Avmedia-Article
Aviation Colour Perception Standard: A Time to
Re-Asses
Arthur
M. Pape, (MBBS).
(This
article was published in September 1997 in „Avmedia“, the journal of the
Aviation Medicine Society of Australia and New Zealand)
(This
paper has been submitted to the Office of Aviation Medicine in response to the
call from Dr. Jeff Brock, the Director, for submissions on the Aviation Colour
Perception Standard)
*
|
Inhalt |
|
|
(1) Introduction |
|
|
(2) Colour
Coding |
(6) Conclusions |
|
(3) The Pilot |
(7) Submissions |
|
|
With the publication in the April issue of AVMEDIA of the article by Dr.
Barry Clark on the subject of the aviation colour perception standard (ACPS),
it is indeed an appropriate time to address this matter from an opposite
perspective. My interest in colour vision is a personal as well as a
professional one, being a colour-defective (deuteranopic) medical practitioner,
and the holder of an Australian CPL and Command (multi-engine) Instrument
Rating. I had the honour in the late eighties of leading the appeals against
the standard in the Administrative Appeals Tribunal in two landmark cases that
forced a major de-restricting of colour-defective pilots in their civil
aviation pursuits in Australia.
The cases referred to are the following:
Dr. Clark’s paper is indeed an excellent tutorial on the clinical types
of colour-defective vision and on the test methods available for the
quantification of those clinical types. But his foray into the field of
aero-medical colour vision requirements is flawed. He is still fighting
yesterday’s fight. Firstly, it is extremely dated, relying on data from old
studies, from an era preceding the significant recent advances in perception
psychology and neuro-physiology of perception. For instance, it is now widely
recognized that neural processing of visual stimuli involves complex
information processing, beginning at the retina and extending throughout the
visual pathway to the primary visual cortex and beyond. This complex processing
involves, it has been proposed, the segregated analysis of aspects of visual
stimuli such as luminance, wavelength, retinal disparity, motion and form. The
retina, with its sixteen million cones is but a part of a highly complex
structure involving billions of neurons that transform light stimuli into the
meaning and structure through which we perceive and respond to our complex
environment.
Dr. Clark advances his essentially political views with disregard for
such advances in knowledge. Not only has he not considered the implication of
these new insights into the physiological basis of visual perception with
respect to simple perceptual tasks, further still he has not considered the
implications for complex tasks, such as piloting an aircraft.
Secondly, his selection of data is selective. He all but ignores the AAT
cases to which I refer, and which have so dramatically influenced the
implementation of the ACPS in this country, and indirectly in others.
Considered together, these
cases represent the most comprehensive review ever conducted anywhere in the
world of the ACPS. There is a link from the AOPA web page at http://www.aopa.com.au to the
text of the corresponding decision documents. Since these pivotal decisions,
the Australian Civil Aviation Safety Authority, under its various previous
names, has issued waivers against the standard, up to and including the CPL,
subject to an „Australian airspace only“ endorsement. In the years since 1989
there have been significant numbers of colour-defective pilots applying for
medical certificates to allow the exercise of ATPL privileges. On a case by
case basis, the Office of Aviation Medicine has granted some of these requests,
sometimes with and sometimes without, specific weight and airspace
restrictions. In all cases, the applicants have had to demonstrate extensive
and outstanding levels of experience.
Through Dr. Jeff Brock, I have been briefed on
international meetings he has attended where the ACPS has been on the agenda. I
am indebted to Jeff for his cooperation in this matter. It is clear from these
briefings that there is no international consensus on this issue. Indeed, the
gap appears to be widening between the Europeans on the one hand and the USA,
Canada and Australia on the other. This gap, I suggest, is a reflection on the
more autocratic nature of the European regulators. There is no instrument in
Europe equivalent to our Administrative Appeals Tribunal, and the European AOPA
bodies are far less pro-active than is Australia’s. The opportunity, therefore,
to force independent review of standards such as the one under discussion is
severely limited in the European environment.
Dr. Clark’s arguments in favour of the ACPS,
which are typical of those used by proponents of the standard, may be
summarized as follows:
Dr. Clark states that „colour coding is
ubiquitous in aviation, with many cases of functional colour coding“. In
support of this broad generalization, he provides an extensive list, which is
quoted verbatim:
Significantly perhaps, he does not mention the tower signal gun (Aldis
Lantern), which employs red, green and white to convey instructions to
non-radio aircraft. This use of colour forms the basis of the „practical test“
used in the USA to allow waivers against the standard.
Apart from the purely military items (NVGs and refueling tankers), the
above list is essentially the same as that which the AAT was asked to consider
in 1989. The AAT decision documents referred to above summarize the evidence
presented to it in relation to each of the listed items. It is important to
stress that in arriving at its decision, the AAT considered evidence contained
in most of the references that Dr. Clark used in preparing his paper (but which
were not published with the paper). I find it most telling that Dr. Clark makes
only the tiniest direct reference in his paper to the entire AAT review of the
colour perception standard, so conscientiously and painstakingly performed.
The above list is typical of many in written papers devoted to justifying
the standard. In visual presentations, a similar but cruder device has been
employed. I recall a presentation at one of the DAME seminars some years ago
where a well-known aviation ophthalmologist flashed some colourful and dramatic
slides of military jets and complex, multi-coloured, instrument panels. This
was followed by the statement that „clearly you would be lost in this
environment if you were a colour-defective.“ The entire exchange lasted no more
than a minute or two. The fate of such ad-hoc testimony in a courtroom is not
difficult to predict.
Dr. Clark makes no attempt to define any aspect of or to explain the
meaning of the terms he uses. For instance, the terms „colour coding“ and
„functional colour coding“, clearly pivotal to any argument on this subject,
are neither defined nor explained. Instead, the reader is left to be swayed by
the briefest excerpts from a very lengthy bibliography, whose word count alone
is over 1700 words, but which is not published alongside the article. Many of
the references were subjected to analysis, however, in the AAT cases already
referred to. Many were found to be entirely irrelevant to the matter under
contention. Others were found to be poor excuses for scientific investigation.
Of special note in the context of Dr. Clark’s
paper is his use as a reference of the work by Cole and Macdonald.
Dr. Clark repeats the claim that this work
demonstrated that“
„Colour vision deficients are slower than colour
normals at responding to redundantly colour-coded EFIS displays, and they make
more errors. Protanopes are especially disadvantaged in responding to red ‚fail’ messages.“
Dr. Barry Clark (1997)
Avmedia-Article
He would, no doubt, be aware of the severe
criticism this project received from expert witnesses and admissions by the
authors themselves that the research was technically flawed. The Cole and
Macdonald papers emerged from the AAT discredited. The work had serious
deficiencies with respect to: candidate selection; failure to properly match
groups in terms of age and educational backgrounds; and the conclusions drawn
were not supported by the data. In addition, the simplistic tasks the subjects
were required to perform bore no resemblance to the practical purpose for which
the equipment under examination was intended. The Cole and Macdonald papers
would make an excellent exercise for students of scientific method.
That Dr. Clark was not persuaded by the eventual
outcome in the AAT is clearly evidenced in his scant reference to these
important cases. His one reference to them is quoted as follows:
„In Australia, six errors with the modified Farnsworth
lantern was once the upper limit for civil night flying eligibility. However,
an Administrative Appeals Tribunal case in 1989 by an emigrant commercial pilot
from New Zealand resulted in dichromats becoming eligible to hold Australian
commercial pilot licences but not airline pilot licences.“
Dr. Barry Clark (1997)
Even this short deference is factually wrong,
which is surprising, given Dr. Clark’s extensive personal participation at the
hearing. Denison was then and is still in fact an Australian citizen, not „an
emigrant commercial pilot from New Zealand“. The reduction of the significance
of these landmark hearings to a few lines betrays the bias that is evident in
Dr. Clark’s public contribution to the debate.
To determine the meaning and value or otherwise
of „colour coding“ in Dr. Clark’s „ubiquitous“ list quoted above, the reader
should first determine exactly what the term means. This defining step, avoided
by Dr. Clark, is essential to progress any argument.
Firstly, the definition of the term „code“
(from Webster’s dictionary) could be:
„Any system of symbols for meaningful communication
[…]. A system of standardized signals for mechanically conveying information
bewteen points seperated by a finite distance.“
Webster’s Dictionary (1997)
Readers would be familiar with the Morse code
system, which employs three elements or „symbols“ (dots, dashes and pauses) to
convey messages by either electronic means or light signals. The code requires
that the sender and the receiver understand the meaning of each of the possible
combinations of the three elements, and for both the sender and receiver to
share a common language. A particular combination of the elements always has
the same unambiguous meaning. There are many combinations used, one for
every letter of the alphabet and for every numeral from zero to nine: a total
of thirty-six combinations. Just like the letters and and numbers of the
language, the code cannot convey meaning until the components are compiled into
a language. So here then is what at first seems a „simple“ code, but which in
reality is a complex entity that can only contribute to meaningful
communication in the context of a language.
With „colour coding“, the „symbol“ is light, either
emitted or reflected, with meaning attributed to the colour (i.e. wavelength)
property of the light.
(It is technically extremely difficult to create light displays that are
iso-luminous, in which the only variable is wavelength. Wavelength
variations in practical displays are always accompanied by changes in
brightness or luminance.)
The simplest of colour codes, which I label CC1,
consists of a single display element such as a single light that
changes colour when the meaning is changed. A CC1 type code is employed in the
battery charger for my cellular telephone. A small, single LED mounted on the
charger has three possible displays:
This device uses colour to provide essential
information if use of the cellular telephone is to be reliable. The meanings
are unambiguous. A simple code for a simple task demanding simple recognition
and response. Or is it? The single light does not exist in isolation. It is
mounted on a structure, made of black plastic, which was supplied at the time
of purchase of the cellular telephone. After the purchase, the handbook was
studied and the item was identified as a battery charger for a very specific
appliance. In other words, the meaning of the simplest of colour code examples
is non-existent without an appreciation of the context in which the code
is used. And what of the response? Understanding the code within its
context is then followed by appropriate action. „If the light is red, wait
until it turns green and then remove the battery from the charger and replace
it in the telephone“ could exemplify appropriate behaviour in response to this
code.
The point to be made is that in this instance
of the simplest possible of colour coding examples, it is the response that is
of the utmost importance, not the recognition of a colour, although the two are
clearly related in CC1 colour usage.
The second type of „colour coding“, which I
shall label CC2, uses a different mechanism. Here the colour of
the light is fixed and meaning is conveyed by the light being either on or off.
There are many examples available from everyday objects: lights on electric
kettles, television and radio sets, dashboard lights of many descriptions in
motor vehicles and so on. In this the choice of colour may be determined by
some convention (for example that red means danger and green safety) but the
essential information is conveyed by the on or off status of the light itself..
Road traffic signals are a complex example of
CC2 coding. Consider the many elements that make up even the simplest of road
traffic signal arrays. As in the example given for CC1 type coding, a lengthy
discussion could be gone into on the questions of context and appropriate
response. Indeed, such a discussion could be the subject of an entire paper in
itself. Such arrays are typically mounted on distinctive backgrounds, on
equally distinctive support structures, and in defined geometric patterns. In
addition there are multiple other components such as „walk“ and „don’t walk“
signs and a host of various arrows for „turn“, „don’t turn“ and so on.. Suffice
it to say that the colours employed in traffic signal systems, have no meaning
unless the the context is first understood and consideration is given to the
many other factors that determine what an appropriate response might be to a
given display. It is the totality of the structure, the perception of which
defines for the observer that the light he is seeing is indeed a traffic signal
light. If a person from a culture where road traffic signals didn’t exist were
to be given instructions to „Go when you see a GREEN light, Stop when you see a
RED light and Be Careful when you see an AMBER light“ the result would
predictably be chaotic.
Thirdly, in what I shall label CC3,
colour is used to delineate and differentiate between elements of a display.
This usage of colour may be seen in maps, circuit diagrams and plans. There is no
specific connotation embodied in the choice of colour, and this usage typically
involves a larger number of colours than is used in examples of CC2 type codes.
And here, once again, the question needs to be considered as to what is the
context of the display, and what action or understanding is required from the
user in interpreting the information
Since this paper is confined to considering the
question of the Aviation Colour Perception Standard, it is necessary to examine
what it is that is expected of pilots. Only once this has been discussed, can
we progress to consideration of the role of colour coding within any meaningful
context. What do pilots do and how do they do it?
Pilots fly aeroplanes. By definition, these
machines operate in a three-dimensional environment, unlike motor vehicles or
ships that are restricted to a navigating on two-dimensional surfaces. In
acquiring the skills needed, pilots study and learn much more than mere
manipulative skills. The learning and training equip the pilot to understand
and deal with a wide variety of normal and unusual flight conditions. At the
end of the training, the pilot will be assessed by other pilots of equal or
greater experience and skill, and will be judged as fit (hopefully) to hold a licence.
The process relies entirely on the demonstration of necessary ability and
knowledge.
In a typical flight, the pilot will begin with
a deal of time in planning the flight, equipped with weather and other briefing
materials. Charts will be studied, the route chosen, the fuel requirements
calculated and the departure and destination airport layouts examined. During
the various phases (take-off, climb, cruise, descent, approach and landing) the
pilot will be busy monitoring aircraft systems, navigation equipment, weather,
performance and will be on the lookout for unusual conditions. Except for
controlled contact with the ground before takeoff and after landing, the
greatest demand is that no accidental contact be made with any solid object
outside of the pilot’s aircraft.
The pilot’s job is indeed a highly complex one,
involving extremes of perception and manipulative skills. It is within this
framework that the role of colour coding must be measured, and conversely the
effective handicap of defective colour vision determined. This is so
fundamental a point that I shall risk laboring the point again. It is simply
invalid under any notion of what does and what doesn’t constitute genuine
„scientific study“ to measure the consequence of defective colour vision in the
work that pilots perform by using experimental settings that are unrelated to
the work-place of the pilot and using subjects that are ignorant of what pilots
do.
The AAT, in coming to the conclusions that it did,
relied extensively on the testimony of practical and experienced pilots. It
rejected contrived and irrelevant „experiments“ that amounted to de-facto
colour vision testing, and that had no regard for the complex environment, the
high level of knowledge and cognitive skills that are inherent in the
occupation of aircraft pilot.
„Normal“ colour vision relies on the unimpaired
function of three distinct cone pigments (hence the term „trichromatic“). Like
the cones themselves, the pigments occur in greatest abundance in the area of
fovea. The plot of the sensitivity of each pigment to various wavelengths of
visible light forms a typical bell shaped curve. One has its peak in the red
zone of the spectrum, the next in the green and the third peaks in the blue.
There is extensive overlap of the three resulting sensitivity plots. Genetic
coding for the „red“ and the „green“ sensitive pigments resides on the
X-chromosome (and is therefore sex-linked), whilst that for the „blue“ sensitive
pigment resides on an autosomal (i.e. non sex-linked) chromosome.
Abnormal colour vision occurs when there a
reduction in function of one or other of these three pigments. When the red
pigment function is reduced, the individual is classed as „Protan“ (derived
from the Greek root PROTOS, meaning „first“). If the degree of
dysfunction is less than total, the sufferer is said to be „protanomalous“.
When total dysfunction is present, the individual is classed as „protanope“.
They are not separate conditions, but variations of degree of the dysfunction.
Dysfunction of the green pigment puts the
individual into the „Deutan“ group (derived from the Greek word for
„second“). In the same manner as above, deutans can be subdivided according to
the severity of the dysfunction into deuteranomal and deuteranope
subgroups, where the latter demonstrate complete green receptor dysfunction.
Exactly the same applies to the blue pigment resulting in the terms tritanomal
and tritanope.
The sex linked (i.e. red and green dysfunctions)
occur in approximately 8 to 10 percent of the male population. The incidence in
females is between 0.6 and 1 percent. (As an aside, I have one family in my practice where the father is a
deuteranope, the mother is a protanope and they have three daughters who each
have normal colour vision. If you can plot the genes in this combination,
you have come a long way towards understanding the genetics of abnormal colour
vision. If they had a son, could he possibly have been „colour normal“?)
Dr. Clark’s opening paragraph in his article
states:
„Practical interest in the topic arises from the fact
that some individuals who may otherwise be excellent candidates for a professional
flying career find themselves at risk of exclusion because their perception of
colours is reliably different from that of most of the population, i.e. their
vision is colour-deficient. They may believe that they can see very well
indeed, and tend to become vocal about rejection at their selection stage. The
test methods and standards are often queried, and appeals are made on the basis
of unfair and unnecessary discrimination.”
Dr. Barry Clarke (1997)
What Dr. Clark is too polite to say is that
colour-defective people often deny they are colour-defective. I was one such
person early in the piece. But denial is not the issue (see below). There are
many instances known to me where the very first time a candidate learned of
being colour-defective was at an aviation medical examination. The point lost
on Dr. Clark is that the denial was based on having lived and navigated through
life successfully in blissful ignorance of the „importance“ of colour coding. We
now have a significant population of colour-defective pilots who can add
successful aviation careers to the list of life’s achievements. One could argue
from first principles for the paramount importance of binocular vision in depth
perception, denying all the while the accuracy and skill of the falcon or
kingfisher in capturing its prey.
One of the reasons colour-defectives have had
such a difficult time historically in aviation is that they typically suffer a
delusion with respect to their colour vision. Prior to my own case in the AAT
in 1987, there were others that had failed. With hindsight, the reason is
plain. In each instance the basis of the appeal by the colour-defective
applicant was that he really wasn’t so terribly colour-defective as was being
made out. In his experience, derived from years of road traffic experience and
from multiple other life experiences where „colour coding“ was considered to be
crucial, the colour-defective felt entirely confident that he could indeed
„see“ the colours, and in turn respond appropriately as the situation demanded.
From the outset, such argument could be easily countered by bringing Ishihara
plates or a Farnsworth Lantern into the courtroom and demonstrating to the
assembled Tribunal members or judges that the applicant was indeed wavelength
crippled.
Add to that the „scientific evidence“ so
typified by Dr. Clark’s paper, and the outcome was a foregone conclusion. In
each and every instance prior to 1987, the applicants received no more than a
vote of sympathy for their efforts. The fundamentals were never even considered.
There is no data that establishes a
causal relationship between colour-defective conditions and increased accident
experience. This applies to both road and aviation studies
I can, and have often done so, take any colour
normal observer for a drive through the most complex traffic situations and
demonstrate without error that I know exactly when the traffic light is
„green“, „red“ or „amber“. The demonstration can include lights at great distances
(well beyond the distance where any practical relevance begins), and include
the many adjunctive components (pedestrian instructions, turn and no turn, give
way and the rest). My driving behaviour would be indistinguishable from that of
any colour normal.
(My apologies for the extensive use of the „first person“ throughout
this paper. Being a deuteranope, a pilot and a medical practitioner, and
having devoted great time and effort in coming to understand this complex
topic, much of the insight has come from personal experience. That is in
itself not an impediment to forming a logical and integrated approach to the
subject matter. I do, however, recognize the risks inherent in such
personal material.).
Yet if the three colours of the traffic signal
system were to be reduced to pin point sources, and presented in a „lantern
test“, I am equally confident I would make significant errors in naming the
colours correctly. Therein lies the paradox.
If
colour-defectives can pass road driving tests and pilot licence tests, and
their behaviour is appropriate in terms of compliance with the rules and
fulfillment of all the practical requirements, then the theoretical paradigm
that underpins the colour perception standard must be questioned.
Yet, the fact is that colour-defectives are
wavelength cripples from the moment of birth. They can never experience
„colour“ as a colour normal would. They confuse colours that are distinctly
different to the colour normal. Their „colour vocabulary“ is small compared to
that of the colour normal. Indeed, colour normals generally find it fascinating
to observe a colour-defective stumble and hesitate over the naming of a colour
that to them is such a simple matter. There is no doubt that colour normals
(and indeed Tribunals) regard colour-defectives with some degree of mistrust.
After all, what does one think of a person who can swear that he „sees the
colours“ when simple demonstrations time and again prove the opposite? I am in
frequent contact with a large number of colour-defectives, and I hear the claim
by colour-defectives repeated over and over that „I can see the colours“. My
first advice to a new colour-defective contact is to cease the denial process.
Then and only then can the relevance with respect to careers be confronted.
It isn’t difficult to see how the mistaken
belief in being able to „see the colours“ might arise. Again, using road
traffic signals as the example, colour-defectives see differences in the
typical three basic lights and respond, as they are required to do. It is
problematic whether the primary „cue“ is wavelength, luminance or position.
What matters is that they understand the intent of the signal, which they do
and do consistently. Just like their colour normal counterparts, they make the
assumption that it is the „colour“ that conveys the meaning of the signal, and
have no regard at a conscious level for the vast amount of other information
that is available and indeed crucial to the outcome of the appropriate
response. Intuitively, this leads the colour-defective to believe that he can
„see the colours“. But if the light of the same wavelength is displayed in a
way that is devoid of context, such as is the case in lantern tests, time and
time again the colour-defective will make errors in correctly naming the
colours.
Lantern tests generally prove the existence of
colour-defective vision. That is what they are designed to do and they do it
well. The many other tests referred to By Dr. Clark confirm and quantify the
extent of the defect. Many papers have been written about the merits of one
lantern over another. For instance, in cases prior to the Pape and Denison AAT
appeals, much reliance was placed on the fact that the Farnsworth Lantern was
not truly representative of „aviation“ coloured lights. It had been designed
for testing prospective submariners, not pilots. This gave candidates some
glimmer of hope that they might obtain relief. But it inevitably proved to be
irrelevant to the main issue.
What distinguished my case and the Denison case
from its predecessors was that we pleaded „guilty“ to being colour-defective
and then fought the case on the basis the irrelevance of defective colour
vision to the things a pilot does.
It is well beyond the scope of this paper to
examine each and every instance provided by Dr. Clark in his long list above.
Again, the AAT did just that in a most comprehensive way. However, the reader
can go through the list and ask, for each example determine what type (CC1, CC2
or CC3) of usage of colour is involved. I shall give a few short examples: