LOW VISION CHAPTER 14 Electronic vision enhancement systems.docx

CHAPTER 14 Electronic vision enhancement systems Classifi cation and defi nition The broad range of electronic vision enhancement systems (EVES) currently on the market has resulted from general consumer demand for the individual components (video cameras and display screens) and the simplicity of...

CHAPTER 14 Electronic vision enhancement systems Classifi cation and defi nition The broad range of electronic vision enhancement systems (EVES) currently on the market has resulted from general consumer demand for the individual components (video cameras and display screens) and the simplicity of connecting them together (Fig. 14.1 \[Plate 15\]). As a result, many of the companies selling EVES do not have a strong interest or background in low vision rehabilitation. Although attempts have been made to classify EVES in terms of performance attributes, such as portability, ease of operation and aesthetic appearance,1 these are diffi cult to quantify objectively. Many systems have more than one option of viewing display and image enhancement features. It is therefore suggested that EVES are classifi ed as outlined in Figure 14.2.2 EVES are often referred to as closed-circuit televisions (CCTVs), because of the direct cable link between the camera imag ing system and the monitor viewing system (in contrast to broadcast television), but this description generally refers to surveillance devices and does not indicate the provision of fea tures, such as enlargement and contrast enhancement, found in devices for the visually impaired. The term 'electronic vision enhancement systems (EVES)' better distinguishes and describes such devices. EVES: advantages and disadvantages Electronic devices that provide electronic (transverse) enlargement were introduced in the early 1970s, although the concept was fi rst described by Potts et al3 in 1959. In 1969, Genensky4 went on to describe modifi cations such as variable magnifi cation, self-focus ing, reverse contrast, high-speed line return and a Gestalt system (to allow a wider fi eld of view, with a small area of interest magni f i ed). Advantages and disadvantages are listed in Table 14.1. 14.2.1 'Mouse'-style devices 'Mouse'-style, more portable, EVES were fi rst described in the 1970s.5,6 They consist of a camera usually mounted on rollers in a small hand-held case that can be moved over the object of interest (Fig. 14.3). They encompass features common to more traditional stand-based EVES (Fig. 14.4), such as contrast reversal and variable magnifi cation (although the range is usually limited), and they may be self-focusing. Mouse-style devices are more portable than con ventional stand-based EVES, but battery power options tend to be heavy and expensive, and they need to be connected to a television set or monitor. Early-model mouse EVES required a fairly fl at surface to be rolled over for good image clarity; tasks such as writing could not be performed beneath them and patients often had diffi culty maintaining their spatial awareness of the text layout.7 Newer models have a larger depth of focus, enabling curved sur faces to be viewed, and they can, in addition, be mounted on a stand or tilted on edge to enable the user to write beneath the camera. Viewing monitors are often integrated in newer versions, but have a limited fi eld of view due to size and weight constrictions Head-mounted devices Head-mounted video magnifi cation devices allow greater porta bility and usage for a wider variety of tasks.11 Video screens are viewed through highly powered positive lenses, which enable the user to focus the screen at a greatly reduced distance, thereby allowing a relatively large fi eld of view. The advent of miniature solid-state electronics has enabled the production of more portable and cheaper electronic magnifi cation aids12 that can be attached to a head-mounted system, such as the low vision image system (LVIS) or low vision enhancement system (LVES). The use of such devices can improve both visual acuity (up to 10-fold) and contrast sensitivity (by up to 1.8 log units), without affecting reading speed or comprehension (Fig. 14.5).13,14 These devices can be time consuming to fi t, can cause problems with claustrophobia and motion sickness, and the user requires steady head and hand control if the system is to be used effectively. Head-mounted display viewing can also decrease task performance compared with magnifi cation and fi eld of view-matched monitor viewing.15 Another important issue is the adequacy of retinal stabilisation during head movements, as at velocities of greater than 20 degrees per second (typical of those experienced when walking) dynamic visual acuity is decreased regardless of the magnifi cation.11 There fore, it has been suggested that presently available head-mounted low vision systems are suitable for only a small number of patients who are highly motivated and mentally alert.11 14.2.3 Comparison of devices Several studies have compared different types of EVES to optical LVAs.13,15--21 The main fi ndings have been that EVES allow reading to be faster and of longer duration than is possible with optical LVAs. Smaller print sizes can also be read. For a basic reading task, no differences in reading speed between a mouse and stand-based EVES have been found.7,15,21 However, head-mounted viewing reduces reading speed, and some tasks are faster to perform with an optical LVA than with EVES, particularly when undertaken by young visually impaired persons.15,2 EVES users and usage The majority of EVES users are young and highly motivated, although children are less likely than adults to use their EVES regularly.9,10 Unfortunately the cost of EVES has excluded them from being commonly prescribed, with reported EVES prescribing in non-Veterans' Affairs low vision clinics ranging from approxi mately 1.7%23 to 25%,24 being highly dependent on funding issues. EVES may be purchased by government departments or charitable organisations for certain needy individuals. In the UK, EVES used to be prescribed, if 'essential to employment', under the Manpower Services Commission Scheme,22 but this now occurs through the 'access to work' scheme, which is part of Employment Services (a government agency). Children usually obtain EVES through the educational authority after formal assessment by suitably trained individuals. The main use of EVES has been to assist the visually impaired with reading. Other uses include dealing with correspondence, school tasks, fi lling in forms, writing and viewing photographs.9,16,25 EVES can also be used for more individual tasks such as performing a hobby, with 42% using their EVES for tasks other than those they had originally intended.26,27 Newer head-mounted systems can be used for a range of different tasks including reading, watching tele vision, computing, sight-seeing, typing, writing and doing repairs.11 Follow-up of patients prescribed EVES by Veterans' Affairs in the USA, after approximately 2 years, has indicated that 85--90% of them still use their EVES, fi nd them benefi cial and demonstrate effi cient use of the device.8,26,27 Age, acuity and aetiology of visual impairment have been found to be unrelated to continued use of EVES.26,27 Interestingly, those with a distance visual acuity better than 6/90 (approximately 1.2 LogMAR) have been found to gain little benefi t from using EVES, many having higher reading speeds with their optical LVA.10 Enlargement As with all magnifi cation aids, the necessary level of enlargement can be predicted (see Ch. 9), although EVES allow the user to modify this level depending on personal preference and the task being undertaken. In general, self-selection of enlargement will tend to be greater than that of optical LVAs used by the patient.18 Enlargement is the measured increase in size of the image on the EVES monitor with respect to the real image (object) size. Moving the patient closer to the monitor produces relative dis tance (proximal) enlargement (see Section 9.4.1). This type of enlargement does not affect the patient's equivalent fi eld of view and may be facilitated by myopia, accommodation or a higher reading addition. However, studies indicate that working dis tances used with stand EVES are generally consistently chosen to be about 40 cm and are always less than 1 m.7,8 Not one patient, in a large survey of EVES users, was found to employ special focus glasses for EVES use.9 Practical advice To measure the chosen enlargement of an EVES, divide the width of the object image on the screen by the width of the true object underneath the camera. Reading speed and fi eld of view As the viewing screen size is fi xed, increases in magnifi cation decrease the fi eld of view and, importantly, the number of letters or words that can be seen at any one time (Fig. 14.6). Increments in magnifi cation enable an initial increase in reading speed up to a maximum, followed by a plateau or decrease with further increases (see Ch. 7).13,17 The window size (fi eld of view) require ment for reading (with page navigation) using EVES has been found to be 10 characters for 85% of maximum reading speed, but more than 20 characters for maximum reading speed to be achieved.28 Research has suggested that visual factors (decoding) rather than navigational demands constrain reading speed in those with visual impairment.7,28 The distance of the eye from the borders of the display fi eld is further than from its centre, an effect exaggerated by the slight overall convexity of cathode ray tubes.29 Thin-fi lm transistor and liquid crystal displays are fl atter and, owing to their thin profi le, more functional ergonomically. Screen resolution is unlikely to be a limiting factor for the visually impaired, but a larger screen size results in a greater fi eld of view for the same enlargement on a smaller screen. Practical advice The visually impaired should be assisted in setting enlargement levels of EVES so that reading speed is maximised. Ideally at least 10 to 20 characters should fi t within the screen width at the chosen magnifi cation. Duration of reading The visually impaired can generally read for longer with EVES than with spectacle lenses or conventional LVAs, but use a higher level of magnifi cation.18 The average reading duration using EVES has been found to be about 2 hours, compared with just 30 minutes with optical LVAs -- with dry eyes and headaches given as the main reasons for cessation of reading.8,25--27 The average duration of use at one sitting decreases with advancing age.9 14.7 Contrast Reading performance with EVES is related more to contrast sen sitivity than to distance or near visual acuity.8,30 If contrast is maximised, the magnifi cation of the print can be reduced while retaining equivalent reading performance.30 For example, reduc ing the contrast of the text by approximately 40% reduces reading rate by about 25%. Although some EVES feature the ability to display monochrome images in contrasting colours (such as white, green and amber), there appears to be no scientifi c basis to this approach apart from some EVES users having a subjective preference.31,32 The blur perceived when EVES are used while moving black-on-white print (owing to the update rate) is increased by reversed-contrast white-on-black print. However, this disadvan tage of using reversed-contrast print must be weighed against the advantages of reduced glare and less picture fl icker than when using black-on-white print.9 Preference for reversed contrast print over black-on-white print seems to be on an indi vidual basis, perhaps infl uenced by the condition causing visual impairment.16,22 Practical advice Contrast reversal should be demonstrated to all potential EVES users, and contrast should be maximised to reduce the required magnifi cation level Image enhancement Image processing can enhance the visibility of text for patients with low vision, although only if based on the individual's loss in visual function.33--35 Digital imaging can be used to add contrast, luminance or coloured fi lters, and to magnify, compress or relocate selected parts of the image to create user-specifi c images. This is not possible with conventional lenses, mirrors and prisms.36 Research on image enhancement of television-type images and face recognition has suggested that moderate benefi t to the visu ally impaired can be achieved with both patient-specifi c and generic fi lters.35,37,38 However, most of these studies have been based on preferences to static rather than true dynamic television images. To date, EVES allow minimal (mainly global contrast) image enhancement, although a recent embodiment (myReader, Pulse Data International) captures text electronically and can present it to users in their preferred format.35 Head-mounted luminance enhancement systems (similar to night vision goggles) have been shown to improve visual performance in terms of visual acuity, contrast sensitivity, motion contrast, mobility and orientation in visually impaired patients with night blindness from conditions such as retinitis pigmentosa.39,40 Training Reading speed and duration have been found to increase in the visually impaired with intensive EVES training in some studies.41,42 However, other studies have found reading speed and task per formance to be no different between (age and sex matched) visu ally impaired novice and experienced EVES users,15 and reading speed and comprehension was not improved with practice in novice head-mounted EVES users.13 There is also no evidence to indicate which components of training are necessary, benefi cial and actively increase the effi cacy of EVES usage. For those studies that have shown an improvement in EVES usage with training (over a period of between a few days and several months), the results do not appear to be related to the patient's age, visual acuity or level of education.41,42 Training programmes usually orient the patient with the EVES physical arrangement; operation of the monitor and camera, such as the enlargement of print size; and ability to track with the X--Y table, to eliminate unnecessary movements. Manual dexterity task training is needed for EVES usage and a hierarchy of increas ingly diffi cult tasks has been suggested as the basis for a training scheme. In addition, teaching to think in units rather than single words, and to learn skimming and previewing skills, may be benefi cial.42--4 Summary Electronic vision enhancement system is a more appropriate term than closed-circuit television (CCTV). EVES have many benefi ts over optical aids, such as allowing greater levels of enlargement, contrast enhancement and a more natural working distance. 'Mouse' cameras and head-mounted displays increase the portability of EVES, although they can adversely affect task performance over traditional 'stand' EVES. EVES generally allow a longer reading duration than optical LVAs, although some visually impaired patients have faster task performance with optical aids 1 Practical pearls EVES are worth considering for anyone who is in education or employment, particularly if they have a progressive sight loss condition. Relatively inexpensive options are available. It is better to demonstrate EVES to a patient and discuss the advantages and disadvantages with them than to presume (often wrongly) that they cannot afford one. With greater access to information through support groups and the internet, patients are likely to question whether EVES would be of benefi t to them at some point -- and perhaps purchase a system without expert guidance. Always calculate the enlargement required for a patient to carry out a particular task and demonstrate how further enlargement can reduce rather than enhance performance