Stand magnifiers for low vision: description,prescription, assessment

Stand magnifiers are still one of the most commonly prescribed classes of low vision devices. Their performance can be difficult to understand because stand magnifiers usually do not give an image at infinity. This review summarises the methods of describing image enlargement for stand magnifiers, emphasising their relationship to equivalent viewing distance (EVD). This is done in terms of the underlying optical equations, and measurement methods, and methods of prescribing. In the past, methods of determining EVD have been somewhat indirect, requiring accurate measurement of lens power, and image position. The use of digital photography provides an alternative, more direct, simpler method of determining EVD, which can be accomplished in-office. This method is described and it is demonstrated how it gives comparable results to older methods with small, clinically non-meaningful differences, that may be due to differences in image distance reference planes. Describing the performance of stand magnifiers in terms of their dioptric power, or in terms of ‘nominal magnification’ or ‘trade magnification’, is imprecise and misleading. It is better to use indices such as equivalent viewing power and EVD, which take into account the magnifier dioptric power, the image position of the magnifier and the distance a patient is from the magnifier. While EVD is a useful index for prescribing stand magnifiers, manufacturers do not always provide sufficient technical details to determine EVD for their stand magnifiers, and available tables of EVDs are more than a decade old and are likely to need updating. Photographic comparison provides a method for determining EVD, and this method can also be applied to other low vision devices.     

New range of stand magnifiers based on standardized image position

Combined Optical Industries Ltd. Of Slough, UK, have released a new range of illuminated stand magnifiers These have been designed with u uniform emerging vergence and art1 specified in terms of equivalent power according to the new standards. This paper describes these magnifiers in terms of their measured optical and ergonomic properties. The expected enlargement ratios achieved with typical presbyopic additions are tabulated.     

Stand magnifiers: an evaluation of new optical aids from COIL

This paper discusses stand magnifiers and the optical parameters that clinicians should know in order to understand the viewing requirements and the expected resolution for low vision patients. We report on measurements made of key optical parameters of a new series of stand magnifiers from Combined Optical Industries Limited (COIL) and comparison is made to the previous series of stand magnifiers from the same manufacturer. These results illustrate that manufacturers' specifications of optical parameters are generally inaccurate and misleading. We urge manufacturers to provide clinically relevant information about their optical products.     

Evaluation of reading speed with four low vision aids

Four commonly prescribed low vision aids of equivalent power were evaluated to determine maximum reading speed of normally sighted young adults. Subjects tested were fully sighted and of similar educational backgrounds. Presentation of the four aids was designed to duplicate common clinical methods of evaluating these categories of aids. Limitations imposed under these conditions on reading speed by the optics of the four systems were determined. Maximum reading speed was attained with microscopes, followed by (in order) hand magnifiers, focusable telescopes, and stand magnifiers. Results have implications on clinicians' initial performance expectations for these categories of optical aids for patients having low vision.     

Thirty years in an urban low vision clinic: changes in prescribing habits of low vision practitioners.

PURPOSES: The purposes of this study were to determine the types of low vision devices (LVDs) prescribed by clinicians in a large optometrist led hospital low vision clinic and to investigate changes in these prescribing habits between 1973 and 2003. METHODS: Data were collected from departmental records for all patients attending the low vision clinic at Moorfields Eye Hospital in London, U.K., during the month of June in 1973, 1978, 1983, 1988, 1993, 1998, and 2003. The primary outcome measure was the type or types of LVDs prescribed to patients attending the clinic for the first time. Data were collected on the age, sex, and diagnosis of all patients attending the clinic. RESULTS: One thousand four hundred thirty-eight patient visits were identified, of which 563 (39%) were first attendances. The median age of the patients examined did not change significantly over this time. Eighty-four percent of the patients attending the clinic for the first time were prescribed at least one LVD. The devices most frequently prescribed to new patients were nonilluminated hand magnifiers, illuminated hand magnifiers, and illuminated stand magnifiers. There was a linear increase in the proportion of hand magnifiers prescribed between 1973 and 2003 (r = 0.62, p < 0.05) and a corresponding decrease in the number of near spectacle-mounted telescopes prescribed (r = 0.70, p < 0.05). CONCLUSIONS: Between 1973 and 2003, the low vision clinic at Moorfields Eye Hospital provided nearly 100,000 low vision consultations. The demographics of patients attending the clinic remained remarkably constant over this time. The number of spectacle-mounted devices prescribed to new patients fell between 1973 and 2003. It is thought this is in part the result of the increased availability of electronic magnification devices and the development of illuminated hand magnifiers. The development of the acrylic bright field magnifier and LED-illuminated LVDs have had particular clinical impact.     

Reading performance of geriatric patients post exudative maculopathy.

This study was performed to evaluate any differences in reading speed and duration of comfortable reading speed in geriatric patients with post disciform stage exudative age-related macular degeneration (ARMD) or ocular histoplasmosis. Reading speed and duration with CCTV, illuminated stand magnifier, and spectacle lenses were compared for 37 patients in the Low Vision Program at the Central Blind Rehabilitation Center. Results indicate significantly faster reading speed with CCTV and spectacle lenses as compared to illuminated stand magnifiers. The difference between reading speeds measured with spectacle reading glasses and CCTVs was borderline significant. Patients read significantly longer with CCTVs than with spectacle lenses or illuminated stand magnifiers. No significant difference was found comparing spectacle lenses or illuminated stand magnifiers with regard to reading duration.     

Low vision aids and the presbyope.

When light rays emerge divergent from a low vision device, a presbyopic patient requires a near addition to focus the image. The magnitude of the divergence and the near addition power used by a presbyope can substantially alter the magnification effects of a device. This study examines the effects of different emerging divergences and near addition powers on the magnification of low vision devices. The emerging divergence of 54 fixed focus stand magnifiers was measured using trial lenses and a telescope. The magnifiers represented a range of powers and manufacturers. The emerging divergence was not predictable. It could not be correlated to magnifier power. Although mathematical formulas exist to calculate the effect of the divergence and near addition on magnification, tables were developed to simplify this determination.     

Thirty years in the life of the Moorfields Eye Hospital Low Vision clinic

Purpose: The Moorfields Low Vision Clinic is thought to be the largest in the world. This presentation will provide a historical perspective of the work performed in the low vision clinic, based on a retrospective review of entries made in the clinic's summary ledger between 1973 and 2003. Methods: Data were collected from the ledger for all patients attending the low vision clinic during the month of June in 1973, 1978, 1983, 1988, 1993, 1998 and 2003. Demographic data and the type of low vision devices (LVDs) prescribed to newly attending patients were recorded. Results: 1438 patient visits were identified, of which 40% were first attendances. The median age, proportion of patients over 60 and sex distribution of the patients did not alter significantly over time. The most frequently prescribed devices were non‐illuminated hand magnifiers, illuminated hand magnifiers and illuminated stand magnifiers. There was a linear increase in the proportion of hand magnifiers prescribed between 1973 and 2003 (r² = 0.62, p < 0.05) and a corresponding decrease in the number of near spectacle‐mounted telescopes prescribed (r² = 0.70, p < 0.05). Conclusions: Between 1973 and 2003, the low vision clinic at Moorfields Eye Hospital provided nearly 100 000 low vision consultations. The demographics of patients attending the clinic remained remarkably constant over this time. The number of spectacle‐mounted devices prescribed fell between 1973 and 2003. It is thought this is due in part to the increased availability of electronic magnification devices, and the development of illuminated hand magnifiers. The development of the acrylic bright field magnifier and LED illuminated LVDs have had particular clinical impact. Reference: Crossland, M.D. and Silver, J.H. (2005) Thirty years in an urban low vision clinic: changes in prescribing habits of low vision practitioners. Optom Vis Sci 82 , 617–622.     

Comparison of reading speed in normal observers through different forms of equivalent power low vision devices

Compensation at near for vision loss can be achieved using a variety of equivalent forms of low vision magnifiers. Each has inherent advantages and disadvantages in terms of field, working distance, etc. We compared the reading speed of 60 trained, normal observers with 4 commonly prescribed types of low vision devices of +12 D equivalent power (spectacles, hand magnifier, stand magnifier, and telemicroscope) to their normal reading speed. Although there was individual variance in performance, reading speed with all the devices was generally within 20% of normal reading speed. Implications are that in terms of differences in reading speed for normals, the form of the low vision device is not as significant as acquiring proficiency in the use of the selected device.     

Tablet and Smartphone Accessibility Features in the Low Vision Rehabilitation

Tablet and smartphone use is rapidly increasing in developed countries. With this upsurge in popularity, the devices themselves are becoming more user-friendly for all consumers, including the visually impaired. Traditionally, visually impaired patients have received optical rehabilitation in the forms of microscopes, stand magnifiers, handheld magnifiers, telemicroscopes, and electronic magnification such as closed circuit televisions (CCTVs). In addition to the optical and financial limitations of traditional devices, patients do not always view them as being socially acceptable. For this reason, devices are often underutilised by patients due to lack of use in public forums or when among peers. By incorporating smartphones and tablets into a patient’s low vision rehabilitation, in addition to traditional devices, one provides versatile and mainstream options, which may also be less expensive. This article explains exactly what the accessibility features of tablets and smartphones are for the blind and visually impaired, how to access them, and provides an introduction on usage of the features.     

On the use of hand-held magnifiers during reading

Little is known about the actual use of magnifiers (or loupes) during reading. In studying the influence of the use of magnifiers on the reading process the reading field is a relevant factor. Four reading fields are distinguished: three horizontal reading fields and the vertical reading field. We examined the influence of a variation in the widths of these fields on the reading behavior of subjects. This was done by measuring two-dimensional loupe movements while subjects read text under conditions that provided a variety of reading field widths. It was found that individual subjects use different reading-field widths depending on loupe dimensions. Most difficulties in reading with a magnifier, especially finding the beginning of a new line, appear to result from the need to move the loupes horizontally in order to read the complete line of text.     

Near Vision Aids In Geriatric Patients With Reduced Visual Acuity

ABSTRACT The usefulness of near vision aids was studied in one hundred and forty-one patients in a geriatric hospital. The patients were over 65 years of age and had reduced vision. Where lens power exceeded +10DS spectacle lenses seldom worked well. Aspheric stand magnifiers were the most useful device in cases where greater magnification was needed. Elderly patients had difficulty in adapting to optical aids over 5X. Impaired cerebral function was the main reason for failure of the low vision aids of lower magnification.     

Eye pathology and low vision aids

The primary cause of impaired vision was retinal pathology for 414 (83%) patients, optic nerve pathology (including glaucoma) for 33 patients (7%), other pathology for 53 (11%) patients. The following optical aids were prescribed: 35 distant spectacle corrections, 159 high-add bifocals, five high-add trifocals, 52 full-size magnifying spectacles, 102 half-eye magnifying spectacles, four stand magnifiers, and 44 paperweight type magnifiers. Of 193 patients evaluated, 122 (63%) were considered to have success with the aid prescribed.     

Low vision in young New Zealanders: a survey of Homai College

The records of 809 students attending Homai College in New Zealand were reviewed. The three major causes of vision loss were congenital cataract (11.0 percent), optic atrophy (9.1 per cent) and albinism (8.8 per cent). The percentage of Homai students with vision loss due to cataract, optic atrophy and retinopathy of prematurity (ROP) was lower than that reported in 1978. Males (59.3 per cent) were more commonly affected than females. The three most commonly used visual appliances were spectacles (39 per cent), distance telescopes (25 per cent) and stand magnifiers (9 per cent). The data are compared with other reports from young visually impaired populations. A future role for optometry in the detection of vision loss in New Zealand children is suggested. Further investigation into the aetiology and racial distribution of vision loss in New Zealand children is needed.     

Low-vision aids in age-related macular degeneration

In spite of the many patients with low vision due to age-related macular degeneration, the year's publications on their rehabilitation are few. The most common complaint of these patients is their inability to read. For this purpose, simple optical devices, eg, overcorrection in reading glasses, hand-held or stand magnifiers, are usually best. With closed-circuit television systems, reading speed may be faster and reading time may be longer but these systems are still too expensive for most users. Results on bifocal intraocular lenses used as a Galilean telescopic system are still unconfirmed. New ideas for many technical devices suitable also for elderly patients have been presented.     

Eye movements and reading with plus-lens magnifiers.

PURPOSE: This study aimed to determine the eye movement parameter(s) that is primarily affected when fully sighted subjects use plus-lens magnifiers for reading. METHODS: Two forms of plus-lens magnifier were used: hand-held and spectacle-mounted. Eye movements were recorded using an infrared limbal reflection system as subjects read passages of text with and without the magnifiers. Data were compared for a young age group and an elderly (education-matched) age group. RESULTS: For both age groups, a sawtooth pattern of eye movements was observed and reading speed was significantly (p = 0.0001) reduced when using the magnifiers compared with reading without. Forward saccade length decreased significantly (p = 0.0001) and time for retrace increased significantly (p = 0.0001), but fixation durations were unaffected. The form of magnifier had no significant effect on reading speed or eye movement parameters. CONCLUSIONS: The reduction in reading speed that occurred when using the plus-lens magnifiers was primarily a result of decreased forward saccade length.     

Determining magnification for reading with low vision

Background : In the past, practitioners have used distance and/or near visual acuity (VA) to calculate required magnification for low vision aids. Magnification was usually under‐estimated when compared with the final magnification prescribed. Recent studies have emphasised the importance of acuity reserve in determining the required magnification for optimum reading rate. Two different approaches have been proposed for the appropriate acuity reserve to use in calculating magnification. These are a fixed acuity reserve of 0.3 log unit or an individual determination of optimum acuity reserve. The aim of this study was to investigate the magnification and reading rates with low vision aids selected by the two methods. Methods : Nineteen low vision subjects with age‐related macular degeneration (AMD) who were experienced magnifier‐users were recruited. Reading rates and near VA with low vision aids determined by the fixed and individual acuity reserve methods were compared with the same measures made with the subjects' own magnifiers. Results : There were no significant differences in reading rate and near VA measured with low vision aids selected by either the fixed or individual acuity reserve methods or the subjects' own magnifiers. Reading rate with low vision aids was not significantly different from reading rate for large print with conventional near additions. Thus, for experienced users, magnifiers do not cause reduced reading rate. Conclusions : The fixed acuity reserve method is simple to apply as only near VA and print size of the target reading task are required. For the individual acuity reserve method, reading rates at different print sizes need to be measured. We recommend the use of a fixed acuity reserve (0.3 log unit) for the calculation of required magnification for low vision patients. If near VA or reading rate are not satisfactory with the magnification calculated by this method, individual assessment of required acuity reserve is necessary.