PREFACE

This work was sponsored by the Federal Aviation Administration's Office
of the Chief Scientific and Technical Advisor for Human Factors
(AAR-100), Integrated Product Team for En Route (AUA-200), and the STARS
Product Team (AUA-310). The authors wish to thank Melissa Bud and Sarah
Miescher for taking the measurements on the SONY monitors and conducting
experiment two at the FAA Technical Center; Sarah Miescher for
conducting experiment three at the Volpe Center; Robert DiSario and
Sarah Miescher for conducting the data analysis; Gary Worrall for his
technical assistance in setting up the program to present the stimuli
and record the data; and Tom Robino for his logistical support. The
authors are also very grateful to Lawrence Arend (NASA/Ames Research
Center), Glen Hewitt (AAR-100), Jay Merkle (AUA-200), and Henry Mertens
(CAMI) for their substantial contributions to an earlier draft of this
report.

**TABLE OF CONTENTS**

[Section]{.underline} [Page]{.underline}

1\. INTRODUCTION 1

> 1.1 Purpose 1
>
> 1.2 Experiments Using the SONY DDM-2801C Monitor 1

2\. CAUTIONS ON THE USE OF COLOR ON ATC DISPLAYS 3

> 2.1 Questions on the Use of Color 3
>
> 2.2 Cautions on the Use of Color 4

3\. HOW SHOULD COLOR BE USED ON ATC DISPLAYS? 7

> 3.1 General Guidelines 7
>
> 3.2 Controllers' Suggestions for Color-Coded Information 9
>
> 3.3 How Many Colors Can be Used Successfully? 10
>
> 3.4 What are the Six Colors That Should be Used? 11
>
> 3.4.1 Colors for the Sony DDM-2801C 11
>
> 3.5 Specific Recommendations 13
>
> 3.5.1 Background 13
>
> 3.5.2 Data Blocks 15
>
> 3.5.3 Alerts and Warnings 16
>
> 3.5.4 Cursor 16
>
> 3.6 Designing a Color Scheme for an ATC Display 16

4.  HOW TO MEASURE THE EFFECTIVENESS OF A SPECIFIC USE OF COLOR

ON AN ATC DISPLAY 19

5\. USE OF COLOR IN AVIATION DISPLAYS 21

> 5.1 Cockpit Displays 21
>
> 5.2 ATC Color Displays in the U.S. 21
>
> 5.3 ATC Color Displays -- Lessons Learned from Other Countries. 22
>
> 5.3.1 Canada 22
>
> 5.3.2 The United Kingdom 23
>
> 5.3.3 ODID (Operational Display Input Development) IV 25

6\. CONCLUSIONS 29

APPENDICES

Appendix A: Determining the Ideal Color Set for the Sony DDM-2801C 31

Appendix B: Summary of Guidelines for the Use of Color on ATC Displays
47

Appendix C: Detailed Comments from the ODID IV Report 49

GLOSSARY 51

REFERENCES 53

# LIST OF FIGURES {#list-of-figures .unnumbered}

[Figure]{.underline} [Page]{.underline}

A-1 Log-log Plot of Input versus Luminance for the Sony DDM-2801C 33

A-2 CIE Plot of the Colors Produced by the Sony DDM-2801C 36

**LIST OF TABLES**

[Table]{.underline} [Page]{.underline}

1.  Colors that Yielded 99% Accuracy in Color Identification and
    Legibility on a

Black Background using the Sony DDM-2801C 12

A-1 Measurements from Nine Screen Positions on Five Sony DDM-2801C
Monitors 34

A-2 Percent Accuracy for Color Identification for Experiment 2 38

A-3 Definition of the Color Sets Tested in Experiment 3 41

A-4 Mean Percent Accuracies of Color Identification and Response Times
(RTs) in

Seconds for Correct Responses in Experiment 3 42

A-5 Color Identification Accuracies from Experiment 2 and Experiment 3
45

# EXECUTIVE SUMMARY {#executive-summary .unnumbered}

Color is an effective and attractive tool for coding information on a
display. It can help to organize and categorize complex information and
can also have a profound effect on our ability to search for specific
information. Color is both attractive and functional. However, caution
must be used in the application of color to displays because it is easy
to do more harm than good. The only thing that is truly obvious about
the use of color on displays is that its benefits and drawbacks depend
upon the task.

The primary purpose of this report is to provide general guidance to the
STARS (Standard Terminal Replacement System) and DSR (Display System
Replacement) program offices on the use of color on air traffic control
(ATC) displays. However, the information in this report will also be
useful to any program considering the use of color on its displays. It
offers general guidelines on how color should and should not be used,
but does not define a specific color-coding scheme. These guidelines are
based on what is known about human vision, display capabilities, the
knowledge gained from lessons learned about the uses of color in the
cockpit and ATC environments, and human factors "best practices." The
report discusses how the effectiveness of color displays can be assessed
and provides an update on studies in progress about specific uses of
color on ATC displays. Finally, the report summarizes what is known
about the general benefits and limitations of the use of color on
displays, the guidelines for use of color in the cockpit, and the
existing recommendations about using color in ATC displays from air
traffic organizations in other countries.

The guidelines presented in the report relating to the use of color in
ATC displays are as follows:

-   Displays must be designed for the tasks they need to support and the
    environment in which they will be used.

-   Whenever color is used to code critical information it must be used
    along with another method of coding.

-   When color is used to assign a unique meaning to specific colors
    (such as red for emergencies or green for aircraft under my
    control), it is imperative that no more than six colors be used.

-   Care must be taken to ensure all color-coded text and symbols are
    presented in sufficient contrast.

-   Cultural color conventions (such as red for danger and yellow for
    warning) should not be violated.

-   Pure blue should not be used for text, small symbols, other fine
    detail, or as a background color.

-   Bright, highly saturated, colors should be used sparingly.

-   Color use needs to be consistent across all of the displays that a
    single controller will use.

-   The specific colors that are selected for a display must take into
    account the ambient environment and the capabilities of the specific
    monitor.

-   The entire set of displays that a controller will use must be
    designed and evaluated as a whole and not as a combination of parts.

-   Any implementation of color needs to be tested in the context of the
    tasks that it is designed to support and the environment in which it
    is intended to be used.

This report also describes a series of experiments that examined the
variability in the color producing characteristics of five "calibrated"
SONY DDM- 2801C (20 x 20) monitors, and defined and validated an ideal
color set for the Sony 20 x 20 monitor.

Data are presented for color identification and legibility for several
different "reds," "greens," "blues," "yellows," "magentas," and "cyans"
on a black, dark grey, medium grey and light grey background. In
general, legibility was excellent with all of the colors at the eight
and 12 pixel sizes tested. Color identification accuracy varied widely
depending on the individual color/background combination. On the black
background, there was at least one example of every color except cyan
that was recognized correctly 99% of the time. Overall, color
identification was very good on the black, dark grey, and medium grey
backgrounds but suffered on the light grey background with only three of
the colors tested (red, white, and magenta) yielding color
identification accuracy of 99%.

#  INTRODUCTION

> **"In the majority of computer installations monochrome displays are
> virtually extinct and color is *de rigeur*, but when color is used in
> ATC displays it can be all too easy to impair controllers' performance
> rather than enhance it. In order to make the best use of color,
> careful consideration must be given to color vision and perception."**

(Reynolds, 1997, p. 185)

Color is probably the most effective, compelling, and attractive method
available for coding visual information on a display. It can be a very
useful tool for organizing and segregating complex information and can
have a profound affect on our ability to categorize and search out
certain types of information. Color is both attractive and functional.
However, this report will show we need to be very careful about using
color on displays for air traffic control (ATC) because it is easy to do
more harm than good. What is most obvious about the use of color on
displays is that its benefits and drawbacks depend upon the task.

## Purpose

The primary purpose of this report is to provide general guidance to the
STARS (Standard Terminal Replacement System) and DSR (Display System
Replacement) program offices on the use of color on ATC displays.
However, the information in this report will be useful to any program
considering the use of color on its displays. It will offer general
guidelines on how color should, and should not, be used, but will not
define a specific color-coding scheme, that is assign colors to
meanings. These guidelines are based on what is known about human
vision, display capabilities, the knowledge gained from the lessons
learned about the uses of color in the cockpit and ATC environments, and
human factors "best practices". The report will discuss how the
effectiveness of color displays can be assessed and will provide an
update on studies that are in progress on specific uses of color on ATC
displays. Finally, it will summarize what is known about the general
benefits and limitations of the use of color on displays, the efforts of
other countries concerning the use of color ATC displays, the guidelines
for use of color in the cockpit, and the existing recommendations about
using color in ATC displays from air traffic organizations in other
countries.

## Experiments Using the SONY DDM-2801C Monitor 

This report also describes a series of experiments conducted
specifically to support the application of color on the Sony DDM - 2801C
(20 x 20) monitor. Extensive research on human performance has
determined that our ability to reliably identify colors is limited to
five or seven. The seven colors that are easiest to identify on a CRT
while using a black or dark grey background are, red, green, blue,
yellow, cyan, magenta, and white. Given that a CRT such as the Sony
DDM-2801C can produce thousands of "reds," "greens," and "blues," how
should these colors be defined to minimize their confusion? Since the
answer to this question depends on the color-producing capabilities of
the specific type of monitor, a series of studies was conducted to
determine the ideal color set for the Sony DDM-2801C. A set of colors is
considered "ideal" if the accuracy of identifying each of the colors is
near 100% (thus minimizing the probability of mistaking one of the
colors in the set for any other color in the set). It is well known that
small patches of color are much harder to identify than large areas of
color-filled areas. For this reason, all of the colors in the set must
be able to be identified in the smallest size that might be color-coded
in actual operations (such as a position symbol). A final requirement is
that none of the colors negatively affects legibility. Certain
combinations of colors and background (such as pure or "royal" blue on a
dark background or yellow on a light background) are very difficult to
read and can induce errors. For this reason, we also need to ensure that
none of the colors in the proposed color set induce errors in
legibility.

Three experiments were conducted. In the first experiment, the
color-producing characteristics of the Sony DDM- 2801C (20 x 20)
monitors was examined and the variability in the color producing
characteristics was measured across five "calibrated" monitors. A set of
colors to be tested was then derived from the colors that the monitor
can produce. In the second experiment this proposed color set was tested
for legibility and color confusability on the Sony monitor. In the third
experiment a wider set of colors was tested. This set included the
deviant measures found in the first experiment and other colors that had
been selected by controllers. These colors were tested on four different
backgrounds: black, dark grey, medium grey and light grey. These data
describe the variability that can be expected within and between
monitors, and describe the performance expected for color identification
and legibility using different definitions of colors on several
different backgrounds. A detailed description of these experiments is
provided in Appendix A.

# CAUTIONS ON THE USE OF COLOR ON ATC DISPLAYS

Clearly, when applying color to ATC displays, what we don't know can
hurt us. There are many unanswered questions, some are more important
than others while some are more complex than others.

## QUESTIONS ON THE USE OF COLOR

Some of the unanswered questions that need to be considered are:

*HOW EFFECTIVELY CAN A CHANGE IN COLOR (e.g., to red) ATTRACT
ATTENTION?*

There is reason to believe that blinking is more effective at attracting
attention particularly under conditions of high workload or when the
alert is in the periphery of the display. In a complex experiment,
Thackray and Touchstone (1990) showed that controllers were able to
notice flashing radar targets significantly faster than they were able
to notice targets that were colored red. Experiments (in progress and
discussed on page 17) examine the efficacy of flashing data blocks vs.
data blocks that change color for attracting attention. The time
required to notice these alerts will be measured under conditions of
high and moderate task load.

*WOULD A CONTROLLER BE LESS LIKELY TO RECOGNIZE AN IMPENDING CONFLICT
BETWEEN TWO AIRCRAFT THAT WERE COLOR-CODED DIFFERENTLY THAN BETWEEN TWO
AIRCRAFT THAT WERE COLOR-CODED THE SAME?*

This is a critical question that needs to be answered before data blocks
are color-coded on ATC displays. It may be the case that color-coding
entire data blocks may have a detrimental effect on the controller's
ability to predict potential conflicts between two aircraft that are
coded in different colors. Color-coding part of the data block (such as
the call sign, altitude, or position symbol) may help the controller to
categorize the data blocks without the potentially detrimental effects
on conflict prediction. This may preserve the ability to predict
conflicts while using a redundant color code to help a particular set of
data blocks stand out from the others. It may be useful in a variety of
situations such as coding "my aircraft" from aircraft not under my
control or separating flows of traffic, such as aircraft heading for
different runways. Clearly, the risks and benefits of this potentially
very useful type of color-coding need to be assessed for different
situations.

*COULD CONTROLLERS BECOME SO DEPENDENT ON THE COLOR-CODED CUES THAT THEY
TEND TO PERCEPTUALLY "FILTER OUT" INFORMATION THAT THEY NEED? IF SO, HOW
WOULD THIS AFFECT THEIR PERFORMANCE?*

Controllers in the ODID (Operational Display Input Development) IV
simulation conducted in Europe "appeared to be looking only for color
\[in the conflict alert tool they were provided\][^1] to indicate a
traffic problem. This appears to have resulted in lack of memorization
of textual and graphical \[flight\] detail which could reduce their
traffic awareness" (Graham et al., 1994 p. 98).

We do not yet have a proven color scheme to help controllers do a better
job than with using a monochromatic (single color) display. In fact,
there is no evidence that adding color to a monochromatic display
enhances controller performance. There is a subjective perception that
color enhances performance and in several simulation studies controllers
*report* that use of color on the situation display helps them do their
job better. However, in the only study that examined both how
controllers *did* and how they *thought they did* using color and
monochromatic displays, controllers thought that they performed better
with the color-coded display, when in fact, they did not (Connolly,
Spanier, and Champion, 1975). One of the color-coding schemes examined
in this study was the color-coding of altitude. Aircraft at level flight
at odd altitudes were presented in green, aircraft at level flight at
even altitudes were presented in yellow, and climbing and descending
aircraft were presented in red. Controllers thought that this color
scheme helped their performance. "Of 30 controllers, all but one were
rather enthusiastic about color in air traffic displays.....Most
controllers reported feeling that color helped them do a more effective
job." (p. 24). However, despite the perception that color helped, this
color scheme failed to have a measurable effect on the controller's
ability to detect and correct potential violations of separation
standards.

All of these questions require scientific investigation before a
responsible application of color to ATC displays within the U.S. can be
formalized. However, there is a vast amount of knowledge on color vision
and use of color on displays that leads to logical conclusions about how
color could be used on ATC displays in the U.S.

## CAUTIONS ON THE USE OF COLOR

**Effects of color-coding are not always intuitive**. Many people
believe that a change in color, for example changing a data block to
red, will automatically attract attention. However, this has not yet
been scientifically proven. Coding mechanisms proven to attract
attention include dramatic differences or changes in contrast such as
blinking. Any display item that blinks will "grab" attention as will any
part of the display that is significantly brighter than surrounding
items. While it may not be possible for a color change to attract one's
attention as effectively as blinking, in all conditions, information
that is color-coded well does stand out from the information that
surrounds it. This makes color a very useful tool for categorizing
information. It is important to remember that there is always an
attentional trade-off in using such coding for alerts; what is effective
at attracting our attention is also distracting if the alert does not
convey useful information. For this reason, it is always recommended
that controllers are able to suppress individual alerts.

**Color-coding schemes will only be helpful if the "right" information
is color-coded.** As an example of how the wrong color scheme can be
worse than no color at all, consider the following. Kinney and DeCicco
(1982) gave 50 controllers at Washington ARTCC in Leesburg, Virginia the
opportunity to use a color display while controlling live traffic. In
the first "color" mode, weather was presented in orange, map information
was presented in red, full data blocks were in green, and limited data
blocks and history information were presented in yellow. In the second
color scheme presented, weather was presented in red, and maps in
orange, with the other coding the same. While using the color mode,
controllers always had the opportunity to select the "all-green" mode,
similar to what they are used to on the PVD. No objective performance
data were taken during this study, but in general, "controllers
considered the four-color mode to be worse than or at best, equal to,
the all-green mode" (p. vii). While some controllers liked the color
mode, most controllers reported difficulty with the red, saying it was
"hard to work with" (p. 2-1) and said that the differences between the
red and the orange and the differences between the orange and the yellow
were too small for easy recognition.

**The appearance of a color is affected by the color around it.** Colors
can appear lighter, darker, or a different "color" (hue), depending on
the color that is next to it ("simultaneous color contrast"), or the
color that you looked at just *before* looking at the next one
("successive color contrast"). All of these factors can subtly affect
the appearance of colors and need to be considered when selecting
specific color schemes for a display.

**Certain colors present special considerations.** Three FAA flight test
pilots encountered an interesting problem when they were examining a
prototype cockpit display. One of them had trouble seeing some small
blue symbols, another had no problem, and the third could see it, but
could also see how someone else might have difficulty. This was puzzling
to them because all three pilots had normal color vision.

The answer to this riddle lies in the age of the three pilots. The pilot
who had the most trouble seeing the small blue symbol was in his early
fifties. The one who had no trouble was much younger. From a display
point of view, pure blue (the blue produced solely by the blue phosphor
of a CRT) is problematic for a number of reasons, particularly for
\"older\" observers. First, if we are looking at a display that contains
several colors, the eye needs to accommodate (change the point of focus
in depth) in order to bring a blue symbol into focus. This means that
when short wavelengths are in focus, all other wavelengths are slightly
out of focus and vice versa. **Small blue symbols or text can appear
fuzzy and difficult to read**. When the blue text or symbols are in
focus, the other information on the display that is not blue would be
slightly out of focus. By approximately age 50, most of us will have
permanently lost a portion of our ability to accommodate. This means
that the blue symbols would be permanently out of focus and appear
fuzzy. For these reasons, **pure blue should never be used for small
symbols, text, fine lines, or as a background color.** (See page 8 of
this document for more detail.) **Red text can also appear fuzzy and
difficult to read (usually due to low contrast).**

There is another interesting age-related factor. As we age - even from
20 to 40 - the cells in the eye that respond to color become less
sensitive and the lens of the eye yellows. As the lens yellows, **light
blues will appear closer to white**.

Even individuals with normal color vision can have difficulty
discriminating certain colors under optimal conditions. **White and
yellow are easily confusable, particularly with small symbols (or,
similarly, with larger symbols viewed from a distance)**[^2]. For
example, it could be very difficult to tell whether a small leader line
was white or yellow.

Waivers have been granted to controllers with known color deficiencies.
**We are uncertain of the exact number of color-deficient controllers
currently in the workforce**. Approximately nine percent of the
population has some sort of color vision deficiency (what is commonly
called "color blindness"). Also, all color deficiencies are not created
equal; there are many different types. The most common form of color
deficiency is the inability to tell the difference between red and
green. This deficiency is much more common among men than women,
affecting eight percent of men and .04 percent of women (Pokorny, Smith,
and Verriest, 1979). While most color vision anomalies are inherited,
some can also be acquired through disease (for example, glaucoma or
diabetes), injury, or as a side effect of certain medications (for
example, streptomycin). There are also some minor color deficiencies
that will not be picked up with traditional color screening tests.
Because of these factors it is not possible to design a color scheme
that can be used as effectively by people with abnormal color vision as
by people with normal color vision. However, by specifying colors that
are maximally discriminable, and by using these colors according to
specified guidelines (such as the use of redundant coding), we can help
to minimize the probability of errors induced by color-coding.

**The appearance of colors on any given monitor will shift over time**.
All monitors are subject to shifts in color over time. At this time, we
do not know enough to be able to predict when or how these shifts will
occur. The report on the FAA/Eurocontrol's ODID IV simulation (that used
a SONY 20 x 20 monitor) noted that the "\...EEC \[Eurocontrol
Experimental Center\] engineers noted that these monitors are frequently
maintained due to color shifts over time." (p. 26). This means that
displays using color must be periodically checked and recalibrated.
**Maintenance procedures** **need to be in place to ensure the
anticipated performance of the monitor and the efficacy of the color
codes.**

**Tower displays deserve special consideration**. The tower (like the
cockpit) environment is especially tricky to design because of its wide
range of ambient lighting conditions. Daytime light levels in the tower
have been measured in the vicinity of 6500fc. (Hannon, 1995). High
ambient light, particularly direct sunlight, affects the appearance of
colors on the monitor (making all colors appear closer to white) and
takes a toll on contrast and readability. Contrast is the key
determinant of legibility and some display technologies are more
successful than others at producing the contrast necessary to make
displays readable in sunlight. The brightness (level of light emitted
from the display) of all displays designed for the tower must be easily
adjustable, and separate daytime and nighttime color configurations are
useful. Glare shields or anti-glare coatings may also be necessary,
depending upon the location of the display in the tower. These may also
affect color appearance. Design of color tower displays also has other
special considerations, many of which are tower specific, such as
physical placement of the display and the effect of the shades (many of
which are blue) on the appearance of the colors. Another factor that
will affect the appearance of the colors on the display is the
characteristics of the sunglasses that many tower controllers wear. All
of these factors combine to form a complex, but manageable, set of
requirements for effective tower displays.

# Like sunglasses, certain types of contact lenses can also change color appearance. Tinted contact lenses, particularly the ones designed to change the appearance of the color of your eyes (e.g., from blue to green or from brown to blue) are known to affect color perception. However, the precise effects of these lenses on color confusions has not been systematically studied and is largely unknown (Bill Wooten, personal communication, 1999).  {#like-sunglasses-certain-types-of-contact-lenses-can-also-change-color-appearance.-tinted-contact-lenses-particularly-the-ones-designed-to-change-the-appearance-of-the-color-of-your-eyes-e.g.-from-blue-to-green-or-from-brown-to-blue-are-known-to-affect-color-perception.-however-the-precise-effects-of-these-lenses-on-color-confusions-has-not-been-systematically-studied-and-is-largely-unknown-bill-wooten-personal-communication-1999. .unnumbered}

# HOW SHOULD COLOR BE USED ON ATC DISPLAYS?

It is clear that color could be a useful tool for categorizing
information (such as inbound versus outbound aircraft) and helping to
search for individual items in that category (such as a particular
outbound aircraft). In the laboratory environment, the advantages of the
use of color on visual displays is typically measured in visual search
tasks. In a visual search task the observer must locate one or more
particular objects, for example, letters or numbers; the time required
to find the object(s) is measured. Almost without exception, the time
required to locate such an object increases with the number of other
objects ("distractors") in the display. In one particular visual search
experiment, the time to find certain objects increased by 108% when the
number of distractors was increased from 30 to 60. However, when the
same items in the display were color-coded, the search time increased by
only 17% as the number of differently colored distractors grew from 30
to 60 (Carter, 1979). It is this type of finding, along with the strong
inherent appeal of color, that suggests an obvious benefit of the use of
color for air traffic control displays.

Color is very useful in helping to separate out what we need to pay
attention to. It is much easier to search for a specific data block if
it is one of several that are color-coded, than if all the data blocks
are the same color, because it narrows the number of data blocks that
need to be examined. It is also easier to identify a group of data
blocks (such as "my aircraft") if this is color-coded. However, this
benefit may come at the price of hindering the controller's ability to
detect potential conflicts. It may be the case that a controller is not
as likely to detect a potential conflict between two aircraft that are
color-coded differently than between two aircraft that are color-coded
the same.

With the implementation of each new ATC system and subsystem, the amount
of information that controllers must process grows at an alarming rate.
It is clear that color may be useful, if not necessary, to help organize
this information and to reduce the amount of physical and cognitive
clutter on the displays. It is also clear that to achieve the expected
benefits of color-coding, color must be used in accordance with known
principles of color vision and human memory. Human factors "best
practices" point to several general guidelines for the use of color on
ATC displays. These guidelines are introduced below.

## GENERAL GUIDELINES

**Whenever color is used on a display, it should be used redundantly
with another means of coding information. This means that there should
be some indication, other than color, about the information that the
color is to convey**. It may be useful to think of redundant coding as a
necessary scheme for color-blind users and a safety net for unknown
color deficiencies. For example, if the activation of conflict alert
caused the data block to turn yellow **and** CA to appear in the data
block, this would be a redundant code (although it may not be sufficient
for color deficient controllers.) If the conflict pair also appeared in
the tabular list, this would give added redundancy.

**Care must also be taken to ensure that color is used consistently
across all of the displays that a single controller will use.** All of
the displays should use the same color conventions and meanings assigned
to individual colors need to be compatible across displays. For example,
if "aircraft under my control" are color-coded in one color on the
situation display, the same colorcoding strategy should be used for \"my
aircraft\" on a conflict probe display.

**When colors are assigned a meaning, such as green for "my aircraft" or
yellow for "caution", the colors should be readily identifiable and each
color should have only one meaning.** This means that colors assigned a
meaning should be able to be identified with near 100% accuracy, no
matter which other color is present (or not present) in the display.
Also, the color-coding scheme should be as intuitive as possible so that
it is easy to use and remember. This does not mean that yellow could not
be used for both Minimum Safe Altitude Warning (MSAW) *and* conflict
alert, (since both are cautions) as long as there was a clear indication
as to which warning was activated.

**The number of colors assigned a different meaning should be limited to
six, so that the colors, and their meanings, are not likely to be
confused**. Due to the limitations of both the color-producing
mechanisms of displays and of our human visual system, we are not able
to identify more than about six colors with 100% accuracy. Having fewer
than seven color-meaning associations also helps to ensure that these
meanings will be remembered accurately. This limitation of six colors,
and which colors should be used, is discussed in detail in Sections 3.3

and 3.4.

**When selecting colors for a display, it is important to consider the
physical differences between the colors and the luminance contrast that
particular colors will yield.** Contrast is a key factor in determining
whether or not items on a display will be legible. The American National
Standards Institute (ANSI) recommends a contrast ratio of 7:1 for
alphanumeric characters, and cites 3:1 as a minimum (ANSI, 1988). To
ensure legibility, the International Civil Aviation Organization
(ICAO,1993) recommends a contrast ratio of 8:1 for items (such as data
blocks) that need to be read. For details that do not need to be read,
such as maps and range rings, a contrast ratio of 3:1 (sometimes even
less) is acceptable. While these guidelines help to ensure legibility,
it should be noted that these contrast ratios may not always be
achievable, particularly in the tower. Also, legibility can be
demonstrated with lower contrast ratios in many conditions. However, in
cases in which these guidelines can not be met, a thorough test of
legibility and operational suitability in all anticipated lighting
conditions is imperative.

**Color-coding schemes should obey cultural color conventions.** Each
culture has certain associations between specific colors and meanings.
In our culture, for example, red means "danger" or "stop" and green
means "safe" or "go." These associations are very strong because of our
constant exposure to them and our responses to them become somewhat
automatic. Because of this they should never be violated in a
color-coding scheme. This means that red should never mean anything but
danger, alert, or warning. In fact, red is best preserved for situations
in which an immediate action is required. This does not mean that all
alerts *must* be red; however, when red *is* used, it should convey
critical information. Similarly, green should indicate an \"OK\" status
and yellow should be reserved for conveying caution.

**From a display point of view, pure blue is problematic** for a number
of reasons, particularly for observers over 50. While blue text is
usually not impossible to read, it is still best to avoid it for text,
small symbols, and fine lines on a dark background. A low luminance or
"dark blue" (which can be thought of as blue mixed with black) would be
useable for a background or for symbols and text on a light background,
as would black text on a very light blue background or light blue text
on a very dark background. In these cases, sufficient contrast can be
created to support reading and other tasks which require resolution of
detail. Pure blue can also be mixed with with green to create "cyan".
While cyan is good for legibility, it can be confusable with blue,
green, or white depending on how it is defined.

**Pure, bright, highly saturated colors should be used sparingly.**
First, this is clearly a case where "less is more"; highlighting is only
effective when there is little of it. Also, these colors should only be
used for critical and temporary information so they are not distracting
or visually disruptive. Finally, saturated red and blue, when presented
simultaneously, can create a false perception of depth.

**Any implementation of color will need to be thoroughly tested in the
environment in which it is intended to be used.** Prototype testing of
individual color schemes, such as a scheme for weather, is highly
recommended, but does not detract from the need to test the display in
its entirety. For example, coding schemes for weather, and special use
airspace may be successful when tested independently, but may be
incompatible and confusing when presented together.

## CONTROLLERS' SUGGESTIONS FOR COLOR-CODED INFORMATION 

We can point to several general guidelines for the use of color on ATC
displays, based on what we know about the workings of the eye, color
perception, and human memory. Unfortunately, when it comes to *specific*
uses of color on ATC displays, more is known about how color should
*not* be used than about color codes that might help controllers do
their job better. Nonetheless, a reasonable approach is to examine what
controllers might like to color code on their display, prioritize this
list, prototype an implementation to address an operational need (such
as a need to segregate traffic on a display or to try to make
overlapping data blocks legible), and then test the prototype.

The "wish list" of what controllers might like color-coded is quite
extensive. When asked to brainstorm about color codes that might be
useful, the STARS Operations working group generated a list that
included, but was not limited to alerts (conflict alert, low altitude
alert, aircraft squawking 7700, radio failure, hijack) inbound and
outbound traffic, arrivals and departures, pointouts, handoffs, aircraft
not under the controller's control versus "owned" aircraft, aircraft
type, maps, and weather (notes from the Nov 3, 1997 meeting of the STARS
Operations working group). There are a total of over 25 different types
of information (characteristics of the aircraft or background
(map/weather)) on the list. If everything on this list were coded with a
different color, the coding scheme would be too complex to be usable.
However, it does provide a useful list of the types of information that
may be beneficial to differentiate on the display.

Another use of color on some controllers' wish list is to try to make
overlapping data blocks legible. Henry Mertens at the FAA Civil
Aeromedical Institute (CAMI) is currently researching this issue.
However, V. David Hopkin, a world-renown ATC human factors specialist in
the U.K., has done research in this area and reports that, "presenting
different labels (U.K. terminology for data blocks) in different colors
seldom makes overlapping labels readable" (Hopkin, 1995, p. 227).

## HOW MANY COLORS CAN BE USED SUCCESSFULLY? 

Given the fact that the "wish list" of what controllers might like to
color code is considerably longer than what can be accommodated, the
obvious question is, "How many colors can be used on a display and not
detract from its effectiveness?" V. David Hopkin (1977) has recommended
that only three or four colors be used on ATC displays, as did Kinney
and Culhane (1978). Weitzman (1986) recommended four colors plus white.
SAE guidelines for use of color on cockpit displays and other
guidelines, such as National Air Traffic Services (NATS) guidelines for
use of color on ATC displays in the U.K., recommend five to seven
colors.

When color is used for the purposes of assigning specific meaning to
specific colors (such as red for emergencies or green for aircraft under
my control) it is imperative that no more than six colors be used. When
more than six colors are used, two problems are likely to result. First,
it is difficult to display the colors so that they are never confusable.
Second, it becomes difficult to remember the entire color-coding scheme.
Using no more than six colors for identification also capitalizes on the
number of colors that are maximally discriminable on a CRT display.

If this limitation of six colors seems restrictive, it may help to
understand that the number of useful colors expands greatly when the
task depends not on absolute identification, but on discrimination, that
is, being able to notice a difference between the colors presented. It
has been estimated that the average person can discriminate, (say that
two colors look different as opposed to looking the same) several
million colors (Chapanis, 1996 p. 221). To illustrate this point,
picture a box of 108 crayons. When held side-by-side, a person with
normal color vision would be able to say that the crayon labeled "mango"
looked different than the crayon labeled "melon". Yet, if asked to
identify either crayon alone, they would probably identify each of them
as "orange". If a person saw only one of these two crayons and was given
the choice of "mango" and "melon" color names, they would probably be no
better than chance at naming either one. When only discrimination, and
not absolute identification is required, such as in reading a
topographical map, the useful number of colors expands greatly. However,
we must be careful not to mistake a task that we think depends on the
ability to tell the difference between the colors for one that actually
requires identification. For example, if weather were to be presented on
a display in six shades of a single color, it could be relatively
straightforward to tell where the levels of weather were - as long as
all six were present. If less than six were present (with no key), the
task would now require absolute identification of a subset of the six
shades of the single color - a task that would be impossible in an ATC
environment. Whenever colors are assigned a meaning, these colors must
be able to be identified no matter what else is (or is not) displayed.

**This does not mean that only six colors can be used on a display with
a black background; it means that no more than six colors (red, green,
yellow, blue, magenta, and white) should be assigned a specific
meaning.** More than six colors can be used successfully on a display if
only color discrimination is required. That is, more than six colors can
be used as long as the additional colors do not need to be identified,
but only recognized as different from the other colors. This could be
true for map lines, sector boundaries, or special use airspace. However,
if more than these five colors are used for identification then the
probability of confusing the meaningful color set on every potential
background color (any color-filled areas such as for weather or special
use airspace) needs to be investigated. Finally, adherence to the
limitations of six colors does not detract from the need to adhere to
the other guidelines (such as consistent use of redundant coding).

## WHAT ARE THE SIX COLORS THAT SHOULD BE USED?

Colors used to convey a specific meaning (such as using green data
blocks to indicate aircraft under my control) should not be confused
with any other color. The six colors that are maximally discriminable on
a CRT are:

-   Red,

-   green,

-   blue,

-   yellow,

-   cyan, and

-   magenta.

In addition the color set could include (depending on the background)
the achromatic colors of:

-   Black,

-   grey, and

-   white.

Yellow is a combination of red and green light. Cyan is a combination of
blue and green light, and magenta is a combination of red and blue.
These colors are chosen because they are the ones likely to be the most
discriminable, based on their physical differences that correspond to
separation in color space. However, precisely how discriminable these
colors are will depend upon their precise definition in display
producing terms. For example, R, G, B values determine the relative
intensity of the red, green, and blue guns in a CRT display (as well as
ambient illumination and other factors). The color-producing
capabilities will vary as a function of the specific technology used to
produce it (for example, flat panel versus CRT or characteristics of the
CRT guns), the manufacturer, and model. For this reason, once the
maximally discriminable color set is defined for a particular display,
these colors should be defined in terms of chromaticity values so that
they can be matched as closely as possible from one type of monitor to
another. This is important whenever the same person is using more than
one color display, so that the red on one display looks like the red on
the other display.

### **Colors for the Sony DDM-2801C**

A series of studies were conducted to determine the ideal color set for
the Sony DDM-2801C (20x20) monitor. These studies are summarized here
and a complete description can be found in Appendix A.

The first study examined five of the SONY DDM-2801C monitors used at the
FAA's William J. Hughes Technical Center to see if there were any
differences in the CIE (*x, y)* coordinates and luminance values of the
RGB guns (that is, characteristics of the color producing mechanisms)
across the different monitors. The next set of studies identified a
proposed "ideal" color set based on the color production capabilities of
the Sony DDM-2801C and what is known about human color vision; this is
referred to as the "derived" color set. A set of colors is considered
"ideal" if it minimizes the probability of mistaking one of the colors
in the set for any other color in the set and none of the colors in the
set adversely affects legibility. Use of additional colors outside of
this set should be thoroughly tested before implementation to ensure
that the colors are not confusable; see Section 3.3 for details. The six
colors are specified in terms of standardized units CIE (*x, y)*
coordinates that can be reproduced by any monitor (that is capable of
producing them). For another CRT, this means mapping the CIE (*x, y)*
coordinates to R,G,B values (for the red, green, and blue guns). The
values tested that were found to result in almost perfect identification
of the colors with no detrimental effect of legibility are shown in
Table 1.

Table 1. Colors that Yielded 99% Accuracy in Color Identification and
Legibility on a Black Background using the Sony DDM-2801C

***[CIE x]{.underline} [CIE y]{.underline} [Luminance
(fL)]{.underline}***

####### Red .630 .341 3.63 {#red-.630-.341-3.63 .unnumbered}

.628 .339 5.17

.546 .357 7.42

.593 .335 6.04

.558 .367 7.53

Green .289 .610 8.21

Blue .149 .066 1.57

.154 .070 3.34

White .298 .281 13.82

.274 .286 26.50

Yellow .444 .488 12.18

Magenta .298 .152 5.33

Coordinates were found for red, green, blue, yellow, white, and magenta
that resulted in at least 99% accuracy in color identification and
legibility when presented on a black background. While a total of three
different coordinates were tested for cyan, the color identification
accuracy on a black background did not exceed 95%. (However,
identification of one of the values of cyan did result in 100% accuracy
on a dark grey background.)

In addition to the derived color set, other values were also tested in
this series of experiments. Deviations (that is, variations in how
programmed colors appeared on different screens) that were found across
the five monitors were tested. The values for magenta and yellow that
were chosen based on preference by controllers working on DSR were also
tested. None of the observed deviations adversely affected legibility on
the black and dark grey background. However, some had a profound affect
on color identification. For example, while the color identification
accuracy on a black background for "Blue 1" was 100%, the accuracy for
"Blue 2" was only 57%. The "preferred" yellow chosen by controllers
resulted in performance that was as good as the "derived" (i.e., chosen
based on mathematical derivation) yellow on a black background. However,
the preferred magenta was only identified as magenta 81% of the time on
a black background, while the derived magenta was identified correctly
98% of the time. For full details on the stimuli tested and their
effects on color identification and legibility, see Appendix A.

## SPECIFIC RECOMMENDATIONS

It is useful to think about a display in terms of background and
foreground, both in terms of the types of information presented and in
the calculation of the primary factor that determines legibility and
contrast. Luminance contrast is a measure of the brightness of the
foreground relative to the brightness of the background. It can be
measured quickly and easily in precise scientific terms. It is even
possible to mathematically determine the effect of introducing ambient
light on the contrast of specific items on a display. This is useful, in
determining whether a particular display would be legible in a tower
environment.

When selecting colors for a display, it is important to consider the
luminance contrast that particular colors will yield. Contrast is the
key factor in determining whether or not items on a display will be
legible. Sufficient contrast can make text that is presented in the same
"color" (hue) as the background - such as light green on dark green -
perfectly legible. If the background (such as weather) is one color and
the foreground (data block) is another, but the contrast between them is
too low, the information will be unreadable. Simply put, the higher the
contrast, the more the information (text, symbol) will stand out.
Therefore, the more conspicuous (or obtrusive) you want the information
to appear, the greater the contrast ratio should be. It is also
important to remember that high luminance, saturated colors need to be
used sparingly for several reasons. First, the ability of these colors
to "stand out" depends on them being rarely used. Clearly, in this case,
less use is more effective. Second, what is good at capturing and
maintaining visual attention can also be distracting and visually
disruptive if used improperly. Third, saturated red and blue can induce
an illusion of depth (this is called "chromostereopsis"), with red
appearing closer to the observer than blue.

Finally, it should also be noted that it is never a good idea for
controllers to be able to select their own color-coding scheme as a
function of personal preference. First, with no conventions, it would be
difficult for the controller to remember the meanings that he or she
assigned. Second, the color-coding might not be meaningful to anyone
else (either a relief controller or a supervisor) who may need to take
over.

### **Background** 

A true background contains no information. Backgrounds are usually
either very dark or very light to achieve maximum contrast. A mid-grey
background may seem appealing, because it allows the use of both lighter
colors and darker colors on top of it. However, the contrast ratios that
are able to be produced with a mid-grey background can never be as great
as with a dark or light background. Black is not usually used as a
background because of the glare problem it can produce. For this reason
a dark grey background is preferred over black. A light background can
have a few advantages over a dark background; glare is not nearly as
noticeable on a light background as on a dark background, and the
subjective contrast that is created with black text on a light
background is greater than that created with light text on a dark
background. However, a wider range of colors will be more easily
identified on a black or dark grey background than a light background.

Another disadvantage to using a light background is that flicker is more
noticeable at higher illuminations. To reduce the probability of flicker
and the associated problems (such as distractions, eye strain,
headaches), displays that use a light background should have a refresh
rate of *at least* 65 cycles per second. In an environment with office
level lighting and a display using a light background, a refresh rate of
75Hz or higher may be required to eliminate flicker for all users[^3].
The careful selection of a background color is an important part of
ensuring that the display is suitably designed for the specific
environment, task, and implementation considerations. For example, a
light background display may not be suitable for the TRACON if the new
displays need to be used next to the old displays during the transition
period, since a new display may emit too much light for an adjacent
phosphor display to be usable.

Displays designed for the tower should have a daytime and nighttime
configuration of background and display colors, and brightness controls
that are easily accessible. Glare shields, or anti-glare coatings may
also be necessary, depending on the location of the display in the
tower. While a dark grey background is suitable for nighttime viewing in
the tower, a light grey background may be preferred for daytime viewing.
The color schemes chosen for the daytime and nighttime configurations
need to be designed with reference to these backgrounds.

While the true background contains no information, it may be useful to
think of the background of everything under the data block. For the
purposes of this discussion, the background will contain information
such as maps, special use airspace, and weather, that the controller
would normally not want to "stand out" or be conspicuous. It should be
presented in low contrast using muted colors. This is not always the
case. For example, there are some tower displays in which the airport
map is a level of detail that should be presented in high contrast to
support the task. Greys (light grey for high ambient light conditions
and dark grey for a dark environment) are good background colors because
they are achromatic (technically colorless). All of these factors must
be considered together. For example, a light grey map on a dark grey
background might provide too much contrast and thus, be more conspicuous
than is desirable. Very light (close to white) or very dark (close to
black) blues can be used as background colors, as long as these colors
are carefully designed. Again, such issues can only be considered in the
entirety of the display design. The decision about how to color-code
certain information cannot be made out of the context of the choices of
background color and colors for other information on the display.

### **Data Blocks** 

The traditional green is a good color for data blocks on a relatively
dark background, because of our natural sensitivity to it, our cultural
association between green and "normal" status, and the fact that
controllers are accustomed to green data blocks. Green also maintains
good brightness contrast over a wide range of saturation. Many
controllers would like to see the aircraft under their control displayed
in a color that is different from the other aircraft displayed on their
screen. This type of color-coding would enable the controller to find
their own data blocks very quickly when they are in an array of data
blocks that are not theirs. While the benefits to this type of
categorization are clear, the potential drawbacks also need to be
considered. There is some evidence (ODID IV, p. 23) that this type of
color-coding may make it more difficult to detect conflicts between
aircraft that are presented in different colors than between aircraft
that are presented in the same color. In effect, the fact that "unowned"
aircraft are presented in a color that is different from "owned"
aircraft makes it easier to unintentionally forget about them - at least
when the entire data block is color-coded.

Color-coding data blocks certainly helps the controller (for better or
worse) to focus on some aircraft and filter out others. However, it
would not necessarily help to alleviate the problems of legibility of
overlapping data blocks. David Hopkin has suggested that, "Although
colour may reduce clutter if applied with care, its success depends on
the successful manipulation of a series of visual layers with the most
important information in the top layer, and it is imprudent to rely
entirely on colour to separate items in this way: presenting different
labels in different colours seldom makes overlapping labels readable"
(Hopkin, 1995, pp. 226-227).

Whichever colors are chosen for the data blocks, it will be important to
examine the color contrast and luminance contrast produced by each color
chosen for data blocks when presented on top of each possible
"background". In this sense, the "background" includes any color in a
filled-in area such as levels of weather or special use airspace.

Data blocks should be able to be presented at medium intensity so that
they stand out from the background. Other conspicuous information, such
as alerts, should be presented at sufficiently higher intensity. This
does not preclude the controller's ability to adjust the intensity of
the data blocks relative to the background. In fact, the intensity of
the maps and data blocks should always be able to be adjusted
independently. It would be helpful if the intensity of one's own data
blocks (coded with the position symbol) were on a separate intensity
control. This would allow the controller to selectively adjust the
intensity of his or her own data blocks while the intensity of
everything else remains the same. This type of tool would help the
controller to increase the intensity of their data blocks to quickly
find his/her own aircraft among many others and to read their own data
blocks (at the expense of the an overlapping one) or to decrease the
intensity of their aircraft data blocks to read an overlapping data
block.

It will also be important to limit the number of different colors within
a data block. It may prove useful to color code portions of a data block
(such as altitude). However, data blocks or elements of data blocks that
are presented in the same color will have a natural grouping effect and
be perceived as a group. The effects of using more than two colors in a
data block are unknown. It is possible that more than two colors within
a data block could perceptually fracture the data block and impair
performance. While multiple colors in a data block may seem appealing,
specific color-coding schemes will need to be tested before they are
implemented.

### **Alerts and Warnings**

Alerts and warnings should be presented at high contrast. If they are
color-coded, the colors used should also be highly saturated. This mode
of presentation should be reserved for critical warnings that require an
immediate response and not be used for routine system messages (such as
"message waiting"). Red and yellow are customarily chosen to present
alerts and warnings because of the learned association of these colors
with danger and caution. While it is not necessary to present alerts and
warnings in yellow or red, these colors should be reserved for alerts
and warnings because of their cultural associations. While controllers
may tend to like the idea of alerts presented in red, it is important to
note that red is a color that usually can only be produced at low
contrast. The implications of low contrast on legibility have already
been discussed. Also, if yellow is being used as an alerting function,
it should be reserved for this purpose alone. If other (non-alerting)
information is displayed in yellow (for example, if the controller draws
a map in yellow) then yellow could lose its effectiveness to indicate an
alert. Finally, the choice as to whether yellow is used as an alerting
color or not should determine the relative luminance used for yellow. If
yellow is used for alerting purposes, the luminance of the yellow should
be higher than if it is used to present routine information.

### **Cursor** 

Since the cursor needs to be visible at all times, it should be
presented at a higher intensity than any other information on the
screen, to provide maximum contrast with the background.

## DESIGNING A COLOR SCHEME FOR AN ATC DISPLAY

Color displays need to be designed for the tasks that they support and
the environments in which they will be used. Ambient lighting is a major
determinant of the characteristics of the colors that should be selected
for a display. Colors are very difficult to see on a CRT in bright
light. Even the brightest colors will appear desaturated ("washed-out")
in bright light; this makes the differences between these colors much
less noticeable and the colors easier to confuse. In these conditions,
bright, highly saturated, colors are necessary, particularly if the
display may need to be used in direct sunlight. High ambient light also
makes a black background undesirable because of the problem with glare.
On the other hand, current TRACONS typically have very low ambient
lighting (although this is not uniform). Because of this, the use of
bright, highly saturated colors should be minimized so as to provide
useful information without being distracting or annoying. The en route
(Display System Replacement, DSR) lighting environment is closer to the
office level lighting than to either the tower or TRACON. While it is
still true that the use of highly saturated colors should be minimized,
this lighting level offers more flexibility in the selection of color
configurations.

Another factor that must be known before choosing a color scheme is the
characteristics of the specific monitor; this will identify the colors
that the display is capable of producing. The information, along with
what is known about the human ability to identify and remember colors,
sets the stage of what is possible. Next, a limited amount of
information must be selected to be color-coded; the fewer the number of
color codes, the more effective the color-coding will be. The
color-coding scheme must be developed as a whole. For example, a coding
scheme for weather needs to be considered in light of all of the other
color-coding schemes (e.g., for conflict alert, special use airspace,
etc) to be used. **The most important component of a plan for choosing a
color-coding scheme for an ATC application is thorough operational
testing using representative users, tasks, environment, and operating
conditions.**

# HOW TO MEASURE THE EFFECTIVENESS OF A SPECIFIC USE OF COLOR ON AN ATC DISPLAY 

The final step in any plan to implement a specific color-coding scheme
should be a thorough test of its effectiveness and operational
suitability. The validity of the test results will depend upon the
appropriateness, completeness, and sensitivity of the measures used. Any
specific application of color can be measured to assess its
effectiveness. For example, a study in progress at the Volpe Center is
examining whether an alert that changes color is as effective as one
that blinks at attracting attention to a conflict alert. In this
part-task simulation, participants control traffic in a modification of
a computer simulation that was designed by CAMI as a screening test.
(Controllers have validated this task realistic \[Dana Broach, personal
communication,1996\].) When conflict alert is activated, it either
blinks, changes color, or both. Participants are instructed to respond
to these alerts as soon as they notice them by selecting one of the data
blocks. In this way, the time required to respond to each type of alert
is measured. We also examine the subjective effectiveness of each type
of alert by asking the participants to rate their effectiveness.

Scientifically speaking, the only way to isolate the effectiveness of
color on a particular display is to have controllers control traffic (in
a simulation) using the color display and using the same display with no
color. However, adding color to a monochromatic display is not the best
way to design a color display. Color displays have special
considerations, such as the sizes and shapes of symbols and
alphanumerics. Still, it is wise to evaluate a particular color scheme,
and make the necessary adjustments, long before it is implemented and
used to control live traffic.

Each application of color should be carefully tested before it is
implemented. The methods used to measure the effectiveness of color
codes on a display are no different than methods used to assess the
effectiveness of any other aspect of a display. If certain information
needs to attract the user's attention immediately, then aspects of the
display, such as contrast, can be measured, and simulations can be
devised to measure the time required to notice such alerts. Legibility
can be assessed by measuring the accuracy of identifying alphanumerics.

All colors on the display must be readily identifiable and the coding
scheme simple and intuitive so that the meanings of the colors are easy
to remember. Everything on the display should be easy to read and
understand; this can be subjectively evaluated by asking controllers if
this is the case.

Finally, the most important measure of the effectiveness of a display is
how well controllers are able to do their job using it. This is best
measured in a full-mission simulation study where controllers are
required to perform all of their usual tasks such as communicating with
pilots and coordinating with other controllers using both objective and
subjective measures of performance. Objective measures of performance
include errors, omissions, and time required to perform certain tasks.
Subjective measures include controllers' estimation of their own
workload and their assessment of the clarity, legibility, and ease of
use of the display. For further information on human factors testing and
evaluation refer to Chapter 10 of *Human Factors in the Design and
Evaluation of ATC Displays* (Cardosi and Murphy, 1995).

#  USE OF COLOR IN AVIATION DISPLAYS

## COCKPIT DISPLAYS

Color has been used successfully in aircraft cockpit displays for many
years. There are many similarities between cockpit and ATC displays.
First, both types of displays present complex information required for
complex cognitive tasks. Second, this information must be presented in a
limited amount of space. Third, both cockpit and tower displays must be
able to be used under ambient lighting conditions ranging from bright
sunlight to nighttime conditions. The displays used at night must emit
light at a low level that does not interfere with the controllers' or
pilots' ability to see in the dark. The similarities between the cockpit
and ATC environments make the lessons learned and knowledge gained from
use of color in the cockpit useful to considerations of use of color in
ATC.

Several experts in the field of color vision and displays have compiled
recommendations on the use of color in electronic aircraft displays
based on lessons learned in the development and use of color displays
for the cockpit. The Society of Automotive Engineers (SAE) 1988
Aerospace Recommended Practice, recommends a conservative and consistent
use of color, using no more than six color codes for symbols: white,
red, green, yellow, magenta, and cyan, while reserving red and yellow
for warnings and cautions. These particular colors were chosen to
maximize the physical differences between them so that they are not
likely to be confused. They note that, "The use of more than six symbol
colors may degrade performance on search, identification and coding
tasks due both to poorer discriminability (especially under high ambient
light) and a loss of organizational value." (p. 5) The document also
describes the poor legibility that characterizes blue phosphors and
recommends that blue not be used for alphanumerics or symbols containing
fine spatial detail.

It is notable that there have been no serious problems reported to the
Aviation Safety Reporting System attributable to the color scheme chosen
for cockpit displays used for navigation and weather avoidance (Mertens,
1997). Again, it is important to remember that the usefulness of a
specific color code is dependent upon what information is coded. In an
experiment that color-coded traffic on a cockpit display of traffic
information (CDTI), pilots were actually slightly more successful at
maintaining self-separation with a monochromatic (single color) display
than when the aircraft were color-coded according to proximity to own
aircraft (Scallen, Smith, and Hancock, 1997). It is likely that
performance in the color-coded condition would have been at least as
good, if not better, as the monochromatic condition if more useful
information (such as closure rate) was coded instead of distance.

## ATC COLOR DISPLAYS IN THE U.S.

The first descriptions of specific colors to be used in the U.S. are for
colors to be used on TRACON displays in the U.S. and can be found in the
Advanced Automation System (AAS) specifications and a later document
referred to as "CDRL EN09\" (or "EN09\" for short). Both of these
documents suggest specific colors (in chromaticity coordinates) but
neither document presents information as to why these particular colors
were chosen, nor did they present any studies to show how these colors
could be operationally useful in an ATC environment. For a detailed
discussion of these two color sets and color display issues,
particularly as they relate to the tower environment (Hannon, 1995).

Color is used on many ATC displays that are used for purposes other than
separating aircraft. For example, the interim situation display (ISD)
used to display calculated aircraft position in oceanic sectors and the
touch screens used for the voice switching and control system (VSCS) are
color displays. Only one civilian facility in the U.S., High Desert
TRACON (HDT) uses color displays to control traffic. Co-located on
Edwards Air Force Base in California, HDT handles a mix of civilian and
military traffic. The color scheme used was designed by controllers at
the facility. The alerting scheme they chose was red for emergencies
(such as hijack) and yellow for conflict alert and minimum safe altitude
warning (MSAW). They also chose to color code the data blocks using gold
for limited data blocks, white for handoffs, cyan (greenish blue) for
aircraft being handled by the controller, and green for aircraft being
handled by other controllers. While controllers who use this display
like it very much, it is important to remember two key points. First, no
studies have been done to determine what effect, if any, this coding
scheme has on performance. Second, what works at one facility may not be
the best display at busier facilities. Each facility has traffic
characteristics that may be different from the needs of other
facilities. For example, while their system was being upgraded, HDT did
not have some functions that other facilities would find it unacceptable
to work without - automated handoffs, conflict alert, and MSAW.

## ATC COLOR DISPLAYS - LESSONS LEARNED FROM OTHER COUNTRIES

### **Canada** 

Canada was the first country to publish a report on their approach to
the issues associated with the use of color in ATC displays. Transport
Canada commissioned Thompson-Hickling Aviation, Inc. to prepare a report
on the use of color in ATC displays (Campbell, White, and Hamilton;
1990). This report examined key aspects of color production and
perception, the activities conducted in other countries on this topic,
and other relevant information, to formulate a list of advantages and
disadvantages of color-coding information on ATC displays and develop
guidelines for the use of color. Their list of potential disadvantages
of color-coding information on ATC displays was as long as the list of
possible advantages.

Advantages included:

-   "Some increased speed of recognition and visual segregation"

-   "Superior recognition and visual segregation of tabulated items in
    alphanumerics"

-   "Perceptual grouping of like-colored items in a random display"

-   "Useful for assisting in attention getting under restricted
    conditions"

-   "May reduce the appearance of visual clutter if coding schemes were
    efficiently used"

-   "Probably more aesthetically pleasing (at least in the short term)"

Disadvantages included:

-   "Colored objects and icons must meet minimum size requirements that
    are generally larger than monochrome objects/icons"

-   "Limited number of useable colors (probably less than five)"

-   "Visual search is not improved if the color-coding of items must be
    ignored in order to perform the search"

The list of recommendations did not include specific color assignments
but consisted of general recommendations such as: using desaturated
colors for areas or backgrounds, a minimum size for colored symbols (15
minutes of arc for red and green symbols and 30 minutes of arc for blue
and yellow symbols), and keeping the number of colors used to a minimum.
Adopting the SAE recommendation, they stated, "More than six
simultaneous colors is thought to detract from performance." (p. 58).
Again, these guidelines were based on sound principles and state of the
art information on the topic, but no independent research on ATC tasks.

Color displays are currently being used to provide air traffic services
in Canada in addition to Australia, the United Kingdom, and other
countries. While controllers prefer the new color displays over the
monochromatic displays, there are no objective data on how the use of
color affects controller performance.

### **The United Kingdom** 

The United Kingdom's Civil Aviation Authority's (CAA), National Air
Traffic Services (NATS) established a group under the Research and
Development Directorate to look at how color should be applied to ATC.
The group presented a comprehensive summary of research findings
relevant to the use of color on CRT displays and various recommendations
of the use of colors on displays. The extensive 1990 document was the
foundation for the 1992 *Guidelines for the Use of Colour on Air Traffic
Control Displays* by Reynolds and Metcalfe. Again, the conclusion was
clear that there was no evidence that color aided performance on ATC
tasks. However, the sound approach taken by this group and their
findings are worth reviewing in detail.

The NATS guidelines present a color palette and display presentation
principles developed by Linda Reynolds of the Royal College of Art and
controllers using en route and traffic management display scenarios. The
colors selected for the display and the "mid-luminance" background were
chosen with "normal office lighting levels in mind" (Section 4.2.1),
which is important to remember when assessing the applicability to our
low illumination TRACONS and towers with their changing light levels. In
fact, the authors note that two sets of display colors may be necessary
for towers because of the extremes in ambient light experienced in the
tower.

The guidelines present a conceptual framework for color-coding ATC
information based on the importance to the controller. These "conceptual
layers" or classifications of information are assigned layers on the
display. There are three basic levels: background, foreground, and
alerts.

These basic levels are further subdivided into seven layers where the
higher the layer number the more important the information and the more
the displayed item must "stand out". The first layer is the background
with sector boundaries indicated either with lines or filled-in areas.
The second layer is the background detail such as range rings and
airways. The third and fourth layers are for radar data; the third layer
is for data on aircraft that the controller is not handling and the
forth is for aircraft that the controller is currently handling. The
fifth layer is for low-level alerts such as aircraft that require
special monitoring. The sixth layer is for aircraft that require the
controller's immediate attention. The seventh and highest layer is for
the cursor, since the cursor must be visible at all times. In this
scheme, muted colors are used except for the alerts.

The procedures given in the NATS document for the use of this color
standard represent a sound, principled, approach to the use of color.
This is reprinted below from Section 3.2:

> \(i\) [Determine High-Level Display Requirements]{.underline}. The
> objective is to identify the display requirements from an analysis of
> the ATC task/application. At this stage the requirements will be at a
> strategic level, i.e., how many levels of alert will be required, what
> information will be required in the track data block, what detail of
> map information is required, etc.
>
> \(ii\) [Derive List of Items/Objects to be Displayed]{.underline}.
> This will be a list of items/objects which will be required to be
> selectable on the display as determined by the ATC task to be
> performed. At this stage, the description will be in terms of the ATC
> requirements and will be independent of display attributes, i.e.,
> airways, airports, beacons, tracks, data blocks (with description of
> actual data required in the block and any variants), etc.
>
> \(iii\) [Divide Identified Items/Objects into Seven
> Layers]{.underline}. This stage is still independent of the display
> attributes. The allocation of an item/object to a particular layer
> will be made according to the visual priority placed on that
> item/object (e.g., background map data on Layers 1 and 2, alerting
> information on Layer 6), and also the graphic form (e.g., background
> infills on Layer 1, background lines/symbology on Layer 2).
>
> \(iv\) [Allocate Colors to Items/Objects in each Layer]{.underline}.
> Using the color specification and guidelines in Section 5, and taking
> into account the high-level requirements identified at step (I),
> allocate colors to the identified items in each level.
>
> \(v\) [Prototyping]{.underline}. Ideally, a prototyping system should
> be available so that the color selection can be checked.

This approach to the use of color was applied to the design of the
displays for the NATS Swanwick en route air traffic control center in
the U.K. The center is not yet operational and the color scheme has yet
to be finalized. However, the color scheme proposes a mid-grey
background and color-coded data blocks. The data blocks are presented as
black text on top of rectangles of color. The exception is if the data
block goes into conflict alert. In that case, the text is white on a red
background (since black text on a red background would not provide
sufficient contrast). The advantage to presenting data blocks on top of
colored blocks is that it makes the colors easier to identify. First, it
is much easier to identify a large area of color than a smaller one.
Second, the color code of the data block remains constant; whereas if
the data block was presented alone (and not on top of a colored
rectangle), the appearance of the color of the data block would be
influenced by the color on the background (which would change as it
moved across the display). Presenting data blocks on filled-in
rectangles keeps this perception constant. The guidelines also note that
"infills of the same color will be more readily perceived as a group
than data blocks presented without infills" (Section 4.3.1). The pros
and cons of such groupings have already been discussed. Another possible
disadvantage is that these filled-in blocks probably take up more space
than the traditional data block, although this has not yet been
measured.

Although the advantages of this particular color scheme has not yet been
proven in the scientific sense, the approach is consistent with a sound
human factor approach to coding information and is an excellent
foundation for applying color to ATC displays.

### **ODID (Operational Display Input Development) IV** 

Another color ATC display that has received a great deal of attention is
the ODID IV. This display is used at the Eurocontrol's Experimental
Center to test advanced ATC tools, such as conflict detection tools and
alerts. When considering this system, it is important to remember that
it is not currently used for air traffic control at any facility. Still,
the results of the simulations using the system provide some useful
information for the use of color in ATC displays.

The first ODID IV simulation (Graham et al., 1994) evaluated some of the
human factors aspects of an ATC system that uses advanced automation
tools and electronic flight strips. This simulation did not record any
data as it relates to controller performance with the use of the color
display, but recorded the controllers' comments with respect to the use
of color in this system. In general, controllers thought that color was
very useful. They particularly liked the use of yellow as a warning
color. However, there was one cautionary note with particularly
interesting and serious implications. Controllers "appeared to be
looking only for color \[in the conflict alert tool they were provided\]
to indicate a traffic problem. This appears to have resulted in lack of
memorization of textual and graphical \[flight\] detail which could
reduce their traffic awareness." (p. 98). More detailed comments from
the report can be found in Appendix B.

Nine out of ten controllers who participated in this ODID simulation
indicated that they were never confused by the meanings of the colors,
but five out of ten controllers said they occasionally experienced
difficulty in reading colored text, such as the radar label or Sector
Inbound Lists (the other five controllers checked the box between
"occasionally" and "never"). These difficulties were primarily
attributed to the grey and red text.

Controllers did find the use of color very helpful in categorizing
information and coding alerts. For example, an aircraft calling in for
the first time would have a pink data block. This would significantly
reduce the time required to find the data block among the others.
However, this useful categorization may come at a high price if this
coding scheme also has the effect of making a controller less likely to
notice a potential conflict between two aircraft with different color
codes than between two aircraft with the same color code, a factor that
was noted, but not investigated, in a later ODID simulation. The
distractions noted with large blocks of color should also be taken into
consideration in color applications.

Another study using ODID was a joint FAA/Eurocontrol simulation to
assess the usefulness of some of the ODID tools for U.S. controllers
(Krois and Marsden, 1997). While it is impossible to separate out the
effects of color-coding from the effects of the use of these tools (such
as short-term conflict alert and medium-term conflict assistance), it is
again useful to look at the controllers' subjective assessment of the
uses of color they saw. Again, this simulation made no attempt to
objectively examine the effect of color on controller performance, nor
did they obtain any objective data that could be used to address this
point. Still, controller opinion is quite valuable, as long as we
remember that controllers may *think* that color helps their performance
in cases where it has been shown to have no effect (Connolly et al.,
1975).

The color scheme in this ODID IV simulation used four different
non-alerting or "state" color codes for data blocks. A description is
presented here, not as guidance, but as background material necessary to
understand the controllers' comments that follow.

GREY - "transferred" (also known as not my aircraft). Grey was used to
indicate an aircraft that was outside the controllers sector or one that
had been handed off to another frequency.

PINK - "advanced information" (and coordination) state. Pink was used to
indicate that the aircraft would be entering the sector in 10 minutes
and the "inbound flight could commence entry negotiations, if required."
(p. 24)

WHITE - "assumed". White was used to indicate that the aircraft was on
the frequency and that the controller had, or was in the process of,
taking control.

MUSTARD - "concerned". "Concerned mustard", as they called it, was used
to indicate that the aircraft had been transferred to another
controller, but was still in the sector (and thus the controller must
still "maintain situational awareness of this traffic").

In addition to these "state" colors, two warning colors were used:

RED - "short term conflict alert". Aircraft call signs within the data
block were coded red when there was an "imminent loss of radar
separation based on a two minute warning". The rest of the data block
stayed the color that it was (Galushka, 1998).

YELLOW - "alert". Aircraft call signs within the data block could turn
yellow for one of two reasons. This alert was automatically activated
when an aircraft had been transferred to the next sector before
receiving a clearance to its exit flight level. Call signs would also
turn yellow when a conflict pair (as detected by the system) was
selected by the controller to be highlighted.

Another use of color was the "Medium Term Conflict Assistance" tool. In
this window, red was used to indicate a conflict, yellow was used to
indicate a risk of conflict and grey was used to indicate a potential
conflict.

*Results of the ODID IV Simulation*

Simulation results mention that "controllers supported the use of color
to assist them in understanding the ATC situation" and that the use of
color displays "contributed to improved effectiveness" (Krois and
Marsden, 1997, p. 77). Again, this is based solely on subjective
assessment and not on any objective performance data. Still, some of the
comments offered by controllers provide useful information on what they
found useful and problematic. Controllers "unanimously endorsed" the use
of color to indicate status such as "not concerned" (not my aircraft)
and warnings. They also liked the use of green to indicate the flight
leg with red to show the area of conflict. There were also a number of
concerns (noted by Krois and Marsden, 1977) with specific applications
of color some of which have serious implications:

"The Mustard labels did not sufficiently attract the attention of some
controllers during their visual scan. For example, *the controller could
climb an aircraft and inadvertently not be aware of another aircraft
because it was not white*" \[emphasis added\] (p. 26) \[White was used
to code "my aircraft".\]

"Grey labels were not conspicuous enough for some controllers and *were
sometimes overlooked* \[emphasis added\] (p. 27). In fact, *one of the
operational errors noted during the simulation was due to a controller
not being aware of a track displayed as grey* (p. 76).

Of the data blocks displayed in pink, "Controllers stated specifically
that they were often unaware of the position of an aircraft at the first
frequency contact as a result of the absence of the handoff facility."
(p. 26) \[In the United Kingdom, controllers do not routinely hand-off
aircraft as we do in the U.S. In fact, in London, handoffs are the
exception, not the rule. It is interesting to note that coding these
aircraft pink was not an adequate substitution for our current handoff
procedures for U.S. controllers.\]

"Some controllers considered the red call signs "fuzzy" and difficult to
read\..." (p. 26)

"Some controllers indicated that there should be fewer number of colors
used to denote aircraft status, for example, not to differentiate pink
and mustard states from white."

# CONCLUSIONS

The effective use of color on a display requires careful attention to
the definitions of the colors used and the ways in which the colors are
used. The development of a coding scheme begins with an understanding of
the tasks to be performed and identification of subtasks for which color
coding may be particularly helpful (such as immediate identification of
certain streams of traffic). From this identification of tasks and the
information required to complete the tasks, we can construct a hierarchy
of display information. Such a hierarchy would include all of the
information to be displayed ranging from the background information
(e.g., a map) that should be present, but unobtrusive, to critical
components of a display (e.g., warnings that require immediate action,
the cursor) that require some form of highlighting.

While this hierarchy of display information will help to determine how
the information is color-coded, the first decision must be a choice of a
background color. As previously discussed, black (or other very low
luminance) backgrounds are very good for color identification and help
to minimize display flicker, but can suffer from noticeable glare. Light
gray backgrounds minimize glare, but color identification can suffer and
flicker is more apparent (and potentially distracting) at higher
luminances. The selection of a background color should consider the task
and operational environment. In a dark environment, a dark background is
usually preferred. In high ambient light (such as in an ATC tower), with
a display that uses minimal color coding, a light gray background may be
preferred. In office level lighting, either light or dark backgrounds
are usable, when properly designed.

Once a background color is selected and a hierarchy of information has
been constructed, color names can be assigned to specific groups of
information. While user input is important at all stages of display
development, an efficient strategy is to have structured user input at
the first stages of design (e.g., the construction of the information
hierarchy) and then at the final stages (evaluating the prototype
displays). Human factors specialists are in a better position than users
to design the prototype color schemes, since their designs will be based
on human factors "best practices" rather than user preferences. There is
a tremendous amount of variation in personal preferences for selection
of display colors. As we have seen, however, the colors that users
prefer are not necessarily the ones that will optimize their
performance. Thus, having a prototype color scheme based on human
factors guidance not only minimizes variability, but also precludes
designs that may have a detrimental effect on performance.

Once a color scheme that matches color names to groups of information
has been developed, these color names will need to be defined to be the
most maximally discriminable for a specific monitor. Once the color set
has been defined, it should be validated by testing to ensure that no
two colors in the set are confusable and none of the colors in the set
adversely affect legibility. Again, these are tasks for human factors
engineers. The next and final steps are a critical review of the color
scheme by users and, finally, operational testing and evaluation.

In developing a prototype color scheme, it is very useful to examine the
color schemes that other air traffic organizations use. It could be
argued that since specific color schemes have been in use at foreign ATC
facilities with no disastrous outcomes, the same color scheme should be
tried in the U.S. *However, since none of these air traffic control
facilities have exactly the same amount, mix, and complexity of traffic
as the U.S. facilities, it could be a mistake to adopt these color
schemes for the U.S. without adequate operational testing.*

It is equally important to consider the concern first raised by V. David
Hopkin in 1977 and noted in the ODID simulation. *Color-coding the
entire data block may be so effective at dividing the aircraft into
groups (departure and arrival) that controllers may not notice potential
conflicts between two aircraft color-coded differently as well as they
would notice potential conflicts between aircraft whose data blocks are
the same color.* This is a serious question that requires further study.

There are many cautions about the use of color that need to be
considered; it is all too easy to do more harm than good. *However,
color remains an effective, compelling, and attractive method for coding
visual information on a display.* The use of color in ATC displays
presents exciting opportunities, as well as challenges. Color, when used
properly, is an extremely useful tool for organizing complex
information. Well-designed color displays for ATC are likely to have
profound advantages over monochromatic displays. The careful design and
testing of specific color-coding schemes with attention to the
guidelines presented in this report will help to realize the potential
benefits of the use of color while minimizing the potential drawbacks.

**APPENDIX A**

**DETERMINING THE IDEAL COLOR SET FOR THE SONY DDM-2801C**

As a result of a request from the FAA's Office of the Chief Scientific
and Technical Advisor for Human Factors (AAR-100), Integrated Product
Team for En Route (AUA-200), and the STARS Product Team (AUA-310), a
series of studies was conducted to determine the ideal color set for the
Sony DDM-2801C (20 x 20) monitor. In the first experiment described,
five of the SONY DDM-2801C monitors used at the FAA's William J. Hughes
Technical Center were assessed for variations in color production
characteristics. The second and third experiments identified and
validated a proposed "ideal" color set based on the color production
capabilities of the Sony DDM-2801C and what is known about human color
vision. A set of colors is considered "ideal" if it minimizes the
probability of mistaking one of the colors in the set for any other
color in the set and none of the colors in the set adversely affects
legibility. Use of additional colors outside of this set should be
preceded by tests to ensure that the colors are not confusable; see
Section 3.3 of this report for details. The recommended colors are
specified in terms of standardized units (i.e., CIE *x,y* chromaticity
coordinates and luminance values) that can be reproduced by any monitor
capable of producing them.

In 1995, the National Information Display Laboratory (NIDL) at the
Sarnoff Research Center conducted a comprehensive evaluation of a single
Sony DDM-2801C monitor (NIDL, 1995). This report identifies many
performance parameters and compares the Sony monitor to other monitors
that were available at the time. The Sony monitor was the largest in
size (19.6 x 19.6 inches) and had the lowest luminance tested (23fL). It
also had a uniform gamma curve (relating input to output luminance) that
was almost straight on a log-log plot from 1 fL to 20 fL (p.x). The
refresh rate is listed in the specifications as 60Hz. Luminance was
found to vary as a function of position on the screen by as much as 27%
at the highest luminance setting. With respect to color, chromaticity
coordinates varied as much as 5% (*x*) and 10% (*y*) with the location
on the screen. This means that the appearance of a color could change
slightly depending where the color was presented on the screen,
particularly at the corners. This study detailed the changes in
chromaticity coordinates and luminance level that were recorded at each
of 12 positions on the screen. However, since measuring human
performance was outside the scope of that study, it could not address
how these changes in chromaticity and luminance might affect color
identification or legibility. It did also not address how the colors on
a monitor might shift over time, or the variability that could be found
from one monitor of the same model to another.

**A.1 EXPERIMENT ONE: ASSESSMENT OF THE VARIABILITY ACROSS SONY
MONITORS**

The purpose of this experiment was to determine the variability that
could be expected across several different Sony DDM-2801C monitors.
Colors are created on a CRT through the excitation of phosphors.
Luminance (roughly correlated to the perception of brightness) is
determined by the amount of excitation of these phosphors. Software
controls which phosphor is excited and the amount of the excitation. Due
to factors beyond the scope of this report, variations in phosphor
output occur over the surface of the screen. This means that the same
software input may result in chromaticity coordinates that are different
for different areas of the screen.

**A.1.1 Method**

Five Sony DDM-2801C monitors used in the integration and
interoperability facility at the FAA's William J. Hughes Technical
Center in Atlantic City, NJ were assessed for variations in color
production characteristics. All of the monitors were reported to be
properly calibrated. Measurements (CIE *x,y* and luminance) were taken
using a Photo Research PR-650 spectraphotometer on each of the guns (Red
\[180, 0, 0\], Green \[0, 180, 0\] and Blue \[0, 0, 180 \]) at nine
different positions on the screen. In this way, the variabilities due to
the location on the screen and the different monitors could be assessed.

## A.1.2 Results  {#a.1.2-results .unnumbered}

Figure A-1 shows the relation between the software control input and the
output luminance observed for one monitor. The resulting slope of 2.72
on log-log coordinates is consistent with what was found by NIDL (1995).
The figure shows the CIE plot of the colors able to be produced by the
Sony monitor. Also shown are the seven colors chosen for the derived
color set. The area bounded by the triangle represents the total color
gamut (in CIE coordinates) of the Sony monitor.

The results of the variations of *x, y* coordinates and luminance values
across the different screen positions are shown in Table A-1 and are
also consistent with the NIDL findings. In addition, variations were
found across the five monitors. Documentation concerning operationally
acceptable tolerances for *x* and *y* coordinates could not be found.
However, the FAA has previously used differences of plus or minus .02
for colors using only one primary (red, green, or blue) and plus or
minus .03 for colors using two or more primaries (yellow, cyan, and
magenta) \[Tarka, 1998, personal communication\]. Using these criteria,
only seven measurements exceeded these tolerances; three "reds," two
"magentas," and one instance each of "green" and "blue." (In Experiment
3, these "deviant" colors were tested to see if they would have a
detrimental effect on performance.)

## A.2 EXPERIMENT TWO: DETERMINATION AND VALIDATION OF THE IDEAL COLOR SET FOR THE SONY DDM-2801C  {#a.2-experiment-two-determination-and-validation-of-the-ideal-color-set-for-the-sony-ddm-2801c .unnumbered}

If an individual color is assigned a specific meaning, then the
information presented in this color is said to be "color-coded."
Information (such as text, symbols, or areas) is color-coded when it is
presented in a color that has been assigned a meaning (such as red for
conflict alert). For this use of color to be successful, a set of
reliably identifiable colors must be established. This means that the
probability of any color in the set being confused with any other color
in the set is near zero when presented on any of the possible
backgrounds, such as color-filled areas.

Extensive research on human performance has determined that our ability
to reliably identify colors is limited to five or seven. The seven
colors that are easiest to identify on a CRT while using a black or dark
grey background are red, green, blue, yellow, cyan, magenta, and white.
Given that a CRT such as the Sony DDM-2801C can produce thousands of
"reds," "greens," and "blues," how should these colors be defined to
minimize their confusability? The answer to this question depends on the
color-producing capabilities of the specific type of monitor.

FIGURE A-1 Log-log Plot of Input versus Luminance for the Sony DDM-2801C

GOES HERE

(Excel File)

TABLE A-1

Measurements from Nine Screen positions on Five Sony DDM-2801C Monitors

GOES HERE

(hard copy -- KC to update Excel file version)

The purpose of this study was to determine the ideal color set for the
Sony DDM-2801C. A set of colors is considered "ideal" if the accuracy of
identifying each of the colors is near 100% (thus minimizing the
probability of mistaking one of the colors in the set for any other
color in the set).

It is well known that small patches of color are much harder to identify
than large areas of color-filled areas. For this reason, it is also a
requirement that all of the colors in the set must be able to be
identified in the smallest size that might be color-coded in actual
operations, such as a position symbol. A final requirement is that none
of the colors negatively affects legibility. Certain combinations of
colors and background (such as pure or "royal" blue on a dark background
or yellow on a light background) are very difficult to read and can
induce errors. For this reason, we also need to ensure that none of the
colors in the proposed color set induce errors in legibility on the
black background. In addition to the black background two dark grey
backgrounds were also tested, since dark grey is often preferred to
black because of the reduction in noticeable glare.

**A.2.1. Method**

*A.2.1.1 Derivation of the Ideal Color Set*

First, the CIE chromaticity coordinates (in *x, y* values) of each of
the guns were mapped to define the colors (in color space) produced by
the monitor[^4] (see Figure A-2). From this information, *x, y*
coordinates of candidate colors were identified for testing.[^5]

*A.2.1.2 Validation of the Color Set*

The resulting color set was validated by testing for confusions of
colors within the set and for detrimental effects on legibility.

*A.2.1.3 Participants*

Seven federal employees of the William J. Hughes Technical Center
participated in the study. They ranged in age from 32 to 51 with an
average age of 42. All had normal or corrected-to-normal vision and had
no known color deficiencies.

*A.2.1.4 Task*

![](media/image1.wmf){width="0.15347222222222223in"
height="0.14930555555555555in"}Each of the colors in the set was used to
display an "x" contained in a circle or a filled-in circle on one of
five positions (four corners and the center) on the Sony monitor at the
Integration and Interoperability Laboratory at the William J. Hughes
Technical Center. Participants were asked to name the symbol and its
color, that is, whether they saw a filled-in circle (  ) or an "x"
enclosed in a circle **( )** and the color of the symbol. Each symbol
subtended .27 degrees of visual angle at a distance of 20 inches. The
participants were given the names of the colors that they could expect
to see and the unfamiliar color terms such as magenta and cyan were
described. (Magenta was described as a reddish blue or "purple" and cyan
was described as a greenish blue or bluish green.) After a response,
another item (filled-in circle or "x" enclosed in a circle) was
presented in another color on another portion of the screen.

FIGURE A-2. CIE Plot of the Colors Produced by the Sony DDM-2801C

GOES HERE

(Excel File)

This procedure was repeated until each person saw each color 50 times
(half as the filled in circle and half as the "x" enclosed in a circle)
on each of the three backgrounds: black, and two variations of grey in
trials blocked by background color. The testing with the black
background was conducted with the room lights off to simulate the
lighting in a typical TRACON. The testing with the grey backgrounds was
conducted with the room lights on.

*A.2.1.5 Stimuli*

The filled-in circle and the x enclosed in a circle were presented in
equal numbers in two sizes - drawn with either eight or twelve pixels.
This was used to simulate the two smallest font sizes currently
available in STARS.

The backgrounds were defined as follows:

[CIE *x* CIE *y* Luminance (fL)]{.underline}

Black not measurable not measurable 0

Grey 1 .300 .287 3.46

Grey 2 .299 .285 7.42

The derived color set tested in Experiment 2 was as follows:

[CIE *x* CIE *y* Luminance (fL)]{.underline}

Red .630 .341 3.63

Green .298 .610 8.21

Blue .149 .066 1.57

White .298 .281 13.82

Yellow .444 .488 12.18

Magenta .298 .152 5.33

Cyan .203 .262 9.74

In addition to this set, an additional value was tested for magenta
(called "Magenta 2" and defined as x = .334, y = .270, fL = 9.01). This
value was tested because it was the one selected by controllers who were
helping to choose the color set for DSR.

**A.2.2 Result**

Legibility (symbol identification) was at least 99% accurate with all of
the colors tested on the black and darker grey (Grey 1) background.
Legibility was at least 99% accurate with all of the colors tested on
the Grey 2 background except for Magenta 2, which yielded an accuracy of
98%.

The accuracy of color identification as a function of color and
background is shown in Table A-2. Color identification on the black
background was at least 99% accurate for all colors except Magenta 2
(93%) and Cyan (95%). Magenta 2 was most likely to be confused with
white and

cyan was most likely to be confused with blue. Results were similar with
the Grey 1 background where color identification was at least 99%
accurate for all colors except Magenta 2 (98%) and Cyan (98%). Color
identification was slightly worse with the Grey 2 background; the same
problems with Cyan (96% accuracy) and Magenta 2 (96%) were found and the
identification of green was also slightly worse (98%). On the Grey 2
background, only red, blue, white, yellow, and magenta were accurately
identified at least 99% of the time.

# Table A-2. Percent Accuracy for Color Identification for Experiment 2 {#table-a-2.-percent-accuracy-for-color-identification-for-experiment-2 .unnumbered}

**BACKGROUND**

**[Color Name]{.underline} [Black]{.underline} [Grey 1]{.underline}
[Grey 2]{.underline}**

Red 100 100 99.7

######## Green 100 100 98.5 {#green-100-100-98.5 .unnumbered}

Blue 100 100 99.2

White 99.8 100 100

Yellow 99.3 99.3 99.2

Magenta 99.3 99.8 99.5

Magenta 2 93.5 97.8 96.5

Cyan 95.3 98 96.5

With the exception of cyan, all of the colors in the derived color set
were validated on the black background with at least 99% accuracy in
color identification and symbol recognition. The magenta proposed for
DSR (based on controller preference) was less accurately identified than
the derived magenta (93.5% vs. 99.3%). Cyan was identified as cyan only
95% of the time. When there were errors, cyan was most often called
"blue" which it is physically very similar to. However, there were not
an equal number of errors calling the blue "cyan". If the errors were
truly a perceptual confusion of blue and cyan, then we would expect
errors in both directions, that is, there would be roughly as many
errors in which "blue" was called "cyan" and "cyan" was called "blue".
The fact that the errors were found only in the labeling of cyan as
"blue" suggests that maybe the unfamiliar color name "cyan" may have
contributed to the error rate. Also, since the colors were not shown to
the participants beforehand, only described, participants had to rely on
their natural (untrained) responses for color naming. It is possible
that fewer errors would have been made in the identification of "cyan,"
had the participants been shown an example beforehand. Experiment 3 was
conducted to determine whether cyan is actually difficult to identify
because of its similarity to blue and green, or simply a difficult name
to recall.

A.3 EXPERIMENT THREE: EXPANSION OF THE RECOMMENDED COLOR SET FOR THE
SONY DDM-2801C

As a result of the variability found across five monitors thought to be
calibrated, the decision was made to test the range of coordinates that
exceeded the FAA tolerance. If these coordinates resulted in criterion
performance for color identification and legibility, they would yield a
range of usable coordinates for the colors in the set. Such a range
would be more useful to designers than a single value for each color
name. Additionally, participants would be shown large patches of the
colors before the testing was conducted and given a list of the colors
that they could expect to see. This would allow us to determine whether
the difficulties found with cyan in Experiment 2 were due to its
similarity to blue and green or because "cyan" was an unfamiliar color
name. At least two sets of values for each color were tested. This
included all of the values observed in Experiment 1 that exceeded the
established tolerances and the DSR "preference" values for cyan,
magenta, and yellow to determine their effect on color identification
and legibility.

**A.3.1 Method**

## *A.3.1.1 Participants*  {#a.3.1.1-participants .unnumbered}

Twelve paid volunteers ranging in age from 19 to 35 (average age of 26)
participated in the study. All had normal or corrected-to-normal vision.
After the color names were listed and described (if necessary), the
participants were given a pre-test in which they were asked to name
large blocks of colors presented on the screen and identify each symbol
("0" or "8") in a pre-test. Two of the participants who suspected they
had a color vision anomaly were unable to successfully name the colors
in the pre-test and their data were not included in the results
presented here. This left a total of 10 subjects on which the data below
are based.

*A.3.1.2 Task*

Each of the colors in the set was used to display a zero (0) or an eight
(8) at either one of the four corners or the center of the display. The
display used for this experiment was a Pixel Link calibrated color
monitor that simulated the colors of interest on the Sony monitor. The
CIE *x, y* coordinates were replicated but the luminance for some of the
values was higher than those produced on the Sony. Participants were
asked to press the space bar when they were ready to say whether they
saw the "0" or the "8" and to name the color it was. After they
responded, another number (zero or eight) was presented in another color
on another portion of the screen. This procedure was repeated until each
person saw each color 50 times (half as zeros and half as eights) on
each of four backgrounds: black, dark grey, medium grey and light grey.
The trials were blocked by background color.

The requirements for this study were to determine the ideal color set
using a black background and low ambient light conditions. In addition,
we also tested the proposed color set in office level lighting on a dark
grey background (which may be preferred because of the reduced glare), a
medium grey, and a light grey background (which may be preferred in high
ambient light environments). The purpose was to provide information on
the effect of increasing the ambient light level (turning up the lights)
on the accuracy of color identification. Testing with the black
background was conducted with the room lights off to simulate a typical
current TRACON environment; testing with the grey backgrounds was
conducted with the room lights on (2fc).

*A.3.1.3 Stimuli*

The "0" and "8" were drawn with eight and twelve pixels to simulate the
two smallest available font sizes in STARS. The height of the "0" and
"8" subtended .27 degrees of visual angle when drawn with eight pixels
and .36 degrees when drawn with twelve pixels at a viewing distance of
20 inches. The colors used are defined in Table A-3. "OD" is used to
indicate that this value was an "observed deviant", meaning this value
was observed in Experiment 1 on one of the Sony monitors measured, and
exceeded the tolerance of +/-.02 for red green or blue, or +/- .03 for
magenta, yellow, and cyan. That is, there was a difference between the
color that was "programmed" to be displayed and the color that was
observed on the display. "DSR" indicates that these were the values
selected by controllers for the DSR program (based on preference) that
were significantly different from the derived values for the same color.

The backgrounds used are also defined in Table A-3. The "Dark Grey"
background produced a contrast of 5:1 for the red targets. The "Medium
Grey" yielded a contrast of at least 2.3:1 for the yellow and red
targets; yellow was brighter than the background (positive contrast) and
red was dimmer than the background (negative contrast). With the "Light
Grey" background, different values for green, yellow, and cyan were
chosen so that all colored targets were presented with negative
contrast, meaning all of the colored "0"s and "8"s were dimmer than the
background.

## A.3.2 Results {#a.3.2-results .unnumbered}

*Legibility, defined as the ability to distinguish between the zero and
eight, was at least 99% accurate with all of the colors tested on all of
the backgrounds. There was no effect of stimulus size (8 or 12 pixels),
meaning that accuracy was no different for the smaller sized numbers
than the larger one.*

*Color identification varied widely with individual colors on the
different backgrounds ([F]{.underline} (3, 33) = 12.96,
[p]{.underline}\<001).* Table A-4 shows the accuracy for color
identification and average response times (RTs) for the correct
responses as a function of the background. Recall that accuracy for
symbol identification was consistently at 99% or above. Generally, color
identification was very good on the black, dark and medium grey
backgrounds but limited with the grey background. On the light grey
background only four colors, red, white, magenta 1 and green were
identified with an accuracy of at least 95%. The effect of individual
colors had a significant effect on the accuracy of color identification
*([F]{.underline} (15,164) = 13.68, [p]{.underline}\<001)* as did the
interaction of the color and the background *([F]{.underline} (45,476)=
3.25 [p]{.underline} \<.001)*. **These results show once again that the
ability to identify colors is a function of both the individual colors
and the background. Colors cannot be chosen for use on a display without
considering the background, nor can the background of a display be
chosen without considering the foreground colors to be used.**

The most dramatic differences within a color name were seen with blue
and magenta. While "Blue 1" was correctly recognized as blue 100% of the
time on a black background, "Blue 2" (an observed deviant) was only
identified as blue 57% of the time. Similarly, "Magenta 1" was correctly
identified 98% of the time on a black background, while "Magenta 2" ( a
color previously selected by controllers based on preference) was only
identified as magenta 81% of the time. Results were similar with a dark
grey background. "Blue 1" was correctly identified as blue 99% of the
time, whereas "Blue 2" was identified as blue only 58% of the time.
"Magenta 2" was identified as magenta only 75% of the time, while
"Magenta 1" was identified as magenta 100% of the time.

# Table A-3. Definition of the Color Sets Tested in Experiment 3 {#table-a-3.-definition-of-the-color-sets-tested-in-experiment-3 .unnumbered}

+-----------------------------------------------------------------------+
| [Color Name CIE x CIE y Luminance (fL)]{.underline}                   |
+-----------------------------------------------------------------------+
| Red 1 .628 .339 5.17                                                  |
+-----------------------------------------------------------------------+
| Red 2 (OD) .546 .357 7.42                                             |
+-----------------------------------------------------------------------+
| Red 3 (OD) .593 .335 6.04                                             |
+-----------------------------------------------------------------------+
| Red 4 (OD) .558 .367 7.53                                             |
+-----------------------------------------------------------------------+
|                                                                       |
+-----------------------------------------------------------------------+
| Green 1 .289 .596 24.20                                               |
+-----------------------------------------------------------------------+
| Green 2 (OD) .313 .555 25.10                                          |
+-----------------------------------------------------------------------+
|                                                                       |
+-----------------------------------------------------------------------+
| Blue 1 .154 .070 3.34                                                 |
+-----------------------------------------------------------------------+
| Blue 2 (OD) .201 .131 7.39                                            |
+-----------------------------------------------------------------------+
|                                                                       |
+-----------------------------------------------------------------------+
| Yellow 1 .388 .522 29.20                                              |
+-----------------------------------------------------------------------+
| Yellow 2 (DSR) .356 .474 29.80                                        |
+-----------------------------------------------------------------------+
|                                                                       |
+-----------------------------------------------------------------------+
| Cyan 1 .219 .299 25.60                                                |
+-----------------------------------------------------------------------+
| Cyan 2 (DSR) .218 .332 26.20                                          |
+-----------------------------------------------------------------------+
|                                                                       |
+-----------------------------------------------------------------------+
| Magenta 1 .261 .136 8.54                                              |
+-----------------------------------------------------------------------+
| Magenta 2 (DSR) .399 .208 6.67                                        |
+-----------------------------------------------------------------------+
| Magenta 3 (OD) .290 .178 10.70                                        |
+-----------------------------------------------------------------------+
|                                                                       |
+-----------------------------------------------------------------------+
| White .274 .286 26.50                                                 |
+-----------------------------------------------------------------------+
|                                                                       |
+-----------------------------------------------------------------------+
| The above colors were used for the following backgrounds:             |
+-----------------------------------------------------------------------+
| [CIE x CIE y Luminance (fL)]{.underline}                              |
+-----------------------------------------------------------------------+
| Black not measurable not measurable 0                                 |
+-----------------------------------------------------------------------+
| Dark Grey .270 .284 1.21                                              |
+-----------------------------------------------------------------------+
| Medium Grey .274 .284 12.1                                            |
+-----------------------------------------------------------------------+
|                                                                       |
+-----------------------------------------------------------------------+
| For the Light Grey Background:                                        |
+-----------------------------------------------------------------------+
|                                                                       |
+-----------------------------------------------------------------------+
| .274 .281 22.3,                                                       |
|                                                                       |
| the values for the Reds, Blues, White, and Magenta were the same; the |
| Green, Yellows, and Cyans were defined as follows:                    |
+-----------------------------------------------------------------------+
| Color Name CIE x CIE y Luminance (fL)                                 |
|                                                                       |
| Green .291 .603 11.00                                                 |
|                                                                       |
| Yellow 1 .412 .509 9.15                                               |
|                                                                       |
| Yellow 2 .411 .509 8.98                                               |
|                                                                       |
| Cyan 1 .212 .305 10.30                                                |
|                                                                       |
| Cyan 2 .228 .365 10.20                                                |
+-----------------------------------------------------------------------+

**Table A-4. Mean Percent Accuracies of Color Identification and
Response Times (RTs) in Seconds for Correct Responses in Experiment 3**

**BACKGROUND**

**Black Dark Grey Med. Grey Light Grey Color**

**Color**

**[Name Accuracy RT Accuracy RT Accuracy RT Accuracy RT]{.underline}**

Red 1 100 .95 100 1.18 99.3 1.15 100 1.07

Red 2 100 .96 100 1.05 99.3 1.51 97.2 1.25

Red 3 100 .93 100 .85 98.6 1.24 99.3 1.21

Red 4 100 .97 100 .95 99.3 1.66 97.9 1.28

Green 1 97.2 .96 98.6 .74 93.8 1.26 95.8 1.41

Green 2 93.8 .94 96.5 .84 94.4 1.12 96.5 1.48

Blue 1 100 .96 98.6 1.21 91.0 1.18 85.4 1.15

Blue 2 56.9 .94 57.6 1.18 86.8 2.10 79.2 1.56

White 100 1.03 99.3 .86 99.3 1.04 99.3 3.98

Yellow 1 97.9 .93 98.6 .75 98.6 .99 65.3 1.47

Yellow 2 97.9 .98 97.9 .77 97.9 1.06 63.9 2.06

Magenta 1 97.9 1.01 100 .88 97.2 2.11 98.6 1.40

Magenta 2 81.3 1.23 75.0 .99 59.7 1.62 70.1 1.53

Magenta 3 97.2 1.01 99.3 .86 92.4 3.67 91.7 1.65

Cyan 1 93.1 .98 96.5 .82 98.6 1.05 66.0 1.55

Cyan 2 91.7 .96 100 .78 98.6 1.06 59.0 1.93

Mean 94.0 .98 94.7 .91 94.0 1.47 85.2 1.62

Standard Deviation 18.6 .51 16.9 .88 18.3 1.39 28.4 1.37

Response time was measured by asking the participants to press the space
bar when they were ready to respond with the number (0 or 8) and color
name. At this point the stimulus disappeared from the screen. Response
times reported here are for correct color identifications only. Response
times increased significantly with the luminance of the background with
the lighter backgrounds associated with longer response times.
([F]{.underline} (3, 33) = 23.94, [p]{.underline} \< .001). Response
times also varied significantly as a function of the individual colors
presented ([F]{.underline} (15,164) = 26.07, [p]{.underline} \< .001)

There was also a significant interaction between individual colors and
backgrounds in response times. ([F]{.underline} (45, 476) = 35.61,
[p]{.underline} \< .001). This means that response times were dependent
upon combinations of colors and backgrounds and were not solely
dependent on either the individual color or the specific background. In
fact, even the nature of the color confusion errors changed as a
function of the background. For example, when errors in identifying the
two values for cyan were made, cyan was mistaken for "white" when
presented on the black background and either "blue" or "white" when
presented on a dark grey or medium grey background. When confusions were
made on the light grey background cyan (using slightly different values
and a much lower luminance than those used for the other backgrounds)
was confused with "blue\" and \"green". Again, the successful use of any
individual color depends as much upon the characteristics of the
background as the characteristics of the foreground. Average response
times for all of the correct color identifications are presented in
Table A-4. (Note: The means and standard deviations at the bottom of the
table are based on all responses.) With the black background, response
times ranged from .93 seconds (for Red 3 and Yellow 1) to 1.23 (for
Magenta 2) seconds with an average of .98 seconds. Performance was
comparable with the dark grey background; average response times ranged
from .74 seconds (for Green 1) to 1.21 seconds (for Blue 1) with an
average of .91 seconds. With the medium grey background, response times
ranged from .99 seconds (for Yellow 1) to 3.67 (for Magenta 3) with a
mean of 1.49 seconds. With the light grey background, response times
ranged from 1.07 (for Red 1) to 3.98 seconds (for White) with a mean of
1.62 seconds.

In actual operations, the accuracy with which colors can be identified
and colored symbols and text can be read is usually more important than
a one second difference in how long this process may take; however, such
issues are task dependent. The data provided in this study will allow
individual questions regarding the use of specific colors on black and
grey backgrounds *for specific uses* to be addressed.

In summary, *the light grey background was associated with longer*
*response times and more errors in color identification compared to the
black, dark grey and medium grey backgrounds. At least one value for
each color except cyan was able to be identified for the Black, Dark
Grey, and Grey 1 backgrounds to yield at least 99% accuracy in color
identification with no detrimental effect on legibility. With the black
background, cyan was the only color for which none of the coordinates
tested resulted in criterion performance of 99% accuracy for color
identification and legibility. It is also the case that the cyan tested
is likely to be confused with white by people with the most common form
of "color-blindness". This suggests that cyan should not be used to code
critical information (i.e., to convey an important meaning). It also
supports the recommendation of using no more than six colors (e.g., red,
blue, green, yellow, magenta, and white on a black background) to code
information.*

The results of Experiments 2 and 3, as shown in Table A-5, provide color
identification accuracies on several different color/background
combinations so that programs can set their own criteria (depending on
the task) and choose the colors accordingly. "Best" color identification
accuracy for each color name/background combination is shown in bold.
The "Mean of "best" instances" is the average percent correct of the
values (one for each color name) that resulted in the highest
identification accuracy. If more than one set of coordinates for a
single color name yielded equally high performance, only one of these
values was used in the calculation of this mean. All of the "best"
instances of colors in the color set for the black background are ones
that can be replicated with the Sony DDM-2801C.

In cases where three or more coordinates for a color name resulted in
criterion performance (of at least 99% accuracy in color naming with at
least 99% accuracy in symbol identification), the values can be plotted
on the CIE diagram to define a recommended color space for that color
name. This means that any values within that space can be assumed to
minimize the probability of color confusions with other colors in the
set and have no detrimental effect on legibility (when presented at the
same contrast tested). However, red was the only color for which three
or more tested values resulted in 99% accuracy on the black background.
Using other performance criteria such as a color identification accuracy
of 97% or 98% would widen the usable color gamut.

Finally, it should be noted that this study found deviations in color
appearance as a function of the location on the screen and as a function
of the variability from monitor to monitor. Some of these differences
were found to result in a decrease in accuracy for color identification.
For example, while the derived blue resulted in 100% accuracy on the
black background, the observed deviant of this blue resulted in 57%
accuracy. *(We must also expect the appearance of colors on any given
monitor to shift over time.)* All monitors are subject to shifts in
color over time. We do not know enough to be able to predict when or how
these shifts will occur. The report on the FAA/Eurocontrol's ODID IV
simulation (that used a SONY 20 x 20 monitor) stated, that the "\...EEC
\[Eurocontrol Experimental Center\] engineers noted that these monitors
are frequently maintained due to color shifts over time." (p. 26).
Displays using color must periodically be checked and recalibrated.
**Maintenance procedures need to be in place to ensure that monitors are
recalibrated on a schedule that sustains reliable color production**.

**Table A-5. Color Identification Accuracies from Experiment 2 and
Experiment 3**

**BACKGROUND**

**Color Experiment Dark Med. Light**

**[Name Number Black Grey Grey 1 Grey 2 Grey Grey]{.underline}**

Red 1 3 **100 100 99.3 100**

Red 2 3 **100 100 99.3** 97.2

Red 3 3 **100 100** 98.6 99.3

Red 4 3 **100 100** **99.3** 97.9

Red 5 2 **100** 100 99.7

Green 1 3 97.2 **98.6** 93.8 95.8

Green 2 3 93.8 96.5 **94.4 96.5**

Green 3 2 **100** 100 98.5

Blue 1 3 **100 98.6 91.0 85.4**

Blue 2 3 56.9 57.6 86.8 79.2

Blue 3 2 **100** 100 99.2

White 1 3 **100** **99.3 99.3 99.3**

White 2 2 99.8 100 100

Yellow 1 3 97.9 **98.6 98.6 65.3**

Yellow 2 3 97.9 97.9 97.9 63.9

Yellow 3 2 **99.3** 99.3 99.2

Mag 1 3 97.9 **100 97.2 98.6**

Mag 2 3 81.3 75.0 59.7 70.1

Mag 3 3 97.2 99.3 92.4 91.7

Mag 4 2 **99.3 99.8 99.5**

Mag 5 2 93.5 97.8 96.5

Cyan 1 3 93.1 96.5 **98.6 66.0**

Cyan 2 3 91.7 **100** **98.6** 59.0

Cyan 3 2 **95.3** 98.0 96.5

**Mean of "best" instances: 99.1 99.3 99.6 98.9 96.9 87.3**

**APPENDIX B**

**SUMMARY OF GUIDELINES FOR THE USE OF COLOR ON ATC DISPLAYS**

*1. DISPLAYS NEED TO BE DESIGNED FOR THE TASKS THAT THEY NEED TO SUPPORT
AND THE ENVIRONMENT IN WHICH THEY WILL BE USED.*

All information presented on the display, including the color-coding
scheme, needs to be designed to support specific tasks. Color displays
also need to be designed for the lighting environments in which they
will be used. Design of tower displays requires special consideration
for optimum legibility in bright sunlight and low ambient light.

*2. USE REDUNDANT CODING.*

Whenever color is used to code critical information (such as Conflict
Alert, handoff status, MOA), there must be some other signal to the
meaning other than color (for example, blinking with \"CA\" or \"H\").

*3. LIMIT THE NUMBER OF COLORS THAT NEED TO BE IDENTIFIED TO SIX.*

When color is used for the purposes of assigning a specific meaning to
specific colors (such as red for emergencies, green for aircraft under
my control), it is imperative that no more than six colors (for example,
red, green, blue, yellow, magenta, and white on a black background) be
used. Beyond this, it is difficult to display the colors so that they
are never confusing and it is difficult to remember the entire
color-coding scheme. This does not preclude the use of more than six
colors on a display. However the use of additional colors should be
limited to tasks that depend on being able to tell the difference
between two or more colors that are always present, rather than tasks
that depend upon identifying any single color.

*4. ENSURE ADEQUATE CONTRAST.*

When selecting colors for a display, it is important to consider the
chromatic and luminance contrast that particular colors (foreground and
background) will yeild. Contrast is a key factor in determining whether
or not items on a display will be legible. For items that need to be
read, such as data blocks, a contrast of 8:1 is recommended (but not
necessary) to ensure legibility. For details that do not need to be
read, such as maps and range rings, a contrast ratio of 3:1 (sometimes
less) is acceptable. These guidelines, originally developed by ICAO
(1993), are sound principles that ensure legibility. While these
contrast ratios may not be achievable in all conditions, especially in
the tower, they point to the need for careful testing to ensure
operational suitability of displays whenever these standards may not be
able to be met.

*5. OBEY COLOR CONVENTIONS.*

Red should not mean anything but danger, alert, or warning. This does
not mean that alerts must be red, only that whenever red is used, it
should be used only to convey critical information. Similarly, green
should indicate an \"OK\" status. Yellow is typically used to convey
caution.

*6. PURE BLUE SHOULD NOT BE USED FOR FINE DETAIL OR BACKGROUND.*

From a display point of view, blue is problematic for a number of
reasons. First, when short wavelengths are in focus, all other
wavelengths are slightly out of focus and vice versa; small blue symbols
or text can appear fuzzy. While blue is usually not impossible to read
(usually because of the amount of white or green combined with it), it
is still best to avoid it for text, small symbols, and fine lines,
particularly on a dark background. Dark blue symbols and text would be
usable on a light background, because the contrast would be sufficient
to support reading and other tasks requiring resolution of fine detail.
It is also necessary to avoid pure blue as a background color, although
very dark blues (close to black) or very light blues (close to white)
could be used as background colors, as long as these colors are
carefully designed.

*7. BRIGHT, HIGHLY- SATURATED COLORS SHOULD BE USED SPARINGLY.*

To preserve the conspicuity of high luminance, highly saturated colors
should be used sparingly. Also, these colors should only be used for
critical and temporary information so they are not visually disruptive.
Finally, saturated red and blue should never be presented simultaneously
to avoid a false perception of depth.

*8. USE OF COLOR NEEDS TO BE CONSISTENT ACROSS ALL OF THE DISPLAYS THAT
A SINGLE CONTROLLER WILL USE.*

All of the displays that a controller will use should use the same color
conventions; meanings assigned to individual colors need to be
compatible across displays. For example, if aircraft under my control
are color-coded in one color on the situation display, the same
color-coding should be used for \"my aircraft\" on a conflict probe
display.

*9. COLOR SET SHOULD BE SELECTED FOR EACH TYPE OF MONITOR AND FOR THE
AMBIENT ENVIRONMENT.*

The precise definition of the colors that are maximally distinct need to
be defined for each monitor to ensure that colors are never confused.
Selection of the background color is also an important consideration.
For dimly lit environments, such as TRACONs, a dark background is
preferred, although an absolute black background should be avoided
because of the glare that it invites. A light background (such as light
grey) can offer better contrast and significantly reduce the problem of
glare. For this reason, it may be preferred for the tower environment
during the day, with the understanding that the number of usable colors
is lower for a light grey background than for a dark grey background.

*10. ALL COLOR SCHEMES SHOULD BE TESTED BEFORE IMPLEMENTATION.*

Any implementation of color will need to be tested in the environment in
which it is intended to be used. Prototype testing of individual color
schemes (such as one for weather) is highly recommended, but does not
lessen the need to test the display in its entirety. Coding schemes for
weather and conflict alert, for example, may be successful when tested
independently, but may be incompatible and confusing when presented
together. The entire display (particularly integrated displays) must be
designed and evaluated as a whole and not as a combination of parts.

**APPENDIX C**

**DETAILED COMMENTS FROM THE ODID IV REPORT**

The first ODID IV simulation (Graham et al., 1994) evaluated some of the
human factors aspects of an ATC system that uses advanced automation
tools and electronic flight strips. This simulation did not record any
data as it relates to controller performance using the color display,
but recorded the controllers' comments relating to the use of color in
this system.

Some of the positive comments with respect to color included:

> "ODID participants felt that the use of color can support the ATC
> task, specifically when determining task priorities and recognizing
> coordination or urgency situations." (p. vii)
>
> "Participants found the role of color-coding in the radar window to be
> positive. The use of color helped to determine task priorities, e.g.,
> when an aircraft makes its first call the controller searches only the
> pink radar labels." (p. VII-8)
>
> "Participants would like to maintain the use of red and yellow to
> distinguish conflict and risk of conflict information." (p. VII-10)
>
> "The consensus of opinion indicates that a color-coded and interactive
> radar label provides a positive and exploitable interface for
> controller/system dialogue." (p. 46)
>
> "The controllers stated that the color-planning states, coordination
> indications, urgency situation warnings, and label shape assisted them
> significantly in their control task." (p. 46)
>
> "The use of red to indicate short-term conflict alert immediately
> attracted the controllers attention." \[Recall, however, that no
> performance data were obtained to verify this subjective report.\]
> "However, the usefulness of this warning was lost due to the
> illegibility of the red call sign. Consideration was given to the
> other colors to overcome this problem, but the participants felt that
> red should be retained due to its special significance and that a
> readable version could be defined." (p. 32)
>
> "The warning color (yellow) was valued as a highlighting tool." (p.
> 32)
>
> "The use of color to indicate significant operating areas was
> considered to be extremely useful and visually "distinctive,"
> requiring little interpretation." (p. 32)
>
> "All of the participants considered the "warning" selection (yellow
> call signs on conflict pairs) to be an extremely useful method for
> highlighting conflicts. It was suggested that this function would be
> improved if the warning could also be deselected on individual call
> signs." (p. 70)

Some of the cautionary comments on the use of color included:

> "The use of too many colors for primary functions can make it
> difficult for the controller to assimilate the meaning of individual
> colors." (p. vii)
>
> "The red call signs resulting from activation of the short-term
> conflict alert function were difficult to read. The red color gave the
> call sign a "halo" effect." (p. VII-7)

"Controllers did not like the use of large blocks of strong color which
were considered to be distracting and without meaning. It was often
difficult to read call sign data due to overlapping text." (p. 98)

> "Controllers in the ODID simulation "appeared to be looking only for
> color \[in the conflict alert tool they were provided\] to indicate a
> traffic problem. This appears to have resulted in lack of memorization
> of textual and graphical \[flight\] detail which could reduce their
> traffic awareness." (p. 98)

In the conclusion, the following comments were offered on the use of
color:

> "The ODID IV participants felt that the use of color supports the ATC
> task, specifically when determining priorities such as identifying an
> aircraft's planning status or recognizing coordination and urgency
> situations." (p. 87)
>
> "The use of too many colors for primary functions can have a negative
> outcome. Controllers found it difficult to assimilate color-planning
> states following the introduction of a fifth-color state in the
> approach area." (p. 87)
>
> "In general, displaying several colors within the radar data block
> appears to be acceptable, but there are potential areas of concern.
> For example:
>
> the use of similar colors (shades) of white to identify the speed
> vector and the lead line (link between data block and position symbol)
> caused confusion in maintaining traffic identity (the speed vector of
> one aircraft could appear to link data blocks and position symbols
> belonging to other aircraft);
>
> the use of white to indicate coordination in the radar label
> occasionally confused the EC \[their equivalent to our radar
> controller\] as it was not apparent without reference to the message
> windows whether the coordination was incoming or outgoing. ECs
> \[controllers\] were observed trying to accept outgoing coordination."
> (p. 88)

**GLOSSARY**

> **ambient light** Light originating from sources other than the
> controller's visual display. The general level of illumination in the
> control room due to sunlight, overhead lights, or individual lamps at
> the workstation.
>
> **brightness** The appearance of a display, or part of a display that
> ranges from "dim" to "bright". The perception that varies (although
> not (linearly) with display illumination.
>
> **color** A visual sensation that can result when light waves of
> various wavelengths strike the retina. The main determinants of this
> perceptual experience are hue, saturation, lightness, and brightness.
>
> **color contrast** A comparison of a foreground image to its
> background based on their relative hue/saturation ratios.
>
> **color discrimination** The task of discerning the difference between
> two or more colors. The ability to differentiate between colors that
> are present at the same time. Compare to *color identification.*
>
> **color identification** The task of associating a unique color name
> with a specific display color whether or not it is the only color
> present**.**
>
> **contrast** (also known as luminance contrast) The difference in
> luminance between foreground objects and their background. See
> *contrast ratio*.
>
> **contrast ratio** A measure of luminance contrast. The luminance of
> the foreground divided by the luminance of the background.
>
> **hue** The component of the experience of color, primarily associated
> with different wavelengths, that is the primary determinant of the
> color name such as red, green, yellow, etc.
>
> **illumination** The amount of light striking a surface. Measured as
> the output level of the light source (in candlepower) divided by the
> square of the distance of the observer from the source. See also
> *ambient light.*

intensity The strength of an input or stimulus. In vision, intensity is
a main determinant of the perception of "brightness"; in audition, it is
associated (again, not 1:1) with "loudness."

> **legibility** The extent to which alphanumeric characters and text
> are easy to read.
>
> **luminance** The amount of light reflected from a surface in the
> direction of an observer.
>
> **redundant coding** Using more than one means or cue to convey the
> same information. For example, if conflict alert was conveyed by
> color-coding *and* blinking the data block, color would be used
> redundantly with blinking.
>
> **refresh rate** The number of cycles per second that the displayed
> contents of the computer screen are periodically regenerated (usually
> by a scanning electron beam).
>
> **saturation** The extent to which a hue is "pure", as opposed to
> mixed with white, black, or grey. Saturated colors such as pure red
> appear "vivid", while desaturated colors such as pink (red mixed with
> a lot of white) appear "washed out."

# REFERENCES {#references .unnumbered}

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Ambient Light Conditions and at User Adjustable Screen Brightness
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1991.

American National Standards Institute (ANSI), 1988. American National
Standard for Human Factors Engineering of Visual Display Terminal
Workstations. ANSI/Human Factors Society Standard No. 100-1988. Santa
Monica, CA: The Human Factors Society, Inc.

Campbell, P., White, R., and Hamilton, A., 1990. *The Use of Colour on
Air Traffic Control Displays.* Transport Canada, Publication Number TP
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the Human Factors Society 23^rd^ Annual Meeting]{.underline}, pp.
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John Wiley and Sons, Inc.

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EN09, *Controller Interaction and Task Analysis.* CDRL EN09. Revised
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Graham, R.V., Young, D., Pichancourt, I., Marsden, A., and Irkiz, A.,
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Crown Communications, Washington, D.C.

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[^1]: As a matter of form, clarifying comments from the author will be
    put in brackets. Thus, any material in brackets is not part of a
    direct quote.

[^2]: You can view this effect, known as small field tritanopia, for
    yourself on page 95 of Cardosi and Murphy (1995) or in the booklet
    of the CD. You can also view it by looking at the arrival/departure
    monitors at the new terminal at Ronald Reagan National Airport in
    Washington, D.C. or on the upper level at Dulles International
    Airport. As of this writing, some of the lines on these monitors are
    printed in white and some are printed in yellow. From a distance,
    these two colors look the same.

[^3]: Whether or not a display will appear to flicker depends on many
    variables, such as the amount of light emitted by the display,
    ambient illumination, age of the observer, and others. Agaki and
    Kelly (1991) found that in an office lighting environment where
    observers were allowed to set their own screen brightness levels, a
    refresh rate of 75 Hz was required for the monitor to appear
    flicker-free for 90% of the observers.

[^4]: You do not have to minor in the CIE color diagram to understand
    the basics of what was done; it is only important to know that it is
    a standardized way of describing colors. However, for those of you
    who may be after my job, a chromaticity diagram and an example of
    its use can be found in Cardosi and Murphy (1985), p. 91-92.

[^5]: The McAdam ellipses were not needed to determine potential
    confusable color spaces because of the separation between the
    candidate colors, although the distances between the colors were
    measured using deltaE\* and found to be acceptable.