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Tactual Displays

Braille
The best known communication system for blind people is Braille which was developed by Louis Braille. A combination of six dots can only have 64 different configurations (Braille dimensions). This gives a problem in that many more than 64 different 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'.

Since one of the greatest deprivations caused by blindness is lack of privacy, the provision of bank statements in Braille has been a very popular service for over 25 years. This system can be totally automated since bank statements are in a fixed format.

Layout is more problematic for mathematics and music. Since Braille mathematics is written on one line, the conversion of the layout on the printed page to a meaningful form in Braille is far from trivial. Braille music is also significantly different in layout from sighted music notation since the Braille reader has to read linearly.

There are about 13,000 people in the UK who regularly read Braille (NB this should be compared to the one million people whose vision is such that they could be registered as blind or partially sighted). For these people it is a very useful communication medium since it can be written as well as read by a blind person. This level of readership, which is typical of developed countries, is partly attributable to the difficulty in learning a new method of communication by people who lose their vision later in life. Another factor is that diabetic retinopathy is a significant cause of visual disability among those of working age, and diabetes usually adversely affects the sense of touch.

Moon
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 blind persons who 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. This type of presentation is also called serpentine or meander; it was used by the Greeks in ordinary writing at about 0 AD. The disadvantage was the perceptual problem of reading alternate lines in reverse.

Tactile graphics
The increasing use of graphics in printed books, particularly school text books, gives problems. Tactile graphics are intended to be read principally by touch rather than vision. 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 a 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. People extract information from tactile graphics in pieces; the diagram is dissected into as much as the fingers can reveal at a time. Various skills are required to 'read' a tactile graphic e.g. logical scanning, tracking horizontally or vertically, size, texture and shape discrimination, angle estimation, and spatial relationships. Lines are discriminable not only by their thickness but also the spacing around them. The discriminability of textures differs to those obvious by vision. Different levels of relief between areas needs to be clearly evident. If something is too small it is difficult to distinguish the details but if it is too large, it can be difficult to comprehend as a whole. Despite this often the graphics are not designed at all but are simply a visual graphic in a raised line version.

As with vision, highlighting a constant invariant feature will provide an 'anchor' from which the person can orientated themselves.

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