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Dual Sensory Impairment - Devices for Deafblind People

Dr. John Gill
February 2001

For the purpose of this document, the term deafblind refers to a person having combined loss of hearing and sight to such degree that he or she cannot make immediate use of facilities for those with impaired hearing or sight alone. This definition therefore includes people who have a combination of a severe hearing loss and low vision.

The estimates for the numbers of deafblind people varies considerably because very different definitions are used in different countries. However using the definition above in developed countries, the numbers will be of the order of 200 per million of the population.

In November 2000, the UK Minister of Health said "We know that at least 23,000 people in this country have combined hearing and visual impairment which is sever enough to cause problems communicating, obtaining information and generally getting about". This higher estimate of numbers is because of a looser definition of deafblindness. However the Royal National Institute of the Blind claim that there are at least 250,000 people who have less severe dual sensory impairment but still have difficulties with access to information and communication, and mobility problems.

Some deafblindness is the direct result of diseases such as Ushers or Lawrence Moon Biedl syndromes of retinitis pigementosa. In some cases it is combined with a cognitive impairment. These diseases may be present from birth.

With other individuals the causes of their visual and hearing impairments may be unrelated. Accidents, such as from explosive devices, can result in deafblindness but may also result in a physical disability.

The numerically largest group of people with a dual sensory impairment are those over retirement age. This group may have a number of other impairments; the net effect of these impairments is often more than the addition of each impairment.

For a device to be useful to a deafblind person, it does not necessarily have had to be designed specifically for the deafblind. However more and more devices for blind people employ audio output, such as synthetic speech, so there are fewer inexpensive devices with tactual output. For instance the number of electronic calculators with Braille output is falling since the cost of synthetic speech output has dramatically reduced in the recent years. These trends have been to the advantage of blind people but have significantly reduced the choice available to deafblind persons.

Since the number of deafblind persons in a country is numerically small and they are not a homogeneous population, the cost of developing and marketing devices to meet their needs is high. In the past the non-profit organisations often subsidised both development and manufacture, but these organisations are now under pressure to minimise their financially loss making activities.

Although there has been a decline in the range of lower price and often low technology devices, the advent of computer systems and the internet has opened up new possibilities for users who can cope with the technology and who can find someone to pay for the purchase and running costs of the equipment. These developments have also required organisations serving deafblind people to develop training and support skills for these high technology systems.

The role of state provision varies considerably from country to country. In the past the socialist countries of Eastern Europe provided relatively generous funds for assistance of deafblind individuals who had above average intelligence; the change in the economic environment has resulted in cutbacks in these services. In western countries the lead is usually taken by voluntary organisations with some subsidy from the state.

Providing assistance to deafblind individuals is a very labour intensive activity and therefore expensive if the carers have to be paid. Also these carers need to be trained in how to communicate with a deafblind person.


The deafblind manual (finger spelling) is similar to the manual alphabet for deaf people but the letters are formed on the deafblind person's hand. Block alphabet, also known as Spartan, involves tracing capital letters on the deafblind person's palm. It takes time to come proficient with either of these methods of communicating, and the speed seems very slow compared with verbal communication. Hands on signing is an adaptation of sign language for people with low vision.

The simplest form of communication device is a magnetic board with raised metal letters (as used on notice boards); this has the advantage of simplicity but it is very slow. A faster device is one where a keyboard is connected to a single-cell Braille display; the disadvantages are that the deafblind person must be able to read Braille, and the cost of the device. There are a number of more sophisticated electronic devices which can be used over the telephone via a modem (a device which converts the digital signals into a form suitable for transmission along a telephone line).

The Braille code was developed by Louis Braille based on an earlier system for communicating with troops after dark. It is based on a 6-dot cell which can have only 64 different configurations. This gives a problem in that many more than 64 characters are used in modern printed texts; so, at times, more than one Braille character is needed to represent one print character. A Braille book is typically 20 times as bulky as the print edition. Therefore a form of shorthand is employed which uses 189 abbreviations and contractions giving a space saving of about 25%.

For computer use it is becoming common to use an eight dot Braille system which can represent 256 characters using only a single cell.

Since there is a shortage of skilled transcribers, computer systems are often used to translate text to contracted Braille which is then output on a special embosser. The algorithms for this translation are not simple since the rules governing the use of contractions depend on pronunciation and meaning. For example, there is a contraction for 'mother' which can be used as part of a longer word as long as it does not bridge a syllable boundary as in 'chemotherapy'.

The Moon code was developed by Dr William Moon of Brighton in 1847. Since it has similarities to ordinary print characters, it is easier to learn by persons who have previously read visually. However it has the disadvantage that it takes about 80 times the volume of the print version. Also the high cost of production has meant that very few books are printed in this medium. The number of Moon readers has dwindled to about 400, most of whom are in the UK.

Up to recently Moon was produced boustrophedon (which literally means turning as oxen in plowing) which meant that the user did not have the problem of backtracking from the end of one line to the beginning of the next. A disadvantage was the perceptual problem of reading alternate lines in reverse.

The increasing use of graphics in printed books gives problems. Although many diagrams can be converted to an embossed form, the process of reading by touch means that a diagram has to be tactually scanned and mental image built of the whole diagram; this is the opposite process to visual reading where one looks at the overall picture and then reads the detail.

Some years ago NASA had a problem with communicating with astronauts during lift-off. The problem was of information overload using visual and auditory communication. Therefore they investigated the use of tactual communication; the project failed, but the research formed the basis of a reading aid for blind and deafblind persons. The Optacon used a hand-held camera connected to 144 piezo-electric elements which gave a vibratory image of the print character; the task of recognising the character was left to the human. Its use has been limited by the low reading speed (typically 40 words per minute after extensive training).

Systems to recognise printed characters have been developed for inputting text to computers. Such systems have immediate application for deafblind persons since the information can be output in Braille. None of these systems can read hand-writing satisfactorily.

Over the years a number of devices have been developed to emulate finger spelling since many deafblind people do not read Braille. Although these devices work well in a laboratory, there have been problems in making them generally available at affordable prices.


The environment in which we live is becoming increasingly complex. Even a journey across a city by bus requires a range of skills including:

  • Being able to avoid obstacles on the pavement
  • To walk in the right direction
  • To safely cross the road
  • To know when you have reached a destination (eg found the correct bus stop)
  • To know which is the right bus
  • To pay the correct fare
  • To find a vacant seat
  • To know when to alight from the bus

These tasks may seem trivial but for a deafblind person they are skills which have to be learnt. Even for someone with low vision, all these tasks are less easy than for someone with normal sight.

The traditional symbol for a blind person is a white stick; the convention in the UK is for a deafblind person to use a white stick marked with red bands. For mobility purposes, this was developed into a long cane which is swept in front of the user to check for a clear path. Since it is rigid it can transmit information about the surface texture. The cane has to be light as well as rigid, so aluminium tubing is often used for construction, but carbon fibre technology has been utilised to reduce weight even further.

A significant disadvantage of the long cane is that it provides no information about obstacles at head height, such as lorry wing mirrors or overhanging holly bushes. This problem can be overcome by the use of a guide dog. Guide dogs have a number of disadvantages in that they cannot be taken everywhere and they require care. For some people, they are the ideal mobility aid but total demand is estimated to be less than 5000 dogs in the UK.

Over the last thirty years, engineers have devoted considerable resources to developing electronic systems to help blind persons avoid obstacles. The most common approach has been to use ultrasonics; as with radar, the range is obtained from the length of time it takes for a pulse to be reflected back to the transceiver. Other systems have used lasers or infra-red.

Many of the devices just provided information about the range of the nearest object; a 'picture' could be built up by moving the sensor from side to side. Other devices have attempted to give a more complete image of the environment but at the expense of providing an excessive amount of information to the user.

The main problems are not in designing the electronic circuitry for a satisfactory mobility aid but in:

  1. Identifying the optimum information needed for independent pedestrian travel.
  2. Displaying this information to the user in a non-visual format.
  3. Manufacturing the device at a reasonable price.
  4. Training the deafblind persons in the use of the device.

The capacities of the sense of touch is very small compared to that of the visual channel for a human. Therefore selecting and processing the information to make best use of the tactual sense is not a trivial task. The sensors in future devices are likely to involve more than one modality (eg both a video camera and an ultrasonic transceiver) in order to obtain the necessary data which can be processed to produce an accurate image of the immediate environment. However the research which has been done on the automatic processing of satellite pictures and the research on neural networks offer hope that significant advances can be made in the next few years.

For deafblind persons, the problem of getting about is not just that of not walking into objects. One problem is that of knowing the layout of the environment; here, an embossed map can help. However embossed maps are not easy to produce or interpret since just embossing a sighted map seldom leads to an intelligible embossed map.

The problem of converting a sighted graphical representation to an embossed one can be illustrated by the problem of indicating direction. Visually it is often shown as an arrow on a line. An embossed arrow gives a sense of direction at only one point on the line and the symbol is unfamiliar to many blind persons. However a line sawtooth in cross-section has an indication of direction over the whole length of the line, and it is easy to associate the symbol with the meaning since the line is smooth in one direction and rough in the other.

Even with an embossed map and a mobility aid, it is still very easy for a deafblind person to get lost. A number of electronic aids have been developed, but few have been widely used because of the cost of modifying the environment.

One type of system uses infra-red transmitters mounted at light-controlled pedestrian crossing; the infra-red signal is modulated so that a receiver, held by the deafblind person, gives a tactual indication of the status of the traffic lights.

Satellite navigation systems, such as the American Global Positioning System (GPS), can be used to determine one's position to a few metres. However this requires line of sight to three or four satellites, which means being outdoors and not close to tall buildings. This position is just given as latitude and longitude, so it needs to be integrated with a detailed digital map of the area.

An alternative method of determining one's position is possible from mobile Telephony by determing the relative signal strengths at different base stations. With the next generation of mobile systems, this has been further developed so that sighted users can be provided with information related to their locality. The advantage of the mobile Telephony system is that it does not require line of sight to satellites, but the accuracy may not be as good as GPS. But, as always, the price charged to deafblind users for the equipment and using the service will be a significant factor in determining its takeup.

Signals and Alarms

These are a number of alarm clocks with raised markings on the dial and an electrically activated vibrator for the alarm. For safety reasons, it is preferable to run the vibrator from a low voltage source (such as rechargeable batteries). The vibrators are typically small electric motors with eccentric weights. Clockwork vibrators are not used in any of the standard products; the reason for this is not clear. The vibrator does not need to be directly connected by a wire to the clock - it can be triggered by a low power radio signal. This means that the vibrator can be worn on the body (eg like a wristwatch).

The simplest doorbell signallers involves a push-button being connected to an electrical device that then transmits a signal to activate a vibrator worn by the deafblind person. Sometimes the transmission is done via a closed loop aerial; this has a number of disadvantages including the high cost of installing the aerial and that the device only operates in the immediate vicinity of the loop. These systems are being superceded by radio devices. The input to the system is not necessarily a push-button; it is possible to use an infra-red detector, but a simpler method is a pressure pad under the carpet (as used as a component in some burglar alarm systems).

A sound indicator is an electronic device that gives out a vibratory signal when it picks up an audio signal (eg a telephone bell) above a pre-set level. Usually the vibratory signal lasts for a fixed minimum time, and the amplitude of the vibration is usually constant (ie independent of the amplitude of the input signal). With some devices it is possible to tune the device to only pick up audio signals at or about a fixed frequency; this is useful in minimising the number of times the device is activated by picking up the wrong audio signal.

The decreasing cost and size of radio paging systems has encouraged the development of modifications to assist deafblind persons. Another significant factor has been the liberalisation of the laws, in many countries, governing the use of very low powered radio transmitters.

Other Low Technology Devices

A small number of devices have been modified to give vibratory output. For example a liquid level indicator, light probe and typing aids. These are numerically very few because the development and manufacturing costs are very high for a small national market.

Developments for the general public are not always to the advantage of deafblind persons. For instance, the standardisation of packaging means that aerosol containers of oven cleaner and hairspray feel the same. One small step was the introduction of an embossed triangle on packaging of dangerous substances. A few items are specially designed to be easy to differentiate by touch to help blind persons; for instance the bank notes in the UK are of different sizes depending on denomination.

There have been a number of proposals for using barcodes to help blind and deafblind persons sort their groceries at home. The barcode gives the product number so it would be necessary to have a databank to relate this number to the product name or label information. However it might be possible to have a barcode reader connected to a database via the telephone or internet, with the output being in Braille. The cost of establishing and running such a service has not made it attractive to potential operators.

In the past the controls on most domestic appliance (eg washing machines, cookers and central heating) could be modified by adding embossed markings to the control panel. However the change from electro-mechanical controls to dynamic visual displays has meant that other solutions must be found. Developments such as Bluetooth may mean that it will be viable to connect a Braille or large visual display to the appliance.

Bluetooth is a radio system for interconnecting systems such as mobile phones, televisions, and central heating controllers. It works at about 2.4 GHz and has a range of about 10 metres.

Access to Information Technology

It was the advent of the personal computer with Braille or magnified visual output that opened up opportunities for a significant increase in access to information for deafblind people. Software for producing large characters on the monitor is relatively inexpensive, but Braille displays have remained expensive.

The DOS text-based operating system is easier for many deafblind people than ones, such as Windows, which use a graphical user interface. However keeping to DOS restricts the choice of software in that most new software is written for the Windows environment.

Fortunately Email: systems are predominantly text-based and therefore relatively easy to use with a Braille display. The world wide web is more problematic in that many sites employ graphical representations without an adequate text alternative. The sites which are fully accessible tend to be ones belonging to government departments, and the ones which are largely inaccessible are the popular home shopping sites. Even with these restrictions the internet has the potential for significantly increasing access to information by deafblind people. What is needed is a range of affordable user friendly terminals which provide access for deafblind people who need non-visual output but who do not read Braille.

The basic mobile telephone has been of limited use to deafblind people. However the introduction of Universal Mobile telecommunications System (UMTS) offers exciting possibilities if the services are affordable. For instance the ability to transmit pictures of where you are to a service centre and receive textual replies, could greatly assist a deafblind pedestrian in an unfamiliar environment.

Related to developments in telecommunications will be radio-based systems for short-range interconnection of both domestic equipment and public terminals. Systems such as Bluetooth have the potential to facilitate the connection of assistive devices to a whole range of equipment. If this technology lives up to the publicity, then deafblind people can anticipate significant improvements for those who wish to live independently.

However not all technological developments are to the advantage of deafblind people. With analogue television it is possible to obtain Braille output of Teletext which gives basic access to the news. However digital Teletext is graphically based so this is no longer possible.


Although technology could greatly increase access to information and the ability of deafblind people to participate in society, this is unlikely to happen unless there is a significant and ongoing investment in developing new devices and systems and making them available at affordable prices.

Further Information

Contains details of special devices for deafblind persons which are commercially available.

Deafblindness Web Resource
A comprehensive site about many aspects of deafblindness.

Equipment for Deafblind People
A list of equipment which has or is being developed for deafblind people.


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