Chapter 8 - Display Devices

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8.2.2 Alphanumeric characters, words, and numbers

Much of the information on visual displays is composed of alphanumeric characters. Many of the display deficiencies in control centers are the result of poorly planned use of characters, in particular, failure to provide for three essential dimensions:(105)

Character size. The determination of character size on pictorial displays is critical, and interacts with distance of observer from the display, and character style, color, and contrast.

Guideline 8/15 Legibility Principle; 130
To be legible, character size should be a minimum of 16 min of arc, and a maximum character size should be 24 min of arc.

The preferred size is 20 to 25 min. The maximum reflects the limits of the eye; the central area of vision imposed by the fovea of the eye is too small to read large text efficiently because too few letters can be recognized in one eye fixation.

Reducing this to more familiar terms: 22 min of visual arc is equivalent to a printed type face of 12 points in height (this handbook is printed in 12 point type), the designation generally used by printers and word processors, when viewed at normal reading distances. A point is approximately 0.35 mm (exactly 1/72 in). The point has no commonly used metric equivalent. A face of 10 to 12 points is preferred for readability at distances of 30 to 40 cm (12 to 16 in).(70, 92)

To compute a given character height, use the following formula:

h = 2dTan(x/2),

where h is the character height, d is viewing distance, and x is the desired angle subtended in radians. One radian equals 3437.747 arc min, or 57.296 arc deg. Table 8-1 shows selected preferred character heights (21 arc min) for selected near viewing distances. Figure 8-1 shows recommended character heights (21 arc min) for viewing at long distances.

Table 8-1. Viewing Distance, Character Size, and Readability.

Viewing Distance


Character Height

mm inches mm inches

















Figure 8-1. Recommended Character Height for Longer Viewing Distances.

Character style. The design style of alphanumeric characters also has an effect on legibility and readability. The wide range of typefaces now available at the touch of a keystroke has made this problem a critical one.

Figure 8-2 gives examples of the relationship between legibility/readability and such factors as stroke width, height to width ratio, and letter, word and line spacing. In many cases, these variables are driven by the defaults of a display. Newer computer systems, however, allow the operator to vary these factors, along with font and color arrangement. In addition, large fixed displays (such as maps and the labels for monitors in an array) require design decisions with respect to character size and arrangement.

Figure 8-2. Recommended Character and Word Dimensions.

The shape of characters — font and other considerations — and their effect on legibility and readability have been studied extensively. The problems in this area of design have been increasing with the variety of font choices. Powerful graphic and word processing packages, combined with laser printers, have provided the designer with a wide selection of confusing typefaces and the temptation to explore inconsistent applications.

Most printed text material uses a Roman-style serif face. This style is very legible and readable, and provides high discrimination between characters. In addition, its wide use has made us comfortable with the Roman "look." But this style is as "fancy" as good design permits. For signs and large displays, simple sans-serif fonts (such as Helvetica, Swiss, or Arial) are acceptable. Table 8-2 shows some undesirable styles.)

Table 8-2. Undesirable Type Styles.

Guideline 8/16 Typeface Characteristics Principle; 130
In particular, avoid typefaces with these characteristics:
  • Letters with extended height-width ratios
  • Odd or stylized shapes, which are easily confused
  • Excessive stroke width or widely different stroke widths
  • Script faces
  • Excessive slope
  • Large serifs

Readability. Readability is improved by the following measures:

Guideline 8/17 Text Reading Level Observation
Design text to be easily read and understood by the least educated staff member.

Generally, for high school graduates, a sixth to eighth grade reading level is recommended. This is about the level of a general circulation newspaper. In general, a lower reading level is achieved by using shorter, more common words and shorter and less complex sentences. For reading during high workload and emergency situations, the lower the reading level, the better.

Guideline 8/18 Common Language Principle; 130
Use common language terms on displays and user aids.

Jargon and acronyms are often hard to avoid, but the effort is usually well spent. For inexperienced staff members with limited detailed technical knowledge, jargon-filled messages may be meaningless or misleading.

Guideline 8/19 Whole Words Principle; 130
Use whole words rather than abbreviations.

Space is sometimes at a premium in displays, and there is a natural tendency to save it by using abbreviated forms. Consider the possible errors and confusion that could result if the abbreviation is not universally understood.

Guideline 8/20 Instrument Labels Principle; 130
To identify an instrument or graphical data presentation, use terms that indicate what the instrument measures.

For example, "Temperature: C" rather than "Thermometer."

Guideline 8/21 Homonyms Principle; 130
Avoid words that have multiple meanings or might be interpreted as verbs or nouns in different contexts.

For example, "file" might refer to a record or to the action of filing information as a record. "Save" better conveys the intended meaning of the latter.

Guideline 8/22 Letter Case Principle; 130
Use capital letters for labels and short instructions. Where full sentences and punctuation are used, employ mixed upper and lower case.

Mixed upper and lower case is significantly easier to read when phrases are assembled in sentences and paragraphs.

Guideline 8/23 Make Instructions Brief Principle; 130
Make instructions as brief as possible while preserving clarity.

To improve readability, use short, common words and arrange them in simply constructed sentences averaging between 10 and 12 words in length.

Guideline 8/24 Confusing Alphanumeric Symbols Principle; 130
Avoid using alphanumeric symbols that can be misinterpreted.

For example, "l" and "1" can be an easily confused pair.


In one TMC, the operators were given a user aid in the form of a printed set of coded keystroke sequences for control of signals in emergencies; the characters were sans-serif (like this text), and the characters "O" and "0" (zero) were indistinguishable. The purpose of the keystrokes was of such importance that confusion of these characters might cause system malfunction. In addition, the use of this procedure was so infrequent that the job aid, not memory, was the major source of information. This further increased the likelihood of error.

Guideline 8/25 Character Orientation Principle; 130
Characters should be oriented left-to-right, not around corners, on the side, or up and down.

For example, a display data field or window on a video display might preserve active data space by labeling the window vertically down one side. It is more difficult, though, to read such labels, particularly if the letters are not rotated.

Guideline 8/26 Manufacturers' Labels Principle; 130
Manufacturers' brandname labels should not overshadow the primary display labels.

Delete or cover manufacturers' brand names on visible parts of equipment if they create visual clutter and distraction.

Guideline 8/27 Consistent Label Positions Principle; 130
Place identification labels for controls and displays in a consistent position (e.g., above the component).

This is particularly important on control consoles where controls and displays are densely arranged. Mistakes are extremely easy to make, for example, in an array of push buttons, a configuration common in older TMCs.

Guideline 8/28 Adjacent Labels Principle; 130
Allow sufficient space to avoid running adjacent labels together.

This most frequently occurs in older TMCs that use push button arrays.

Guideline 8/29 Size or Color Code Principle; 130
Use size coding and/or color coding to help the operator differentiate between levels of importance.

A malfunctioning signal controller may be more urgent than a malfunctioning loop detector and, if so, this difference should be indicated by a difference in color or size of the associated icon on the screen.

Guideline 8/30 Resistance to Damage Principle; 130
All labels should be as permanent as possible for resistance to damage and wear.

This is particularly important for labels that will be touched by operators' fingers on a regular basis, such as labeled buttons and entry keys. In particular, labels that have painted or dry-transfer characters are likely to wear excessively. If possible, labels should be engraved.

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