The following section presents some brief recommendations on the requirements, which should be placed on CRTs, currently the most common form of information presentation aid in the computer control system. The basic design features of a CRT are that the information shall be visible and easy to read. It is important to choose a suitable coding and presentation of information for particular applications. These points are dealt with in Section 3.4. The following recommendations assume a screen with a dark background and light text. This form of CRT demands very careful planning of the lighting (see Chapter 7). There is much to suggest that screens with dark text on a light background give a better result if one can avoid the problems of flicker, which easily occur on light screens (Berns and Herring, 1985).
Some approximate guidelines are given below:
a. Luminance—Minimum 85 cd/m2 for characters (250 asb), with an optimum of 171 cd/m2 (500 asb).
b. Contrast—The optimal character contrast is 94%, but down to 90% is acceptable.
c. Light/dark characters—If flickering can be avoided, dark characters on a light background are preferable. Otherwise, and if other environmental conditions are suitable, light characters on a dark background can be accepted.
d. Flickering—For the optimal luminance level of 171 cd/m2, a frame regeneration rate of at least 50 Hz is required. Certain types of phosphor (e. g., p-20) may need higher frame rates, up to and even over 60 Hz (frequencies of 60 Hz may interfere with mains electrical frequencies of 50 Hz in Europe and other places).
a. Character size—The height varies between 16 and 27 minutes of arc (visual angle), a width-to-height ratio of 0.75, and a ratio of height to thickness of the strokes making up the characters of between 10:1 and 6:1.
b. Character shape—A dot matrix using 9 x 7 points produces the best symbols. There are many different ways of designing letters and numbers and it is not always clear which is the best. It is, however, important to choose types that do not cause confusion between letters and numbers, as in O and Q, T and Y, S and 5, I with 2 and K, I and 1, O with 8 and 0.
c. Resolution—Ten raster lines per character height and more for nonalphanumeric characters.
d. Character separation—50% of character height and 100% between lines.
e. Reading distance—For characters of 3.2 mm in height, a reading distance of 69 cm is acceptable. In most practical applications, the reading distance is determined by the height of the characters. The screen should be read from directly in front; angles of more than 30° from the optimum considerably reduce legibility.
f. Colour— Colours that lie well outside the optimum visual spectrum must be avoided, particularly those in the blue ultraviolet (UV) region. Colours in the yellow-green range are good.
g. Flashing (e. g., to gain attention)—A 3-Hz flashing frequency has no adverse effect on legibility.
h. Cursors—A rectangular cursor with a 3-Hz flashing rate is thought to be best.
Coding improves performance considerably, especially for simple information processing tasks.
a. Colour coding—Best for localisation, calculation, and comparison of different information.
b. Alphanumeric codes—For recognition and identification of information, these are the best. Colour coding is the next best. Coding is of little or no help in quantifying or size estimation.
c. Components—A code should not comprise more than seven different components (e. g., seven letters or numbers). The fewer components the code uses, the better. The efficiency of the coding must be seen in relation to the total construction of pictures on the screen.
d. Code groups—Different groups of codes, each consisting of components, should, if possible, not contain the same components.
4. Construction of Picture on the Screen
The basic issue in the determination and design of the picture is the function that is to be fulfilled. The following information may be used to provide certain guidelines (see also Section 3.3):
a. Digital presentation is best for quantitative information.
b. Pointers on circular scales are best for showing changes.
c. Vertical and circular scales with moving pointers are good for check readings.
d. Chart recorders are valuable for seeing instrument faults and for obtaining a rapid impression of the system’s response.
Diagrams are often preferable to graphs. Histograms are especially easy to read. Curves, however, are preferable for reading trends. Several curves can be presented to advantage for comparisons between several variables. Here, semigraphic screens are better, but x/y screens are to be preferred above all.