Electronic Vision Enhancement Systems
Optical magnifying systems are limited practically to a maximum magnification of about 20×. If using a plus-lens, this would require a +80.00DS lens with a working space of only 1.25 cm (equal to the anterior focal length of the lens). Such a high-powered system would also have considerable aberrations and be severely restricted in field-of-view. Transverse or ‘real-image’ magnification is available up to extremely high levels and does not require a change in the working space from the patient’s preferred or habitual position.
The most efficient way to provide real image magnification is electronically, using a video camera to create a magnified image on a screen. The patient can be the same distance from the image on the screen as from the original object, so the existing refractive correction will be equally appropriate. Such a Closed-Circuit Television (CCTV) system was first proposed in 1959 ( ), but much credit for the development from prototype to commercial production model must go to Sam Genensky, a mathematician with visual impairment who worked at the Rand Corporation ( ; ). These systems are usually used for near or intermediate tasks, with the image presented on a screen close to the patient, although the camera can be pointed at a distant object, for example, a lecture screen or whiteboard. Flat panel LCD screens were incorporated into CCTVs in the early 2000s, making them smaller and lighter, and portable electronic video magnifiers have been used for about 20 years.
Since the early 1990s, researchers have attempted to develop wearable or head-mounted electronic magnifiers, such as the Low Vision Enhancement System (LVES) designed in conjunction with NASA ( ), although it is only since the availability of smaller lightweight screens that these have been used more widely ( ).
As electronic magnifiers have become more diverse, and to avoid confusion with security cameras, Peterson and colleagues suggested the more appropriate term Electronic Vision Enhancement Systems (EVES) is used for all electronic systems which use image manipulation such as magnification and contrast enhancement ( ).
Magnification and Field-of-View
The magnification of EVES is expressed as a direct increase in the linear size of a feature as measured on the display screen, compared to that of the original object:
M=linearsizeofimageonscreenlinearsizeoforiginalobject
There is no theoretical limit to the magnification which an electronic system can provide: up to 70× or even more is not unusual. In the early days of EVES, this extremely high magnification was emphasised, and it was suggested that patients who could not be helped with optical aids would be able to read with an electronic system. Nowadays, it is more common for the devices to be used by patients who use optical aids for some tasks, but revert to EVES when a long working space is needed, or longer duration tasks must be performed.
It may take some time for patients to become familiar with EVES ( ), and their ability to use it can increase with practice ( ). Desktop and portable EVES (p-EVES) seem to allow faster reading than optical magnifiers ( ), although this improvement is not seen with wearable systems ( ).
All EVES require a certain degree of manipulative skill, as indeed do optical aids, but in addition, the patient with limited sight must be able to distinguish all the controls: if these are all located on the same panel, they should be of different shapes or have tactile markings. When reading, text must be manipulated below the camera in a predictable and regular fashion. For desktop systems, this is usually achieved by using an X-Y platform to move the material in the X (left-right to read along the line and then return to the beginning) or Y (towards-away to move to the beginning of the next line) planes without any oblique movement. These platforms often have stops and adjustable resistance to movement to prevent overshoot of the area of interest. It is technically possible, though rare, to have semi-automatic hand or foot-operated push-button control of the platform movement or to have text presented digitally one line, or one word, at a time.
Field-of-View
If the field-of-view is defined as the area of the task (e.g. the number of words of text) visible at a given time, then this will not be influenced by the monitor-to-eye distance. The number of characters seen simultaneously on the screen depends on the magnification and the screen size. Increasing the screen size will obviously increase the field-of-view, but will give a heavier, bulkier system which may be difficult to transport or house. Patients are generally advised to use the minimum magnification possible, which will maximise the available information on the screen: given a free choice, they will usually select much higher magnification than used in optical magnification, even though this must restrict the field-of-view. Patients can also produce additional magnification by using a very close viewing distance. People with low vision habitually obtain magnification using a combination of reduced viewing distance and increased print size, with the latter being the primary method for most people ( ).
Encouraging the patient to get closer to the screen to maximise this ‘relative distance magnification’ whilst using the minimum real image magnification on the screen, will optimise the field-of-view. For example, the same retinal image size will be produced by viewing a screen with 10× magnification at 40 cm, as by viewing a screen with 5× magnification from 20 cm. In the second case, however, twice as many letters will be seen simultaneously.
Contrary to intuitive impressions, there is experimental evidence that reading can be very fast with a field-of-view of only 4 characters ( ) but in these cases the text was being automatically scanned. When the patient has to perform their own ‘page-navigation’, optimal performance may require 15 characters or more ( ).
Advantages and Disadvantages
These can be summarised as follows.
Advantages
- 1.
The use of zoom lenses permits a rapid change of magnification without altering focus. This allows the use of low magnification for overall assessment of the task before higher magnification is selected for examination of detail; for example, scanning newspaper headlines before zooming in on the text of an article.
- 2.
When compared to optical aids, EVES usually increase reading speed, reduce the number of errors made when reading and can be used for longer durations without fatigue ( ).
- 3.
The systems typically have magnification adjustable over a wide range, so the patient can use it for a wide variety of tasks, and can continue to use it if their vision changes.
- 4.
EVES might be psychologically more acceptable than optical aids, particularly in a school or work environment where screens are on every desk.
- 5.
Binocular viewing of the screen from a ‘normal’ working distance is usually possible, so there are fewer restrictions on posture or convergence.
- 6.
Many patients with severe field restrictions and reduced visual acuity can read more efficiently with EVES than with the same magnification produced by an optical aid. This appears to be because they can fixate on a single area of the screen and use the X-Y platform to scan the image through this area. In contrast, when using an optical aid they appear to use refixation saccades and miss out lines or words. Although this ‘steady eye strategy’ can be taught with optical aids, it is more difficult to learn (see section ‘Central Field Loss’ in Chapter 13 ).
- 7.
Most EVES allow contrast reversal—electronic alteration of the polarity of the image on the screen, to transform black-on-white text to a white-on-black image. This is particularly useful for patients with media opacities because the average intensity of the image is considerably reduced, and thus light scatter within the eye is decreased.
- 8.
EVES provide a higher contrast image than optical systems. This particularly benefits patients with poor contrast sensitivity for whom the optical aid provides insufficient ‘contrast reserve’ (see section ‘Visual Requirements for Reading’ in Chapter 3 ). Some systems allow a wide choice of background and text colour combination. There does not appear to be a systematic strategy to the selection of these, but some users claim to find them beneficial.
- 9.
Desktop EVES may be useful for people with other comorbidities, such as those with tremor who find it difficult to hold optical aids steady.
- 10.
Text can be underlined on the screen, and electronic windows can be created to blank out unwanted areas of the image.
- 11.
Some EVES allow split screen presentation to enable two different tasks to be viewed simultaneously. This could be used, for example, for distance and near, such as taking notes from a whiteboard.
Disadvantages
- 1.
In comparison to optical aids, these systems are usually bulkier and more obtrusive.
- 2.
Those less familiar with technology may require more practice with EVES than with optical aids to become proficient in their use ( ), although formal training is usually not required ( ).
- 3.
EVES are usually more expensive to buy than optical magnifiers, and may not be funded by low vision clinics.
- 4.
They may be difficult to repair in the case of malfunction.
- 5.
There is some persistence of the image on the display, causing ‘smearing’ of the image as it moves across the screen. This can be worse with white-on-black images than with black-on-white and can limit the maximum reading speed attainable.
- 6.
EVES are often provided ‘off-the-shelf’ without a full low-vision assessment. They do need to be prescribed like any other aid to ensure that they are suitable, and to be backed-up by optical aids for the tasks for which EVES are unsuitable. In some cases, a patient may purchase an EVES without realising a simpler and cheaper optical aid can perform the task just as well.
- 7.
The depth-of-field is limited, and scanning across the page of a thick book can cause the image to go out of focus. The autofocus on some systems can take about a second to adjust to different object distances.
- 8.
Viewing quickly moving images on EVES, particularly on wearable systems, can induce nausea and dizziness ( ).
- 9.
At very high levels of magnification, pixels may be visible on the screen, limiting image quality.
- 10.
EVES typically take longer to turn on and setup than optical magnifiers, and portable and wearable systems are often limited by short battery life.
- 11.
Some systems have standard component or control positions that are not suited to a left-handed user.
Classification
The term ‘electronic vision enhancement system’ applies to a wide range of devices, which can be classified by object distance, portability, power source and other parameters ( Table 8.1 ; Fig. 8.1 ). Broadly, these systems can be grouped into those that are desktop-based; those that are easily portable; and those which are worn, such as head-mounted devices.
| Object Distance | Portability | Power Supply | Camera | Display | Field of View | Image Magnification Type | Image Magnification Range | Image Manipulation |
|---|---|---|---|---|---|---|---|---|
| Distance | Fixed location | Mains | Independent | Standard TV | Small/restricted | Fixed | Low (≈2× – 3×) | Contrast reversal |
| Intermediate | Transportable | Rechargeable | Rotatable | Standard monitor/laptop | Large | Steps | High (≈10×) | Colour change |
| Near | Handheld | Mouse-style | Head mounted | Continuous | Extreme (≈100×) | Image capture/freeze frame | ||
| Variable | Wearable | Incorporated opposite display | Incorporated | Line windowing/marking | ||||
| Incorporated in-line | Electronic reader | Mirror reversal | ||||||
| Text reformatting | ||||||||
| Adding fixation marker | ||||||||
| Contrast enhancement | ||||||||
| Edge enhancement | ||||||||
| Stabilisation |
Desktop EVES
Desktop EVES consist of a camera, a processor and a screen ( Fig. 8.1 , left ). Modern systems typically incorporate a camera mounted underneath the screen, pointing down towards the x,y table on which text is placed. A large working space is available under the camera, which makes it suitable for use in hobbies or handicrafts—for example, building models or repairing domestic appliances. The patient must learn to look at the magnified view of their hands performing the task on the screen, rather than directly: this becomes more difficult as the screen is positioned further away from the task area.
Diagonal screen size is typically 12 to 24 in (30–60 cm), and magnification can be increased to about 75×. Controls for magnification level and image manipulation are usually placed beneath the screen or in front of the x,y table. More advanced systems include optical character recognition, text-to-speech and other sensory substitution options (see Chapter 15 ).
Although usually designed for use in one location, smaller desktop EVES may fold up for transportation. Transportable magnifiers usually have a screen of about 10 to 16 in (around 25–40 cm).
Some desktop and transportable EVES can also be used for distance viewing, using a different camera or by rotating the camera to point at a distant object. These were used widely in schools and colleges, although an electronic system to relay the whiteboard image on to a tablet computer or laptop is now more common. They are sometimes used for intermediate tasks such as DIY or applying makeup (and for this purpose may allow a ‘reversed’ image, as would be seen looking in a mirror).
Television and Mouse-Based Magnifiers
A cheaper option is for patients to use an electronic system with their existing television screen or computer monitor. In these devices, the camera is mounted in a handheld unit which looks like a computer mouse, connected to the screen by a cable. Magnification is typically fixed or variable between a few levels and will be dependent on the size of the screen used. These may have some simple image manipulation modes. It can be difficult to trace a line of text with these systems, and holding the camera straight can be hard for some users. However, they were the first genuinely ‘cheap’ electronic magnifiers, typically costing less than one-tenth the price of desktop EVES.
Portable EVES
In the last 20 years, dozens of portable electronic vision enhancement systems have become available. p-EVES incorporate a camera, light source, processor and screen in a single handheld device. Screen size varies between about 2 in and about 8 in (5–20 cm), although the point at which a p-EVES device becomes a transportable device is not well defined. p-EVES usually weigh less than 500 g and have a battery life of up to 5 hours. They have a more modest magnification range than desktop EVES, generally having a maximum level of about 10×, which may be stepped or on a continuous zoom function. They generally offer reversed contrast and may provide some other colour options, although this will typically be more restricted than on a full-featured desktop system. Manipulation of a p-EVES requires practice to learn to follow lines of text and return to pick up the next line. The action required, however, is not that different to scanning an optical magnifier across text, and most patients master the skill relatively easily: placing a strip of wood across the page and pushing the camera against it whilst moving across the page may be useful in the early stages. Some systems have a stand into which the camera can be clipped to convert it to ‘conventional’ operation if portability is not required.
Unlike desktop EVES, portable systems do not improve reading speed when compared to optical aids ( ), although they do improve near visual acuity. In Taylor’s study, people could read for longer with p-EVES than with optical devices, but optical low vision aids (LVAs) were used more frequently and for more tasks ( ). The limited screen size and reduced field of view is likely to be the major factor in reducing reading ability with portable devices: it is not physically possible to magnify text to the very high levels possible on a large-screen desktop EVES.
Wearable and Head-Mounted EVES
As cameras, screens, batteries and processors have got smaller and lighter, wearable EVES have become a more acceptable form of LVA for some patients. The principle of these systems is similar to desktop or p-EVES, although the camera is generally optimised for distance viewing. Wearable EVES comprise a camera mounted on a pair of spectacles, which also include an LCD or organic light-emitting diode (OLED) screen to display the magnified or manipulated image ( Fig. 8.2 ). The battery may be included on the spectacles or connected by a cable. Magnification of up to 25× is available and is typically adjusted using a wired or wireless remote control. Some systems also have a minification mode for people with restricted visual fields. Image processing options may include reversed contrast, edge enhancement and adjustable brightness. The first commercially available head-mounted system—the LVES, introduced in 1994—weighed nearly a kilogram; but modern equivalents are nearly 10 times lighter ( ).
