1、 rmm Page 1 Recommendation T/SF 40 E (Nice 1985) concerning the human factor aspects of visual display terminals for telecommunication services Recommendation proposed by Working Group T/WG 7 “services and facilities” (SF) Text of the Reconmendation adopted by Commission “Telecotimitiicatiotis”: “Th
2、e European Conference of Posts and Telecommunications Administrations, Considerittg - that display terminals may or may not be provided to customers by the Administrations; - that the provision of different types of visual means e.g. display screen, small digit display, lamps, etc., for a variety -
3、that the provision of such means and services provided by them are becoming cheaper and stimulate demand; - that even though the use of e.g. optical fibres is becoming widespread, the cost of wideband services long distance transmission may remain high in the near future; - that various forms of vis
4、ual services are likely to be introduced in the PABX and office environment initially, and will subsequently need to be switched via the public networks and will also subsequently be used by customers in the public networks; of telecommunications purposes are now technically possible; O - that the c
5、apability for interworking between all versions of the visual services is desirable; - that the evolving IDN/ISDN networks provide a future possibility to integrate these services into the same network, into the same customer access/line and into the same functionally integrated terminal(s) where th
6、e display screen module can be common for a number of visual services; - that it would be advantageous for both customers and the Administration to have common standards for terminals and their modules providing visual services; - that guidance should be given regarding the operational parameters, s
7、ervices, features and interworking require- ments so as to avoid Administrations introducing unnecessarily widely diverging types of visual service and displays providing them. Recommends that therefore visual display terminals should be developed on the basis of the user requirements and operationa
8、l characteristics given in Annex A.” Edition of March 15, 1986 CEPT T/SF*40*E 85 E 2326434 0009373 2 E 1 T/SF 40 E Page 3 Annex A . Contents 1 . INTRODUCTION 5 2 . TERMINAL CATEGORIZATION . 5 3 . GENERAL REQUIREMENTS FOR VISUAL DISPLAY TERMINALS 5 3.1. Display requirements 5 5 3.1.2. Character requi
9、rements . 6 3.1.3. Presentation of ifformation . 8 3.2. User input requirements 8 3.2.1. Physical requirements of the keyboard 8 3.2.2. Key requirements . 9 3.2.3. Keyboard lay-out . 9 3.3. Additional terminal requirements . 10 3.3.1. Automatic adjltstemetit of lighting 10 3.3.2. Connecting point fo
10、r recording devices 10 3.3.3. Power feeding security . 10 3.3.4.Noise 10 3.4. Control procedures . 10 3.5. Workplace requirements 10 3.6. Environmental requirements . 10 4 . REQUIREMENTS FOR SPECIFIC TELECOMMUNICATION SERVICES 10 4.1. Teleconferencing . 10 4.2. Videophone 11 4.2.1. General . 11 4.2.
11、2. Selfview . 11 4.2.3. Optical control 11 4.2.4. Camera kill 11 4.2.5. Graphics and document mode 11 4.2.6. Direrent types of lenses 11 4.2.7. Usage. . 11 4.2.8. Numbering 11 a 4.2.9. Acoustic condition . 11 4.3. Telephony 11 4.3.1. Visual indications . 11 3.1.1. Physical requirements of the screen
12、 a Appendix 1 13 Edition of March 15. 1986 e 1. 2. 3. 3.1. 3.1.1. 3.1.1.1. 3.1.1.2. 3.1.1.3. TISF 40 E Page 5 INTRODUCTION This Recommendation concerns human factor aspects of visual display terminals used in telecommunica- tions applications. Chapters 1-3 of the Draft Recommendation are intended to
13、 cover all services while chapter 4 includes Additional requirements for specific telecommunication services. Visual display terminals for telecommunications will be employed for both professional, continuous usage and domestic, casual usage. Generally all of the appropriate Recommendations of this
14、document apply to both situations. An indication is given where it is considered that a particular recommendation is not appropriate to the Domestic environment. TERMINAL .CATEGORIZATION Visual display terminals can be divided into: 1. Supplementary services telephone terminals with displays which a
15、re able to indicate the status of supplementary telephone services and simple data services (alpha-numerical) and present system re- sponses and guidance messages. 2. Multifunction terminals which are intended to provide a variety of voice and non-voice services, including supplementary services, an
16、d where display screen provides visual means for, in addition to those services mentioned in point one above the following services: - teletex and word processing; - videotex; - directory enquiry; - local computing; - drawing, graphics transmission; - text telephone (for deaflspeech retarded persons
17、); - videoservices for - videophone, - teleconferencing. Recommendations given in the following chapter 3 are valid for terminals and services mentioned above, although some aspects may not be relevant for all terminals and services. In addition recommendations which are specific to one type of tele
18、communication service are included in chapter 4. GENERAL REQUIREMENTS FOR VISUAL DISPLAY TERMINALS Display requirements In this section consideration is given to the physical requirements of the screen unit of visual display terminals, including character requirements and presentation of information
19、. Physical requirements of the screen Screen size and number of characters per line Screen size (number of horizontal and vertical character lines) is specifically related with the task to be performed. For terminals with one or two line display a minimum of 16-20 characters should be used. For task
20、s in which alternatives have to be chosen or forms have to be filled in, a screen size of 12-16 lines of 40 characters each is suitable. With other tasks such as textprocessing, programming, etc., the numbers of characters and lines should be greater (e.g. 24 lines of 80 characters each). Aspect rat
21、io The aspect ratio i.e. the ratio between the height and width of the viewing area of the VDT screen should be 3 : 4 for visual display terminals in which the screen is horizontally mounted. For terminals in which the screen is vertically mounted, the ratio should be 4: 3, which corresponds approxi
22、mately to the proportions of an A4 sheet of paper. This recommendation only holds for rectangular viewing areas. Screen angle (tilt) Ideally the screen should be positioned such that the focus of the users attention is at right angles to the line of sight. As the point of focus can vary with the tas
23、k between the top and the bottom of the screen, it is desirable for the screen angle to be adjustable. Adjustment controls should be easy and positive in operation. Edition of March 15, 1986 CEPT T/SF*40*E 85 m 232bYL4 O009373 b m TISF 40 E Page 6 3.1.1.4. 3.1.1 $5. 3.1.1.6. 3.1.1.7. 3.1.1.8. 3.1.1.
24、9. Viewing height of the screen Correct screen height is important in the prevention of potential postural problems. It may be defined in several ways, of which the specification of the visual down angle seems functionally the most appropriate. Adjustment of screen height can be provided in the term
25、inai or furniture, In the case of intensive utilization of the terminal the centre of the screen should be adjustable between 905 and 1 148 mm from the floor. Using e.g. a 720 mm high desk, this leads to a range of 185 and 482 mm for the height of the centre of the screen above the working surface.
26、Adjustment controls should be easy and positive in operation. Screen swivel This is a desirable facility when it is necessary to frequently rotate the screen around its vertical axis. The adjustment controls must be easily accessible and the operation should be simple, positive and safe. Screen manu
27、vrability To enable the user to place the screen unit in the optimum position, the unit should be easily manuvred, yet stable when positioned. The weight of the screen unit and the frictional characteristics of its base should fulfil this requirement. Screen finish The finish of the housing material
28、 of the screen unit is important in determining possible sources of visual discomfort and distraction in the users field of view. The reflection factor of the housing material should be 20-50% and the surface should be matt finished. Adjacent surfaces within the field of view should preferably have
29、luminance ratios no greater than 3 : 1, Screen homogeneity The screen unit should be such that no perceivable distorsion of the characters is present at the screen edges. Lines should give the impression of running straight, Screen reflectance Bright reflections from the screen should be avoided as
30、much as possible. They can be reduced by (a) coating or (better) frosting the screen (b) lowering the luminance of objects in the environment (c) limiting the sideward radiation from ambient lighting by application of parabolic reflectors. Screen reflectance should be maximally 0.2 times character l
31、uminance. In this respect it is important to have a careful position of the screen unit in relation to lights, windows, etc. P 3.1.1.1 O. Display resolution Generally speaking, the more scan lines (in CRT technology), the finer the resolution of the character shapes and the more readable the. inform
32、ation is for the user. The minimum number of scan lines depends upon the number of text lines including line spacings and the number of vertical dots of the character matrix, the requirements of which depend largely on the task requirements and kind of service. For professional and frequent terminal
33、 use, a greater number of scan lines might be required. Note: Similar requirements will apply for non-CRT technology displays. In order to keep the image visible on the screen, the screen must be continually regenerated or refreshed. This frequency or refresh rate is technology dependent. Using shor
34、t to medium persistance phosphor it is recommended to be 80 Hz minimally. However, a frequency of 50 or 60 Hz is often used for economic reasons. 3.1.1.1 1. Screen refresh rates 3.1.2. Character requirements 3.1.2.1. Character set It should be possible to display the following types of characters: -
35、 numeric, alphanumeric; - lower case, upper case; - special graphics, symbols. A full range of characters is not necessary for all applications. In addition, possibilities for different character styles (e.g. inverse display, italics, underlining, double character dimensions) are desirable. The norm
36、al light adapted eye is maximally sensitive to 550 nm: the greenlyellow part of the spectrum. Although colour choice is a matter of personal choice largely, the green/yellow colours are recommended by most sources. However, adequate luminance and colour contrast are considered to be of greater impor
37、tance for readability. As regards character and background colour, there are no uniform recommendations. Dark symbols on light background (e.g. black or blue on white) or similar character and background colour (yellow/green) seem the most acceptable ones. Different shades of the same colour for cha
38、racters and background are also acceptable. 3.1.2.2. Colour 0, 7 Edition of March 15, 1986 TISF 40 E Page 7 3.1.2.3. Character display Dot matrix character generation is the technique which is commonly used for visual display terminals. At first, a 5 x 7 dot matrix seemed to be sufficient for displa
39、ying letters and numerals. However, to generate both upper and Iower case with true ascenders and descenders, a 5 x 9 dot matrix is considered to be a minimal requirement. Because there should be a separation between characters and lines of characters, it is necessary to reserve a 6 x 10 dot matrix
40、for each character. The next figure should clarify this It is recognized that other types of generation techniques exist e.g. seven or sixteen segment display for alpha numerics. Character dimensions are important for good legibility and readability and are derived from considerations of viewing dis
41、tance and the visual acuity of the eye. Character height shouId subtend a visual angle of 16 to 22 minutes of arc. A height of 3 mm is considered to be the minimum. Preferred viewing distances for naive users are 550 to 690 mm. The angular dimensions given above lead to a character height of 2.6 to
42、3.5 mm at a viewing distance of 550 mm and 3.2 to 4.4 mm at a viewing distance of 690 mm. Character width (capital letter) should be 60-80% of the character height. Stroke width (the thickness of a line segment) should be 10-17% of character height. The horizontal (inter character) spacing should be
43、 0.25-0.5 times the character width (capital letter). The horizontal spacing between words should be 1.5-2.0 times the character width (capital letter). The vertical spacing between lines should be such that the descenders should not interfere with any part of the character below. The recommendation
44、s vary from 10% of the character height to 150% of the character height. 3.1.2.6. Character differentiation Confusion between characters primarily occurs because of similarity of shape and construction. The following characters are most frequently confused and should therefore be examined for mis-id
45、entification. MUTUAL ONE WAY 0,OY Q C called G T, y D called B SY 5 H called M, N I, L J, T called I X, K K called R I, 1 2 called Z 3.1.2.4. Character size 3.1.2.4.1. Character height e 3.1.2.4.2. Character width 3.1.2.5. Character spacing e B called R, S, 8 3.1.2.7. Cursor A cursor directs the use
46、rs attention to the appropriate position on the screen. It should be easily located at any random position in the display, easily tracked as it is moved, not interfere with the reading of the symbol, not be distracting and be unique. The two most common cursor types are underscore and a box with inv
47、erse display. Preference depends on the task to be performed but is merely a personal matter. A blinking facility can be added to both types, with recommended blink rate of 3 Hz. The possibility to switch off the blinking is recommended. Luminance is the physical attribute of light which produces th
48、e sensation of brightness. It is measured in terms of light emitted per unit area: candelas per square meter (cd/m2). The following recommendations should be considered in relation with the lighting conditions of the workplace (see chapter 4). 3.1.2.8. Luminance e Edition of March 15, 1986 CEPT T/SF
49、*4O*E 85 W 2326434 0009375 T W TISF 40 E Page 8 3.1.2.8.1. Background luminance Background is defined here as the area of screen luminance with no character information. It is important for both the resistance of the screen to reflections and the clarity of the characters. A background luminance of 10-20 cd/m2 is recommended, when light characters on a dark background are used. A minimum character luminance of 45 cd/m2 is given for light characters on a dark background. Lumi- nances of 80-160 cd/m2 are preferred. Brightness and contrast controls enable the user to adjust both luminan
