John Barbur

Marisa Rodriguez-Carmona

Applied Vision Research Centre

Northampton Square

London, EC1V 0HB, UK

Sally Evans

Civil Aviation Authority

Gatwick Airport South

Gatwick, RH6 0YR, UK

Nelda Milburn

Civil Aerospace Medical Institute

Federal Aviation Administration

Oklahoma City, OK 73125

June 2009

Final Report

Minimum Color Vision Requirements 

for Professional Flight Crew, Part III:

Recommendations for New 

Color Vision Standards

DOT/FAA/AM-09/11

Office of Aerospace Medicine

Washington, DC 20591

OK-09-0434-JAH

Federal Aviation

Administration
NOTICE

This document is disseminated under the sponsorship 

of the U.S. Department of Transportation in the interest 

of information exchange. The United States Government 

assumes no liability for the contents thereof.

___________

This publication and all Office of Aerospace Medicine 

technical reports are available in full-text from the Civil 

Aerospace Medical Institute’s publications Web site: 

www.faa.gov/library/reports/medical/oamtechreports


Technical Report Documentation Page 

1. Report No. 2. Government Accession No. 3. Recipient's Catalog No. 

DOT/FAA/AM-09/11 

4. Title and Subtitle 5. Report Date 

Minimum Color Vision Requirements for Professional Flight Crew, June 2009 

Part III: Recommendations for New Color Vision Standards 6. Performing Organization Code 

 

7. Author(s) 8. Performing Organization Report No. 

Barbur J1, Rodriguez-Carmona M1, Evans S2, Milburn N3 

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS) 

 

11. Contract or Grant No. 

1

Applied Vision Research Centre 

Northampton Square 

London EC1V 0HB, UK 

2

Civil Aviation Authority 

Gatwick Airport South 

Gatwick, RH6 0YR, UK 

3

FAA Civil Aerospace Medical Institute 

P.O. Box 25082 

Oklahoma City, OK 73125 06-G-003 

12. Sponsoring Agency name and Address 13. Type of Report and Period Covered 

 

 

 

Office of Aerospace Medicine 

Federal Aviation Administration 

800 Independence Ave., S.W. 

Washington, DC 20591 14. Sponsoring Agency Code 

15. Supplemental Notes 

Work was accomplished under approved task AM-521. 

16. Abstract 

This report describes the findings of the third phase of the project sponsored by the United Kingdom Civil 

Aviation Authority on “Minimum Color Vision Requirements for Professional Flight Crew.” This third part of 

the project, “Recommendations for New Color Vision Standards,” involved collaboration and co-sponsorship by 

the Federal Aviation Administration. 

Minimum color vision requirements for professional flight crew have been established by assessing the level of 

color vision loss above which subjects with color deficiency no longer perform the most safety-critical, colorrelated tasks within the aviation environment with the same accuracy as normal trichromats.

The new CAD (Color Assessment & Diagnosis) test provides accurate assessment of the applicant’s color vision. 

The results of the test establish with high specificity whether the subject’s red-green and yellow-blue color vision 

performance falls within the normal range and the class and severity of color vision loss in subjects with color 

deficiency. The results of the test also indicate whether the applicant’s color vision meets the minimum 

requirements for safe performance that have emerged as necessary from this investigation.

If the new, experiment-based, pass/fail color limits were adopted as minimum requirements for professional flight 

crew, 36% of deutan subjects and 30% of protan subjects would be classified as safe to fly. Given the higher 

prevalence of deutan deficiencies, these findings suggest that 35% of color deficient applicants would be classified 

as safe to fly.

 

17. Key Words 18. Distribution Statement 

 

 

Color Vision, Pilot Screening, Colour Deficiency, CAD 

Test, PAPI Lights, Signal Lights 

Document is available to the public through the 

Defense Technical Information Center, Ft. Belvoir, VA 

22060; and the National Technical Information 

Service, Springfield, VA 22161 

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price 

Unclassified Unclassified 47 

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized 

iii

Acknowledgments

We thank Dr. Anthony Evans (ICAO) for his active participation in this project and continued support with many 

aspects of this investigation. We also acknowledge the valuable contribution each of the persons listed below has made 

to the project through progress review meetings, valuable criticism, and comments on the final report: 

Ms. J. Birch City University, UK

Mr. A. Chorley CAA Chief Optometrist, UK

Dr. D. Connolly Qinetiq Ltd., UK

Dr. H. Courteney CAA, UK

Dr. A. Evans ICAO Montreal, Canada

Mr. S. Griffin CAA, UK

Mr. A. Harlow City University, UK

Dr. I. Moorhead Qinetiq Ltd., UK

Ms. M. O’Neill-Biba City University, UK

Dr. J. Pitts CAA Consultant Ophthalmologist, UK

Dr. R. Scott RAF Consultant Ophthalmologist, UK

Mr. S. Williams CAA, UK

Executive Summary

This report describes the findings of the third phase 

of the project sponsored by the United Kingdom Civil 

Aviation Authority (CAA) on “Minimum Color Vision 

Requirements for Professional Flight Crew.” Parts I and 

II have already been published and cover “Use of Color 

Signals and Assessment of Color Vision Requirements,” 

and “Visual Task Analysis,” respectively. This third part 

of the project, “Recommendations for New Color Vision 

Standards,” involved collaboration and co-sponsorship 

by the Federal Aviation Administration (FAA). 

Minimum color vision requirements for professional 

flight crew have been established by assessing the level of 

color vision loss above which subjects with color deficiency 

no longer perform the most safety-critical, color-related 

tasks within the aviation environment with the same 

accuracy as normal trichromats. The new CAD (Color 

Assessment & Diagnosis) test provides accurate assessment of the applicant’s color vision. The results of the 

test establish with high specificity whether the subject’s 

red-green and yellow-blue color vision performance falls 

within the normal range and the class and severity of 

color vision loss in subjects with color deficiency. The 

results of the test also indicate whether the applicant’s 

color vision meets the minimum requirements for safe 

performance that have emerged as necessary from this 

investigation. If the new, experiment-based, pass/fail 

color limits were adopted as minimum requirements 

for professional flight crew, 36% of deutan subjects and 

30% of protan subjects would be classified as safe to fly. 

Given the higher prevalence of deutan deficiencies, these 

findings suggest that 35% of color deficient applicants 

would be classified as safe to fly. 

Background

The use of color in aviation for coding of signals and 

information is important, hence the need to set adequate 

color vision requirements to ensure that flight crew are 

able to discriminate and recognize different colors, both 

on the flight deck and externally. Concern has, however, 

been expressed during the past few years that the current color vision standards, at least within JAA (Joint 

Aviation Authorities) member states, are not appropriate 

since most tests and pass limits only screen for normal 

red/green color vision. Since the incidence of congenital 

yellow/blue deficiency is extremely low (see Table 1), the 

absence of red/green deficiency is virtually equivalent 

to normal trichromacy. Subjects with minimal color 

deficiencies often fail normal trichromacy tests, and the 

great majority are therefore prevented from becoming 

pilots, although many of these subjects may well be able 

to perform safety-critical tasks, as well as normal trichromats, when presented with the same, suprathreshold color 

signals. In principle, these subjects should be allowed to 

fly. To include some individuals with minimum color 

deficiency that may well be safe to fly, some authorities 

have either relaxed the pass limits on tests designed to 

screen for normal color vision (e.g., Ishihara, Dvorine) 

or they have introduced less demanding tests that applicants with mild color vision deficiency can pass. This 

approach does justice to some applicants but not to others. 

Existing, conventional color screening tests employed by 

most authorities cannot be used to quantify accurately 

the severity of color vision loss, and this makes it difficult 

to set reliable pass/fail limits. With very few exceptions, 

no red/green color deficient applicants pass either the 

Ishihara or the Dvorine color screening tests with zero 

errors. The same applies to anomaloscope matches when 

strict criteria are employed (e.g., when the applicant sets 

an appropriate red/green mixture field to match the color 

appearance of a yellow, monochromatic field, as in the 

Nagel anomaloscope). In this respect, these tests are excellent, but as has been shown in several studies, neither 

the anomaloscope results (Barbur et al., 2008) nor the 

Dvorine/Ishihara plates (Squire et al., 2005) can be used 

to quantify reliably the severity of color vision loss. 

When the pass limits are relaxed, the outcome of such 

tests no longer guarantees normal trichromatic performance in the most safety-critical, color-related tasks. The 

FAA guidelines for aviation medical examiners (www.faa.

gov/about/office_org/headquarters_offices/avs/­offices/

aam/ame/guide/) in relation to color vision tests, test-

ing procedures, and pass limits are different from those 

practiced in Europe. The JAA member states employ the 

Ishihara screening test to identify applicants with red/green 

deficiency. No errors on any of the first 15 plates of the 

24-plate Ishihara set are allowed in order to pass. Most 

color deficient observers (both deutan and protan) fail 

this stringent use of the Ishihara test, except for a very 

small number of minimum deuteranomalous that pass. 

In addition, ~15% of normal trichromats (an estimate 

based on 202 normal trichromats examined at Applied 

Vision Research Centre (AVRC)) also fail the Ishihara, 

when one employs the strict CAA/JAA pass/fail criteria. 

Secondary tests such as the Holmes-Wright Type A (HW) 

lantern (used in the United Kingdom) are employed, 

and although some color deficient observers (mostly 

deuteranomalous subjects, see discussion section) pass 

these tests, the severity of their color vision loss remains 

unknown. One advantage of using the HW lantern as a 
vi

secondary test is that normal trichromats pass this test 

and are therefore not disadvantaged when they fail the 

Ishihara test. Mild deuteranomalous subjects that pass 

the lantern tests cannot be distinguished from normal 

trichromats on the basis of these tests. These subjects 

(i.e., the deutan applicants that pass the HW lantern) are 

therefore allowed to fly. All protan subjects fail the HW 

test, but some may have sufficient chromatic sensitivity 

to carry out safety-critical color tasks, as well as normal 

trichromats. It is, therefore, clear that, at least in the UK 

(which employs the HW lantern as the secondary test), 

protans are very likely to be excluded. 

The current procedures within JAA are therefore 

unsatisfactory for at least two reasons. First, there is no 

guarantee that the deutan subjects that pass secondary 

tests can cope with safety-critical, color-related tasks since 

the severity of their color vision loss remains unquantified. Second, many color deficient subjects that can carry 

out such tasks safely fail the lantern tests and will not be 

allowed to fly. There are also other additional problems. 

The pass/fail variability of different conventional, color 

screening tests is high (Squire et al., 2005). Although 

subjects with minimum color deficiency may sometimes 

pass these tests, the results provide no reliable information 

as to the minimum color vision requirements that can be 

considered safe within the aviation environment. 

Another important, practical aspect of regulatory test-

ing of color vision is that aspiring pilots are often highly 

motivated to pass a screening test. The context in which 

the test is undertaken is therefore very different to the 

clinical setting. It is known that, in order to pass the 

Ishihara test, or similar pseudoisochromatic tests, color 

deficient applicants have been known to have learned 

the correct responses, so as to maximize their chances 

of passing the test. It is, for that reason, important to 

eliminate any opportunity of learning the right answers 

to pass a screening test. When used in the recommended 

clinical settings, most of the popular occupational color 

tests exhibit large within-subject and inter-subject variability, even within normal trichromats (Squire et al., 

2005). The recommended surround, ambient viewing 

conditions, measurement procedures, and interpretation of results can vary significantly from country to 

country, even when the same tests are employed. Many 

ICAO member states have different requirements for 

color vision assessment and use different tests. Within 

the JAA, the HW (United Kingdom), the Spectrolux 

(Switzerland), the Beyne (France, Belgium, and Spain) 

lanterns, and the Nagel anomaloscope (Germany) are 

recognized secondary tests. Since the correlation between 

the outcomes of different tests is poor (Squire et al., 2005), 

it is not uncommon for pilot applicants to fail the color 

vision assessment in one country and to pass in another. 

Although such occurrences have not passed unnoticed, no 

adequate solutions have emerged to set minimum limits 

of color vision sensitivity that can be considered operationally “safe” within specified environments. The lack 

of adequate solutions to this problem explains why some 

authorities unknowingly insist on normal trichromacy 

(which is largely what is achieved when current pass/fail 

limits are employed with Ishihara as the primary test and 

HW lantern as the secondary test). 

The FAA guidelines are more liberal and allow for the 

use of various pseudoisochromatic plates (e.g., Ishihara, 

Dvorine, AOC-HRR, Richmond 1983-edition, 15 plates, 

etc.) with relaxed pass/fail limits (e.g., the applicant has to 

make seven or more errors in order to fail the Ishihara or 

Dvorine tests). Alternative tests such as Farnsworth lantern 

(FALANT), Keystone Orthoscope, etc., can also be used 

as acceptable substitutes. As far as the FAA guidelines are 

concerned, the analysis of results is restricted to the FAA 

approved tests that have been included in this investigation (i.e., Ishihara and Dvorine). In addition, the Aviation 

Lights Test (i.e., a modified Farnsworth lantern that was 

developed to screen air traffic controllers, see Fig. 7 in 

full report) was also included in this investigation. 

New Developments

Advances in understanding human color vision 

(Barbur, 2003) and the development of novel methods 

to measure accurately the loss of chromatic sensitivity 

(Barbur et al., 1994) have prompted the CAA to sponsor new studies to examine how color vision loss can be 

measured accurately and also to establish minimum color 

vision requirements for civil aviation professional pilots. 

As a result of the progress made in these studies, it is now 

possible to define the variability that exists within normal 

color vision and to detect with confidence and classify 

accurately even the smallest congenital color vision deficiencies that sometimes pass undetected in conventional, 

occupational color vision tests. More importantly, it is 

now possible to achieve the aim of the project, i.e., to 

quantify the severity of color vision loss and to recommend 

minimum color vision requirements by establishing the level 

of color vision loss when color deficient observers can no longer 

perform the most safety-critical, color-related tasks with the 

same accuracy as normal trichromats. 

A number of developments that have emerged from 

the studies carried out during the last few years have made 

it possible to achieve the aim of this project:

A Color Assessment and Diagnosis (CAD) test that 

employs novel techniques to isolate the use of color 

signals and measures accurately both red-green (RG) 

and yellow-blue (YB) chromatic sensitivity has been 

developed and validated (see Fig. 9).

•
vii

A study that compared outcome measures in the most 

common, occupational color vision tests, in both 

normal trichromats and in a large number of color 

deficient observers, has improved our understanding 

of current limitations. The findings from this study 

also justify the need for a test that can be used to accurately measure the subject’s chromatic sensitivity 

and the variability expected within the color normal 

population (Squire et al., 2005).

The establishment of color discrimination limits for 

normal vision (i.e., the standard normal CAD observer 

based on RG and YB color detection thresholds measured in ~250 normal trichromats) provides a template 

for detection of abnormal sensitivity (RodriguezCarmona et al., 2005; see Fig. 8). In addition, similar 

measurements in over 300 color deficient observers that 

participated in several projects related to color vision 

provided the data needed to describe the differences in 

the severity of color vision loss within deuteranomalous 

and protanomalous observers (see Fig. 12).

Identification of the most important, safety-critical, 

color-related tasks for pilots, and faithful reproduction of such tasks in the laboratory made it possible to 

establish experimentally the safe limits of color vision 

loss. The visual task analysis carried out as part of this 

study identified the Precision Approach Path Indicator 

(PAPI) as the most important, safety-critical task that 

relies largely on color vision. At some airports, color 

signals are also used for aligning the aircraft when approaching the parking area, and in such cases, correct 

color recognition is critical to the safe accomplishment 

of this task. There are many other tasks that involve 

the use of color signals, but they involve larger stimuli. 

The viewing is under more favorable conditions of light 

adaptation and other cues make the color coding less 

critical. In the case of the PAPI lights, it is essential 

that the pilot distinguishes accurately the number of 

“white” and “red” lights. Moreover, the pilot needs 

to recognize the four adjacent lights as “white,” when 

too high, and as “red,” when too low. The PAPI lights 

task is demanding since the lights can be very small 

(i.e., subtend a very small visual angle at the eye) and 

are often seen against a dark background (see Fig. 

18) when color discrimination sensitivity is known 

to be poor. 

Color discrimination limits (based on the CAD test) 

that can be classified as safe for pilots in civil aviation 

have been established. This was achieved by measur-

ing and relating PAPI task performance and color 

discrimination sensitivity as assessed on CAD, signal 

lights, and a number of other color vision tests (see 

description below) in 40 protanomalous, 77 deuteranomalous, and 65 normal trichromats. There are other 

•

•

•

•

visual tasks that can be classed as safety-critical, but 

these generally involve larger and brighter lights, and 

are therefore easier to carry out. These tasks either rely 

on color discrimination (such as the red-green parking 

lights) or, in some cases, the tasks benefit from the use 

of color signals as redundant information (such as the 

“green” runway threshold lights). In addition to the 

red-green parking and the green runway threshold 

lights, there are also a number of other runway lights: 

the red-white centerline lights, the green-yellow lead-off 

lights, and the red stopway lights. The tasks that involve 

these additional lights have not been simulated in the 

laboratory, but, as argued in the main report, they are 

either less demanding in terms of color discrimination, or the color signals are only used to reinforce the 

functional significance of the lights. 

Data showing correlation between PAPI scores and 

CAD sensitivity thresholds are shown in Fig. 23 

for normal, deuteranomalous, and protanomalous 

observers.

Principal Conclusions

Subjects with red/green congenital color deficiency 

exhibit an almost continuous loss of chromatic sensitivity. The loss of sensitivity (when expressed in 

Standard Normal (CAD) units (SN units) is greater 

in protanomalous than deuteranomalous observers 

(Fig. 12). 

When the ambient level of light adaptation is adequate, 

normal aging does not affect significantly either RG or 

YB thresholds below 60 yrs of age (see Fig. 13).

Analysis of PAPI results shows that the use of a modified 

“white” light results in significant, overall improvements in PAPI performance, particularly within 

normal trichromats and deuteranomalous observers. 

The modified (or color corrected white) is achieved 

simply by adding a color correction filter to the standard white lights produced by the source. The filter 

employed in this study decreased the color temperature 

of the standard white (used in PAPI systems) by 200 

MIREDS (micro reciprocal degrees). 

The deuteranomalous subjects investigated in this 

study with CAD thresholds <6 SN units and the 

protanomalous subjects with CAD thresholds <12 

SN units perform the PAPI test, as well as normal 

trichromats.

43 of the 77 deuteranomalous subjects failed the PAPI 

test. 29 out of the remaining 34 subjects that passed 

the PAPI test had CAD thresholds <6 SN units. 

20 of the 40 protanomalous subjects failed the PAPI 

test. 13 out of the remaining 20 subjects that passed 

the PAPI test had CAD thresholds <12 SN units. 

•

•

•

•

•

•

•
viii

A small number of deuteranomalous (5) and protanomalous (7) observers with thresholds higher than 

6 and 12 SN units, respectively, passed the PAPI test. 

All these subjects do, however, exhibit poor overall 

RG chromatic sensitivity in all the other color tests 

employed in the study and are therefore likely to be 

disadvantaged in many other suprathreshold visual 

performance tasks that involve color discrimination. 

The results suggest that subjects with minimum 

color deficiency that does not exceed 6 SN units for 

deuteranomalous observers and 12 SN units for protanomalous observers perform the PAPI test as well as 

normal trichromats. If these findings were adopted 

as pass/fail limits for pilots ~35% of color deficient 

applicants would be classified as safe to fly. 

The administration of the CAD test eliminates the 

need to use any other primary or secondary tests. It 

is proposed that a rapid, reduced version of the CAD 

test (labelled fast-CAD) is administered first to establish whether the applicant passes with no errors the 

•

•

•

6 SN limit established for deutan subjects. Deutans 

represent ~6% of color deficients and 36% of deutan 

subjects pass the recommended CAD limit (see Table 

3). When one includes normal trichromats, ~94% of 

all applicants will pass the fast-CAD screening test and 

be classified as safe to fly. This process is very efficient 

since the fast-CAD test is simple to carry out and 

takes less than 30 seconds to complete. The definitive 

CAD test (which takes between 6 to 8 minutes for RG 

sensitivity) is administered only when the applicant 

fails the fast-CAD screening test. The latter establishes 

the class of color deficiency involved and whether 

the applicant’s threshold is below the pass/fail limit 

established for protan subjects. In addition, the CAD 

test provides the option to test the applicants YB color 

vision. This option reveals whether the applicant’s YB 

discrimination sensitivity falls within the normal range. 

In view of the increased use of color in aviation, test-

ing for normal YB thresholds can also be of relevance 

to aviation safety. 
ix

Contents

1.0 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1.1 The Use of Color in Aviation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1.2 Current Color Vision Requirements and Assessment Methods in Aviation. . . . . . . . . . . . . . . . . 1

1.1.3 Problems Identified With Current Assessment Methods and Procedures. . . . . . . . . . . . . . . . . . 2

1.1.4 A New Approach Based on Recent Advances in Color Vision Testing. . . . . . . . . . . . . . . . . . . . 2

1.1.5 A New Color Vision Test. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2 Identification of the Most Safety-Critical and Demanding Color Vision Tasks. . . . . . . . . . . . . . 2

1.2.1 Other Uses of Signal Lights Within the Aviation Environment. . . . . . . . . . . . . . . . . . . . . . . . . 4

a. Parking Lights. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

b. Runway and Taxiway Lights. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.2.2 Analysis of the PAPI Lights Task. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.2.3 Disability Discrimination. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.3 Brief Description of the Most Common Occupational Color Vision Tests. . . . . . . . . . . . . . . . . 6

1.3.1 Ishihara Plate Test. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.3.2 Dvorine Plate Test. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.3.3 Nagel Anomaloscope. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.3.4 Aviation Lights Test (ALT). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.4 The CAD Test. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.5 Summary of Congenital Color Vision Deficiencies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

1.6 Acquired Color Vision Deficiencies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.0 Subjects and Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.1 PAPI and PSL Simulator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.2 Testing Procedure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

3.0 Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3.1 Computing an Index of Overall Chromatic Sensitivity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

4.0 Discussion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

4.1 Color Vision Concerns in Aviation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

4.2 Advances in Assessment of Color Vision. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

4.3 Safe Color Vision Limits in Aviation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

4.4 Benefit Analysis of Using the New Approach. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

a. Analysis Based on CAA/JAA Pass Criteria and Guidelines. . . . . . . . . . . . . . . . . . . . . . . . . . 30

b. Analysis Based on FAA Pass/Fail Criteria and Guidelines. . . . . . . . . . . . . . . . . . . . . . . . . . . 31

5.0 Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

6.0 References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
List of Figures 

1a: The Precision Approach Path Indicator signal lights and Visual Approach Slope Indicator lights.. . . . . . . . . . . . . . . . . 3

1b: The Precision Approach Path Indicator signal lights and Visual Approach Slope Indicator lights.. . . . . . . . . . . . . . . . . 3

 2a: Photograph of PAPI lights and parking lights.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2b: Photograph of PAPI red and green parking lights.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3: Ishihara pseudoisochromatic plates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

4: Dvorine pseudoisochromatic plates.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

5: Photograph of the Nagel anomaloscope (Model I, Schmidt & Haensch, Germany) and illustration of the Nagel 

anomaloscope split field.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

6: Scatter plot of Nagel matching midpoints versus RGI for 231 observers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

7a, b: Schematic representation and photograph of the Aviation Light Test. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

8: Frequency distributions of the yellow-blue and red-green chromatic thresholds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

9: Data showing the 97.5 and 2.5% statistical limits that define the “standard” normal CAD test observer. . . . . . . . . . . 12

10: Chromatic thresholds for two color vision deficient observers with minimal color vision deficiency.. . . . . . . . . . . . . . 13

11: Chromatic thresholds for two color vision deficient observers with severe color vision deficiency... . . . . . . . . . . . . . . . 13

12: Graph showing red-green and yellow-blue thresholds expressed in CAD Standard Normal units.. . . . . . . . . . . . . . . . 14

13: Effect of age on the YB and RG chromatic thresholds for normal trichromats under normal daylight conditions.. . . . 15

14: Examples of subjects with acquired loss of chromatic sensitivity.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

15: 1931 CIE-x,y color space diagram showing the recommended chromaticity boundaries for the colors of light signals. . 17

16: Schematic representation of PAPI simulator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

17: Graphs showing the CIE-x,y chromaticity coordinates of the red and white PAPI lights under the effect of neutral 

density filtering and current setting of the lamp. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

18: Schematic representation of the Precision Approach Path Indicator simulator test and PAPI Signal Lights Test.. . . . . 19

19: The number of plates read correctly on the Ishihara test (24 plates) is compared to performance on the PAPI 

simulator test separately for normals, deutan and protan color vision observers.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

20: PAPI % correct scores plotted as a function of the number of plates read correctly on the Dvorine test for normals, 

deutan, and protan observers.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

21: The number of presentations identified correctly on the Aviation Light Test compared to performance on the 

PAPI simulator test. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

22: PAPI test scores plotted against an index of red-green chromatic sensitivity based on the Nagel anomaloscope range. 23

23: Graphs showing performance of normal, deutans, and protan observers on the PAPI (standard white) versus CAD 

test sensitivity (1/threshold).. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

24: Graphs showing comparisons between standard and modified PAPI white versus the CAD test sensitivity values.. . . . 25

25: Graphs showing R=W and W=R errors only made on the PSL versus the RG CAD sensitivity.. . . . . . . . . . . . . . . . . . 26

26: Justification for proposed limits on the CAD test. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

27: Summary of deutan subjects’ results if the proposed pass/fail criteria of 6 RG CAD threshold units is accepted.. . . . . 29

28: Summary of protan subjects’ results if the proposed pass/fail criteria of 12 RG CAD threshold units is accepted.. . . . 29

List of Tables

1: Percentage of color deficient observers that fail Ishihara and HW tests.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

2: Predicted outcome per thousand applicants using current CAA/JAR guidelines.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

3: Predicted outcome per thousand applicants using the new, CAD based pass/fail limits.. . . . . . . . . . . . . . . . . . . . . . . . 31

4: Percentage of color deficient observers that fail Ishihara, Dvorine, and ALT tests (using FAA pass/fail criteria).. . . . . . 32

5: Contingency tables showing results of Ishihara, Dvorine, and ALT tests and the corresponding pass/fail PAPI scores.. . . 32

6: Pass/fail scores on Ishihara and Dvorine compared against the ALT.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

7: Predicted outcome per thousand applicants when using Ishihara, Dvorine, and ALT tests.. . . . . . . . . . . . . . . . . . . . . . 33
xi

Definitions

For a relevant list of definitions refer to a previous CAA Paper 2006/04 (CAA 2006a). 

Abbreviations

AGL -------- Aeronautical Ground Lighting

ALT--------- Aviation Light Test 

ATC -------- Air Traffic Control 

ATCS------- Air Traffic Control Specialist

AVRC ------ Applied Vision Research Centre (City University)

CAA -------- Civil Aviation Authority

CAD-------- Color Assessment and Diagnosis test

CIE --------- Commission Internationale de l’Eclairage

CS ---------- Chromatic Sensitivity

FAA--------- Federal Aviation Administration

ICAO ------ International Civil Aviation Organization

JAA --------- Joint Aviation Authorities

JAR --------- Joint Aviation Requirements

L-cones----- Long-wavelength sensitive cones

LC ---------- Luminance Contrast

MIRED---- Micro Reciprocal Degrees 

M-cones---- Medium-wavelength sensitive cones

NTSB ------ National Transportation Safety Board

PAPI-------- Precision Approach Path Indicator

RG---------- Red-Green

S-cones----- Short-wavelength sensitive cones

SI------------ Système International d’Unités (International System of Units)

SN ---------- Standard Normal

UK---------- United Kingdom

YB ---------- Yellow-Blue

Nomenclature

°- - Degrees

cd m-2- - Candelas per square meter

λ- - Wavelength (lambda), nm

λmax- - Maximum (peak) wavelength of V(λ)

%- -- -- -- -- Percent

2’ arc- - 2 minutes of arc

A- - Ampere (amp) unit of electric current

km- - Kilometer

K- - Degrees Kelvin

mm- - Millimeters (1 mm=10-3 of a metre)

M- -- -- -- -- Mired (unit of measurement to express color temperature = 106 K)

nm- - Nanometers (1 nm=10-9 of a metre)

s- - Second (time)

μ- - Micro=x10-6

V(λ)- -- -- -- Standard photopic luminous efficiency (for high ambient illumination) (CIE, 1924)

V’(λ)- - Standard scotopic luminous efficiency (when very low light levels are involved) (CIE, 1951)

Minimum Color Vision Requirements for Professional Flight Crew, 

Part III: Recommendations for New Color Vision Standards

1.0 Introduction

Occupational color vision standards were introduced 

in aviation in 1919 by The Aeronautical Commission of 

the International Civil Air Navigation Authority. These 

standards reflected both the needs and the methods available for color vision assessment at the time. Concern has 

been expressed during the last few years that the current 

JAR (Joint Aviation Requirements) color vision standards 

may be too stringent and, at the same time, also variable. 

The tests employed do not always reflect the tasks pilots 

encounter in today’s aviation environment. An examination of current standards and techniques employed to 

assess color vision requirements suggests the need for a 

more unified color vision test to provide a measure of color 

vision loss that relates directly to the most safety-critical, 

color-related tasks within the aviation environment (Cole, 

1993). The current color vision standards and accepted 

JAA (Joint Aviation Authorities) color vision tests for 

professional flight crew have been reviewed by the United 

Kingdom Civil Aviation Authority (UK CAA). This report follows other CAA documents published in 2006: 

“Minimum Color Vision Requirements for Professional 

Flight Crew: Part 1. The Use of Color Signals and the 

Assessment of Color Vision Requirements in Aviation, 

and Part 2. Task Analysis.”

1.1.1 The Use of Color in Aviation

The use of color in the aviation environment is 

important since it makes possible the efficient coding 

of signals and information and this, in turn, enhances 

visual performance, provided the subjects can make use 

of color signals. Humans with normal trichromatic color 

vision possess three distinct classes of cone photoreceptors. These contain short (S), middle (M), and long (L) 

wavelength sensitive photopigments with appropriate 

peak absorption wavelengths (λmax). Variant L- and/or 

M-cone genes can cause significant shifts in λmax and this, 

in turn can cause large changes in chromatic sensitivity. 

In addition to λmax changes, other factors such as the 

amount of pigment present in photoreceptors can also 

affect chromatic sensitivity. Red/green deficiency is the 

most common type and is caused by either the absence 

of or the abnormal functioning of L- or M-cones. The 

corresponding condition is normally described as protan 

or deutan deficiency, respectively. Color vision deficiency 

affects approximately 8% of men and less than 1% of 

women (see table in section 4.4).

Aviation accidents have high social and economic 

costs, especially if the accident involves large passenger 

aircraft. Rigorous safety standards have been established 

over decades to decrease the probability of aviation accidents. An important strategy in achieving high levels 

of safety in aviation is to build redundancy in equipment 

and the interpretation of signals and other information 

by pilots and other personnel. Color is used extensively 

to code information in the aviation environment, and 

pilots are normally expected to have good color discrimination. Even when other cues are also available, the 

ability to use color information increases redundancy, 

and in some tasks, this improves considerably the level 

of visual performance that can be achieved. Some accidents have been linked to loss of color vision (National 

Transportation Safety Board, 2004). There is also some 

evidence to suggest that the likelihood of accidents is 

increased in pilots that are color deficient (Vingrys & 

Cole, 1986). Other studies have shown that subjects 

with color vision deficiencies make more errors and are 

slower in recognizing aviation signals and color coded 

instrument displays (Vingrys & Cole, 1986; Cole & 

Maddocks, 1995; Squire et al., 2005). There are also a 

small number of tasks in which color information is not 

used redundantly; therefore, the correct interpretation 

of color signals becomes very important.

1.1.2 Current Color Vision Requirements and Assessment Methods in Aviation

The International Civil Aviation Organisation (ICAO) 

requires member nations to maintain a color vision standard to ensure pilots can recognize correctly the colors of 

signal lights used in aviation: “The applicant shall be required to demonstrate the ability to perceive readily those colors 

the perception of which is necessary for the safe performance 

of duties” (ICAO, 2001b). Many ICAO member states 

have different requirements for color vision assessment 

and employ different tests as the standard.

In Europe, there is agreement among the 38 members 

of the JAA to apply the same standards, at least in terms 

of primary tests. The current JAA color vision requirements (Section 1; JAR-FCL 3, 2002) use the Ishihara 

pseudo-isochromatic test (section 1.3.1 of this report) 

as a screening test for color vision. The JAA use the first 

15 plates of the 24-plate version of the Ishihara pseudo-

isochromatic test, with no errors as the pass criteria. If 

the applicant fails this test then either a lantern test or 

the Nagel anomaloscope test is used. The three lanterns 
recommended by the JAA are the Holmes-Wright Type 

A (United Kingdom), the Spectrolux (Switzerland), and 

the Beyne (France, Belgium and Spain). The subjects pass 

when they make no errors on the corresponding lantern 

test. For the Nagel anomaloscope (section 1.3.3 of this 

report), “This test is considered passed if the color match is 

trichromatic and the matching range is 4 scale units or less...” 

(See Appendix 14 to subpart B; JAR-FCL 3, 2000). The 

tests currently employed by JAA member states and the 

corresponding pass/fail criteria are fully described in the 

report by the CAA (Civil Aviation Authority, 2006a).

In the U.S.A., the Federal Aviation Administration 

(FAA) guidelines are more liberal, and the approved 

primary tests include Ishihara, Dvorine, AOC-HRR, 

Richmond, etc. The pass limits are also more relaxed 

which favors some applicants with mild color deficiency. 

Other tests include the Farnsworth Lantern, Keystone 

Orthoscope, etc. In exceptional cases, this can then be 

followed by the more practical Signal Light Gun Test 

(SLGT), usually carried out in an airport tower. This 

approach does justice to some applicants but not to 

others (see section 4). The disadvantage of this more 

liberal approach is that when the pass limits are relaxed, 

the pass/fail outcome of the screening tests no longer 

guarantees normal performance in the most safety-­critical, 

color-related tasks.

Follow-up color vision tests may be carried out for renewal of Class 1 medical certificates. Within the JAA, the 

Ishihara plates which screen only for red/green deficiency 

are normally used for this assessment. Any yellow-blue 

loss (either congenital or acquired) will not therefore be 

picked up by this test (section 1.6 of this report). Since 

changes in chromatic sensitivity are often indicative of 

early-stage systemic (e.g., diabetes) or ocular diseases 

(e.g., glaucoma, age-related macular degeneration), it 

is recommended that both red-green and yellow-blue 

color sensitivity should be assessed with every medical 

examination and any significant changes noted. The data 

can then be used to detect when the progression of any 

inherent condition yields color thresholds that fall outside 

the range established for normal vision.

1.1.3 Problems Identified With Current Assessment 

Methods and Procedures

Current color vision requirements vary from country 

to country, even within the JAA member states. The correlation between the outcomes of different tests is poor; 

therefore, it is not uncommon for pilot applicants to fail 

the color vision assessment in one country and to pass 

in another (Squire et al., 2005). This is not completely 

unexpected given the large inter-subject variability, the 

different factors that can contribute to loss of chromatic 

sensitivity, and the different characteristics of the various 

color vision tests. The lack of standardization often causes 

confusion among applicants and provides the opportunity 

to attempt several tests in order to pass one of the many 

color vision standards.

1.1.4 A New Approach Based on Recent Advances 

in Color Vision Testing

Advances in the understanding of human color vision 

(Barbur, 2003) and the development of novel methods to 

measure accurately the loss of chromatic sensitivity (Barbur 

et al., 1994) have prompted the UK CAA and the FAA 

to sponsor new studies to examine how color vision loss 

can be measured accurately and to establish minimum 

color vision requirements for professional pilots. As part 

of the CAA funded study, the current accepted JAA color 

vision requirements for professional flight crew have been 

reviewed and the variability associated with the most 

common occupational color vision tests assessed, both 

in normal trichromats and in subjects with red/green 

color deficiency. The aim of the current project was to 

establish minimum limits of color vision sensitivity that 

can be considered to be operationally “safe” within the 

aviation environment. This joint report follows other 

CAA documents published in 2006 (CAA 2006/04): 

“Minimum Color Vision Requirements for Professional 

Flight Crew: Part 1. The Use of Color Signals and the 

Assessment of Color Vision Requirements in Aviation, 

and Part 2, Task Analysis.”

1.1.5 A New Color Vision Test

Ideal color vision assessment requires a test that (i) 

provides true isolation of color signals and quantifies 

the severity of color vision loss, (ii) is based on data that 

describe the statistical limits of color discrimination in 

“normal” trichromats, so as to be able to differentiate 

minimal color vision loss due to congenital and acquired 

deficiencies from fluctuations expected within normal 

trichromats, (iii) has enough sensitivity to detect “minimal” deficiencies and to classify accurately the class of 

deficiency involved, and (iv) can be used to detect and 

monitor “significant changes” in color sensitivity over 

time. The Color Assessment and Diagnosis (CAD) test 

has been developed and improved over several years to 

fulfill these requirements (Section 1.4 of this report).

1.2 Identification of the Most Safety-Critical and 

Demanding Color Vision Tasks

An important aspect of this study was to investigate 

whether subjects with minimal color vision loss were 

able to carry out the most demanding, safety-critical, 

color-related tasks with the same accuracy as normal 

trichromats. If the findings indicate that “normal” color 

vision is not required to carry out such tasks, it then 
becomes important to establish the limits of color vision 

loss that can still be considered safe within the aviation 

environment.

The approach adopted in this investigation was to 

relate the accurate assessment of color vision loss to the 

subject’s ability to carry out the safety-critical, color-based 

tasks within a specified environment when the use of cues 

other than color was minimized. A visual task analysis 

was carried out (CAA, 2006a) to identify and characterize 

the most important safety-critical, color-related tasks for 

flight crew. The PAPI and the parking signal lights were 

found to be the most safety-critical, color-related tasks 

when no redundant information is available to carry out 

the task. An earlier study by Cole and Maddocks (1995) 

also identified the PAPI lights as the most safety-critical, 

color-related task. The PAPI lights provided the pilot 

with accurate glide slope information on final approach 

to landing using four lights, each of which can be either 

red or white. Two whites and two reds indicate correct 

approach path, too many reds indicate that the approach 

height is too low, and too many whites indicate that the 

approach height is too high. The geometry of the PAPI 

signal system is shown in Fig. 1a (see also Fig. 2a).

An alternative system, the VASI (Visual Approach 

Slope Indicator), is sometimes used in North America and 

Australia. The VASI is more expensive and requires more 

space. There are several versions of the VASI system, but 

the main task of the pilot remains the discrimination of 

horizontal bars of well defined red and white lights (see 

Fig.1b). The more favorable geometry and the greater 

angular separation of the lights make the VASI color 

discrimination task less demanding than the PAPI. In the 

T-VASI version of the system, the changing geometry of 

the lights provides the required approach slope information, hence the color coding is used redundantly. The PAPI 

lights system is visually more demanding. The angular 

subtense of each of the four red/white lights corresponds 

to the smallest retinal image that can be produced by the 

Too low

Slightly low

On slope

Slightly high

Too high

Figure 1a: The Precision Approach 

Path Indicator (PAPI) signal lights 

are used to inform pilots of the 

correct glide path for landing. This is 

an efficient system since it consists 

only of four lights, each of ~ 8” 

diameters, spaced 9m apart. The 

angular subtense of the lights 

remains relatively unchanged 

beyond ~ 800m, but the retinal 

illuminance of the lights decreases. 

The output of the lamps can be 

adjusted using 6 preset lamp current 

settings. The disadvantage is that 

the correlated color temperature of 

the lamp changes significantly and 

this affects mostly the color of the 

“white” lights (see section 2). 

Figure 1b: The

Visual Approach 

Slope Indicator 

(VASI) and the more 

sophisticated TVASI systems. The 

correct angle of 

approach is 

indicated as red over 

white. The T-VASI 

requires more space 

and is more 

expensive, but the use of color provides only redundant information.

too high

too low

on slope

on slope

slightly high

very high

high

slightly low

well below

glide path

low

very low

VASI T-VASI

too high

too low

on slope

on slope

slightly high

very high

high

slightly low

well below

glide path

low

very low

on slope

slightly high

very high

high

slightly low

well below

glide path

low

very low

VASI T-VASI
optics of the eye, and the red/white color coding is used 

non-redundantly. This project has, therefore, focused on 

the PAPI lights as the most safety-critical, color-coded 

task for pilots.

1.2.1 Other Uses of Signal Lights Within the Aviation Environment

There are many other color signals that are used in 

the aviation environment to enhance conspicuity, code 

information, and group objects of interest together. These 

situations are less safety-critical, involve the use of larger 

stimuli under more favorable conditions of light adaptation, and the same information is also available in some 

other ways (e.g., text or audible signals). The PAPI signal 

system, on the other hand, offers no redundancy – at 

night there is no other unique cue in addition to color 

discrimination to help the pilot recognize the red and 

white light signals reliably to visually determine whether 

the aircraft is on the correct approach path for landing.

a. Parking Lights

Parking the aircraft requires the correct alignment of 

the aircraft with the line of approach. The pilot is aided 

in this task by the red-green parking lights. Both lights 

are seen as green when the aircraft is positioned correctly 

for approach. A red-green combination (see photograph 

of parking lights in Fig. 2b) signals that the aircraft has 

to veer slowly (towards the green light) to ensure that the 

pilot sees two green lights.

This is a color-related, safety-critical task simply 

because no other redundant cues are available; however, 

the lights are bright, the color difference between the 

lights is large, and the lights subtend a large visual angle 

at the eye. Consequently, the color discrimination task 

is less demanding, and it is expected that observers with 

minimum color vision deficiency may be able to carry out 

this task with the same accuracy as normal trichromats. 

This task was not investigated in this study.

b. Runway and Taxiway Lights

The lighting of runways and taxiways involves the use 

of color signals, but the use of color for coding is often 

redundant. The correct information is also provided by 

the geometry of the lights. Runway lighting is used for 

landing and take-off. On approach, the lighting of the 

runway provides essential information that yields outline 

views of the geometry of the runway. On touch-down, 

the lights form unique geometric lines and shapes that 

convey specific information. A particular runway may 

have some or all of the following lights:

Runway Edge Lights are white (or amber) and run the 

length of the runway on each side.

Runway Threshold Lights are green and indicate the 

starting point for the available landing distance.

Runway End Lights are red and delineate the extremity 

of the runway that is available for maneuvering.

Runway Centerline Lights start white, become red-white 

intermittent and then red only, towards the end of 

available runway for take-off.

Touchdown Zone Lights consist of rows of white 

light bars (with three in each row) on either side of 

the centerline over the first 914 m of the runway (or 

to the midpoint, whichever is less).

Stopway Lights are four unidirectional red lights equally 

spaced across the width to mark the end of any stopway 

associated with a runway used at night.

Runway edge lights provide perspective cues on 

approach and are less demanding than the PAPI light 

system; runway lighting becomes visible from several 

kms and often aids the pilot’s visual search to locate the 

•

•

•

•

•

•

Figure 2a: Photograph of PAPI lights viewed from ground level. Photograph taken at 

Sussex Flight Centre, Shoreham Airport (December 2006). 
PAPI lights. The color of the runway threshold lights is 

largely redundant because these green lights cannot be 

confused with any other similar lights in terms of location, 

geometry, and shape, but the green color may reinforce 

their function. Touchdown zone lighting is added in 

order to improve texture and perspective and to give 

the flight crew an indication of the area within which 

a landing must be initiated. The geometry and location 

of the runway end lights in relation to other lights is, 

again, sufficient to indicate their function. The color of 

the runway centerline lights changes from “white” on 

touchdown to alternating “white-red” lights and then to 

“red” lights when the aircraft advances towards the end 

of the runway. The color of the lights indicates the position of the aircraft on the runway, and this information 

is important in some situations (e.g., when take-off has 

to be abandoned, especially in conditions of poor visibil-

ity). Runway signal lights tend to be larger and brighter 

than PAPI lights, and this makes the discrimination of 

color differences less demanding. It has therefore been 

assumed that if the applicants can discriminate the red 

and white PAPI lights from 5 kms, they should be able 

to easily discriminate the red and white lights on the 

runway centerline.

For night operations, taxiways at most airports are 

equipped with lights that may include some or all of 

the following:

Taxiway Edge Lighting is blue to outline the edges 

of taxiways during periods of darkness or restricted 

visibility.

Taxiway Centerline Lighting is green and provides 

centerline guidance on taxiways and aprons and when 

entering or vacating a runway.

Stop-bar Lights are a single row of red, flush, or semiflush inset lights installed laterally across the entire 

•

•

•

taxiway showing red towards the intended direction 

of approach. Following the controller’s clearance to 

proceed, the stop-bar is turned off, and the centerline 

lead-on lights are turned on.

Runway Guard Lights are either a pair of elevated flash-

ing amber lights installed on either side of the taxiway, 

or a row of in-pavement yellow lights installed across 

the entire taxiway, at the runway holding position 

marking at taxiway/runway intersections.

Taxiway lights are seen from much shorter distances 

when the aircraft moves slowly on the ground. The 

discrimination by the pilot of the center taxiway line as 

green and the edge as blue is not an essential requirement 

to carry out the task safely, but the use of appropriate 

colors may well emphasise the function of the lights. 

Therefore, there is little doubt that an acceptable level 

of color discrimination is needed which can enhance 

the conspicuity of light signals, even when color is used 

redundantly and the tasks are less demanding or safety 

critical. The stop-bar and runway guard lights both play 

an important role in preventing runway incursions. In 

addition, the flashing aspect of the guard lights adds 

conspicuity to these signals but may also distract the 

pilot from interpreting other signals. The most common 

causes of runway incursions do not involve the incorrect 

interpretation of color signals since color is used redundantly and simply adds to the conspicuity of the lights. 

Other factors such as lack of communication between 

controller and pilot, lack of familiarity with airport layout, 

tiredness, lack of attention, and poor cockpit procedures 

for maintaining orientation in low visibility conditions 

(ICAO NAM/CAR/SAM Runway Safety/Incursion Conference, Mexico City, October 2002) can all contribute to 

runway incursions.

•

Figure 2b: Photograph of the red and green parking lights 

that are used at airports to indicate to the pilot the correct line 

of approach for parking the aircraft at the stand. The pilot 

sees two green lights when the aircraft is positioned correctly 

for approach. A red-green combination (as shown in the 

photograph) signals that the aircraft has to veer slowly right 

(towards the green light) whereas a green on the left and a 

red on the right signals the need to veer slightly to the left 

(again towards the green light). This task description 

illustrates clearly that the pilot has to be able to discriminate 

between the red and green lights. The angular subtense of

the parking lights is much larger than the PAPI and the retinal 

illuminance generated is also higher. This color discrimination 

task is therefore likely to be less demanding. 
1.2.2 Analysis of the PAPI Lights Task

The PAPI task is a simple, efficient, red-white, twocolor code (and the white and red lights generate both redgreen and yellow-blue color signals in the eye). Red/green 

color deficient observers will continue to have full use of 

their yellow-blue channel, although the properties of this 

channel will differ between deutan and protan subjects. 

The PAPI system is efficient since it takes a small amount 

of space, and the size of the image of each light generated 

on the retina remains largely unchanged as the viewing 

distance increases beyond ~0.8 km (although the lights 

become less bright as the viewing distance is increased). 

The geometry of the lights carries no information and 

hence the need to use colored signals. It has been suggested that dichromats (who exhibit severe red/green color 

vision loss) may be able to correctly interpret differences 

between two colors, at least under some conditions (Heath 

& Schmidt, 1959). In addition, color deficient observers 

can usually recognize red signals with few errors (Vingrys 

& Cole, 1993; for a review, see Cole, 2004). It is likely 

that some subjects with severe color deficiency may be 

able to carry out the PAPI task with no errors, but it is 

essential to ensure that the subjects recognize and name 

all four lights as red when too low and as white when too 

high. Any simulation of the PAPI test must include all 

conditions and must also ensure that the use of brightness difference cues is minimized. On the other hand, the 

recognition of the red and white PAPI lights is not always 

an easy task. Atmospheric scatter and the use of reduced 

lamp current settings at night to dim the lights can shift 

the white signal toward the yellow region of the spectrum 

locus (see Fig. 17). This often causes problems for color 

normal observers and may cause even greater problems 

for color deficient observers. In the case of large passenger 

aircraft, the PAPI lights are seen from large distances (>5 

kms) at night when both the angular subtense of each 

light and the angular separation between adjacent lights 

is very small. Adjacent lights tend to overlap visually, and 

this is particularly troublesome at night when the pupil 

size is large. Subjects with large, higher-order aberrations 

and increased light scatter in the eye will be disadvantaged 

at night. Although most subjects will have high visual 

acuity (<1 min arc) under photopic conditions, subjects 

with large higher order aberrations and scattered light may 

have very poor visual acuity under mesopic conditions 

when the pupil size is large. Visual acuity at low light 

levels in the mesopic range is not normally assessed for 

certification purposes. Partial overlap of adjacent lights 

makes the task of discriminating the number of red 

and white lights even more difficult. These additional 

factors explain why the PAPI task (which involves only 

two colors) is considered to be more critical than other 

color based tasks.

1.2.3 Disability Discrimination

There are further considerations that justify the need 

to establish safe, minimum requirements for color discrimination (when appropriate) and to avoid the easier 

alternative (from a regulatory viewpoint) of requiring 

every applicant to have normal color vision. The recent 

UK Disability Discrimination Act (2004) has, to a certain extent, exposed weaknesses in the current standards 

and procedures. Companies need to justify refusal to 

employ an applicant on the basis of his/her defective 

color vision, and this requires scientific evidence to 

demonstrate convincingly that the applicant will not be 

able to carry out necessary occupational tasks that involve 

color vision with the accuracy and efficiency expected of 

normal trichromats. In view of these arguments, we have 

developed a PAPI simulator and a PAPI Signal Lights test 

that can be used under controlled laboratory conditions. 

The simulators reproduce both the photometric and 

the angular subtense of the real lights under demanding 

viewing conditions when the lights are viewed against a 

dark background. The aim was to correlate the measured 

loss of chromatic sensitivity on the CAD test with the 

subject’s performance on the most safety-critical, colorrelated task identified in the aviation environment. 

Since other color-related tasks (such as seeing the color 

of the parking lights or the discrimination of runway, 

centerline, and red and white lights) are less demanding, 

it is assumed that the pilot will also be able to correctly 

perform these tasks. In principle, this approach should 

make it possible to recommend pass/fail limits based on 

the observer’s ability to carry out the most safety-critical 

and demanding PAPI task.

1.3 Brief Description of the Most Common Occupational Color Vision Tests

For a full description of color vision tests used in aviation, please refer to CAA Paper 2006/04 (2006a), and for 

a list of tests accepted by the FAA, see the FAA Guide for 

Aviation Medical Examiners (2008). The following color 

vision tests will be described here since they have been 

used along with the CAD test in this study. These are 

the Ishihara and Dvorine pseudoisochromatic plate tests, 

Nagel anomaloscope, and the Aviation Lights Test (ALT). 

Measures of color discrimination performance computed 

from the results of these tests will be examined and compared with the subject’s scores on the PAPI simulator. The 

same PAPI simulator was also used to produce a more 

demanding signal lights test that required the subject to 

name one of six different colored lights, as described in 

section 2 of this report. The latter will be referred to as 

the PSL (PAPI Signal Lights) test.
1.3.1 Ishihara Plate Test

The Ishihara pseudoisochromatic plate test consists 

of a series of numbers outlined by different colored dots 

as shown in Fig. 3. This is the most widely accepted 

screening test for red/green color deficiency and uses camouflage to exploit the color confusions of color deficient 

observers (Sloan & Habel, 1956; Belcher et al., 1958; 

Frey, 1958; Birch, 1997). The Ishihara test consists of 

single or double-digit numbers that have to be identified 

verbally and pathways for tracing for those who cannot 

read numbers. The 24-plate test version consists of the 

following: plate 1 for demonstration of visual task, plates 

2-15 for screening, plates 16-17 for protan/deutan classification. The Ishihara test employs a range of designs 

such as transformation, vanishing, or hidden digit. In the 

vanishing type plate (Fig. 3, middle) a figure is seen by 

color normals but not by color deficients; the reverse of 

this, the hidden figure design, is harder to design and not 

always so effective. More complex patterns are contained in 

transformation plates (Fig. 3, left), with careful placement 

of the color dots giving an apparent transformation of the 

perceived figure; normal trichromats see one figure, and 

color deficient people see a different figure in the same 

design. Positive evidence of color deficiency is given by 

transformation designs, whereas vanishing designs give 

negative evidence. In the classification plate design (Fig. 

3, right), protans only see the number on the right side of 

each plate and deutans only see the number on the left. 

The test is limited to red/green deficiency and cannot be 

used to assess loss of yellow-blue sensitivity.

The test is viewed at about two-thirds of one meter 

(arm’s length) distance using a Macbeth easel lamp for 

illumination. The first 25 plates of the 38-plate test version 

were used in this investigation. The book is placed in the 

tray beneath the lamp and the illumination, equivalent 

to CIE Standard Illuminant C (representing average daylight), is incident at an angle of 45° to the plate surface. 

The illuminant used is important because the selected 

reflectances of the patches on the plates have been chosen 

with reference to this illuminant. The examiner instructs 

the person being tested to report the number they can 

see as the pages are turned and warns the subject that 

on some occasions they may not see a number. The first 

introductory plate is used to demonstrate the visual 

task. This plate is designed so that anyone, including 

color deficient subjects, should see this number. With a 

viewing time of about 4 seconds allowed for each plate, 

undue hesitation on the part of the subject is the first 

indication of color deficiency.

1.3.2 Dvorine Plate Test

The Dvorine test is based on pseudoisochromatic 

principles and is similar to the Ishihara test. It has 15 

numeral plates, consisting of 1 initial plate that demonstrates the visual task, 12 plates for screening, and 2 

plates for protan/deutan classification (see Fig. 4). These 

plates are of the vanishing type. The font of the numerals 

is slightly different to the Ishihara plates and is believed 

to be easier to read.The Dvorine test is administered 

in a similar manner to the Ishihara test. The plates are 

positioned at arm’s length, perpendicular to the line of 

sight, under daylight illumination or a Macbeth easel 

lamp. The subject is instructed to read the numerals (all 

plates have a numeral).

Pseudoisochromatic plate tests provide a simple, read-

ily available, inexpensive, and easy to administer screener 

mostly for red/green deficiencies. However, plate tests 

tend to be relatively easy to learn, and this encourages 

cheating. The spectral quality of the light source illuminat-

ing the plates is also important. Plates may be degraded 

by fingerprints, dust, and excessive light exposure. In 

general, subjects with minimal color vision loss tend to 

Figure 3: Ishihara pseudoisochromatic plates; left, transformation design; middle, vanishing 

design; right, protan/deutan classification plate. Please note that these may not be 

reproduced accurately as the printed color and the viewing illuminant will be different. 
show greater variability on repeated tests by comparison 

with normal trichromats and on some occasions can even 

pass these tests (Squire et al., 2005).

1.3.3 Nagel Anomaloscope

The Nagel anomaloscope (Fig. 5) is based on color 

matching and is the standard clinical reference test for 

identifying and diagnosing red/green color deficiency 

recommended by the National Research Council - National Academy of Sciences (NRC-NAS) Committee on 

Vision (1981). This instrument produces a disc stimulus 

that consists of two half fields and is viewed in an optical 

system. The top half of this disc is illuminated by a mixture of spectrally narrow red and green wavelengths, and 

the lower half is illuminated by spectrally narrow yellow 

light. Two control knobs are used, one to alter the redgreen color mixture ratio of the top field, and the other 

to alter the luminance of the yellow lower field (see Fig. 

5). The test is administered in two stages. Usually only 

Figure 4: Dvorine pseudoisochromatic plates; left and right, vanishing design; 

middle, protan/deutan classification plate. Please note that the color of the plates 

may not be reproduced accurately in this document or in print since the printed 

reflectances and the viewing illuminant will be different. 

Figure 5: Photograph of the Nagel anomaloscope (Model I, Schmidt and 

Haensch, Germany) and illustration of the Nagel anomaloscope split field. The 

percentage mixture of red to green in the top half and the luminance of the 

yellow bottom field can be changed until a match of the two fields can be 

perceived.

Red

670 nm

Green

546 nm

Yellow

589 nm 2 degrees

one eye (i.e., the dominant eye) is fully tested and the 

other eye is then checked to ensure the same deficiency. 

This confirms that the loss of color vision is congenital. 

Following familiarization with the instrument controls, 

the subject is then asked to alter both the control knobs 

until the two halves of the circle match completely in both 

color and brightness. The subject is not asked to name 

the colors. A few matches are made, with the examiner 

“spoiling” the match after each setting. About ten seconds 

are allowed for each match and then, to minimize the 

effect of chromatic after images, the subject looks away 

from the instrument into the dimly lit room for a few 

seconds before the procedure is repeated. The second 

stage of the test is to determine the limits of the matching 

range. The initial matches made by the subject are used 

as a guide by the examiner to set the red/green mixture 

ratio near to the estimated limits of the range. The subject must alter the luminance of the lower yellow half of 

the field and see if an exact “match” in both color and 
brightness can be made with the set red/green mixture in 

the upper half. The ratio of the red/green mixture field 

is altered systematically by the examiner until the limits 

of the matching range are found. The matching range 

is recorded from the matching limits on the red/green 

mixture scale and the midpoint calculated.

Ideally, the red/green “match” parameters should provide enough information to determine whether a person 

has normal or defective red/green color vision, whether 

color deficiency is deutan or protan, and whether the 

subject is a dichromat (absence of a cone-type) or anomalous trichromat (anomalous cone-type). The size of the 

red-green matching range is often taken as an indicator 

of chromatic sensitivity loss. The red-green discrimination index (RGI), a parameter relating to the matching 

range, has been introduced in this study and provides an 

indication of the subject’s ability to discriminate red-green 

color differences:

RGI=1– , where rsubject is the subject’s mean 

matching range.

 rsubject 

 74

0 10 20 30 40 50 60 70

Nagel Midpoint on RG scale 

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

DA

PA

N

Dichromat

G R

Dichromats

RGI=0

Extreme

Anomalous

trichromat

Minimal

anomalous

trichromat

RGI

0 10 20 30 40 50 60 70

Nagel Midpoint on RG scale 

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

DA

PA

N

Dichromat

G R

Dichromats

RGI=0

Extreme

Anomalous

trichromat

Minimal

anomalous

trichromat

RGI

Figure 6: Scatter plot of matching midpoints versus RGI for 231 observers; 70 subjects 

formed a cluster that is separated from all other subjects by having a midpoint around 40 

and a high RGI. The abbreviations in the legend refer to deuteranomalous (DA), 

protanomalous (PA), and normal trichromat (N) subjects. The value of the luminance 

setting on the yellow scale provides additional information to separate deutan and protan 

color deficient observers. The data show clearly that according to the Nagel test, many 

deutan and protan subjects have RG chromatic sensitivity that is indistinguishable from the 

range of values measured in normal trichromats. 

The RGI ranges from a value very close to 1 for normal 

sensitivity, to 0 for a dichromat that accepts any red/green 

mixture setting as a match to the yellow field. A more 

appropriate measure of red/green sensitivity based on the 

Nagel is obtained simply by dividing the mean normal 

matching range (rmean) obtained by averaging results for 

a large number of normal trichromats by the subject’s 

range (rsubject). The mean normal matching range for the 

Nagel anomaloscope used in this study is approximately 

4 scale units. Hence, the new measure of chromatic 

sensitivity becomes:

Nagel sensitivity = 

A scatter plot of Nagel midpoints on the red-green 

scale versus RGI allows one to separate a clear cluster of 

subjects with midpoints between 36 and 44 units on the 

red/green mixture scale, which are likely to be normal 

trichromats (see Fig. 6). Dichromats will accept the full 

range of red/green mixtures as a match with the yellow 

field (i.e. RGI=0), as they have only one photopigment 

r

mean

r

subject
10

in the spectral range provided by the instrument. Deuteranopes are distinguished from protanopes as the intensity 

of the yellow they set for both ends of the red/green scale 

is fairly similar, whereas protanopes set the luminance of 

the yellow very low to make a match at the red end of the 

scale and much higher at the green end. This is because 

protans tend to see red as fairly dark, as they have reduced 

long wavelength sensitivity. If a color match within the 

normal range is not achieved, the subject is classed as an 

anomalous trichromat. Two separate distributions are 

formed either side of the normal range, as protanomalous trichromats require significantly more red light in 

their color mixture, and deuteranomalous trichromats 

require more green (see Fig. 6). The RGI or match-

ing range provides some measure of the severity of the 

color discrimination deficit on the Nagel anomaloscope, 

although it is well known that the correlation between 

the size of the matching range and the subject’s ability 

to discriminate colors under more natural conditions is 

generally poor (Wright, 1946).

The principal advantage of the anomaloscope is that, 

unlike the previous tests, the parameters of the yellow 

match can be used to classify accurately the type of color 

deficiency involved.

1.3.4 Aviation Lights Test (ALT)

The Aviation Lights Test is a modified Farnsworth 

Lantern (Milburn & Mertens, 2004) designed to meet 

the FAA’s signal color (USDOT-FAA, 1988) and International Civil Aviation Organization (ICAO, 1988) 

specifications for the red, green, and white signal light 

colors on aircraft. The chromaticity coordinates of the 

ALT are shown plotted in Fig. 11. Originally the test was 

employed for secondary screening of air traffic control 

specialist applicants.

Nine vertically separated pairs of colored lights (see 

Fig. 7) are presented to the examinee, who is seated 8 ft 

(~2.4 m) away from the lantern. The constant vertical 

separation of the 2 apertures is 13 mm, or 18.3 minutes 

of arc at the recommended viewing distance. Each pair 

of lights subtends a visual angle of 3 min arc. There are 

three colors: red, green, and white. Each series of nine 

pairs was presented three times in random order, making 

a total of 27 presentations.

Before the ALT test is carried out, the subject is shown 

each of the three test light colors. Light pairs numbered 1 

and 2 are shown (see Fig. 7a) while saying: “this is green 

over red” and “this is white over green,” respectively. The 

examinee has to name correctly the colors of the lights 

shown (with a pass criterion of not more than one error) 

in all 27 presentations. If the color of either or both lights 

in a pair was identified incorrectly, this was counted as 

one error. The ALT is administered in a very dim room 

that approximates the light level of the air traffic control 

(ATC) tower cab at night.

1.4 The CAD Test

The Color Assessment Diagnosis (CAD) test has been 

described in an earlier CAA report (CAA, 2006a). The 

CAD test is implemented on a calibrated visual display 

and consists of colored stimuli of precise chromaticity 

and saturation that are presented moving along each of 

the diagonal directions of a square foreground region 

made up of dynamic luminance contrast (LC) noise. The 

subject’s task is to report the direction of motion of the 

color-defined stimulus by pressing one of four appropriate 

buttons. Randomly interleaved staircase procedures are 

used to adjust the strength of the color signals involved 

according to the subject’s responses to determine the 

thresholds for color detection in each direction of interest 

to establish reliable estimates of red-green and yellow-blue 

color thresholds. The CAD test has a number of advantages over conventional tests, both in terms of isolation 

of color signals, as well as sensitivity and accuracy: 

(a) (b) 

Figure 7: (a) Schematic representation of the different pairs of lights presented in the 

Aviation Lights Test (ALT). The lantern can show 9 different combinations (as shown) three 

times giving a total of 27 presentations, (b) Photograph of the ALT lantern. 

1 2 3 4 5 6 7 8 9

G/R W/G G/W G/G R/G W/R W/W R/W R/R
11

Isolation of Color Signals

It is very important to isolate the use of color signals 

by masking any luminance contrast cues. Although the 

colored stimuli generated are isoluminant for the standard 

CIE observer, the large variation in L:M spatial density 

ratio within normal trichromats (i.e., 0.6 to 13; Carroll et 

al., 2002) and the variation in cone spectral responsivity 

functions in color deficient observers introduce variations 

in the perceived luminance contrast of most colored 

stimuli. This is simply because the resulting luminance 

efficiency function V(λ) is likely to vary both among normal trichromats and within color deficient observers. The 

CAD test employs dynamic LC noise, and this effectively 

masks the detection of any residual luminance contrast 

signals that may be present in the colored test target. The 

mean luminance of the foreground remains unchanged, 

both spatially as well as in time, and equal to that of the 

surround background field. The technique isolates the 

use of color signals and ensures that the subject cannot 

make use of any residual LC signals. The dynamic LC 

noise does not affect the threshold for detection of color 

signals but effectively masks the detection of luminance 

contrast signals (Barbur et al., 1994; Barbur, 2004).

Measurement of Chromatic Detection Thresholds

An efficient, four-alternative, forced-choice procedure is used to measure subject’s chromatic detection 

thresholds in a number of carefully selected directions 

in the CIE – (x,y) chromaticity chart. Thresholds are 

measured along 16 interleaved directions in color space. 

These are grouped together to test red-green (RG) and 

yellow-blue (YB) color sensitivity. Threshold ellipses are 

•

•

computed and plotted using the standard CIE 1931 

chromaticity chart. The use of 16 randomly interleaved 

color directions makes the technique statistically robust 

and eliminates any other possible cues, so the subject has 

to rely entirely on the use of color signals. The output of 

the CAD test also accurately diagnoses the class of color 

deficiency involved. If the latter is not needed, one only 

needs to test two color directions to screen for red/green 

color deficiency.

The Statistical Limits of Chromatic Sensitivity 

Within “Normal” Trichromats

Chromatic discrimination thresholds have been 

measured in over 450 observers, including 250 normal 

trichromats and 200 color defective observers (Fig. 12). 

Fig. 8 shows the distribution of YB and RG chromatic 

thresholds obtained in the 250 normal trichromatic subjects. Fig. 9 shows the statistical limits for the “standard 

normal” (SN) observer on the CAD test plotted in the 

1931 CIE-x,y color chart (Rodriguez-Carmona et al., 

2005; Rodriguez-Carmona, 2006). The variability in 

both RG and YB thresholds is shown by the grey shaded 

ellipse, which represents the region of the CIE chart where 

we expect to find 95% of normal trichromats. The 2.5% 

and 97.5% limits define the boundaries of this region. 

The median chromatic discrimination threshold ellipse 

is also plotted. The median threshold value is important 

since it represents the Standard Normal (SN) observer. 

A subject’s thresholds can then be expressed in SN units, 

and this makes it possible to assess the severity of color 

vision loss (i.e. an observer with a RG threshold of 2 SN 

units requires twice the color signal strength that is needed 

•

Figure 8: Frequency distributions of the YB and RG chromatic thresholds 

obtained in 250 observers with ‘normal’ trichromatic vision. The mean, standard 

deviation (SD) and median are shown. 

0 5 10 15 20

YB CAD thresholds

0

10

20

30

40

50

N

o. of o

b

s

erv

ers

Mean = 11.4

SD = 2.67

Median = 11.1

2 3 4 5 6 7

RG CAD thresholds

0

10

20

30

N

o. of o

b

s

erv

ers

Mean = 4.50

SD = 0.78

Median = 4.37

0 5 10 15 20

YB CAD thresholds

0

10

20

30

40

50

N

o. of o

b

s

erv

ers

Mean = 11.4

SD = 2.67

Median = 11.1

2 3 4 5 6 7

RG CAD thresholds

0

10

20

30

N

o. of o

b

s

erv

ers

Mean = 4.50

SD = 0.78

Median = 4.37
12

by the average standard CAD observer). Fig. 9 is an extremely useful representation in that it provides a CAD 

test template for the SN observer. Any subject’s results 

provide instant diagnosis of either normal or deficient 

color vision when plotted on this template.

Detection of Color Vision Loss That Falls Outside 

Normal Range

The distribution of thresholds along the directions 

examined provides enough information to classify even 

minimal deficiencies that would otherwise remain undetected using conventional color vision tests. For example, 

Fig. 10 shows results of two minimal color vision deficient 

observers that fall just outside the normal range indicated 

by the shaded grey area. The subject on the left (subject 

CC) has a Nagel range of 16-18 and passes the Ishihara, 

whereas the subject on the right (subject SH) has a Nagel 

•

range of 40-42 units but fails the Ishihara with 2 errors. Although both subjects are classified as minimum 

deuteranomalous on the CAD test, their classification 

on the other two tests is less clear. The first subject is 

classified as normal on Ishihara and deuteranomalous 

on the Nagel (but with a mixture range smaller than 

the average normal trichromat). The second subject is 

classified as normal on the Nagel, but slightly red/green 

deficient on the Ishihara.

Diagnosis of the Type of Deficiency Involved

The CAD test identifies the type of deficiency involved by the elongation of the subject’s results, either 

along the deuteranopic (Fig. 11, left) or protanopic 

(Fig. 11, right) confusion bands. In the case of absolute minimum deuteranomalous deficiencies, the 

distribution of the thresholds is as shown in Fig. 10. 

In the case of minimum protanomalous deficiencies, 

the thresholds are much larger and extend sufficiently 

along in the protanopic direction indicating a diagnosis 

of minimum protanomaly with no ambiguity. The 

agreement with the Nagel for screening and classification of the class of congenital red/green deficiency 

is ~99%.

Quantifying the Severity of Color Vision Loss

The severity of red-green and yellow-blue loss of 

color vision is proportional to the color signal strength 

needed for threshold detection. For example, subjects 

in Fig. 11 show more severe loss (i.e., higher thresholds or lower chromatic sensitivity) than the subjects 

shown in Fig. 10. The severity of color vision loss can 

be quantified with respect to the standard normal 

observer (Fig. 8 and 9). Chromatic sensitivity varies 

greatly within color deficient observers from complete 

absence of red-green discrimination, in the case of 

dichromats, to almost normal sensitivity in subjects 

with thresholds not much larger than 2 SN units. Fig. 

12 shows the subject’s RG threshold in SN CAD units 

along the abscissa, plotted against the YB threshold 

along the ordinate in 450 observers. The results show 

that the RG thresholds vary almost continuously from 

very close to “normal” to extreme values which can be 

25 times larger than the standard normal threshold. The 

YB thresholds, on the other hand, vary little, as expected, 

in the absence of yellow-blue loss or acquired deficiency. 

Interestingly, the RG thresholds show some correlation 

with YB thresholds in normal trichromats, suggesting 

that subjects with high RG chromatic sensitivity are also 

likely to exhibit high YB sensitivity. The loss of sensitivity 

(when expressed in Standard Normal (CAD) units (SN 

units) is greater in protanomalous than deuteranomalous 

observers (Fig. 12).

•

•

0.28 0.29 0.3 0.31 0.32 0.33

x

0.3

0.31

0.32

0.33

0.34

0.35

y

Deuteranope

Protanope

Tritanope

2.5% < P < 97.5%

"Standard Observer"

Normal trichromat

CIE - (x, y)

Figure 9: Data showing the 97.5 and 2.5% statistical 

limits that define the “standard” normal CAD test 

observer. The dotted, black ellipse is based on the 

median RG and YB thresholds measured in 250 

observers. The grey shaded area shows the limits of 

variability of 95% of these observers. The 

deuteranopic, protanopic and tritanopic confusion 

bands are displayed in green, red and blue, 

respectively. The background chromaticity (x,y) is 

indicated by the black cross (0.305, 0.323). The 

colored symbols show data measured for a typical 

normal trichromat. 
13

0.28 0.29 0.3 0.31 0.32 0.33

x

0.3

0.31

0.32

0.33

0.34

0.35

y

Deuteranope

Protanope

Tritanope

2.5% < P < 97.5%

"Standard Observer"

subject's data

CC

0.28 0.29 0.3 0.31 0.32 0.33

x

0.3

0.31

0.32

0.33

0.34

0.35

y

SH

0.28 0.29 0.3 0.31 0.32 0.33

x

0.3

0.31

0.32

0.33

0.34

0.35

y

Deuteranope

Protanope

Tritanope

2.5% < P < 97.5%

"Standard Observer"

subject's data

CC

0.28 0.29 0.3 0.31 0.32 0.33

x

0.3

0.31

0.32

0.33

0.34

0.35

y

SH

Figure 10: Chromatic thresholds for two color vision deficient observers 

with minimal color vision deficiency. The data for the average normal 

trichromat is shown as a black contour. 

0.19 0.23 0.27 0.31 0.35 0.39

x

0.21

0.25

0.29

0.33

0.37

0.41

y

Protanope

Tritanope

Deutanope

2.5% < P < 97.5%

"Standard Observer"

subject's data

0.19 0.23 0.27 0.31 0.35 0.39

x

0.21

0.25

0.29

0.33

0.37

0.41

y

0.19 0.23 0.27 0.31 0.35 0.39

x

0.21

0.25

0.29

0.33

0.37

0.41

y

Protanope

Tritanope

Deutanope

2.5% < P < 97.5%

"Standard Observer"

subject's data

0.19 0.23 0.27 0.31 0.35 0.39

x

0.21

0.25

0.29

0.33

0.37

0.41

y

Figure 11: Chromatic thresholds for two color vision deficient observers 

with severe color vision deficiency. The largest chromatic displacements 

away from background chromaticity, as set by the isoluminant condition 

and the limits imposed by the phosphors of the display, are shown as grey 

lines. The extent of color vision loss is related to the elongation along the 

protanopic or the deuteranopic confusion band and suggests that the 

greater the elongation, the lower the level of chromatic sensitivity. 
14

Effects of Light Level and Stimulus Size

Both the ambient light adaptation level and the size 

of the colored stimulus can affect chromatic sensitiv-

ity. In general, as the light level is reduced and/or the 

stimulus size is decreased, the RG and YB thresholds 

increase. The YB thresholds are affected most at lower 

light levels. Both background luminance and stimulus 

size have been optimized for the CAD test so that no 

significant improvement in chromatic sensitivity results 

by increasing either the light level or stimulus size. Any 

small variations in either light level or stimulus size will 

not therefore alter significantly the subject’s RG and YB 

thresholds (Barbur et al., 2006). However, older subjects 

are likely to show more rapid effects as the light level is 

reduced because the retinal illuminance in these subjects 

is already low as a result of small pupil sizes and increased 

pre-receptoral absorption of blue light.

The Effects of Aging on Red-Green and Yellow-Blue 

Loss of Chromatic Sensitivity

The effect of aging for YB and RG chromatic thresholds in normal trichromats is shown in Fig. 13. These 

results show that up to the age of 60 years there is little 

correlation between the subject’s age and chromatic sensitivity. A small effect can be observed when examining 

YB thresholds (but the correlation with age remains very 

poor) and virtually absent in the case of RG thresholds. 

The age range examined is representative of the typical 

working life of pilots. Color vision is usually assessed in 

demanding occupational environments. Loss of color 

•

•

vision later in life is described as acquired color deficiency 

and can be caused by a number of factors including both 

systemic diseases and specific diseases of the eye (such as 

diabetes, glaucoma, age-related macular degeneration.). 

Since loss of chromatic sensitivity usually precedes the 

reliable detection of any structural changes using fundus 

imaging, regular screening for acquired color vision loss 

may be of great clinical value. In view of these findings, it 

makes sense to recommend that in addition to assessing 

color vision at the start of the working career, periodic 

re-assessments should also be done as a way of testing 

for acquired deficiencies.

1.5 Summary of Congenital Color Vision 

Deficiencies

Congenital color vision deficiencies remain unchanged 

throughout life and are largely determined by changes 

in the spectral responsivity of cone photoreceptors that 

are determined genetically. There are a number of other 

factors that can affect chromatic sensitivity, such as the 

optical density of photoreceptors, post-receptoral amplification of cone signals, or pre-receptoral filters that 

are spectrally selective and reduce the amount of light 

that reaches the cone photoreceptors in the eye (Alpern 

& Pugh, Jr., 1977; Alpern, 1979; Neitz & Jacobs, 1986; 

Barbur, 2003). These factors are all likely to contribute 

to the variability measured within normal and color 

deficient observers.

Congenital yellow/blue color vision deficiency is very 

rare (with an incidence of 1 in 13000 to 65000; Sharpe 

0 5 10 15 20 25

RG threshold units

0

1

2

3

4

5

Y

B thre

s

h

o

dl u

nits

Type of colour vision:

Normals

Deutans

Protans

0 1 2 3

0

1

2

3

0 5 10 15 20 25

RG threshold units

0

1

2

3

4

5

Y

B thre

s

h

o

dl u

nits

Type of colour vision:

Normals

Deutans

Protans

0 1 2 3

0

1

2

3

Figure 12: Graph showing Red-Green (RG) and Yellow-Blue (YB) thresholds 

expressed in CAD Standard Normal units for the population of subjects tested as part 

of this study. The spread of the data along the abscissa illustrates the large variation 

that exists among subjects with deutan- and protan-like deficiencies. 
15

et al., 1999) and usually implies the absence of S-cones. 

Loss of YB sensitivity with age, on the other hand, is 

very common and is often associated with toxicity or 

disease (see below).

1.6 Acquired Color Vision Deficiencies

Acquired deficiencies tend to affect both RG and YB 

color discrimination, although frequently the YB loss is 

greater and more apparent. Acquired color deficiencies are 

most commonly caused by systemic diseases (e.g., diabetes, 

multiple sclerosis) and other diseases that are specific to 

the eye (e.g., glaucoma, age-related macular degeneration, 

optic neuritis). Acquired deficiency affects predominantly 

older subjects. Acquired loss can sometimes be expressed 

in subjects with congenital color deficiencies. If congenital 

color deficiency is present, the identification of acquired 

color deficiency and the classification of the congenital 

component are more difficult. In such cases, the use of 

larger stimuli and dynamic luminance contrast noise that 

achieves a high level of luminance contrast masking with 

saturated colors can reveal both the type of congenital 

deficiency and the acquired loss of chromatic sensitivity 

(Barbur et al., 1997). There are other differences as well. 

Acquired loss of color sensitivity is generally non-uniform 

over the retina in the same eye and often affects one eye 

more severely than the other. One can also separate the 

congenital and the acquired loss by carrying out the 

CAD test in each eye both at the fovea and in the near 

periphery of the visual field or/and by using more than 

one stimulus size. The congenital component remains 

largely unchanged, whereas the acquired component 

varies with stimulus size, retinal location, and eye tested. 

Since the yellow-blue sensitivity is most affected, the 

CAD is particularly suitable for investigating acquired 

deficiency since it tests for both red-green and yellowblue loss. Fig. 14 shows examples of acquired color vision 

deficiencies.

2.0 Subjects and Methods

Summary of tests employed in this study:

Ishihara plate test

Dvorine plate test

Nagel anomaloscope

CAD test

Aviation Lights Test (ALT)

PAPI simulator test

PAPI Signal Lights test (PSL)

The PAPI and PSL simulators were designed and constructed specifically for this investigation. A full assessment 

of color vision using all these tests takes between 1.5 to 2 

hours per subject. The order in which the different tests 

were carried out varied randomly, and the testing took 

place in three different rooms, allowing the subject to take 

short breaks between tests. We examined 182 subjects in 

this investigation: 65 normal trichromats and 117 subjects 

with deutan- and protan-like color deficiencies. The age 

of the subjects ranged from 15 to 55 years (mean 30.2 

years, median 27 years).

1.

2.

3.

4.

5.

6.

7.

10 20 30 40 50 60

Age (years)

0

1

2

3

4

R

G thre

s

h

o

dl s

RG threshold (r^2=0.005)

1

2

3

4

Y

B thre

s

h

o

dl s

YB threshold (r^2=0.06)

Figure 13: Effect of age on the YB and RG chromatic thresholds for 

normal trichromats under normal daylight conditions. The best-fit line is 

shown for both sets of data. The correlation coefficients (r2

) for the YB 

and RG thresholds are 0.06 and 0.005, respectively. 
16

2.1 PAPI and PSL Simulator

The PAPI system consists of four lights arranged in a 

horizontal line and installed at right angles to the runway 

with the nearest light some 15 m away from the edge. 

The lights are approximately 30 cm in diameter with an 

inter-light separation of 9 m. The unit nearest the runway 

is set higher than the required approach angle at 3°30’, 

with progressive reductions of ~20 minutes of arc further 

out field: 3°10’, 2°50’ and 2°30’ (for a 3° approach). Usually each unit contains three light projectors (in case one 

fails). The light system has an intensity control for day 

and night use, with up to six luminous intensity settings: 

100%, 80%, 30%, 10%, 3%, and 1% (CAA, 2004).

The units direct a beam of light, red in the lower 

half and white in the upper half, towards the approach. 

The different elevation angles give a combination of red 

and white for an on-slope signal, all-red if the aircraft is 

too low, and all-white if it is too high (see Fig. 1). The 

chromaticities of the lights should follow the ICAO 

specification for Aerodrome Ground Lighting (AGL) 

(see Fig. 15). The light intensity of the white signal is 

required to be no less than twice and no more than 6.5 

times as bright as the red signal. The recommended 

intensities for the white and red light are 85000cd and 

12750cd, respectively, at the maximum of their light 

intensity distribution (CAA, 2004).

0.26 0.28 0.3 0.32 0.34 0.36

x

0.28

0.3

0.32

0.34

0.36

0.38

y

Diabetic subject

BL (binocular)

0.26 0.28 0.3 0.32 0.34 0.36

x

0.28

0.3

0.32

0.34

0.36

0.38

y

ARMD subject

JW (left eye)

0.26 0.28 0.3 0.32 0.34 0.36

x

0.28

0.3

0.32

0.34

0.36

0.38

y

Diabetic subject

BL (binocular)

0.26 0.28 0.3 0.32 0.34 0.36

x

0.28

0.3

0.32

0.34

0.36

0.38

y

ARMD subject

JW (left eye)

0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

x

0.18

0.23

0.28

0.33

0.38

0.43

0.48

0.53

y

CG (Acquired + Congenital)

Right eye Left eye

Figure 14: Examples of subjects with acquired loss of chromatic sensitivity. The data 

shown on the top left graph is from a subject with diabetes; top right shows data from the 

left eye from a subject with Age Related Macular Degeneration (ARMD); and bottom 

graph shows data from a subject with both congenital and acquired color vision loss 

(note the difference between the two eyes). 
17

A schematic of the laboratory set-up developed to 

simulate the PAPI and PSL tests is shown in Fig. 16. A 

four-channel optical system was developed using an airfield 

halogen lamp (JF6.6A100W/PK30d) as the single light 

source. The light is then split up and channeled using two 

beam-splitters (BS) to generate four beams. Each beam 

of light passes through two motorized filter wheels: color 

(CW) and neutral density (NDW) wheels. The CW has 

six different filters: red, modified white (~3900K), blue, 

green, yellow, and standard white (~2400K). Each NDW 

has neutral density filters with the following optical density (OD) values: 0.0, 0.3, 0.6, 1.0, 1.3, and 1.6. During 

the calibration procedure, the luminous intensity of each 

beam was measured with each filter in place to account 

for the actual and not the nominal absorption of each 

filter. The angular subtense of each light was 1.36’ at a 

viewing distance of 4 m. Beyond ~0.8 km the angular 

subtense of the real PAPI lights approaches the diffraction limit of the eye. The size of each light on the retina 

remains relatively unchanged as the approach distance 

is increased, but the light flux captured from each light 

is decreased. On approach, the PAPI lights are first seen 

as a small continuous line until the angular separation 

between adjacent lights is resolved by the eye (typically less 

than 2’, taking into consideration pupil size and optical 

aberrations). In order to reproduce the geometry of the 

real PAPI lights in the laboratory for a viewing distance of 

4 m, the adjacent lights (center to center) were separated 

by ~6.5 mm. This corresponds to an angular separation of 

5.5’ which translates to an approach distance of 5.54 km, 

in the case of the real PAPI lights. This design therefore 

requires the pilot to locate and recognize the white and 

red PAPI lights from 5.54 km when the size of the image 

of each light on the retina is determined by the pointspread-function (PSF) of the eye. We did not choose a 

longer approach distance in order to minimize the effects 

that higher order aberrations and increased scatter in the 

eye have on the retinal images of the lights. When the 

pupil size is large, the higher order aberrations in the eye 

can be quite large, and this causes the PSF to broaden 

and the visual acuity to decrease. The light distribution 

in adjacent PAPI lights can overlap significantly, and 

this, in turn, makes it more difficult for the subject to 

process the color of each light. Since a longer approach 

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

x

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

y

Signal lights: 

Green

Red

Blue

CC White

Yellow

White

Effect of current

ALT

700

600

550

520

480

450 1931 CIE-x,y 

Figure 15: 1931 CIE-x,y color space diagram showing the 

recommended chromaticity boundaries for the colors of light 

signals (CIE, 2001a; CIE, 2001b). The signal colors used in the 

laboratory set-up are also plotted and the effect of varying the 

current (i.e., the output intensity of the laps) is shown for the white 

light.
18

distance would produce even more overlap, a distance 

of 5.54 km, which is considered to be safe, was selected 

for the study.

The optical fiber heads form a line located at the center 

of a black plate, which provides a dark uniform surround 

(see Fig. 16). The whole system is encased and ventilated 

by two fans to prevent overheating and to reach a steady 

state temperature which is needed for stable lamp operation. The intensities of the red and white lights were 

adjusted using ND filters so that the simulated PAPI 

lights appeared as intense as the real PAPI when viewed 

in the dark from a distance of 5.54 km. These calculations 

assumed that in the absence of significant ambient light, 

the pupil of the eye would in general be large (~6 mm). 

In addition, the intensities of the colored lights varied 

randomly by ±0.3 OD with respect to the nominal values 

to eliminate the detection of brightness cues.

The effect of the different intensity settings and ND 

filters was investigated to establish the extent to which 

the chromaticities of the white and red lights changed 

with lamp current setting and/or the use of ND filters 

(see Fig. 17). The results show that the ND filters caused 

only small changes in the chromaticity coordinates of 

the white, and even less for the red light. Changes in 

lamp current caused larger changes in the chromaticity 

of the white light, but in spite of these changes the white 

remained within the “variable white” area indicated on 

the CIE diagram, as appropriate for AGL (see Fig. 15 and 

17). In the case of real PAPI lights, other factors, such as 

atmospheric absorption, can also affect the chromaticity 

of the white, with very little effect on the red.

2.2 Testing Procedure

The four horizontal lights are presented for 3 seconds 

and the subject’s task is to simply report the number of 

red lights in the display. There are five possible combinations of red and white lights that are presented randomly 

(Fig. 18, left). When carrying out the PAPI simulator test, 

Figure 16: Schematic representation of PAPI simulator designed and constructed for this 

study. Light emerging from each arm of the lamp house is divided into two channels via beam 

splitters (BS) to produce four independent channels. Each channel has a color wheel (CW) 

and a neutral density wheel (NDW) which are controlled by the computer. After passing 

through appropriate filters, the light from each channel is focused into an optical fiber head 

which are attached to the viewing panel so as to simulate the PAPI lights. 
19

PAPI test

1

0

2

4

3

PAPI test

1

0

2

4

3

PSL test

W

CC

R

Y

G

B

PSL test

W

CC

R

Y

G

B

Figure 18: Schematic representation of the Precision 

Approach Path Indicator (PAPI) simulator test (left) and 

PAPI Signal Lights (PSL) test (right). The PAPI test 

presents 5 different conditions (as shown) twelve times 

giving a total of 60 presentations. The PSL presents 6 

different conditions (as shown) twelve times giving a total 

of 72 presentations. The intensities of the colored lights 

varied randomly by ±0.3 OD with respect to the nominal 

values to eliminate the detection of brightness cues. 

0.3

2

0.68 0.69 0.7 0.71 0.72

x

0.28

0.29

0.3

0.31

0.32

y

0.3

2

0.44 0.49 0.54 0.59

x

0.35

0.4

0.45

0.5

y

+ND

+ND

6.6

2.57

0.68 0.69 0.7 0.71 0.72

x

0.28

0.29

0.3

0.31

0.32

y

6.6

2.57

0.44 0.49 0.54 0.59

x

0.35

0.4

0.45

0.5

y

CIE-x,y diagram CIE-x,y diagram

The effect of ND filtering

The effect of current setting

CIE-x,y diagram CIE-x,y diagram

+current

4.00A

Figure 17: Graphs showing the CIE-x,y chromaticity coordinates of the Red 

and White lights under the effect of neutral density (ND) filtering and the effect 

of the current setting (intensity) on the lamp in amps (A). 
20

observers were instructed to report the number of red lights 

using the following names: one, two, three, four, or zero 

(to avoid confusing “none” with “one”). Prior to the test, 

observers were allowed to dark adapt to the low mesopic 

surround and then were presented with a practice run. A 

low power lamp was placed behind the test equipment to 

provide low mesopic conditions of ambient illumination. 

The black, immediate surround around the PAPI lights, 

was dark (i.e., mean luminance ~0.005 cd/m2). Subjects 

were encouraged to respond only after an auditory cue 

signaled the end of the 3-second viewing period. The 

PAPI test was carried out twice, once with the standard 

white (~2400K) and once with a modified white light 

(higher color temperature of ~3900K).

The PSL uses similar parameters to the PAPI 

lights test. In this test six possible colors are presented 

(standard white, modified white, red, green, blue, and 

yellow). The chromatic properties of the lights lie 

within the boundaries for the recommended signal 

lights for AGL (CAA, 2004) as shown in Fig. 15. The 

PSL addresses the issue of correct color naming when 

all lights have the same chromaticity, as opposed to 

the ability to distinguish and categorize some of the 

four lights as red and the others as white on the basis 

of some perceived differences between the lights. The 

PSL tests whether the applicant can recognize and 

name reds as “red” and whites as “white’ for the same 

conditions and geometry as the PAPI lights, but when 

all the lights are of the same color. The conditions when 

all four PAPI lights have the same color to indicate 

“far too low” (all reds) or “far too high” (all whites) 

are clearly very important. Observers were instructed 

to report the color of the lights as either: red, green, 

yellow, blue, or white.

There were two whites: the standard white, as produced by the lamp, and a modified white, produced by 

raising the color temperature of the standard white by 

200 MIREDS. This is achieved by using a color correction filter that modifies the spectral content of the 

tungsten light to make it more like daylight. Prior to 

the test, observers were presented with a practice run. 

All the colors were shown to the subject and named by 

the examiner during the practice run, and the subject 

was allowed to review any of the lights and to ask the 

examiner to confirm the color. The results for the PAPI 

and PSL are recorded as the percent correct.

3.0 Results

The color vision of each subject was examined us-

ing five different color vision tests, as well as the PAPI 

and PSL simulator tests. Results from each of the five 

tests were then examined, in relation to the subject’s 

performance on the PAPI, to establish which test yields 

the best prediction of performance in the PAPI task. 

Performance on the PAPI task is computed as number of 

correct reports out of a total of 60 presentations.

The results summarized in Fig. 19 show that normal 

trichromats can also make errors, both on the PAPI and 

the Ishihara tests (i.e., five subjects produce one error, 

one subject produces two errors, and one other subject 

produces three errors on the PAPI). The rest of the normal 

80 85 90 95 100

0

20

40

60

80

100

PAPI %( c

orre

ct)

0 20 40 60 80 100

0

20

40

60

80

100

PAPI %( c

orre

ct)

0 20 40 60 80 100

Ishihara percent correct (all 24 plates)

0

20

40

60

80

100

PAPI %( c

orre

ct)

Normals

Deutans

Protans

Ishihara (% correct) 11 28

2

Figure 19: The number of plates read correctly expressed 

as a percentage on the Ishihara test (24 plates) is 

compared to performance on the PAPI simulator test 

separately for normals, deutan and protan color vision 

observers. The x-axis for the top graph has been 

expanded to show clearer the errors made by normals.
21

subjects score 100% correct on the PAPI test. Results for 

deutan color deficient observers reveal that all subjects 

with scores >70% (i.e., 16 or more correct plates out of 

24 on the Ishihara 24-plate test) pass the PAPI with a 

score of 100% correct. Results for protan observers show 

that four subjects with scores greater than 40% pass the 

PAPI test. Overall, the results reveal very poor correlation 

between the subjects’ performance on the Ishihara and 

the PAPI test scores. Many of the subjects that pass the 

PAPI task can score anything from 0 to 95% correct on 

the Ishihara test.

Comparisons of data from the Dvorine plate test with 

the PAPI simulator show similar results to those obtained 

with the Ishihara test (Fig. 20). Three normals obtain less 

than 100% on the Dvorine test (but pass the PAPI with 

no errors). Deutan and protan color deficient subjects 

need more than 65 and 50%, respectively, on the Dvorine 

plate test to achieve 100% on PAPI. Since the prediction 

of the class of deficiency (protan or deutan) involved is 

poor with both Ishihara and Dvorine tests, it is difficult 

to know which of the two limits one should apply to any 

color deficient subject.

Fig. 21 plots the PAPI scores against the subjects’ 

performance on the ALT test. All normals secure 100% 

score on the ALT, but not on the PAPI test. Results for 

deutanobservers show that out of 77 subjects tested, 

only 18 passed the ALT (with a pass criterion of one 

or no errors). Fourteen of the 18 subjects that passed 

the ALT also passed the PAPI. Results for protans 

show that all 40 subjects tested had failed the ALT 

test and that only one subject achieved a score higher 

than 80%, although just over 50% of protans passed 

the PAPI.

Fig. 22 compares PAPI scores with a measure of RG 

sensitivity based on the Nagel anomaloscope range. 

Only a few deutan and protan observers pass the 

PAPI with Nagel sensitivity >0.6 (deutan) and >0.4 

(protan). The Nagel anomaloscope test is excellent 

at distinguishing between deutan- and protan- like 

deficiencies, but fails to quantify reliably the severity 

of color vision loss, and does not test for yellow/blue 

deficiency.

PAPI test scores in Fig. 23 are plotted against the 

corresponding CAD based measure of RG sensitivity. 

The top section shows the performance in normal 

trichromats. Most normal trichromats perform 100% 

correct in the identification of the red and white lights. 

However, 7 out of the 65 normal subjects made some 

errors. This could be due to lack of attention and/or 

reduced visual acuity at low light levels caused by 

increased higher order aberrations when the pupil 

size is large. The errors were found to be distributed 

with equal probability among the five conditions. The 

blue dotted line in Fig. 23 shows the 95% confidence 

interval. The lower sections compare the performance 

on the PAPI test with the corresponding CAD measure of RG sensitivity for subjects with deutan- and 

protan-like deficiencies. The sensitivity limits beyond 

which deutans and protans perform the PAPI test, as 

well as normal trichromats, are 0.17 (RG threshold ~6 

SN units) and 0.085 (RG threshold ~12 SN units), 

respectively. These limits are indicated by dotted 

vertical lines in Fig. 23.

80 85 90 95 100

0

20

40

60

80

100

PAPI %( c

orre

ct)

0 20 40 60 80 100

0

20

40

60

80

100

PAPI %( c

orre

ct)

0 20 40 60 80 100

Dvorine percent correct (all 14 plates)

0

20

40

60

80

100

PAPI %( c

orre

ct)

Normals

Deutans

Protans

Dvorine (% correct)

3

41

4

Figure 20: PAPI percent correct scores plotted as a 

function of the number of plates read correctly on the 

Dvorine test (expressed as a percentage) for normals, 

deutan and protan observers. 
22

80 85 90 95 100

0

20

40

60

80

100

PAPI %( c

orre

ct)

0 20 40 60 80 100

0

20

40

60

80

100

PAPI (%

c

orre

ct)

0 20 40 60 80 100

ALT (% correct)

0

20

40

60

80

100

PAPI (%

c

orre

ct)

Normals

Deutans

Protans

ALT (% correct) 59

10

Figure 21: The number of presentations identified 

correctly on the Aviation Light Test (ALT) from a total of 

27 presentations is compared to performance on the 

PAPI simulator test separately for normals, deutan and 

protan color vision observers.
23

0 0.5 1 1.5 2

0

20

40

60

80

100

PAPI (%

c

orre

ct)

0 0.5 1 1.5 2

0

20

40

60

80

100

PAPI (%

c

orre

ct)

0 0.5 1 1.5 2

Nagel (4/rg range)

0

20

40

60

80

100

P

A

PI %( c

orre

ct)

Normals

Deutans

Protans

Nagel sensitivity

2 3 11 15 19 6

Figure 22: PAPI test scores plotted against an index of 

red-green chromatic sensitivity based on the Nagel 

anomaloscope range. Results are again shown 

separately for normals, deutan and protan observers. 

Numbers next to some symbols indicate number of 

subjects with overlapping data points. 
24

0 0.1 0.2 0.3 0.4 0.5 0.6

0

20

40

60

80

100

PAPI %( c

orre

ct)

0 0.1 0.2 0.3

CAD sensitivity (1/threshold)

0

20

40

60

80

100

PAPI %( c

orre

ct)

0.4 0.6 0.8 1 1.2 1.4 1.6

0

20

40

60

80

100

PAPI %( c

orre

ct)

Normals

Deutans

Protans

CAD sensitivity

95 % C.I.

2 2 3

Figure 23: Graphs showing performance of normal, 

deutans and protan observers on the PAPI (standard 

white) versus CAD test sensitivity (1/threshold). Note the 

scale of the x-axis is different for the three graphs for 

clarity of presentation. 
25

Fig. 24 shows the benefit of replacing the standard 

white light in the PAPI test with the color-corrected 

(CC) white. Higher PAPI performance scores were 

obtained with the CC white in all subject groups. All 

but one normal trichromat scored 100% correct on the 

PAPI test when using the CC white. Fig. 24 shows the 

overall improvement observed in the deutan group. All 

deutans obtain 80% correct or higher, and many score 

100% correct with the CC white. The improvement is, 

however, very small among protanomalous observers, 

particularly among subjects with severe deficiency (i.e. 

those with RG sensitivity less than 0.05 units). The 

improvements were statistically significant in normal 

trichromats (p=0.019) and deutans (p=0.000). Protans 

also showed slightly higher PAPI performance scores 

with the CC white, but the improvement failed to reach 

statistical significance (p=0.224).

The PSL test was introduced to examine the condition 

when all four PAPI lights have the same color. In this 

condition the subjects can no longer make use of perceived differences between two adjacent colors presented 

simultaneously without being able to name the correct 

color. The results in Fig. 25 show that subjects do not 

often confuse reds with whites or whites with reds. Four 

protan subjects with severe loss of chromatic sensitivity 

(i.e., CAD RG sensitivity less than 0.05 units) make W=R 

and R=W errors, and six deutan subjects with CAD RG 

sensitivity less than 0.15 units make errors on W=R.

3.1 Computing an Index of Overall Chromatic 

­Sensitivity

The threshold signal needed to just see the colored 

stimulus is expressed by direct comparison with the median 

threshold for normal trichromats. This approach has the 

advantage that a threshold <1 indicates color discrimination better than the standard normal trichromat, whereas 

values >1 indicate precisely the increase in threshold signal 

with respect to the normal observer. Sensitivity is usually 

0.4 0.6 0.8 1 1.2 1.4 1.6

0

20

40

60

80

100

0 0.1 0.2 0.3 0.4 0.5 0.6

0

20

40

60

80

100

0 0.1 0.2 0.3

CAD Sensitivity (1/threshold)

0

20

40

60

80

100

Normals

Deutans

Protans

Standard White Modified White

0 0.1 0.2 0.3 0.4 0.5 0.6

0

20

40

60

80

100

PAPI %( correct)

0 0.1 0.2 0.3

CAD sensitivity (1/threshold)

0

20

40

60

80

100

PAPI %( correct)

0.4 0.6 0.8 1 1.2 1.4 1.6

0

20

40

60

80

100

PAPI %( correct)

3

Figure 24: Graphs showing comparisons between standard and modified white (colorcorrected white) versus the CAD test sensitivity values. 
26

defined as the reciprocal of the signal strength needed to 

just see the stimulus. The normalized CAD threshold falls 

within the range ~0.7 (i.e., better than normal sensitivity) 

to a maximum of 23.7 (a value limited by the phosphor 

limits of the display). Therefore, it is reasonable to use the 

reciprocal of the CAD threshold as a measure of chromatic 

sensitivity, which ranges from just above 1 for subjects 

with better than normal sensitivity, to ~0.04 for subjects 

with very slight or absent chromatic sensitivity. In the 

case of the most common occupational color vision tests, 

loss of chromatic sensitivity is more difficult to quantify 

because the tests do not measure the smallest color signal 

strength needed to detect the stimulus. Instead, most occupational tests yield scores of correct responses that are 

indicative of the subject’s overall chromatic sensitivity. In 

spite of these limitations, it is of great interest to derive 

an index of mean chromatic sensitivity based on the 

subject’s performance in several color vision tests. With 

this aim in mind, we used the parameters of each test to 

derive the best measure of average chromatic sensitivity, 

which ranges from around 1 for normal trichromats, to 

close to zero for subjects with very limited or complete 

absence of color discrimination:

Ishihara: Fraction of plates named correctly

Dvorine: Fraction of plates named correctly

ALT: Fraction of stimulus pairs named correctly

Nagel: See definition of RGI (i.e., red-green 

discrimination index)

CAD: Reciprocal of threshold signal when measured in standard normal CAD units

Using this approach, we were able to quantify the 

subject’s average chromatic sensitivity derived from 

the above tests in which we take the best available 

measure of the subject’s overall ability to cope with a 

variety of color vision tasks. This measure of average 

chromatic sensitivity has been used to further justify 

the selection of minimum color vision requirements 

that can be classified as safe within the aviation environment and the exclusion of the very few subjects 

with poor overall RG chromatic sensitivity that pass 

the PAPI test.

Fig. 26 shows that the very few subjects that pass 

the PAPI with CAD sensitivities less than 0.2 (deutan) 

and 0.1 (protan) have poor overall chromatic sensitivity. The results also show that the pass/fail limits 

proposed on the basis of the CAD test ensure that 

the color deficient subjects that pass have an overall 

chromatic sensitivity greater than ~0.7 (deutans) and 

greater than ~0.5 (protans).

4.0 Discussion

4.1 Color Vision Concerns in Aviation

Color enhances object conspicuity and can also be 

used to code information. In overcrowded displays or 

complex visual scenarios, color allows segmentation and 

grouping operations which enhance and speed up the 

acquisition and processing of visual information (Firth, 

2001). The primary signal colors in aviation are red, 

green, and white, with blue and yellow as supplementary 

colors (CAA, 2006b). ICAO requires member nations to 

maintain a color standard to ensure pilots can recognize 

the colors of signal lights used in aviation (ICAO, 2006). 

Further, it acknowledges that the requirements are open to 

interpretation; and at present, there is no clearly defined 

line that specifies the class and severity of color deficiency 

beyond which the applicant is no longer safe to fly (for 

a full review, see CAA, 2006a). This is partly due to the 

varied task requirements and the technological advances 

in the use of color in the aviation industry.

There are a number of currently approved JAA and 

FAA color vision tests that applicants have to pass to 

be licensed to fly. In this investigation, we assessed 182 

subjects using several color vision tests, and the results 

reveal both inter-subject variability and poor consistency 

0 0.2 0.4 0.6

0

20

40

60

80

100

P

S

L

o

n

yl R&W

%( c

orre

ct)

0 0.1 0.2 0.3

CAD Sensitivity (1/threshold)

0

20

40

60

80

100

P

S

L

o

nly R&W

%( c

orre

ct)

Deutans

Protans

CAD sensitivity

Figure 25: Graphs showing R=W and W=R errors only 

made on the PSL versus the RG CAD sensitivity. 
27

0 0.1 0.2 0.3

CAD sensitivity (1/threshold)

0

20

40

60

80

100

PAPI %( c

orre

ct)

Protans

0 0.1 0.2 0.3 0.4 0.5 0.6

CAD Sensitivity

0

20

40

60

80

100

PAPI (%

c

orre

ct)

Normals

Deutans

0 0.2 0.4 0.6 0.8 1

Mean colour sensitivity

0

20

40

60

80

100

P

A

PI %( c

orre

ct)

0 0.2 0.4 0.6 0.8 1

Mean colour sensitivity

0

5

10

15

20

25

C

A

D

R

G thre

s

h

old

u

nits

Figure 26: Justification for limits chosen. The top two diagrams compare PAPI (% 

correct) versus CAD test (sensitivity units=1/RG CAD threshold); color deficient 

subjects to the right of the dotted line perform the PAPI test as well as normal 

trichromats do. CAD sensitivity of 0.2 and 0.1 corresponds to 6 and 12 CAD 

threshold units, respectively, for deutans and protans. Note the change of 

horizontal scale units for protan observers. The bottom diagrams compare: left, 

CAD RG threshold units and right, PAPI performance, versus the “mean” or overall 

color sensitivity as computed by averaging the subject’s performance on the 

Ishihara, Dvorine, ALT and Nagel anomaloscope tests. 
28

among the various tests. Comparison of the outcome 

from Ishihara and the Dvorine tests shows that significant 

variability remains even when, at least in principle, the 

tests are very similar. These findings expose the facts that 

color deficient subjects can produce very different scores 

on the two tests and that normal trichromats can make 

more errors on one test, but not on the other. Further, 

the pseudoisochromatic tests designed for screening are 

not suitable to either accurately diagnose the class of 

deficiency, or to quantify the severity of color vision loss 

(Belcher et al., 1958; Birch, 1997).

The measures of chromatic discrimination on the CAD 

and Nagel anomaloscope reveal very poor correlation. 

Some color deficient observers that require a lot of red or 

green in the match can only accept a narrow, red-green 

range that is a characteristic of normal red/green color 

vision. Fig. 22 confirms well-known observations (Wright, 

1946) in that the measure of chromatic sensitivity based 

on the Nagel range overestimates the subject’s ability 

to discriminate red-green color differences under more 

natural conditions of illumination. Subjects with severe 

loss of chromatic sensitivity, as measured on the CAD 

test, can have RGI (Nagel) values that are similar to those 

measured in normal trichromats. The JAR pass criterion 

of 4 scale units (RGI=0.95) suggests that some normals 

are likely to fail, and of greater concern, that some severe 

color deficient subjects are likely to pass.

Fig. 21 shows comparisons between the ALT and the 

PAPI simulator tests. The results suggest that the ALT 

is a more challenging test, i.e. the average percent of 

correct scores are lower on the ALT than on the PAPI 

test. However, normals were found to make no errors on 

the ALT. The fact that color deficient subjects perform 

worse on the ALT than on the PAPI could be due to the 

increased number of colors presented on the ALT (three 

instead of two) and the very dark immediate surround 

in which the apertures of the lights presented are encased 

in the ALT.

These discrepancies among the various screening tests 

can be attributed partly to the differences in the methods 

of assessing color vision. These include differences in 

stimulus configuration, background lighting conditions, 

and also the different instructions given to subjects. 

The results from this study justify the recent concerns 

expressed by ICAO (1999) and CAA (2006a) in relation 

to some aspects of color assessment in aviation. The results confirm earlier findings from a study that examined 

three lantern types accepted by the JAA and found that 

the same individual can pass some of the tests and fail 

others (Squire et al., 2005). The study also showed that 

some normal trichromats that fail the initial screening 

may also fail the secondary test, and that the outcome of 

regulatory assessment depends on which secondary color 

vision test is used, which varies between countries. The 

observed inconsistency in results between the currently 

approved color vision tests is therefore unsatisfactory, both 

in terms of flight safety issues, as well as being potentially 

unfair to some pilot applicants.

4.2 Advances in Assessment of Color Vision

Red-green chromatic sensitivity varies from “normal” 

performance to total “color-blindness”, with an almost 

continuum of color impairment between these two extremes. Among congenital color deficient observers, the 

loss of RG color sensitivity varies along a continuous scale 

(Fig. 12). This is the most common type of color vision 

deficiency, affecting 8% of the male population (<1% 

females; see table in section 4.4). Yellow-blue deficiencies affecting the S-cone are very rare (1 in ~20,000) and 

are most often related to acquired color vision defects, 

as a result of eye disease, or as a side effect of toxicity 

or medication (see section 1.6 in this report). Acquired 

deficiencies tend to be age-dependent and when unnoticed, may compromise safety within certain occupations. 

Follow up color vision tests are carried out for some Class 

1 renewal of medical certificates (JAR-FCL 3, 2000). 

Current civil aviation tests are not designed to detect or 

measure YB sensitivity, so any anomalies due to YB loss 

or disease are likely to remain undetected. Yellow-blue 

discrimination has also become more important because 

of the extensive use of different colors in the modern 

aviation environment.

4.3 Safe Color Vision Limits in Aviation

The PAPI system built for this investigation reproduces 

the red and white lights under conditions simulating 

the spectral composition, the angular subtense, and the 

retinal illuminance of real PAPI lights viewed from 5.54 

km. Comparisons between the performance on the PAPI 

and CAD tests reveal the minimum level of chromatic 

discrimination, below which subjects with red/green color 

deficiency no longer perform the PAPI task with the same 

accuracy as normal trichromats. Fig. 23 shows that deutans 

and protans with a RG threshold of less than 6 and 12 

CAD units, respectively, perform the PAPI simulator test 

in a similar way to normal trichromats. The PSL test addresses the question as to whether subjects can also name 

correctly the color of the PAPI lights when all lights are 

of the same color, either red or white. Fig. 25 shows how 

the ability to correctly name the white and red lights on 

the PSL test relates to the subject’s RG discrimination 

sensitivity on the CAD test. Color deficient subjects do 

not have difficulty with this condition, and no subject 

with RG threshold units less than 6 and 12 for deutans 

and protans, respectively, confuse R=W or W=R when 

all four lights are the same color. The PAPI and PSL tests 
29

also include a CC white to investigate whether the use 

of a more “bluish” white can improve performance. The 

results show that this is indeed the case for both PAPI 

and PSL tests in all subject groups.

Using the proposed pass and fail limits of 6 and 12 RG 

CAD threshold units for deutans and protans, respectively, 

29 out of 77 deutans and 13 out of 40 protans would pass 

the PAPI simulator test using the standard white (Fig. 27). 

However, five deutans and seven protans (Fig. 28) with 

RG thresholds larger than the proposed safe limits also 

pass the PAPI. An important question is whether these 

subjects are disadvantaged unfairly if the new pass/fail 

limits were to be adopted. There is little doubt that these 

subjects have severely reduced RG color discrimination 

(as revealed in all the color vision tests employed in this 

investigation). The overall loss of chromatic sensitiv-

ity becomes increasingly more severe as the subject’s 

thresholds increase beyond the recommended limits. 

These subjects are therefore likely to be disadvantaged 

in other, less safety-critical, visual tasks that involve color 

discrimination. By computing an average chromatic discrimination performance on a battery of color vision tests, 

we are able to examine whether these subjects have poor 

overall color discrimination performance. Fig. 26 shows 

PAPI (standard white)

77

Overall < 6 > 6

pass

fail

CAD threshold range 

0

43 43

34

5

29

PAPI (standard white)

77

Overall < 6 > 6

pass

fail

CAD threshold range 

0

43 43

34

5

29

PAPI (modified white)

77

Overall

pass

fail

< 6 > 6

CAD threshold range

0

26 26

57

28

29

PAPI (modified white)

77

Overall

pass

fail

< 6 > 6

CAD threshold range

0

26 26

57

28

29

Figure 27: Summary of deutan subjects’ results if the proposed pass/fail criterion of 6 RG CAD 

threshold units is accepted. 

Overall < 12 > 12

CAD threshold range

40

pass

fail

PAPI (modified white)

0

17 17

23

10

13

Overall < 12 > 12

CAD threshold range

40

pass

fail

PAPI (modified white)

0

17 17

23

10

13

Overall < 12 > 12

CAD threshold range

40

pass

fail

PAPI (standard white)

0

20 20

20

7

13

Overall < 12 > 12

CAD threshold range

40

pass

fail

PAPI (standard white)

0

20 20

20

7

13

Figure 28: Summary of protan subjects’ results if the proposed pass/fail criterion of 12 RG CAD 

threshold units is accepted. 
30

that the very few subjects that pass the PAPI with CAD 

threshold limits greater than 6 (deutan) and 12 (protan) 

units have poor overall chromatic sensitivity. The results 

also show that the pass/fail limits proposed on the basis 

of the CAD test ensure that the color deficient subjects 

that pass have an overall RG chromatic sensitivity greater 

than ~0.7 (deutans) and greater than ~0.5 (protans).

The use of the color-corrected white condition increases 

quite significantly in the number of subjects that passed 

the PAPI test. For deutans (Fig. 27) with thresholds 

higher than 6 units, 28 subjects passed instead of only 

five with the standard white condition. Similarly, for 

protans (Fig. 28), ten passed in the modified condition, 

instead of seven for the standard white condition. Overall, 

the CC white condition improves performance of the 

PAPI test for all subject groups. This improvement is, 

however, more significant for deutan than protan color 

deficient observers.

The analysis shown graphically in Figs. 27 and 28 

reveal that 37.6% of subjects with deutan-like deficiency 

and 32.5% of subjects with protan-like deficiency would 

be classified as safe to fly under the new research-based 

pass/fail limits. When using the current PAPI systems 

(i.e. PAPI with the standard white light), 44% of deutan 

subjects passed the PAPI test and, according to the proposed pass/fail limit of 6 SN CAD units, 85% of these 

subjects (i.e., the deutans that passed the PAPI) would 

be classified as safe to fly. Similarly, 50% of subjects with 

protan-like deficiency passed the PAPI test, and according 

to the pass/fail limit of 12 SN CAD units, 65% of these 

subjects would be classified as safe to fly.

So far, 386 color deficient observers have been examined on the CAD test as part of this study, as well as 

other ongoing studies. Two hundred fifty-five (255) of 

these subjects had deutan and the remaining 131 had 

protan deficiencies. If the proposed limits of 6 and 12 SN 

CAD units for deutan and protan subjects, respectively, 

were applied to this larger group, 36.1% of deutans and 

29.8% of protans would pass these limits. These findings 

suggest that 35% of the total number of color deficient 

subjects investigated should be certified as safe to fly. The 

percentages shown above are very close to those estimated 

from the smaller number of color deficient subjects (n = 

117) that participated in the PAPI study.

The proposed pass/fail limits relate accurate measurements of chromatic sensitivity to the subjects’ performance on the most critical color vision task in aviation. 

The limits ensure that the subjects that pass can also 

perform the PAPI task with the same accuracy as normal 

trichromats. In addition, the proposed limits ensure that 

all subjects that fail according to these limits have poor, 

overall chromatic discrimination sensitivity.

4.4 Benefit Analysis of Using the New Approach

a. Analysis Based on CAA/JAA Pass Criteria and 

Guidelines

Table 1 shows the different classes and relative distribution of color deficient subjects that make up ~8% of 

the male population (Sharpe, et al. 1999).

The data in Table 2 are based on the strict CAA/JAA 

pass criteria for the Ishihara test. A similar analysis using 

the FAA pass/fail criteria for Dvorine, Ishihara, and ALT 

tests will be shown separately. Interestingly, only 7% of 

color deficients pass the Holmes-Wright (HW) lantern, 

and there is no clear evidence to suggest that all of these 

subjects are safe to fly.

One hundred sixty-three (163) color deficients (i.e., 

110 deutan and 53 protan subjects) have been investigated 

using the HW lantern in all of our studies. All protans 

failed, but only 99 of 110 deutan subjects failed the HW 

lantern. These findings make it possible to predict the 

number of color deficient subjects that are classified as 

safe to fly using current assessment procedures.

Table 3 shows the predicted outcome when the same 

1,000 applicants were examined on the CAD test and 

the pass/fail criteria employed are based on the findings 

Table 1: Percentage of color deficient observers that fail Ishihara and HW tests. 

Protanope Deuteranope Tritanope P-nomalous D-nomalous T-nomalous Total

1 1.1 0.002 1 4.9 0 8.002

†Gegenfurtener, K.R. & Sharpe, L.T. "Color Vision, from Genes to Perception" : Cambridge University Press.

 Percentage of normal trichromats that fail the Ishihara test* 15.8

 Percentage of protan subjects that fail the Ishihara test 100

 Percentage of deutan subjects that fail the Ishihara test 99

 Percentage of normal trichromats that fail the Holmes-Wright lantern test 0

 Percentage of protan subjects that fail the Holmes-Wright lantern test 100

 Percentage of deutan subjects that fail the Holmes-Wright lantern test 90

Accepted Prevalence of Color Vision Deficiencies†

Other facts based on normal trichromats and colour deficient subjects studies at AVRC

*Results based on 202 normal trichromats when employing the strict CAA/JAA pass criteria (i.e., no 

errors, no misreadings in the first 15 plates of the 24-plate version). 
31

from this study. The current procedures that employ a 

secondary test whenever the applicant fails the primary 

test results in 22% of the applicants having to take the 

secondary test. The new procedure based on the CAD 

pass/fail limits does not require any secondary test. The 

analysis in Table 3 shows that 36% of deutan and 30% 

of protan subjects meet the pass/fail criteria established 

experimentally and can therefore be classified as safe to 

fly. Given the higher prevalence of deutan subjects within 

the male population, these findings suggest that 35% of 

all color deficient subjects pass the new guidelines and 

are therefore safe to fly.

If the HW secondary test is replaced by the Nagel 

anomaloscope test (using the current JAA pass/fail limits 

for midpoint and range), the percentage of color deficient 

subjects that are classified as safe to fly decreases to less 

than 2%. If the HW lantern test is replaced with other, 

less demanding secondary tests, the percentage of color 

deficient subjects that pass can increase to more than 10%. 

There is, however, no guarantee that the color deficient 

subjects that pass these secondary tests can carry out the 

most safety-critical, color-related tasks.

b. Analysis Based on FAA Pass/Fail Criteria and 

Guidelines

The FAA pass/fail criteria are more liberal and were 

selected deliberately to be fair to those color deficient applicants that may well be able to carry out safety-­critical, 

color-related tasks with the same accuracy as normal 

trichromats. The criteria are not, however, based on the 

PAPI task and employ a large number of recommended 

tests that fail to correlate well when used to quantify the 

severity of color vision loss. This means that some of the 

applicants fail some tests and pass others. In this investigation, we have included two of FAA’s most popular 

tests, the Ishihara and the Dvorine pseudoisochromatic 

plates. The ALT (i.e., a Farnsworth Lantern (FALANT) 

with modified filters designed to produce lights with 

chromaticities specified for aviation signal lights (Milburn 

& Mertens, 2004)) was included in the study. Normal 

Table 2: Predicted outcome per thousand applicants using current CAA/JAR guidelines. 

No. that fail No. that fail No. classed as

Applicants 1000 Ishihara HW safe to fly

Normals 920 145 0 920

Deutans 60 59 53 6

Protans 20 20 20 0

Total 1000 225 73 926

22

10

0

7

 Percentage of applicants that undergo secondary tests = 

 Percentage of deutan subjects that pass secondary tests =

 Percentage of protan subjects that pass secondary tests =

 Percentage of total color deficient subjects that pass = 

Predicted outcome per 1000 applicants using current assessment methods

Table 3: Predicted outcome per thousand applicants using the new, CAD based 

pass/fail limits. 

No. that pass No.that fail set No. classed as

Applicants 1000 CAD as normals CAD limits safe to fly

Normals 920 920 0 920

Deutans 60 0 38 22

Protans 20 0 14 6

Total 1000 920 52 948

0

36

30

35

** % deutan subjects that pass CAD (pass < 6 SNU) = 36.1

** % protan subjects that pass CAD (pass < 12 SNU) = 29.8

 Percentage of of deutan color deficients that pass = 

 Percentage of total color deficient subjects that pass = 

 Percentage of protan color deficients that pass =

Predicted outcome per 1000 applicants using CAD pass / fail criteria**

 Percentage of applicants that undergo secondary tests = 
32

trichromats pass both the FALANT and the ALT lanterns, 

but no data are available to describe how the deutan and 

protan subjects examined in this study perform on the 

FALANT test. The following analysis is, however, of great 

interest since it provides a useful comparison of expected 

outcomes when FAA guidelines are followed.

Table 4 lists the percentage of normals, deutan, and 

protan subjects that fail each of the three tests: Ishihara, 

Dvorine, and ALT, according to the FAA criteria. The 

first observation of interest is that the Dvorine test passes 

the largest number of deutan and protan subjects. The 

Ishihara is more demanding for both deutan and protan 

subjects, and all protan subjects fail the ALT test. In the 

following sections we examine the correlation between 

the outcome of each of these tests and the corresponding 

PAPI pass/fail scores for the 182 subjects investigated.

The contingency tables below (i.e., Table 5) show that 

only one deutan subject (i.e., less than 1%) passed the 

Ishihara according to the FAA criterion and failed the 

PAPI. Not unexpectedly, the same subject also passed 

the Dvorine test and failed the PAPI. Of equal interest 

is the observation that 31% of color deficients (deutan 

and protan) failed the Ishihara but passed the PAPI. The 

results also show that only 24% of the color deficients 

that failed the Dvorine test passed the PAPI. It is also of 

interest to investigate the outcome if the ALT were used 

as a secondary test for those deutan and protan pilot 

applicants that fail either Ishihara or Dvorine tests using 

the FAA pass/fail criteria.

This analysis should be strictly carried out using the 

applicant’s performance data on the FALANT test, but 

in the absence of such data, the ALT test provides the 

Table 4: Percentage of color deficient observers that fail Ishihara, Dvorine and ALT tests 

(using FAA pass/fail criteria for the Dvorine and the 24-plate Ishihara test). 

0

90

 Percentage of deutans that fail the Ishihara test 81.8

 Percentage of normal trichromats that fail the Dvorine plates 0

 Percentage of protans that fail the Dvorine test 82.5

 Percentage of deutans that fail the Dvorine test 75.3

 Percentage of normal trichromats that fail the ALT 0

 Percentage of protans that fail the ALT 100

 Percentage of deutans that fail the ALT 77.9

Other percentages based on the FAA pass / fail guidelines*

 Percentage of normal trichromats that fail the Ishihara test

 Percentage of protans that fail the Ishihara test

*The results listed in the table are based on the FAA pass limits for Ishihara/Dvorine tests (fail equals 

7 or more errors). Percentages based on 65 normal trichromats and 117 color deficients (i.e., 77 

deutan and 40 protan subjects). All protans failed the ALT test, but only 60 out of 77 deutans failed 

the same test (pass classification on the ALT requires no more than one error on 27 trials).

Table 5: Contingency tables showing results of Ishihara, Dvorine and ALT tests and the 

corresponding pass/fail PAPI scores. 

Pass Fail P F P F

Pass 62 3 P 4 0 P 13 1

Fail 0 0 F 16 20 F 20 43

P F P F P F

P 62 3 P 7 0 P 18 1

F 0 0 F 13 20 F 15 43

P F P F P F

P 62 3 P 0 0 P 17 0

F 0 0 F 20 20 F 16 44

Ishihara

PAPI

IshiharaIshihara

PAPI Normals (65) PAPI

PAPI

DvorineDvorine

PAPI

Protans (40)

ALT

ALTALT

Dvorine

PAPI PAPI

Number of subjects that pass/fail each test (i.e., a pass requires less than 7 errors on 

Ishihara; less than 7 errors on Dvorine; one or no errors on ALT)

Deutans (77)

Protans (40)

Protans (40) Deutans (77)

Deutans (77)

Normals (65)

Normals (65) PAPI

PAPI
33

nearest, useful substitute. Findings from a recent study 

(Cole & Maddocks, 2008) suggest that some subjects that 

pass the FALANT test can confuse red and white signals 

and may not therefore perform the PAPI task as well as 

normal trichromats. Table 6 compares the subjects pass/fail 

results on Ishihara and Dvorine against the corresponding 

pass/fail results on the ALT. The results show that it is 

possible to pass Ishihara or Dvorine and then to fail the 

ALT. In the case of Dvorine, six deutan and seven protan 

subjects pass the test only to fail the ALT. Interestingly, 

the reverse to this also happens. Four deutans that fail 

the Dvorine pass the ALT and six deutans that fail the 

Ishihara also pass the ALT.

These findings clearly reveal the lack of agreement that 

exists when using different occupational color screening 

tests and the difficulties involved if one wished to use 

such data to predict performance in other tasks, such 

as the PAPI.

Table 7 shows the predicted outcome per thousand 

applicants when using Ishihara, Dvorine, and ALT tests. 

Although the FALANT is an accepted FAA alternative 

test, the ALT test (used by FAA for assessing air traffic 

controllers) was employed in this study. The ALT fails all 

protan subjects and passes only 16% of deutan applicants. 

The results are of interest since they show clearly that an 

applicant is most likely to pass the Dvorine test, followed 

by ALT, and Ishihara when only deutan subjects are 

considered. In the case of protan deficiency, an applicant 

is most likely to pass the Dvorine, followed by Ishihara, 

but all protans fail the ALT test.

5.0 Conclusions

The aim of this project was to develop new methods 

for accurate assessment of color vision and to provide 

evidence-based guidelines for minimum color vision 

requirements for flight crew. The current diversity in 

color vision testing methods and standards demonstrates 

the need to adopt more objective assessment techniques 

internationally and to set minimum color vision requirements that are both safe and fair to the applicants.

The CAA/JAA guidelines are strict, and consequently 

only 10% of deutan applicants are classified as safe to 

fly (see Table 2). The applicants that pass are likely to 

perform the PAPI task as well as normal trichromats. The 

current CAA/JAA procedures have some disadvantages. 

A large percentage of applicants fail the primary tests and 

have to take lengthy secondary tests that differ in various 

member states. This introduces some variability in that 

the same subjects can pass some secondary tests and fail 

Table 6: Pass/fail scores on Ishihara and Dvorine compared against the ALT. 

Pass Fail P F P F

Pass 65 0 P 0 4 P 11 3

Fail 0 0 F 0 36 F 6 57

P F P F P F

P 65 0 P 0 7 P 13 6

F 0 0 F 0 33 F 4 54

ALT

DvorineDvorineDvorine

IshiharaIshihara

Normals (65) ALT Protans (40) ALT Deutans (77)

Number of subjects that pass/fail each test (i.e., a pass requires less than 7 errors on 

Ishihara; less than 7 errors on Dvorine; one or no errors on ALT)

Normals (65) ALT Protans (40) ALT Deutans (77) ALT Ishihara

Table 7: Predicted outcome per thousand applicants when using Ishihara, Dvorine and ALT tests. 

No. that fail No. that fail No. that fail Safe to fly Safe to fly Safe to fly

Applicants 1000 Ishihara Dvorine ALT Ishihara Dvorine ALT

Normals 920 0 0 0 920 920 920

Deutans 60 49 45 47 11 15 13

Protans 20 18 17 20 2 3 0

Total 1000 67 62 67 933 938 933

18.3 25.0 21.7

10.0 15.0 0.0

16.3 22.5 16.3

Percentage of deutan subjects classed safe to fly according to each test

Percentage of protan subjects that are classed safe to fly

Percentage of all color deficient subjects classed safe to fly

Predicted outcome per 1000 applicants using the FAA criteria 
34

others. Some of these subjects may not necessarily be able 

to carry out the PAPI tasks as normal trichromats. Equally 

important, a large percentage of color deficients that can 

carry out the safety-critical, color-related tasks fail the 

tests and are, therefore, unfairly disadvantaged.

The FAA guidelines are more liberal, and as a result, 

more color deficient applicants pass. In the case of Dvorine 

and Ishihara tests, less than 1% of the applicants that pass 

these tests fail the PAPI. All color deficient applicants that 

pass the ALT test also pass the PAPI (see Table 5). The 

current FAA guidelines have some disadvantages. The 

color vision tests employed provide only an approximate 

measure of the severity of color vision loss, and the pass/

fail limits have not been validated against the PAPI. The 

FAA accepts 14 different tests (FAA Guide for Aviation 

Medical Examiners, 2008). The use of so many tests that 

may not correlate well with each other (when used to assess the severity of color vision loss) increases variability 

and gives rise to odd outcomes when an applicant passes 

one test and fails another. Although the number of color 

deficient applicants that pass can increase significantly, this 

increase is test specific and often dependent on the use 

of the Signal Light Gun test that has not been validated 

against the PAPI task.

The results also show that one cannot regard subjects 

with deutan- and protan-like deficiencies as equivalent. 

Therefore, minimum color vision requirements must be 

set separately for each class of deficiency.

The principal findings of this study can be summarized 

as follows:

Subjects with red/green congenital color deficiency can 

exhibit an almost continuous distribution of chromatic 

sensitivity loss.

The loss of red/green chromatic sensitivity is greater 

in subjects with protan congenital color deficiency 

when compared to the deutan class. Unlike many 

conventional color vision tests, the CAD test cannot 

be learned, and hence the outcome depends entirely 

on the subject’s chromatic sensitivity. The test provides 

the means to classify protan and deutan subjects and 

also quantifies the severity of color vision loss.

Ninety-four percent of all applicants are likely to 

complete the CAD test in less than 20 seconds (using 

the fast-CAD option) which is based on screening for 

minimum color sensitivity in the deutan category. The 

remaining applicants are expected to fail fast-CAD. 

The definitive-CAD test takes 12 to 15 minutes to 

complete, and provides the information needed to 

establish whether the subjects that fail (i.e., ~4% of 

all applicants) have protan color deficiency and have 

residual chromatic sensitivity, within the established 

pass limit for protan subjects.

•

•

•

Below ~60 years of age, normal aging does not significantly affect either RG or YB thresholds, provided adequate levels of ambient illumination are employed.

Use of a modified “white” light results in significant, 

overall improvements in PAPI performance, particularly within normal trichromats and deuteranomalous 

observers.

A comparison between the PAPI and CAD tests shows 

that deutan subjects with CAD thresholds <6 SN 

units and protan subjects with CAD thresholds <12 

SN units can perform the PAPI test as well as normal 

trichromats.

A small number of deutan and protan observers with 

thresholds higher than 6 and 12 SN units, respectively, 

passed the PAPI test, but these subjects exhibited poor 

overall chromatic sensitivity and are, therefore, likely 

to be affected unfavorably in other visual performance 

tasks that involve color discrimination.

If these findings were adopted as pass/fail limits for 

pilots, ~35% of color deficient applicants would be 

certified as safe to fly.

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