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Colour vision deficiency part 3--occupational standards.

In this final part in the series on colour vision, occupational requirements are discussed along with the various methods of assessment and an overview of recent developments.

Course code: C-34766 | Deadline: February 28, 2014

Learning objectives To be able to explain the occupational implications of colour vision defects to patients (Group 1.2.4)

Be able to identify the tests suitable for occupational colour vision screening and interpret the results (Group 3.1.4)

Learning objectives

To be able to identify the occupational implications of colour vision defects to patients (Group 1.2.4)

Introduction

Colour vision standards for marine watch-keepers and train drivers were introduced following two fatal accidents in the nineteenth century. Ten people were killed in July 1875 when a tug collided with a steam ship off the coast of Virginia in the US. The tug failed to give-way and the captain was later found to confuse port and starboard navigation lights. In November of the same year, two passenger trains collided head-on near the town of Lagerlunda in Sweden; both drivers and seven passengers were killed. (1) Colour deficiency was assumed to be the cause but there was no evidence of this. However, following these incidents, the Holmgren Wool Test was adopted to examine railway personnel and recruits for the armed services. The examination procedure was similar to that used to select colour matchers in the textile industry and involved matching shades of wool. Pigment tests based on camouflage patterns composed of confusion colours were developed in Germany in 1876, with the Ishihara pseudoisochromatic test introduced in 1917. Other pseudoisochromatic screening tests and occupational lanterns were produced in different countries, and it became routine to use a pseudoisochromatic test to identify colour deficiency followed by a secondary occupational lantern test to determine colour-naming ability.

Colour vision lanterns

Tests that examined general colour-naming ability were thought to be more appropriate for transport workers and several colour vision lanterns were produced before 1895. (2) For example, the Edridge-Green Lantern, devised in 1891, included blue and purple as test colours --although these are not used for transport signalling. The Board of Trade (BOT) approved a dedicated occupational lantern in 1913 for use in the merchant marine service. This lantern displayed nine pairs of red, white and green signal colours separated horizontally. (3) The angular subtends and separation of the lights is equivalent to ship navigation lights viewed at a distance of 2,000 yards (two nautical miles) in scotopic viewing. The BOT lantern was replaced by the Martin lantern in 1939 and subsequently by the Holmes-Wright (H-W) lantern Type B in 1974. (4,5) The new lanterns reproduced the original design but had improved mechanical construction and modern light sources. A second version of the Martin lantern was made for rail transport and included yellow/amber as a test colour. (6) A colour vision requirement for aircraft pilots was proposed in 1919 but correct naming of red and green flags or flares, which indicated permission to land, was probably all that was required prior to 1945.2 The Martin lantern was subsequently adopted by the Civil Aviation Authority (CAA) and by the Ministry of Defence (MoD) until the H-W lantern Type A came into service. (5) The examination procedures for the H-W lanterns are clearly documented to ensure that consistency is maintained. These lanterns are no longer manufactured but are robust and can continue in service if the calibrated light source is replaced after 1,000 hours of operation. Replacements for the H-W lantern have been produced but have not been validated.

[FIGURE 1 OMITTED]

Recommended colour vision requirements for transport

A review, which aimed to harmonise colour vision standards in international maritime, air and road transport, was made by the Commission Internationale d'Eclairage (CIE) in 2001. (7) Three new standards were proposed with the Ishihara test, the Farnsworth D15 (D15) test and an approved lantern test recommended for implementation (see Table 1). The Ishihara test was recommended for screening with three errors as the fail criterion. (8) More than two diametric crossings on the D15 results diagram was the recommended fail criterion. (7) This criterion fails about 40% of colour deficient people but favours protans because perceived luminance contrast can be used to obtain a good result. About 60% of colour deficient people fail if a circular results diagram with no diametric crossings is needed to pass. (9) Only the Beyne Lantern (France), the Farnsworth Lantern (FALANT) (USA) and the H-W lanterns, Type A and B (UK) were approved. The TriTest 13 and the Optec 900 are recent replacements for the Beyne lantern and the FALANT respectively. The Nagel anomaloscope was recommended to confirm the identification of protan deficiency if required. The Spectrolux lantern (Switzerland) is not listed in the CIE report although the Joint Aviation Authorities (JAA) approves it as a secondary test for Joint Aviation Requirements (JAR). In the test, (12) pairs of specified red, green and white signal lights are displayed.

The aim of the CIE to harmonise international standards cannot be achieved, primarily because the approved lanterns are differently designed and examination procedures vary. (10) For example, the Beyne lantern shows five single colours, including yellow and blue, derived from narrow wavelength bands. The H-W and the FALANT both display nine pairs of red, green and white lights separated vertically (see Figure 1). This configuration is not used in transport systems and neither lantern is dedicated to the needs of a particular occupational task. The H-W Type A displays specified signal colours, approved by the CIE, but the FALANT displays red, yellow-green and yellowish-white lights that have x, y chromaticity co-ordinates within a common protan/deutan isochromatic zone. (3) The FALANT is, therefore, a grading test intended to identify significant isochromatic colour confusions. The D15 test has a similar aim but a pass on the D15 does not predict whether a person will be successful on the FALANT. (11) The recommended fail criterion for a lantern test is two errors on two runs (sequences) of nine colour pairs in photopic viewing but this can only be applied to the H-W lanterns and the FALANT. Between 25% and 30% of colour deficient people are likely to pass using this criterion but some individuals may have severe deficiency. (12-14) Neither the number of errors nor the number of qualitative error categories are a reliable guide to the severity of deficiency. (14,15) Sensitivity of the H-W Type A at high brightness in photopic viewing is 97% if three runs are completed and a single error is a fail. (14) Results obtained from 125 colour deficient men who completed three runs at high brightness in photopic viewing showed that green/white errors (green called 'white' or white called 'green') were the most common--70% of subjects made red/white errors and 43% made red/green errors. (15) Green/white discrimination is needed by marine watch keepers and correct red/white discrimination is critical for safety in aviation.

New developments

The UK Disability Discrimination Act (1995) (DDA), extended by the Disability Discrimination Order (2006) (DDO), aimed to limit discrimination against disadvantaged groups in the work place with similar laws passed in other developed countries.

Restrictive colour vision standards were particularly targeted for review. The background necessary for setting a new occupational colour vision standard is outlined in Table 2. The DDA required employers to modify colour codes and important tasks to enable colour deficient people to work as normal. Exclusion from employment remained lawful if this could not be done.

Colour codes have different functions--connotative codes provide information by colour recognition alone and are used in electrical work as well as transport systems whereas denotative codes segregate items of information and are widely used in business and industry. Text, number or shape differences are usually available in denotative codes and the use of colour is said to be 'redundant' because information can be obtained in another way. However, colour recognition helps to retrieve information quickly and a delayed reaction might compromise safety or efficiency. Guidelines issued by the Health and Safety Executive (HSE) provide good advice to employers on how to identify important tasks and consider whether redundant features can be added that would enable colour deficient people to work as normal. (16) As a result, considerable improvements have been made to colour codes on stands and taxiways at airports and some equipment used by fire fighters has been withdrawn. Breathalysers used by police constables now display digital information and traffic wardens take photographs rather than write down the vehicle colour. Guidelines on colour vision examination produced by the HSE are less helpful: (17)

1 Knowledge of the needs of the occupation and work environment with awareness of the consequences of error or slow working

2 Identification of the most difficult safety critical task that must be performed in the unfavourable viewing conditions

3 Knowledge of the characteristics of different types of inherited red-green colour deficiency

4 Assessment of the ability of a colour deficient person to complete the most safety critical task with the same accuracy as a normal trichromat

5 Selection of an appropriate validated test or examination procedure that ensures that a new standard is implemented fairly and consistently

[FIGURE 2 OMITTED]

A review of colour vision requirements in aviation and rail transport

The need for a radical review of colour vision standards for employees in aviation and rail transport was emphasised by major accidents in 1996, and 2002. In 1996 two passenger trains collided head-on near Secaucus, New Jersey. Three people were killed, including one of the drivers, and 69 people were injured. The cost of the damage was estimated at more than $3.3m. The cause of the accident was attributed to the known acquired colour deficiency of the deceased driver. (18) In 2002, a FedEx Boeing 737 landed in trees short of the runway at Tallahasse Airport, Florida and was destroyed by fire with all three crew members seriously injured. (19) The pilot had severe congenital red-green deficiency but had passed an examination with the FALANT. The official accident report ordered a review of colour vision examination procedures and recommended that the FALANT be discontinued. Poor interpretation of the Precision Approach Path Indicator (PAPI) code was identified as the primary cause of the accident. A subsequent study found that 10 of 52 colour deficient subjects that passed the FALANT were unable to perform a simulated PAPI task as normal. (20)

[FIGURE 3 OMITTED]

PAPI consists of four lights in a horizontal line at the side of the runway (see Figure 2). The correct approach path is shown by two white and two red lights and must be maintained until the aircraft has landed. Commercial airline pilots must identify these lights at a distance of four miles (5.5 km) and adjust altitude to achieve the correct angle of approach.

This discrimination task was replicated in a laboratory investigation at City University London. Results for 60 random presentations of red and white light combinations were obtained for a representative group of 111 colour deficient men (40 protans and 71 deutans) and 64 normal trichromats. (21) The results were then compared with RG thresholds measured with the Colour Discrimination and Diagnosis (CAD) test; this test is performed on a colour calibrated visual display unit and presents stimuli of precise chromaticity and saturation as moving targets within a dynamic background of neutral grey dots that mask luminance contrast. The target moves along one of four diagonal directions and the subject presses a button to indicate the direction of motion. Protans who obtained R/G CAD thresholds less than 12 SN (standard normal) units and deutans that obtained thresholds less than 6 SN units were able to perform the PAPI test as normal and could safely begin pilot training (see Figure 3). Acceptance of these criteria particularly favours minimal/slight deuteranomalous trichromats that would have failed the lantern test and been rejected. Use of the CAD test was accepted by the CAA as a replacement for the H-W Type A in 2009. (22) There are considerable advantages in setting an evidence-based occupational standard derived from results obtained with a single objective test that has a validated pass criterion linked to satisfactory completion of the most safety critical task. A computerised assessment procedure removes examiner variance and ensures that the same pass/fail decisions are always made. The CAD test has also been accepted by more that 50 National Airline companies that use the CAA pilot licensing facilities and by the National Air Traffic Society (NATS) as a screening test for air traffic controllers. (23)

New lanterns

Equal opportunity laws in Australia and Canada require colour vision standards to be implemented with a dedicated occupational test that replicates the most safety critical task. As a result, two new dedicated lanterns have been developed for rail transport that display the chromaticities and configuration of trackside signals in these countries. (24,25) Both lanterns display red, yellow/amber and green lights. Only the inability to see a red light at all or naming a red light incorrectly results in failure of the Australian RailCorp (LED) lantern. This criterion passes a higher percentage of colour deficient people than the FALANT and about 50% of those that pass the D15. The CNLAN displays 22 triplicates of red, yellow and green lights. This is a difficult test for normal trichromats and up to five errors must be allowed as a pass. Only deutans with minimal/slight deficiency are likely to be successful.

The Ministry of Defence

The colour perception (CP) standards in the Armed Forces are under review but continue to be implemented with the Ishihara test and the H-W Type A (see Table 2). Different CP classifications are required which are dependent upon the role.

Colour vision standards in the fire service and the police force

A new standard was introduced for firefighters in 1996 after a review of important tasks. (26) Applicants that fail the Ishihara plates and deutans that pass the Farnsworth D15 test without error were accepted. More recent online information prepared for fire service and police recruits are ambiguous and show poor background knowledge. The advice offered is based on the assumption that clinical tests are able to distinguish anomalous trichromats and dichromats when this is not the case. It is also suggested that anomalous trichromats can be relied upon to develop individual 'coping strategies'. Some colour deficient people may be employed as police officers on this basis but may be unaware that normal colour vision may be needed for special police roles, such as pursuit driving and armed surveillance, which require additional training. (27)

The Merchant Marine Service

The Marine Coastguard and Safety Agency (successors to the BOT) aims to implement the colour vision standards recommended by the CIE. (7) Watch keepers must pass the Ishihara test. An examination with the H-W Type B is given to rejected applicants on appeal.

MORE INFORMATION

References Visit www.optometry.co.uk/clinical, click on the article title and then on 'references' to download.

Exam questions Under the new enhanced CET rules of the GOC, MCQs for this exam appear online at www.optometry.co.uk/cet/exams. Please complete online by midnight on February 28, 2014. You will be unable to submit exams after this date. Answers will be published on www.optometry.co.uk/cet/exam-archive and CET points will be uploaded to the GOC every two weeks. You will then need to log into your CET portfolio by clicking on 'MyGOC' on the GOC website (www.optical.org) to confirm your points.

Reflective learning Having completed this CET exam, consider whether you feel more confident in your clinical skills--how will you change the way you practice? How will you use this information to improve your work for patient benefit?

Jennifer Birch was formerly a senior lecturer in Clinical Optometry at City University London, and is now a senior research fellow in the Henry Wellcome Research Laboratory in the Department of Optometry. She was a founder member of the International Research Group on Colour Vision Deficiencies and has written extensively on clinical aspects of colour deficiency and on occupational colour vision requirements. She was appointed to an Honorary Life Fellowship of the College of Optometrists in 2012.
Table 1 CIE Colour vision standards for transport

Standard               Test results          Application

Standard 1             Pass the Ishihara     High risk activities
Normal Colour          test (or pass The     when correct recognition
Vision                 H-W lantern Type B)   of colour signals or
                                             other codes is safety
                                             critical

Standard 2             Fail the Ishihara     Low risk activities
Defective Colour       test and pass an      needing the ability
Vision A Slight        approved lantern      to recognise signal
deutan deficiency      test (the Nagel       lights at a
Protans are excluded   anomaloscope to       'moderate' distance
                       identify protans
                       if needed)

Standard 3             Fail the Ishihara     Low risk activities
Defective Colour       test and pass the     needing the correct
Vision B Slight/       Farnsworth D15 test   recognition of
moderate red-green                           pigment colours or
deficiency                                   large signal lights
                                             at short distances
                                             in photopic viewing

Table 2 Colour Perception (CP) standards that may be
applied in the Armed Forces

Standard                  Test specification

CP 1: Superior            No errors on a Holmes-Wright
  colour discrimination     lantern at low brightness
                            in scotopic viewing
CP 2: Normal              No errors on the first 17
  colour vision             plates of the Ishihara
                            test 38 plate edition
CP 3: Slight red-         No errors on the H-W Lantern
  green deficiency          Type A at high brightness
                            in scotopic viewing
CP 4: Adequate colour     Correct recognition of
  discrimination            coloured wires, resistors
                            or stationery tabs used
                            in different trades
CP 5: Severe colour       Unable to obtain any of the
  deficiency (Royal         above CP classifications
  Navy only)
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Title Annotation:1 CET POINT
Author:Birch, Jennifer
Publication:Optometry Today
Geographic Code:4EUUK
Date:Jan 31, 2014
Words:2944
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