CONTROL ROOM DESIGN AND ERGONOMICS

Comprehensibility of Codes

In this section, we look at the ease of understanding visual codes. It is, however, important to remember that sound codes can be just as useful in many instances as visual codes. One advantage of sound codes is that it is not necessary to be in a par­ticular workplace in order to notice them, and also faster reactions may be expected to sound signals. Hearing can be said to be the dominant sense in terms of noticing, recognising, and identifying patterns. The ability of a musician to recognise very small variations in a piece of music (which is a pattern presented in the form of sound) is outstanding. The amount of information and the total number of possible different messages that could be presented in this way is immense. Sound also has the advan­tage in that it is not necessary to focus attention on the equipment supplying the infor­mation material, but one is free to move around while listening for the information.

Another information channel that gives almost the same degree of freedom is the sense of touch. There have been experiments to produce a language for the deaf, which is transmitted through touch. There has been an attempt (see Ivergard,

■ Substitute nonfigurative code = Belongs to Lathe 1/2 (adjective)

1982, for example) to produce a ‘hearing glove’; this has an instrument mounted against each finger, which produces a pressure at different frequencies on the fin­gers. Sound—for example, from someone talking—is converted electronically and transferred to this hearing glove. It is known that at least four different frequencies can be sensed, so with this method one can transfer at least 4 x 4 different symbols (information units). Similar methods have also been used by placing the vibration devices on the chest and other parts of the body.

The sense of touch is not only useful as an information transmission channel for the deaf but it can also be used for people who are overloaded with visual or sound information. Pilots, for example, are often overloaded by the large number of instru­ments they have to read. If we wish to give the pilot even more information, vibrators could be used to transfer information through the sense of touch instead. Like hear­ing, touch is better than sight for use as a warning signal. Whereas one can close the eyes to remove the sense of vision, the sense of hearing or feeling cannot be shut off. Four variables can be used in the transferral of information by touch: the positioning, frequency, amplitude, and variation of the stimulation.

The visibility of visual codes will not be discussed in detail here except to say that they must be (a) large enough, (b) bright enough, and (c) have sufficient contrast, in order to be visible.

Experience from Gestalt psychology can be useful regarding the suitability of codes and for general rules on their design. In addition, there are a number of percep­tual psychological grounds that form the basis of the number of elements which can be used in different codes in order to avoid confusion. In particular, there is a body of specialist knowledge on the design of letters and numbers.

Table 3.3 summarises the number of possible variants that can be obtained with different types of stimuli for designing codes. It is important to remember that the estimations given are only approximations. The maximum number only refers to con­ditions where the person performing the reading has special education or training for that particular form of stimulus. In most normal applications—for example, work on VDUs—the recommended number of alternatives must be used. Table 3.3 also gives the number of absolutely recognisable units of a particular stimulus. If comparisons are possible, the number of recognisable units becomes considerably greater.

Table 3.4 identifies letters and numbers that are easily confused with each other, including also those that are difficult to read. The risk of confusion on VDU screens is especially great, as the letters and numbers are built up in a fairly simple and lim­ited manner. Figure 3.14 shows the number of seconds it takes to read a particular number compared with the grouping of the digits. It is clear from Figure 3.14 that groups of three digits are the quickest to read. Suggested forms of grouping are given in Table 3.5, which gives examples of how signs in codes of different lengths should be grouped.