**Table of contents**

[5 Displays and printers 5-1](#displays-and-printers)

[5.1 Displays 5-1](#displays)

[5.1.1 General 5-1](#general)

[5.1.2 Location and arrangement 5-2](#location-and-arrangement)

[5.2 Cathode ray tube displays 5-7](#cathode-ray-tube-displays)

[5.2.1 General 5-8](#general-1)

[5.2.2 Luminance and contrast 5-10](#luminance-and-contrast)

[5.2.3 Glare control 5-12](#glare-control)

[5.2.4 Resolution 5-13](#resolution)

[5.3 Flat-panel displays 5-13](#flat-panel-displays)

[5.4 Liquid crystal displays 5-15](#liquid-crystal-displays)

[5.5 Gas plasma displays 5-17](#gas-plasma-displays)

[5.6 Electroluminescent displays 5-17](#electroluminescent-displays)

[5.7 Large-screen displays 5-18](#large-screen-displays)

[5.7.1 General 5-18](#general-2)

[5.7.2 Large-screen optical projection displays
5-19](#large-screen-optical-projection-displays)

[5.8 Stereoscopic displays 5-21](#stereoscopic-displays)

[5.9 Printers 5-22](#printers)

[5.10 Plotters and recorders 5-23](#plotters-and-recorders)

[5.11 Accommodating people with disabilities
5-25](#accommodating-people-with-disabilities)

[Glossary 5-27](#__RefHeading___Toc26772320)

[References 5-27](#refheading___toc26772321)

[Index 5-32](#__RefHeading___Toc26772322)

**List of exhibits**

[**Exhibit 5.1.2.7 Display face not tilted more than 45° to user's line
of sight 5-3**](#__RefHeading___Toc18406077)

[Exhibit 5.1.2.10 Optimum vertical and horizontal visual fields
5-4](#__RefHeading___Toc18406078)

[Exhibit 5.3.3 Stroke width for pixel-generated characters
5-14](#__RefHeading___Toc18406079)

[Exhibit 5.3.4 Height-width relationship for upper case pixel-generated
characters.....5-14](#__RefHeading___Toc18406080)

[Exhibit 5.3.8 Example of positive and negative polarity.
5-15](#__RefHeading___Toc18406081)

#  Displays and printers

This section presents information on different means of presenting
output information to the user visually. In particular, this chapter
covers different types of monitors (also known as Video Display
Terminals (VDTs), Video Display Units (VDUs), or Cathode Ray Tubes
(CRTs)), projection displays, stereoscopic displays, printers and
plotters.

## 5.1 Displays 

This section covers visual displays used for information output. It does
not address touch screen displays or visual indicators such as LED
displays and lights accompanying switches and controls. Touch screen
displays are in the input devices chapter (Chapter 9) and visual
indicators are addressed in the chapter on controls and visual
indicators (Chapter 6).

### 5.1.1 General

-   **5.1.1.1** **Make displays function under operational conditions.**
    Visual displays should function under any circumstance corresponding
    with the operational and use philosophies of the system. \[Source:
    Department of Defense (MIL-STD-1472F), 1999\]

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-   **5.1.1.2** **Make displays legible under all conditions.** Visual
    displays shall be legible under all anticipated viewing conditions.
    \[Source: MIL-STD-1472F, 1999\]

**Discussion.** Factors affecting the **legibility** of a display
include the nature and characteristics of the display itself, ambient
lighting, and viewing distance.

-   **5.1.1.3** **Avoid unnecessary markings.** Trademarks, company
    names, and other markings not needed to identify a display or aid in
    performing tasks should not be displayed on a panel face. \[Source:
    MIL-STD-1472F, 1999\]

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-   **5.1.1.4** **Ensure adequate refresh rate.** Displays that require
    refreshing of information such as CRTs, heads up displays, and
    collimated displays, shall be refreshed at a rate that insures that
    the displayed information is sufficiently current for the users to
    perform their tasks. \[Source: MIL-STD-1472F, 1999\]

**Definition. Refresh rate** is the rate in cycles per second (Hz) at
which the displayed contents of a computer screen are periodically
regenerated.

-   **5.1.1.5** **Provide adjustable contrast and brightness.** Visual
    displays should provide users with the capability of adjusting
    contrast and/or brightness. \[Source: Vanderheiden & Vanderheiden,
    1991\]

-   **5.1.1.6** **Controls should conform to Chapter 6.** Controls for
    displays should conform to the design criteria contained in Chapter
    6, Controls and visual indicators.

### 5.1.2 Location and arrangement

-   **5.1.2.1** **Locate displays to be readable without assuming
    uncomfortable positions.** Displays shall be located so that a user
    can read them to the degree of accuracy required without having to
    assume an uncomfortable, awkward, or unsafe position. \[Source:
    MIL-STD-1472F, 1999\]

**Discussion.** Locating visual displays so that the users can see the
information can include positioning the displays so they are readable to
users in wheelchairs. \[Source: Vanderheiden & Vanderheiden, 1991\]

-   **5.1.2.2** **Make readable without special equipment.** A user
    should be able to read a visual display without the use of a ladder,
    a flashlight, or other special equipment. \[Source: MIL-STD-1472F,
    1999\]

-   **5.1.2.3** **Locate directly in front of user** A screen should be
    directly in front of the user when the user is in his or her normal
    working position. \[Source: Department of Energy (DOE-HFAC 1),
    1992\]

**Discussion.** Locating the screen off-center can cause the user to
adopt uncomfortable or awkward positions, leading to pain. Exceptions to
this rule are when the users look at the monitor infrequently.

-   **5.1.2.4** **Place the top of the screen below eye level.** The top
    of the screen should not be above the viewer\'s eye level. \[Source:
    DOE-HFAC 1, 1992\]

**Discussion.** By placing the top of the screen below eye level, the
user can view the information on the screen using a downward gaze. When
people view the displays at eye level, they tend to blink less and eyes
can get dry and irritated. A downward gaze exposes less of the eye to
the atmosphere, preventing dry eyes. \[Source: Tsubota & Nakamori,
1993\].

-   **5.1.2.5** **Make line of sight below horizontal.** The line of
    sight from the viewer\'s eyes to the center of the screen should be
    between 10° and 60° below horizontal, preferably allowing the user
    to set the angle most comfortable for him or her within this range.
    \[Source: DOE-HFAC 1, 1992; Balci & Aghazadeh, 1998; MIL-STD 1472F,
    1999, American National Standard Institute (ANSI), 1988\]

**Discussion.** The resting position of the eyes, considered to be the
most comfortable position, is 15° below the horizontal. However, a lower
monitor position improves the ability to accommodate and facilitates
convergence (the turning in of eyes to focus on a nearby object),
particularly for persons over 40 years old. Additionally, a monitor
location 40° below horizontal can reduce discomfort in the neck,
shoulders, forearms, and wrists for users wearing bifocals. \[Source:
Balci & Aghazadeh, 1998; Krimsky, 1948; Morgan, Cook, Chapanis & Lund,
1963; Ripple, 1952; Tyrrell & Leibowitz, 1990;\]

-   **5.1.2.6** **Orient perpendicular to line of sight.** Display
    screens should be positioned so that the face is perpendicular to
    the user\'s line of sight whenever feasible. \[Source: DOE-HFAC 1,
    1992\]

**Discussion.** Tilting the monitor downward can lead to increased
discomfort both physically and visually, particularly in the neck area,
versus a monitor tilted back a little. \[Source: Ankrum, Hansen, &
Nemeth, 1995; Ankrum & Nemeth, 1995\]

> []{#__RefHeading___Toc18406077 .anchor}**Exhibit** **5.1.2.7** Display
> face not tilted more than 45° to user's line of sight \[Source:
> MIL-STD-1472F, 1999\]

![](media/image1.png){width="4.0159722222222225in"
height="2.7506944444444446in"}

-   **5.1.2.7** **Avoid excessive tilt.** The display face shall not be
    tilted more than 45° from the normal line of sight as illustrated in
    Exhibit 5.1.2.7. \[Source: MIL-STD-1472F, 1999\]

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-   **5.1.2.8** **Group task-related displays together.** All displays
    necessary to support a user\'s activities or sequence of activities
    should be grouped together. \[Source: MIL-STD-1472F, 1999\]

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-   **5.1.2.9** **Arrange according to function and sequence.** Displays
    shall be arranged in relation to one another according to their
    sequence of use or the functional relations of the components they
    represent. \[Source: MIL-STD-1472F, 1999\]

**Discussion.** In general, it is beneficial if displays are arranged
sequentially within a functional group so that they provide a
left-to-right or top-to-bottom information flow within the group because
this is how people read.

> []{#__RefHeading___Toc18406078 .anchor}**Exhibit 5.1.2.10** Optimum
> vertical and horizontal visual fields

![](media/image2.png){width="4.103472222222222in" height="5.38125in"}

-   **5.1.2.10** **Locate critical** **displays in central visual
    field.** Critical or frequently used displays shall be located in
    the **central visual field**, as illustrated in Exhibit 5.1.2.10,
    and occupy a privileged position in that field (e.g., the top or
    left-most position). \[Source: MIL-STD-1472F, 1999\]

**Discussion.** Focal vision is the central 30° of the visual field,
pictured as the shaded area in the top panel of the Exhibit 5.1.2.10,
(along with the range of eye movements with the head stationary). This
is the area that people use to look at objects in the world, moving
their eyes as needed to bring images of the object on to the **fovea**,
which is the area of highest acuity. When an object is outside of the
focal area, a person will usually turn the head rather than simply move
the eyes. The range of head rotation is illustrated in the second panel
of Exhibit 5.1.2.10. The combined range of combined head and eye
movement is illustrated in the third panel of Exhibit 5.1.2.10. By
locating frequently used displays in the central 30° of visual field,
the user is not required to move his or her head to bring the
information into the focal area, presumably minimizing neck strain.

-   **5.1.2.11** **Arrange displays consistently.** The arrangement of
    displays within a system shall be consistent from application to
    application. \[Source: MIL-STD-1472F, 1999\]

-   **5.1.2.12** **Distance for monitors with controls.** If there is a
    control associated with a display, the viewing distance from the eye
    reference point of a seated user to the associated display and
    control shall not exceed 635 mm (25 in). \[Source: MIL-STD-1472F,
    1999\]

**Discussion.** The reach distance for a 5^th^ percentile female is 26
inches, so setting the distance at 25 inches insures that the 5^th^
percentile female can reach the controls. The 5^th^ percentile female
measurements and reach ranges represent the customary minimal design
criteria. Designing to accommodate the minimal reach range will increase
the likelihood that the entire user population can reach the controls.

-   **5.1.2.13** **Maintain at least a minimum viewing distance.** The
    viewing distance from the eye to a display shall not be less than
    330 mm (13 in), unless the periods of viewing will be short or if
    dim signals must be detected, in which case the minimum can be 250
    mm (10 in). \[Source: MIL-STD-1472F, 1999\]

**Discussion.** In general, a further viewing distance is preferred.
Earlier recommendations were to locate the monitor at the same viewing
distance as the distance for reading hard copy 307 mm- 410 mm (12-16
inches). However, new research has found that eyestrain is not increased
when the document and monitor were viewed at different distances. Less
eyestrain is reported for 980 mm (38 in) viewing distances than 660 mm
(26 in). Users judged 50 cm (20 in) as too close for computer work and
performance suffered at this distance over a 100 cm (40 in) distance.
Users with presbyopia (common in people over 40) may require corrective
lenses to view displays further than 16 in from the eye. \[Source: ANSI,
1988; Owens & Wolf-Kelly, 1987; Jaschinski-Kruza, 1990\]

-   **5.1.2.14** **Determine maximum viewing distance by legibility.**
    Maximum viewing distance for displays should be determined by the
    legibility of the displayed information. \[Source: MIL-STD-1472F,
    1999\]

**Discussion.** The further the screen is from the viewer, the more
difficult it is for the eyes to resolve fine detail. This can often be
fixed by displaying screen items at larger resolution.

-   **5.1.2.15** **Modify displayed information when viewing distance
    exceeds 20 inches.** Information on displays that are located at
    viewing distances greater than 20 in should be appropriately
    modified in aspects such as display size, symbol size, brightness
    ranges, and resolution to ensure legibility of displayed
    information. \[Source: MIL-STD-1472F, 1999\]

**Discussion.** A general rule of thumb is that items viewed at twice
the distance will appear half as large. As the preferred character size
for readability is 20-22 arc minutes, the size of the characters on the
display will need to be larger to maintain the preferred character size
at greater distances. \[Source: ANSI, 1988\]

-   **5.1.2.16** **Allow users to set viewing distance where possible.**
    Although there may be a set normal viewing distance, workplace
    design should not restrict observers from viewing a CRT from other
    distances. \[Source: MIL-STD-1472F, 1999\]

**Example.** The set distance between the viewer's eye and the CRT
screen may be 20 in, but the viewer may occasionally want to lean
forward to take a closer look at the information displayed on the
screen.

**Discussion.** Users have individual differences in their preferred
viewing distances. Particularly for workstations with a single user, it
is preferable for users to adjust the viewing distance to their
preferred distance. Users report more visual strain when they are forced
to work at a distance shorter than their preferred distance. \[Source:
Jaschinski, Heuer, & Kylian, 1998\]

-   **5.1.2.17 Do not allow** **vibration to interfere with tasks.**
    Vibration of visual displays shall not hinder users in the
    performance of their tasks. \[Source: MIL-STD-1472F, 1999\]

## 5.2 Cathode ray tube displays

A **Cathode Ray Tube** or CRT is the tube of a television or computer
monitor in which rays of electrons are beamed onto a phosphorescent
screen to produce images. CRT is often used as a generic term for a
computer monitor. Three key specifications for a CRT are the maximum
resolution it will display (determined by dot pitch and resolution), the
refresh rate, and whether it is interlaced or non-interlaced. An
**interlaced** display produces a video image by displaying alternate
scan lines. A **non-interlaced** display produces a video image by
displaying all lines in a frame in one pass from top to bottom before
the next frame appears. Non-interlaced displays are preferred to reduce
the perception of flicker. (**Flicker** is the appearance of flashing or
wavering light that occurs in a computer display.) Negative aspects of
CRTs are that they use a lot of electricity and take up a lot of space.

### 5.2.1 General

-   **5.2.1.1** **Refresh rate.** CRT displays shall be refreshed at a
    rate of at least 65 Hz, preferably more than 100 Hz to avoid the
    perception of flicker on monitors for photosensitive users.
    \[Source: ANSI, 1988; Bauer & Cavonius, 1980; Cardosi & Murphy 1995,
    DOE-HFAC 1, 1992; Vanderheiden & Vanderheiden, 1991\]

**Discussion.** A refresh rate of 100 Hz on a non interlaced monitor is
considered sufficient to ensure that flicker is well beyond the
threshold of perception even in the peripheral vision where sensitivity
is higher. (A 100 Hz refresh rate on an interlaced monitor refreshes a
given line only 50 times a second, causing noticeable flicker.)
Individual differences exist in flicker sensitivity. The perception of
flicker increases in the peripheral vision, so the bigger the monitor
is, the higher the refresh rate will need to be to ensure flicker is not
a problem. \[Source: Bauer & Cavonius, 1980\]

-   **5.2.1.2** **Avoid display induced seizures.** Displays shall be
    designed to maximize the number of people who can view them without
    experiencing a seizure. \[Source: Vanderheiden & Vanderheiden,
    1991\]

    **Discussion.** Seizures may be induced by flashing screen cursors
    or by flickering displays, particularly when the flash rates are
    between 10 and 25 Hz, with peak levels occurring between 15-20 Hz.
    Flicker sensitivity becomes greater with increases in light
    intensity and the proportion of the visual field that is flickering.
    Some users experience seizure responses when exposed to flicker
    rates of 15-20 Hz even when their eyes are closed. \[Source:
    Vanderheiden & Vanderheiden, 1991\]

-   **5.2.1.3** **Flicker.** CRT displays shall have no apparent flicker
    to at least 90 % of a sample of the user population when viewed
    under the expected conditions of use. \[Source: ANSI, 1988; DOE-HFAC
    1, 1992; Cardosi & Murphy, 1995\]

**Discussion.** The refresh rate is one of several factors that can
cause the user to perceive flicker. The perception of flicker increases
in the peripheral vision, so the bigger the monitor is, the higher the
refresh rate will need to be to ensure flicker is not a problem.

Ambient illumination, display contrast, colors used, and the
**phosphor** persistence of the monitor also contribute to the
perception of monitor flicker. One of the traditional ways of minimizing
flicker is by keeping the screen as dark as possible.

A longer decay rate on the phosphor (high persistence phosphor)
decreases the flicker perception but can produce trails or after images
behind moving elements. There are also individual differences and age
differences in the ability to perceive flicker, with younger people
generally being more sensitive.

-   **5.2.1.4** **Use medium persistence phosphors.** General purpose
    CRTs should use medium-persistence phosphors. \[Source: DOE-HFAC 1,
    1992\]

    **Discussion.** High-persistence phosphors tend to produce trails or
    after images behind moving elements, and low-persistence phosphors
    are more likely to result in a perceptible flicker. \[Source:
    DOE-HFAC 1, 1992\]

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-   **5.2.1.5** **Ensure color fringes do not affect performance.**
    Color fringes on images on CRT displays, if perceptible, shall not
    have an adverse effect on a user\'s perception or performance.
    \[Source: MIL-STD-1472F, 1999\]

    **Definition.** **Color fringes** are the pixels along the border of
    an object that contain a combination of the selection and background
    colors.

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-   **5.2.1.6** **Avoid jitter.** Deviations in the location of a
    displayed element shall be equal to or less than .0002 mm per mm of
    viewing distance over the period of a second. \[Source: ANSI, 1988\]

    **Definition. Jitter** is a departure from geometric stability. It
    occurs when pixels in displayed objects move instead of remaining in
    a fixed position.

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-   **5.2.1.7** **Maintain uniform element size.** The size of a display
    element (for example, an alphanumeric character or symbol) should
    not vary by more than 10 % regardless of its location within the
    display. \[Source: ANSI, 1988\]

**Discussion.** If you type the capital letter E such that it fills the
screen, all of the Es, from the ones in the center to those on the outer
edge, should appear to be the same size.

-   **5.2.1.8 Minimize element displacement.** The displacement of a
    single display element's position shall vary by less than 5 % of the
    display element box height relative to those above and below, or
    right and left of it. \[Source: ANSI, 1988\]

-   **5.2.1.9 Maintain column and row linearity.** Rows and columns
    shall be parallel and orthogonal to each other with the linearity of
    any column or row not varying by more than 2 % of the length of the
    column or row. \[Source: ANSI, 1988\]

### 5.2.2 Luminance and contrast

-   **5.2.2.1** **Minimize luminance variation across the display.**
    Luminance shall not vary by more than 50 % from the center to the
    edge of the display. \[Source: ANSI, 1988\]

**Definition. Luminance** is the physical measure of the amount of light
emitted by or reflected in a given direction from the display. Luminance
is expressed in candela per meter squared or foot lamberts. **Foot
lamberts** is a measure that has been corrected for the visual system's
differential sensitivity to different wavelengths, giving an
approximation to perceived brightness. \[Source: Murch, 1987\]

-   **5.2.2.2** **Luminance.** Either characters or their background,
    whichever has higher luminance, shall have a luminance of at least
    35 cd/m^2^ (10 fL). \[Source: MIL-STD-1472F, 1999\]

**Discussion.** Increasing luminance reduces pupil diameter, reducing
distortions and improving speed of accommodation and depth of field,
especially for users over 40 who tend to lose some of their ability to
accommodate. However, higher luminance increases sensitivity to flicker.
\[Source: Bauer & Cavonius, 1980\]

-   **5.2.2.3** **Screen luminance.** The ambient screen illumination
    shall not contribute more than 25 % of screen brightness through
    diffuse reflection and phosphor excitation. \[Source: MIL-STD-1472F,
    1999\]

-   **5.2.2.4** **Provide a control for adjusting luminance.** A control
    shall be provided to vary the CRT luminance from 10 % of minimum
    ambient luminance to full CRT luminance. \[Source: MIL-STD-1472F,
    1999\]

-   **5.2.2.5** **Provide a control for adjusting contrast ratio.** A
    control shall be provided to vary the foreground-background contrast
    ratio. \[Source: MIL-STD-1472F, 1999\]

**Definition. Contrast ratio** is the luminance of the foreground
divided by the luminance of the background. It indicates how much
brighter a pure white output would be than a pure black output. The
greater the contrast, the sharper the image will be. Also called
luminance ratio, contrast ratios for CRTs are generally better than
those for active matrix LCDs, which are better than those for passive
matrix LCDs.

-   **5.2.2.6** **Facilitate detection of faint signals.** When the
    detection of faint signals is required, and when the ambient
    illumination may be above 2.7 lux (0.25 ft-c), CRTs shall be hooded,
    shielded, or recessed. \[Source: MIL-STD-1472F, 1999\]

-   **5.2.2.7** **Ensure** **CRT luminance exceeds adjacent surfaces.**
    With the exception of emergency indicators, no light source in the
    immediate surrounding area shall be of greater luminance than the
    CRT signal. \[Source: MIL-STD-1472F, 1999\]

-   **5.2.2.8** **Provide adequate ambient illumination.** The ambient
    lighting levels in areas of the CRT shall be appropriate for other
    visual functions such as setting controls, reading instruments, and
    maintenance, but shall not degrade the visibility of signals on the
    CRT display. \[Source: MIL-STD-1472F, 1999\]

**Discussion.** Automatic adjustment of CRT brightness may be used if
the adjustment is a function of ambient illumination and the range of
adjustment is adequate for the full range of ambient illumination.
\[Source: MIL-STD-1472F, 1999\]

-   **5.2.2.9 Provide controls to modulate ambient lighting.** If
    ambient illumination in an area where a CRT display is used is
    variable, controls shall be provided to dim all light sources,
    including illuminated panels, indicators, and switches in the
    immediate vicinity of the CRT. \[Source: MIL-STD-1472F, 1999\]

### 5.2.3 Glare control

Glare may be of two types, diffuse or specular. **Diffuse** **glare** is
caused by the general environmental illuminance, which effectively
reduces the display contrast. **Specular glare** is the appearance of
unwanted images (reflections) on the display surface. The most effective
method of glare control is to design the workplace so that neither type
is produced. Other methods include screen meshes placed over the display
surface, etched faceplates, anti-reflective coatings, and bonded
quarterwave filters. All of these degrade both contrast and resolution
to some degree.

-   **5.2.3.1** **Minimize or eliminate glare.** Glare shall be
    eliminated or minimized.

    Discussion. Some of the methods that can be used to eliminate or
    minimize glare are

a.  proper placement of the CRT relative to light sources,

b.  use of an anti-glare treatment, such as a diffusing surface or an
    optical coating, or

c.  filter control of the light sources. \[Source: ANSI, 1988;
    MIL-STD-1472F, 1999\]

    Hoods are often unacceptable for this purpose because they tend to
    restrict the viewing angle of screens. Do not allow the use of
    anti-glare treatments to violate the requirements for luminance,
    contrast, and resolution contained in this section. \[Source: ANSI,
    1988; MIL-STD-1472F, 1999\]

-   **5.2.3.2** **Make adjacent surfaces matte.** Surfaces adjacent to
    the CRT shall have a dull, matte finish. \[Source: MIL-STD-1472F,
    1999\]

-   **5.2.3.3** **Minimize reflections.** Displays shall be constructed,
    arranged, and mounted to prevent interference from reflections of
    illumination sources, windows, and other visual displays. \[Source:
    MIL-STD-1472F, 1999\]

**Discussion.** If necessary, shields, filters, or other techniques may
be used to ensure that system performance is not degraded. \[Source:
MIL-STD-1472F, 1999\]

### 5.2.4 Resolution 

-   **5.2.4.1** **Allow discrimination of similar characters.** When
    presenting alphanumeric characters, displays shall allow
    discrimination of similar characters. \[Source: MIL-STD-1472F,
    1999\]

    **Example.** The letter \"l\" and the number \"1\", or the letter
    \"Z\" and the number \"2\" are easily confused.

-   **5.2.4.2 Resolution for CRTs.** CRTs for displaying simple
    alphanumeric text shall have a resolution of at least 0.8 resolution
    elements per mm (20 per in). \[Source: DOE-HFAC 1, 1992\]

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-   **5.2.4.3** **Minimum resolution for alphanumeric characters.**
    Alphanumeric characters should have at least 10 resolution elements
    per character height. \[Source: DOE-HFAC 1, 1992\]

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-   **5.2.4.4** **Resolution for high reading speed.** When high reading
    speed is required, high resolution monitors with at least 35 pixels
    per mm (90 dpi) shall be used. \[Source: DOE-HFAC 1, 1992\]

    **Discussion.** Higher resolution displays (greater than 90dpi) are
    associated with increases in visual performance and decreases in
    visual fatigue in visual search tasks. \[Source: Ziefle, 1998\]

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-   **5.2.4.5** **Resolution for complex symbols.** CRTs for displaying
    complex symbols and graphic detail should have at least 100
    resolution elements per inch. \[Source: DOE-HFAC 1, 1992\]

## 5.3 Flat-panel displays

In flat-panel displays, images are formed from discrete, nonoverlapping,
rectangular pixels. These images can differ from images on CRTs in
character-to-character spacing, interline spacing, character and symbol
design, the effect of ambient illumination, image polarity, and failure
mode. \[Source: Avery & Bowser (DOE HFDG ATCCS V2.0), 1992\]

-   **5.3.1** **Character formation \-- vertical orientation.**
    Characters in a vertical orientation should be formed from a matrix
    of at least 9 by 13 pixels. \[Source: DOE HFDG ATCCS V2.0, 1992\]

-   **5.3.2** **Minimum character formation for nonvertical
    orientation.** Characters in a nonvertical orientation should be
    formed from a matrix of at least 8 by 11 pixels, preferably 15 by 21
    pixels. \[Source: DOE HFDG ATCCS V2.0, 1992\]

-   **5.3.3** **Character stroke width.** Character stroke width should
    not exceed the minimum and maximum values given in Exhibit 5.3.3.
    \[Source: DOE HFDG ATCCS V2.0, 1992\]

**Definition: Stroke width** is the thickness of the lines used to make
up the number or letter.

> []{#__RefHeading___Toc18406079 .anchor}**Exhibit** **5.3.3** Stroke
> width for pixel-generated characters. All numbers are in pixels.

  ------------------------- ----------------------- ---------------------
  Upper case character      Minimum stroke width    Maximum stroke width
  height                                            

  7 to 8                    1                       1

  9 to 12                   1                       2

  13 to 14                  2                       2

  15 to 20                  2                       3

  21 to 23                  2                       4
  ------------------------- ----------------------- ---------------------

-   **5.3.4** **Minimum and preferred character height-width
    relationship.** The width of characters of a given height should not
    exceed the minimum and maximum values given in Exhibit 5.3.4. The
    exhibit also includes the preferred values. \[Source: DOE HFDG ATCCS
    V2.0, 1992\]

> []{#__RefHeading___Toc18406080 .anchor}**Exhibit** **5.3.4**
> Height-width relationship for upper case pixel-generated characters.
> All numbers are in pixels.

  ------------------ ------------------- ---------------- ----------------
  Character height   Minimum width       Preferred width  Maximum width

  7                  4                   5                5

  8                  4                   6                7

  9                  5                   6                8

  10                 5                   7                9

  11                 6                   8                10

  12                 6                   9                11

  13                 6                   9                12

  14                 7                   10               13

  15 or 16           8                   11               14
  ------------------ ------------------- ---------------- ----------------

-   **5.3.5** **Image formation time** **when motion artifacts are
    unimportant.** If motion artifacts are not important, image
    formation time should not exceed 55 msec. \[Source: DOE HFDG ATCCS
    V2.0, 1992\]

**Definition.** **Motion artifacts** are the after images (made up of
the previously drawn object that have not yet decayed enough to
disappear) that one sees that appear to follow a moving display object.
They often look like the tail of a comet following the object in motion.

-   **5.3.6** **Image formation time when motion artifacts are
    important.** If motion artifacts are important, image formation time
    should not exceed 10 msec. \[Source: DOE HFDG ATCCS V2.0, 1992\]

-   **5.3.7** **Minimize pixel failure rate.** Displays should be
    selected and maintained so that the pixel failure rate does not
    exceed 1%. \[Source: DOE HFDG ATCCS V2.0, 1992\]

-   **5.3.8** **Use positive polarity when all else is equal.** If
    character stroke width, modulation, and luminance values are
    approximately equal for both polarities, the positive polarity (dark
    characters on a light background) should be used. (see Exhibit
    5.3.8). \[Source: DOE HFDG ATCCS V2.0, 1992\]

**Discussion.** Reflections are less visible on a bright background than
on a dark background.

> []{#__RefHeading___Toc18406081 .anchor}**Exhibit 5.3.8** Example of
> positive and negative polarity.

![](media/image1.wmf)

-   **5.3.9** **Treat displays to minimize reflections.** All flat panel
    displays should incorporate a first-surface treatment to diminish
    specular reflections. \[Source: DOE HFDG ATCCS V2.0, 1992\]

## 5.4 Liquid crystal displays

**Liquid crystal displays** **(LCDs)** are a type of flat-panel display
that works by suspending liquid crystals between two transparent sheets
of polarizing material. An electric current passes through the liquid
causing the crystals to act like a shutter, either permitting light to
pass through or blocking the light so that it cannot pass through. Most
LCDs are backlit to make them easier to read in bright environments.
LCDs are especially suited for information display in environments where
ambient illumination is high. Their advantages include excellent
contrast, long life, rugged design, low voltage, and low power
consumption (except when backlit). Their disadvantages include slow
speed, limited color capability, limited temperature range, and
manufacturing problems for larger panels with higher resolution.
\[Source: DOE HFDG ATCCS V2.0, 1992\]

-   **5.4.1 Use only with adequate illumination.** LCDs should be used
    only with adequate levels of ambient illumination. \[Source: DOE
    HFDG ATCCS V2.0, 1992\]

    **Discussion.** LCD reading performance improves as ambient
    illumination increases over the range 20 to 1500 lx. \[Source: DOE
    HFDG ATCCS V2.0, 1992\]

-   **5.4.2** **Polarity.** For reflective LCDs, the image should be
    light characters on a dark background; for backlighted
    (transmissive) LCDs, the image should be dark characters on a light
    background. \[Source: DOE HFDG ATCCS V2.0, 1992\]

-   **5.4.3** **Minimize backlighting.** The amount of backlighting used
    should be minimized. \[Source: DOE HFDG ATCCS V2.0, 1992\]

    **Discussion.** LCD reading errors increase as backlighting
    increases over the range of 0 to 122 cd/m^2^. \[Source: DOE HFDG
    ATCCS V2.0, 1992\]

-   **5.4.4** **LCDs used in low ambient illumination.** If LCDs are
    used in the presence of low ambient illumination, users should be
    able to adjust the viewing angle and the amount of backlight.
    \[Source: DOE HFDG ATCCS V2.0, 1992\]

-   **5.4.5** **Minimize off-axis viewing of backlit LCDs.** If used,
    backlit LCDs should be located so that off-axis viewing is
    minimized. \[Source: DOE HFDG ATCCS V2.0, 1992\]

    **Discussion.** The readability of many LCD screens drops off
    considerably as does the brightness particularly with a 45 degree
    off axis viewing angle. This may not be as much of a problem with
    newer LCD technology that claims a higher off axis viewing angle.

-   **5.4.6 Incident illumination.** The incident illumination on the
    surface of LCD displays should be at least 35∏/R lux, with R
    equaling reflectivity of the display at its most reflective state.
    \[Source: ANSI, 1988\]

## 5.5 Gas plasma displays

A **gas plasma display** is a type of flat-panel display that works by
placing neon gas between one plate coated with vertical conductive print
and another plate coated with horizontal conductive print to form a
grid. A point of light (pixel) is created at the intersection of a
horizontal and vertical line that has been charged with an electric
current. A major advantage of gas plasma displays is the availability of
very bright, high-resolution displays, some of which can be viewed in
direct sunlight. Other advantages are uniformity, high resolution, large
size, long life, ruggedness, excellent viewing angle, and the absence of
flicker. Some disadvantages are high voltage and power requirements,
complex drive circuits, low luminous efficiency, relatively large pixel
size, and limited color capability.

## 5.6 Electroluminescent displays

**Electroluminescent displays** **(ELD)** may be used where display
visibility from multiple viewer positions, high display uniformity, high
MTBF, high resolution, low display volume, low heat dissipation, and low
power consumption are more important than display of multicolored
objects, high brightness images, or sunlight readability. They are also
lightweight, and provide display flexibility, therefore, they may be
used instead of mechanical instruments. They may also be used where
sudden lamp failure could result in catastrophic consequences. \[Source:
MIL-STD-1472F, 1999\]

-   **5.6.1** **Alphanumeric character and symbol size.** The height of
    alphanumeric characters and geometric or pictorial symbols shall
    subtend a visual angle of at least 15 min. \[Source: MIL-STD-1472F,
    1999\]

    **Definition. Visual angle** is the angle subtended by objects
    measured in minutes of arc. It represents an apparent size of an
    object based on the relationship between an object\'s distance from
    the viewer and its size (perpendicular to the viewer\'s line of
    sight). For example, if an object that is size ***h*** is at a
    distance ***d*** from the retina, the visual angle subtended
    (***x***) is equal to arctan (***h***/***d***).

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-   **5.6.2** **Evaluate** **through prototyping.** The use of ELDs
    should be evaluated through prototyping before being incorporated in
    a new system. \[Source: DOE HFDG ATCCS V2.0, 1992\]

-   **5.6.3** **Verify acceptability by users.** The acceptability of
    ELDs should be verified by the expected users. \[Source: DOE HFDG
    ATCCS V2.0, 1992\]

## 5.7 Large-screen displays

The selection or design of a large-screen display, especially a
projection display, may be more complex than that of other workstation
displays. The effects of ambient illumination, observer location, type
of data to be displayed, visual acuity for symbol size and contrast,
screen size, screen format, symbol luminance, and screen gain are all
important factors.

### 5.7.1 General

-   **5.7.1.1** **When to use.** Large-screen displays should be used
    when

a.  more than one user needs to refer to the same displayed information,
    but space or other constraints make the use of a single, common
    display preferable to many, individual displays.

b.  one or more members of a team of users need to be able to move
    about, yet still need access to displayed information. \[Source:
    MIL-STD-1472F, 1999\]

-   **5.7.1.2** **When not to use.** Large-screen displays shall not be
    used if the spatial and environmental conditions do not allow all
    users to have appropriate visual access in terms of viewing
    distance, angle, and lack of interference from intervening objects,
    personnel, and ambient lighting. (If the display is optically
    projected, see Paragraph 5.9.2). \[Source: MIL-STD-1472F, 1999\]

-   **5.7.1.3** **Viewing distance.** The display shall be near enough
    that the most distant viewers can resolve the critical details
    presented, but not closer to any viewer than 1/2 the display width
    or height, whichever is greater. \[Source: MIL-STD-1472F, 1999\]

-   **5.7.1.4** **Locate so that view is not obscured.** A large-screen
    display shall be located so that its critical users do not have
    their view of it obscured by persons moving about in their normal
    traffic patterns. \[Source: MIL-STD-1472F, 1999\]

-   **5.7.1.5** **Ensure critical information cannot be deleted.**
    Control of large-screen group display systems shall ensure that
    critical information cannot be modified or deleted inadvertently or
    arbitrarily. \[Source: MIL-STD-1472F, 1999\]

-   **5.7.1.6** **Place display changes in control of users.** Changes
    in the group display shall be under the control of designated users
    who operate according to pre-established procedures, on command of a
    person in charge, or both. \[Source: MIL-STD-1472F, 1999\]

-   **5.7.1.7** **Separate display.** When a user must make changes that
    are of interest only to him or her, a separate, remote display shall
    be provided. \[Source: MIL-STD-1472F, 1999\]

### 5.7.2 Large-screen optical projection displays

-   **5.7.2.1** **When to use.** When ambient light can be properly
    controlled, optical projection displays shall be used for
    applications requiring group presentation, pictorial and spatial
    information, past history versus real-time presentation,
    synthetically generated pictures, simulation of the external world,
    or superimposition of data from more than one source. \[Source:
    MIL-STD-1472F, 1999\]

-   **5.7.2.2** **When to use rear projection.** Rear projection shall
    be used where physical obstructions to front projection result in
    poor visibility, or where work areas require high ambient
    illumination for other activities. \[Source: MIL-STD-1472F, 1999\]

-   **5.7.2.3** **Viewing distance and screen size.** The ratio of
    viewing distance to screen size (measured diagonally) shall be not
    more than 8:1 and not less than 2:1. The optimum ratio is 4:1; the
    preferred range is not less than 3:1 or more than 6:1. \[Source:
    MIL-STD-1472F, 1999\]

-   **5.7.2.4** **Viewing angle for groups.** The angle off center-line
    for viewing a large-screen display shall not be greater than 30° for
    groups. The optimum viewing angle is 0°; and the preferred limit is
    20°. \[Source: MIL-STD-1472F, 1999\]

-   **5.7.2.5** **Viewing angle for individuals.** The angle off
    center-line for viewing a large-screen display shall not be greater
    than 10° for individuals. \[Source: MIL-STD-1472F, 1999\]

-   **5.7.2.6** **Image luminance with no film in the projector.** The
    image luminance with no film in the operating projector shall be not
    less than 17 cd/m^2^ (5 fL), and not more than 70 cd/m^2^ (20 fL).
    \[Source: MIL-STD-1472F, 1999\]

-   **5.7.2.7** **Luminance ratio across the screen.** The ratio of
    maximum to minimum luminance across the screen shall be not greater
    than 3:1. The optimum ratio of maximum to minimum luminance across
    the screen is 1:1, and the preferred limit is 1.5:1. \[Source:
    MIL-STD-1472F, 1999\]

-   **5.7.2.8** **Luminance ratio as a function of viewing location.**
    The ratio of maximum to minimum luminance as a function of viewing
    location shall be not greater than 4:1. The optimum ratio of maximum
    to minimum luminance as a function of viewing location is 1:1, and
    the preferred limit is 2:1. \[Source: MIL-STD-1472F, 1999\]

-   **5.7.2.9** **Luminance ratio of ambient light to brightest image.**
    The ratio of ambient light to the brightest part of an image shall
    not be greater than 1:10 for black and white images and 2:10 for
    images with gray scale or color, while maintaining optimum image
    luminance. The optimum ratio of ambient light to the brightest part
    of the image is 0:1, and the preferred range is 1:100 to 1:500.
    \[Source: MIL-STD-1472F, 1999\]

**Discussion.** The optimum image luminance is 35 cd/m2 (10 fL), and the
preferred range is from 27 to 48 cd/m2 (8 to 14 fL).

-   **5.7.2.10** **Luminance of screen center at the maximum viewing
    angle.** The luminance of the screen center at the maximum viewing
    angle shall be at least half its maximum luminance. \[Source:
    MIL-STD-1472F, 1999\]

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-   **5.7.2.11** **Luminance ratio.** Under optimal ambient lighting
    conditions, the luminance ratio for optically projected displays
    should be 500:1. \[Source: MIL-STD-1472F, 1999\]

    **Definition.** **Luminance ratio** is the ratio of the luminance of
    an object to that of its surrounding field or background. \[Source:
    MIL-STD-1472F, 1999\]

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-   **5.7.2.12** **Minimum luminance ratio for viewing charts and
    text.** The minimum luminance ratio for viewing charts, printed
    text, and other line work shall be 5:1. \[Source: MIL-STD-1472F,
    1999\]

-   **5.7.2.13** **Minimum luminance ratio for images with limited range
    of detail.** The minimum luminance ratio for images that contain
    limited shadows and detail with a limited luminance range, such as
    animation or photographs, shall be 25:1. \[Source: MIL-STD-1472F,
    1999\]

-   **5.7.2.14** **Minimum luminance ratio for images with a full
    range.** The minimum luminance ratio for images that contain a full
    range of colors, or grays in black-and-white photographs, shall be
    100:1. \[Source: MIL-STD-1472F, 1999\]

-   **5.7.2.15** **Minimize distortion.** Distortion of the projected
    image shall be minimized by ensuring that the screen is as nearly as
    possible perpendicular to the light beam from the projector.
    \[Source: MIL-STD-1472F, 1999\]

## 5.8 Stereoscopic displays

**Stereoscopic displays** generate the sensation of three dimensions,
height, width and depth within the human visual system.
Three-dimensional display technology may be \"stereoscopic,\" which
requires that users wear special glasses that provide different images
to the two eyes, or \"auto stereoscopic,\" which does not require any
special viewing aids. There are situations in which three-dimensional
images can enhance user performance or increase the \"naturalness\" of
the presentation of complex spatial data. Emerging technology in this
area is likely to have the disadvantages of limiting the field of view,
the number of viewers, and the nature of data that can be displayed.
\[Source: DOE HFDG ATCCS V2.0, 1992\]

-   **5.8.1** **Use only if third dimension is meaningful.**
    Three-dimensional displays shall be used only if the third dimension
    conveys a real benefit to the user. \[Source: DOE HFDG ATCCS V2.0,
    1992\]

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-   **5.8.2** **Do not slow system performance.** The three-dimensional
    presentation of information should not slow information display,
    degrade image quality, or degrade other aspects of system
    performance. \[Source: DOE HFDG ATCCS V2.0, 1992\]

-   **5.8.3** **Avoid interocular crosstalk.** There should be no
    interocular crosstalk, that is, the left eye should not see the
    images intended for the right eye, and vice versa. \[Source: DOE
    HFDG ATCCS V2.0, 1992\]

-   **5.8.4** **Avoid saturated primary colors.** Secondary colors
    should be used in coding stereoscopic images; saturated primary
    colors should be avoided. \[Source: DOE HFDG ATCCS V2.0, 1992\]

    **Discussion.** Saturated primary colors can produce depth
    perceptions by themselves, which might interfere with the
    stereoscopically produced depth perceptions. **Primary colors** for
    light emitting displays such as CRTs are Red, Green, and Blue (RGB)
    and **secondary colors** are Cyan, Magenta, and Yellow (CMY). This
    is different than the primary colors for subtractive color mixture
    (like mixing paints), which are Yellow, Red, and Blue. \[Source: DOE
    HFDG ATCCS V2.0, 1992\]

-   **5.8.5 Temporal modulation for dynamic displays.** If dynamic
    three-dimensional displays are used, the temporal modulation of
    stereopsis should be approximately 1 Hz to ensure the most accurate
    perception of stereo motion. \[Source: DOE HFDG ATCCS V2.0, 1992\]

**Definition.** **Stereopsis** (also called stereoscopic vision) is
three dimensional depth perception based on retinal disparity. As the
eyes are slightly separated, each eye sees a slightly different image,
when these images are fused in the brain. The result is a perception of
depth or stereoscopic vision.

-   **5.8.6** **Separate depth-coded objects.** Depth-coded objects
    should be separated spatially to eliminate disparity averaging,
    crowding, and repulsion. \[Source: DOE HFDG ATCCS V2.0, 1992\]

**Definition.** **Disparity** is the computation of depth values based
on the lateral distance between corresponding picture elements in both
image planes of stereo vision.

-   **5.8.7** **Scale images according to disparity.** Image size should
    be scaled according to the disparity of the image. \[Source: DOE
    HFDG ATCCS V2.0, 1992\]

-   **5.8.8** **Provide individual size scaling if critical to task
    performance.** If accurate size perception is critical to task
    performance, size scaling should be done for each observer.
    \[Source: DOE HFDG ATCCS V2.0, 1992\]

-   **5.8.9** **Co-modulate luminance and stereopsis.** Luminance should
    be co-modulated with stereopsis. \[Source: DOE HFDG ATCCS V2.0,
    1992\]

**Discussion.** Brightness is also a depth cue, with brighter objects
being perceived as nearer. \[Source: DOE HFDG ATCCS V2.0, 1992\]

## 5.9 Printers 

-   **5.9.1** **When to use.** Printers should be used if a visual
    record of data is necessary or desirable. \[Source: MIL-STD-1472F,
    1999\]

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-   **5.9.2** **Minimum contrast.** A minimum luminance contrast of 3.0
    shall be provided between the printed material and the background on
    which it is printed. \[Source: MIL-STD-1472F, 1999\]

-   **5.9.3** **Provide illumination where needed.** If the printed
    matter would not be legible in the planned operational ambient
    illumination, the printer shall be provided with internal
    illumination. \[Source: MIL-STD-1472F, 1999\]

-   **5.9.4** **Provide a take up device.** A take-up device shall be
    provided for printed material. \[Source: MIL-STD-1472F, 1999\]

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-   **5.9.5** **Allow for annotation.** If applicable, printers should
    be mounted so that the user can write or mark on printed matter
    easily while it is still in the printer. \[Source: MIL-STD-1472F,
    1999\]

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-   **5.9.6** **Ensure legibility.** The print shall be free from
    character line misregistration, character tilt, and smear. \[Source:
    MIL-STD-1472F, 1999\]

-   **5.9.7** **Ensure readability of printed tapes.** If information is
    printed on tapes, the information on tapes shall be printed in such
    a manner that it can be read as it is received from the machine
    without requiring the cutting and pasting of tape sections.
    \[Source: MIL-STD-1472F, 1999\]

-   **5.9.8** **Controls, feedback, and normal operations.** Printers
    shall conform to the rules in this standard with respect to

a.  the controls and displays used to start, stop, and adjust the
    machine and its critical operating elements,

b.  giving a positive indication of the remaining supply of materials
    such as paper and ink,

c.  operations performed by the user, such as inserting, adjusting,
    removing, replenishing, and replacing supplies and materials without
    requiring disassembly or special tools, and

d.  operations performed on site by a technician, such as adjustments
    and replacements not ordinarily performed by the user. \[Source:
    MIL-STD-1472F, 1999\]

-   **5.9.9** **Locate printed outputs within reach.** Printed output
    should be located within easy reach of those who need it. \[Source:
    Vanderheiden & Vanderheiden, 1991\]

**Discussion.** Improperly located printouts may not be reachable to
users in a wheelchair because of the location of the printer. \[Source:
Vanderheiden & Vanderheiden, 1991

## 5.10 Plotters and recorders

-   **5.10.1** **When to use.** Plotters or recorders should be used if
    a visual record of continuous graphic data is necessary or
    desirable. \[Source: MIL-STD-1472F, 1999\]

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-   **5.10.2** **Do not obscure critical graphics while plotting or
    recording.** Critical graphics (those points, curves, or grids that
    must be observed while a recording is being made) shall not be
    obscured by the pen assembly, arm, or other hardware elements.
    \[Source: MIL-STD-1472F, 1999\]

-   **5.10.3** **Provide adequate contrast.** Luminance contrast between
    a plotted function and the background on which it is drawn shall be
    at least 1.0. \[Source: MIL-STD-1472F, 1999\]

-   **5.10.4** **Provide a take up device.** If necessary or desirable,
    a take-up device shall be provided for plotted material. \[Source:
    MIL-STD-1472F, 1999\]

-   **5.10.5** **Provide an overlay if needed.** If it is critical to
    the proper interpretation of graphic data as they are being
    generated, a graphic overlay shall be provided. \[Source:
    MIL-STD-1472F, 1999\]

-   **5.10.6** **Do not obscure data.** Graphic overlays shall not
    obscure or distort the data. \[Source: MIL-STD-1472F, 1999\]

-   **5.10.7** **Resist smudge and smear.** Output from plotters shall
    be resistant to smudging and smearing under operational use.
    \[Source: MIL-STD-1472F, 1999\]

-   **5.10.8** **Allow annotation.** If applicable, plotters and
    recorders shall be designed or mounted so that the user can write on
    or mark the paper while it is still in the plotter or recorder.
    \[Source: MIL-STD-1472F, 1999\]

-   **5.10.9** **Controls, feedback, and normal operations.** Plotters
    and recorders shall conform to the rules in this standard with
    respect to

a.  the controls and displays used to start, stop, and adjust the
    machine and its critical operating elements,

b.  giving a positive indication of the remaining supply of materials
    such as paper and ink,

c.  operations performed by the user, such as inserting, adjusting,
    removing, replenishing, and replacing supplies and materials without
    requiring disassembly or special tools, and

d.  operations performed on site by a technician, such as adjustments
    and replacements not ordinarily performed by the user. \[Source:
    MIL-STD-1472F, 1999\]

## 5.11 Accommodating people with disabilities

Accessibility in design extends general design principles to cover those
individuals who are faced with either temporary or permanent limitations
in some dimension of human ability (sight, hearing, physical mobility,
etc.). Although these rules are meant to make systems more accessible
and thus make systems available to an increased number of users, it is
not possible to design everything for use by everyone. However, there
are often adaptations that can significantly increase system
accessibility and usefulness. The goal of this section is to make
systems more accessible and thus maximize the number of potential users.

-   **5.11.1** **Maximize the number of people who can see output.**
    Visual displays intended to be accessible should be designed to
    maximize the number of people who can clearly see the presented
    output. \[Source: Vanderheiden & Vanderheiden, 1991\]

**Discussion.** For instance, users with decreased visual abilities may
have difficulty seeing small output or complex fonts and graphics. Other
users may have difficulty seeing objects if there is insufficient
contrast between the object and the background or may be especially
sensitive to glare. \[Source: Vanderheiden & Vanderheiden, 1991\]

-   **5.11.2 Ensure that visual outputs are not missed.** Equipment
    intended to be accessible should be designed to minimize the number
    of people who will miss important information if they cannot see.
    \[Source: Vanderheiden & Vanderheiden, 1991\]

**Discussion.** Visual output, for example, information presented on
screens, paper printouts, warning lights, and dials, may not be seen at
all by some users. \[Source: Vanderheiden & Vanderheiden, 1991\]

-   **5.11.3 Provide connection points for alternative output devices.**
    Computers and computer systems should provide a point to which an
    alternative output device can be connected. \[Source: Scadden &
    Vanderheiden, 1988\]

> **Discussion**. The connection point might be a standard serial or
> parallel port. Alternative output devices include speech synthesizers
> and Braille display devices. \[Source: Scadden & Vanderheiden, 1988\]

-   **5.11.4 Provide speech output compatibility.** Computers and
    computer systems should provide a built-in speech output capability
    or provide a point to which a speech synthesizer can be connected.
    \[Source: Scadden & Vanderheiden, 1988\]

-   **5.11.5** **Facilitate the manipulation of printouts.** The
    manipulation of printouts should be facilitated by providing
    reaching and grasping devices such as reachers, artificial hands or
    hooks, and mouthsticks with clasps attached if the printouts are not
    within easy reach of the user. \[Source: Vanderheiden &
    Vanderheiden, 1991\]

-   **5.11.6** **Provide redundant auditory output.** Redundant auditory
    output should be provided in addition to a visual display if the
    visual display cannot be made physically accessible to a person in a
    wheelchair. \[Source: Vanderheiden & Vanderheiden, 1991\]

-   **5.11.7** **Attach larger displays or accommodating assistive
    devices.** If a visual display is not sufficient for users with
    difficulty seeing small output, a means should be provided of either
    attaching larger-image displays or utilizing other special assistive
    devices, such as an electronic magnifier, a voice synthesizer or a
    braille printer. \[Source: Vanderheiden & Vanderheiden, 1991\]

-   **5.11.8 Provide redundant visual information.** A system should
    provide warnings, cues, and all other critical visual information
    redundantly in audible or tactile form if it is intended for use by
    a visually impaired person or a user who may have their attention
    focused elsewhere. \[Source: Vanderheiden & Vanderheiden, 1991\]

[]{#__RefHeading___Toc26772320 .anchor}Glossary

**Cathode Ray Tube** **(CRT)** - A vacuum tube of a television or
computer monitor in which the inner surface is coated with phosphors
which glow and produce light when hit by an electron beam. CRT is often
used as a generic term for a computer monitor.

**Central visual field** - Central visual field (sometimes referred to
as the focal area) is the central 30° of the visual field. This is the
area that people use to look at objects in the world, moving their eyes
as needed to bring images of the object on to the fovea, which is the
area of highest acuity. When an object is outside of the focal area, a
person will usually turn their head rather than simply move their eyes.

**Color fringes** - Are the pixels along the border of an object that
contain a combination of the selection and background colors.

**Contrast** - The range between the lightest tones and the darkest
tones. The lower the number value, the more closely the shades will
resemble each other. The higher the number, the more the shades will
stand out from each other.

**Contrast ratio** - The luminance of the foreground divided by the
luminance of the background. It indicates how much brighter a pure white
output would be than a pure black output. The greater the contrast, the
sharper the image will be. Also called luminance ratio.

**CRT** - See Cathode Ray Tube

**Diffuse glare** - A type of glare caused by the general ambient
luminance, which effectively reduces the display contrast without
producing significant specular reflection.

**Disparity** - The computation of depth values based on the lateral
distance between corresponding picture elements in both image planes of
stereovision.

**ELD** - See electroluminescent displays

**Electroluminescent displays (ELD)** - A flat-panel display, the type
typically used for laptop computers, that works by placing a thin
phosphorescent film between one plate coated with vertical wires and
another plate with horizontal wires, to form a grid. When an electrical
current passes through a horizontal and a vertical wire, the
phosphorescent film at this intersection glows, creating a point of
light (pixel).

**Flicker** - The appearance of flashing that occurs in a computer
display when the display is not refreshed frequently enough causing the
phosphor to begin to decay prior to being refreshed.

**Focal vision** - The central 30° of the visual field. It is the area
that people use to look at objects in the world, moving their eyes as
needed to bring images of the object on to the fovea, which is the area
of highest acuity.

**Foot lamberts** - Is a measure that has been corrected for the visual
system's differential sensitivity to different wavelengths, giving an
approximation to perceived brightness.

**Fovea** - The small central region of the retina that exhibits the
greatest sensitivity to detail and color.

**Gas plasma displays** - Is a type of flat-panel display that works by
placing neon gas between one plate coated with vertical conductive print
and another plate coated with horizontal conductive print to form a
grid. A point of light (pixel) is created at the intersection of a
horizontal and vertical line that has been charged with an electric
current.

**Interlaced** - A display that produces a video image by displaying
alternate scan lines.

**Interocular crosstalk** - When the left eye can see the images
intended for the right eye, and vice versa.

**Jitter** - A departure from geometric stability, which occurs when
pixels in displayed objects move instead of remaining in a fixed
position.

**LCD** - See liquid crystal display

**Legibility** - The extent to which the user can decipher or read
alphanumeric characters or text.

**Liquid crystal display (LCD)** - Is a flat-panel display that works by
suspending liquid crystals between two transparent sheets of polarizing
material. An electric current passes through the liquid causing the
crystals to act like a shutter, either permitting light to pass through
or blocking the light so that it cannot pass through.

**Luminance** - Is the physical measure of the amount of light emitted
by or reflected in a given direction from the display. Luminance is
expressed in candela per meter squared or foot lamberts.

**Luminance ratio** - The luminance of the foreground divided by the
luminance of the background. It indicates how much brighter a pure white
output would be than a pure black output. The greater the contrast, the
sharper the image will be. It is also called contrast ratio.

**Motion artifacts** - The after images made up of the previously drawn
object that have not yet decayed enough to disappear that one sees that
appear to follow a moving display object. They often look like the tail
of a comet following the object in motion.

**Non interlaced** - A display that produces a video image by displaying
all lines in a frame in one pass from top to bottom before the next
frame appears.

**Phosphor** - A luminescent substance, used to coat the inside of a
CRT, which emits visible light when illuminated by electrons within an
evacuated glass tube.

**Polarity** - The relationship between the brightness of the background
and an image. A bright image on a dark background is negative polarity
and a dark image on a bright background is positive polarity.

**Primary colors** -- Primary colors for displays are Red, Green, and
Blue (RGB) for light emitting displays such as CRTs. This is different
than the primary colors for subtractive color mixture (like mixing
paints), which are Yellow, Red and Blue.

**Refresh rate** - The rate (in cycles per second or Hz) at which the
displayed contents of a computer screen are regenerated.

**Resolution** - The number of pixel elements per square inch.

**Secondary colors** -- Secondary colors are Cyan, Magenta, and Yellow
(CMY) for light emitting displays such as CRTs.

**Specular glare** - The appearance of unwanted images (reflections) on
the display surface.

**Stereopsis** - (also called stereoscopic vision) Three dimensional
depth perception based on retinal disparity. As the eyes are slightly
separated, each eye sees a slightly different image, when these images
are fused in the brain. The result is a perception of depth or
stereoscopic vision.

**Stereoscopic display** - A method used to generate the sensation of
three dimensions within the human visual system. Three-dimensional
display technology may be \"stereoscopic,\" which requires that users
wear special glasses that provide different images to the two eyes, or
\"auto stereoscopic,\" which does not require any special viewing aids.

**Stereoscopic vision** - See Stereopsis

**Stroke width** - The thickness of the lines used to make up the number
or letter.

**Viewing angle** - The angle off the center line from which a displayed
will be viewed

**Visual angle** - The angle subtended by objects measured in minutes of
arc. It represents an apparent size of an object based on the
relationship between an object\'s distance from the viewer and its size
(perpendicular to the viewer\'s line of sight). For example, if an
object that is size ***h*** is at a distance ***d*** from the retina the
visual angle subtended, ***x***, is: ***x*** = arctan (***h***/***d***).

[]{#refheading___toc26772321 .anchor}References

American National Standards Institute. (1988). *American national
standard for human factors engineering of visual display terminal
workstations* (ANSI/HFS Standard No. 100-1988). Santa Monica, CA: The
Human Factors Society, Inc.

Ankrum, D. R., Hansen, E. E., and Nemeth, K. J. (1995). The vertical
horopter and the angle of view. In A. Grieco, G. Molteni, B. Piccoli &
E. Occhipinti (Eds.), *Work with display units '94* (pp.131-136).
Amsterdam: Elsevier.

Ankrum, D. R., & Nemeth, K. J. (1995). Posture, comfort and monitor
placement. *Ergonomics in design*, April, 7-9.

Avery, L. W., & Bowser, S. E. (Eds.).(1992). *Department of Defense
human-computer interface style guide (Version 2.0)* (DOE HFDG ATCCS V2.0
also known as DOD HCISG V2). Washington, DC: Defense Information Systems
Agency.

Balci, R., & Aghazadeh, F. (1998). Influence of VDT monitor positions on
discomfort and performance of users with or without bifocal lenses.
*Journal of human ergol:* *Tokyo.* 27 (1-2), 62-69.

Bauer, D., & Cavonius, C. (1980). Improving the legibility of visual
display units through contrast reversal. In E. Grandjean & E. Vigliani
(Eds), *Ergonomic aspects of visual display terminals.* London: Taylor &
Francis.

Cardosi K., M. & Murphy E. D. (1995). *Human factors checklist for the
design and evaluation of ATC systems*. (DOT/FAA/RD-95/3.1,
DOT-VNTSC-FAA-95-3.1). Washington, DC: United States Department of
Transportation, Office of Aviation Research.

Department of Defense. (1999). *Design criteria standard -- human
engineering.* (MIL-STD-1472 F). Philadelphia, PA: Navy Publishing and
Printing Office.

[]{.mark}

Department of Energy. (1992). *Human factors engineering design
criteria* (DOE-HFAC 1). Washington, DC: United States Department of
Energy.

Jaschinski, W., Heuer, H., & Kylian, H. (1998). Preferred position of
visual displays relative to the eyes: A field study of visual strain and
individual differences. *Ergonomics, 41* (7), 1034-1049.

Jaschinski-Kruza, W. (1990). On the preferred viewing distances to
screen and document at VDU workplaces. *Ergonomics, 33* (8), 1055-1063.

Krimsky, E. (1948). *The management of binocular imbalance*.
Philadelphia, PA: Lea and Febiger.

Morgan, C., Cook, J., Chapanis, A., & Lund, M. (Eds.) (1963). *Human
engineering guide to equipment design.* New York: McGraw-Hill.

Murch, G. M. (1987). Colour graphics -Blessing or ballyhoo? in R.M.
Baecker & W.A.S. Buxton (Eds.), *Readings in Human-Computer
Interaction - A Multidisciplinary Approach* (pp. 333--341). Los Altos,
CA: Morgan Kaufmann.

Owens, D. & Wolf-Kelly, K. (1987). Near work, visual fatigue, and
variations of oculomotor tonus. *Investigative Ophthalmology and Visual
Science*. 28, 743 - 749.

Ripple, P. (1952). Variation of accommodation in vertical directions of
gaze. *American Journal of Ophthalmology*, 35, 1630-1634.

Scadden, L. A., & Vanderheiden, G. C. (1988). *Considerations in the
design of computers and operating systems to increase their
accessibility to persons with disabilities.* Madison, WI: Trace Research
& Development Center.

Tsubota, K., & Nakamori, K. (1993). Dry eyes and video display
terminals. *New England Journal of Medicine*, *328* (8), 584.

Tyrrell, R., & Leibowitz, H. (1990). The relation of vergence effort to
reports of visual fatigue following prolonged near work. *Human Factors,
32* (3), 341-357.

Vanderheiden, G. C., & Vanderheiden, K. R. (1991). *Accessible design of
consumer products: Guidelines for the design of consumer products to
increase their accessibility to people with disabilities or who are
aging*. Madison, WI: Trace Research and Development Center.

Ziefle, M. (1998). Effects of display resolution on visual performance.
*Human Factors*, 40 (4), 554-568.

[]{.mark}

[]{#__RefHeading___Toc26772322 .anchor}**Index**

3

3D displays 5-21

A

Ambient lighting 5-11

B

Backlighting 5-16

C

Cathode ray tube displays 5-7

refresh rate 5-8

Character formation 5-13

Character height 5-14

Chromatic aberration 5-9

Color fringes 5-9

Contrast ratio 5-11

adjusting 5-11

Critical displays

location 5-5

Critical graphics 5-24

CRT 5-7

D

Disparity 5-22

Displays 5-1

arrangement of 5-3

positioning 5-2

tilt 5-4

viewing distance 5-5

Distortion 5-20

E

ELD 5-17

Electroluminescent displays 5-17

Element

displacement 5-10

size 5-10

Evaluation 5-17

F

Faint signal detection 5-11

Flat-panel displays 5-13

Flicker 5-7, 5-8, 5-9

G

Gas plasma displays 5-17

Glare 5-12

Diffuse 5-12

minimizing 5-12

Specular 5-12

I

Image formation time 5-14

Interocular crosstalk 5-21

J

Jitter 5-9

L

Large-screen displays 5-18

Large-screen optical projection displays 5-19

LCD 5-15

Legibility 5-1

Line of sight 5-3

Liquid crystal displays 5-15

Luminance 5-10, 5-19

adjusting 5-11

ratio 5-19, 5-20

surrounding light sources 5-11

M

Motion artifacts 5-14

P

People with disabilities 5-25

alternative output devices 5-25

mobility 5-26

speech 5-25

visual 5-25

Phosphor 5-9

Pixel failure rate 5-15

Plotters 5-23

Polarity 5-15, 5-16

Printed output 5-23

Printer

controls 5-24

Printers 5-22

Projection displays 5-19

R

Readability 5-2

Recorders 5-23

Reflections 5-12

Refresh rate 5-1, 5-8

Resolution 5-13

S

Screen luminance 5-10

Seizures, display induced 5-8

Size

alphanumeric 5-17

symbols 5-17

Stereopsis 5-22

Stereoscopic displays 5-21

color usage 5-21

temporal modulation 5-22

Stroke width 5-14

T

Three-dimensional displays 5-21

V

Vibration 5-7

Viewing angle 5-19

Viewing distance 5-6, 5-7, 5-18, 5-19

Visual angle 5-17