lecture 32 colour vision assessment II

Question 1

what are examples of occupations that have normal colour vision requirements ?

Answer 1

• aviation
• maritime
• railway
• fireservices
• police force
• armed force
• electrical engineers

Question 2

how has the colour vision test evolved in occupational environments ?

Answer 2

. colour vision tests have been largely developed for use in different occupations when the primary need was to detect the presence of a congenital deficiency

. recently, it has become more important to quantify the severity of colour vision loss both within specific occupational environments and clinical applications to detect small changes as a result of a disease or treatment managements

Question 3

why is the use of colour vision in occupational environments important?

Answer 3

• the use of colour vision in occupational environments is important since it enables efficient coding of signals and information and this, in turn enhances visual performance
• good colour perception is an important dimension of visual processing that provides additional information on the physical properties of the environment
• colour adds object conspicuity

Question 4

what does the absence of colour signals lead to?

Answer 4

• the absence of colour signals can affect how we see objects and hence our visual performance

Question 5

what does abnormal colour vision cause ?

Answer 5

• reduced effective contrast or conspicuity of coloured targets
• delayed recognition of colour signals and extended decision time
• confusion of colour signals and its consequences

Question 6

what are examples of colour coding of information?

Answer 6

• colour coding in traffic lights , colour conveys a message very rapidly
• air traffic control , colour-coding is used to transmit information of caution and warning for aircraft in traffic complex and for weather conditions
• specific colours can be used for segmenting or grouping similar objects together in crowded scenes

Question 7

how is the Ishihara test used in colour occupation?

Answer 7

• the normal practice is to allow a small number of errors when screening for functionally safe colour vision
• the number of errors is often taken as a measure of the subject’s severity of RG colour loss and colour safe certification within different environments can involve anything from 0 to 6 areas depending on edition of test
• different occupations use different Ishihara test edition

Question 8

how does the Ishihara test work ?

Answer 8

• the Ishihara test has high sensitivity for detecting RG loss of chromatic sensitivity,, but this is achieved at the expense of specificity
• in order to allow 100% of normals to pass you need to allow up to four errors , this means that if you allow four errors 10% of deutan will pass and 1% of protans will pass
• if you decide to set the pass criteria at 0 errors only 98.9% of normals are passing

Question 9

what has prompted the use of the plates failed as a direct measure of the severity of colour vision loss?

Answer 9

• mild deuteranomalous subjects tend to make fewer errors than dichromats

Question 10

explain the the use of Ishihara vs CAD ?

Answer 10

• the plot is a plot of RG threshold measured on the CAD test against the number of errors the subject makes on the Ishihara test plates
• the results reveal extremely poor correlation between the measured RG threshold and the number of errors the subject makes on the Ishihara test plates within each type of subject group
• subjects with high RG thresholds make more errors ( not always case )
• there is a small range of RG thresholds for both protans and deutan subjects that exhibit large differences in Ishihara test error rates
  e.g. subject with 5 CAD units make 0 errors on Ishihara test , while subjects with 5 cad units who make 23 errors on Ishihara test
  . this suggests extremely poor correlation between the measured RG colour detection thresholds and the number of errors the subjects make on the Ishihara plates within each subject group

Question 11

why is it not always good to use Ishihara test to judge suitability for the job?

Answer 11

• the number of errors on the Ishihara test does not reflect the same severity of colour vision loss for deutan and protan colour deficients
• the number of errors the applicant makes on the Ishihara plates should not be used by occupational medical advisors to judge suitability for the job
• there is a need for a new approach in colour vision assessment in occupational medicine that provides evidence- based guidelines specific to the occupational environment

Question 12

what do evidence - based guidelines mean?

Answer 12

• this means that the colour vision requirements for a specific occupation are based on colour tasks that are required to perform in that specific occupation

Question 13

explain the aviation case study?

Answer 13

• concern was expressed by CAA that the colour vision standards are too stringent , the use of colour vision test do not quantify well the severity of colour vision loss
• in 2009, CAA introduced more accurate colour assessment using CAD test and adopted pass/fail limits based on the applicant’s ability to carry out the most demanding colour-related tasks with the same accuracy as normal trichromats
• as a result, 35% of subjects with congenital colour vision deficiency are allowed to become pilots

Question 14

what did the aviation study involve?

Answer 14

the new method involved setting minimum colour vision requirements in any professional environments includes a number of stages :

1. visual task analysis to establish the most important, visually unfavourable colour related tasks when the use of colour is safety critical
2. produce accurate stimulation of the most demanding tasks in the lab and method for quantifying subjects performance on these functional tasks
3. assess severity of colour vision loss using a test that quantifies accurately the loss of chromatic sensitivity
4. relate subject’s performance on functional tasks to their colour thresholds and use theses findings to establish pass/fail limits which ensure equivalent performance to that measured in normal trichromats

Question 15

how was the visual task analysis in aviation carried out?

Answer 15

• identification of most critical tasks when no redundancy is involved and the discrimination of colour difference is most difficult
• used the PAPI system
• the visual task analysis identified the position approach path indicated system as one of the most critical tasks which involve colour and these are lights that are either on the right or the left runway and it’s a two colour code system which indicates whether the pilot or the plane is on the correct path to landing
• if the pilot is on the correct path (i.e. on slope) , the pilot should see two reds and two white lights
• if the pilot is too low, the number of red lights increases
• if the pilot is too high then more white light
• parking lights were also used which indicate whether the pilot is left or right of parking space
• runway light also use colour , however theses two signals use redundancy and are less critical
• the visual task analysis identified the PAPI as the most critical task that involved colour in aviation
• a lab setup was used using a PAPI stimulator

Question 16

how was a lab setup used using a PAPI stimulator ?

Answer 16

• same lamp house was used in aviation
• the source of lamb was split into four channels using a beam splitter and the mirror
• the subject sat at a certain distance , viewing distance of 4m and visual angle of 1.4 min of arc which corresponds to an approach distance of 5.54km
• the subject will see a schematic of the PAPI lights and there’s five conditions that subject saw
• subjects had to indicate how many red lights they will see (0 , 1,2 ,3 4)
• the comparison between the functional test, the PAPI stimulator test and the CAP test
• most normals had 100% correct , but some normals made some errors
• deutans - up to threshold up to 6 CAD units all deutan performed similarly to normals
• protans - up to 12 CAD units have similar performance to normals

Question 17

explain the summary of results in deutans ?

Answer 17

out of 77

• 34 passed
• 43 failed
• threshold less than 6 ( 29 pass and no fail )
• threshold more than 6 ( 5 pass and 43 fail)
• 37.6% of deutans are safe to fly

Question 18

explain the summary of results on protans?

Answer 18

out of 40

• 20 pass
• 20 fail
• threshold less than 12 (13 pass and none failed)
• threshold more than 12 ( 7 pass and 20 fail)
• 32.5% of protans are safe to fly

Question 19

explain the summary of results ?

Answer 19

• the development of sensitive colour vision test that isolate the use of colour signals and provide accurate estimates of both RG and YB chromatic sensitivity have important repercussions , both within occupational environments and in the detection of acquired loss of chromatic sensitivity
• the establishment of functional colour vision limits that are evidence-based, job- specific and can be implemented through objective testing without having to rely on arbitrary limits or normal trichromacy can be extended to other occupational environment where colour is important for signalling information
• a positive correlation between chromatic sensitivity loss and disease severity has been demonstrated in several retinal conditions