Tao Scientific Apparatus, HyderabadSoham Low Vision Centre, Kolkata.


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