Lecture 9 - Colour Vision Assessment Flashcards
Why assess CV in someone
Job applications - high alert/policman/fireman
What are the different ways to assess CV in practise
- Screen for normal (RG and YB) colour vision
- Classify class/type of colour vision
a. Normal trichromatic colour vision
b. Congenital colour deficiency (Deutan, Protan or Tritan)
c. Acquired RG and / or YB colour deficiency
d. Acquired loss on top of congenital deficiency
- Quantify severity of RG and YB loss - how bad is it
What does screening do
Detects if theres deficiencies or not
How many types of congential colour deficeincy are there
3 - deutan, protan, tritan
Depends on which one it effects
What causes acquired CV loss
Result of disease later on in life, affect from retina to virtual cortex
What does Holmes - Wright Lantern test do
Shines light and have to name R, G or B light
What does Nagel Anomaloscope do
Look down circular telescope - see field - illuminating half and half - match the 2 halves - identify
What does City University (2nd ed) with
daylight illumination do
5 dots - 1 in centre - 14 outside
Say which is closest in colour to one in middle
CAD test
CAD test measurements carried out in a large number of observers reveal a large variability in RG colour vision
What happens where you’re colour blind
See in black and white
How does CVD vary
- Varies on a continous scale, not same as having threshold of 3 = 3x worse than average normal of threshold of 25.
- Normals: 1,1 for RG and YB
- Variables can happen genetically and depending on how pigment in eye or how big or narrow cones are
What does CAD test do
- Isolation of colour signals
- Dynamic luminance contrast noise masks effectively the detection of LC-defined motion, but has no
effect on the detection of colour-defined motion.
How was CAD test developed and when
Marisa’s PHD
2002
What is important when assesing for CV
To isolate colour signals as this is what you want to assess. If you show CV test with luminance i.e. with colour and very bright - might not need colour vision to need stimulus but see how bright it is
What is important when assesing for CV
To isolate colour signals as this is what you want to assess. If you show CV test with luminance i.e. with colour and very bright - might not need colour vision to need stimulus but see how bright it is
- Make sure colour detectors are not using luminance signals to detect stimulus
How do congentials deficiencies differer from normal
Congenital’s CD’s have different luminance sensitivities than normal
What did CAD test involve
- Moving target within this static background of text
- Put luminance target to find luminance background of dyanmic noise - cant see it
- Then use colour target - moving in static noise - no effect
- Then move central target in dynamic noise = no effect
- If you had target defined by luminacne moving in dyanmic noise it would mask luminance
Results for CAD test
- Static pattern - could see it -doesnt matter how much noise you put - can’t see it all the time
- But when you have dynamic noise, you can mask luminance target using dynamic noise - for colour it doenst make any difference - doenst matter how much noise you put on - can always see the colour
What does Ishihara test do
Isolate the use of colour signals
Uses isochromatic confusion lines as a base for its design
How does ishihara test work
Involves numbers on background of different colours
e.g. red/orange 8 on green shades background: Some people see a 3 because this green is similar to orange - same confusion axis
What happens in normal trichromat
Have 3 cones
What does relative luminous efficiency show
Combination of signals coming from L + M cones
Luminance signals in traffic lights
Traffic lights designed to be seen by everyone because they have different luminance signals from 3 different colours
What happens in Deuteranopia - Luminance discrimination
- Relative luminous efficiency virtually normal as it will follow the L cone - based on 2 L + M cones -
- Reduced colour discrimination
- M cone - green missing
- Someone with green colour deficiency - luminance detection similar to someone with normal colour vision, but have reduced colour discrimination because missing M cone
What happens in Protanopia- Luminance discrimination
- Note reduced luminous efficiency for long wavelengths (reds look dim) - Reduced colour discrimination - luminance function affected - L cone missing - red lights look dim
What happens in Protanomaly- Luminance discrimination
Relative deficiency function is according to whatever pigment they have
Isochromatic confusion lines
CIE-1931 (x,y) colour diagram Typical chromatic discrimination of a normal trichromat Show isochromatic confusion loci
What is important for efficient assessment of chromatic sensitivity
- A true isolation of colour signals is obtained
- Appropriate illumination ( specific lighting ) is used – Daylight D65 or equivalent,
- It is based on data that describe the variance in colour discrimination amongst
“normal” trichromats, - Has adequate sensitivity and specificity to detect “minimal” deficiencies and to
classify them - Can be used to detect and monitor “significant changes” in colour discrimination
over time.
How can you get an isolation of colour signals
Avoid use of luminance signals - CD’s have different to someone with normal CV
Why is specific lighting for assessing CV important
If you use different types of white light - all look white but spectral composition is very different and will affect colours and colours available to subject doing CV test
D65 light
Has all complete wavelengths, but fluorescent has peaks of wavelenths, not a spread
Why use normal trichromats
Need to know how normals behave on CV test to assess someone with congential CV deficiency, to compare - see if someone is abnormal
Case Study: aviation
- Concern was expressed by the Civil Aviation Authority UK that the original colour
vision standards were too stringent and that the pass/fail limits had been arbitrarily
set using colour vision tests that do not quantify well the severity of colour vision loss. - In 2009, the CAA UK introduced more accurate colour assessment procedures using
the 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 now allowed to
become pilots.
Why did pilots do Ishihara test
To see if its safe to look at colours on runway and land plane
If they passed - straight through pilot
If failed - 5/6 different CV tests applicants could do and all has different results
What are the minimum colour vision requirements
- Visual task analysis to identify the most demanding colour-critical
tasks - Assess severity of colour vision loss using a test that quantifies accurately
the loss of chromatic sensitivity - Relate subjects’ performance on functional tasks to their colour thresholds
and use these findings to establish pass/fail limits which ensure equivalent
performance to that measured in normal trichromats - Produce accurate simulations of the most demanding tasks in the laboratory
and method for quantifying subjects performance on these functional tasks
What is visual task analysis
- Looked at environment pilots have to work in and identified which colour signals are crucial for them to do their job
- Identification of most colour critical tasks when no redundancy
is involved and the discrimination
of colour differences is most
difficult.
What visual signals that make use of colour were used in visual task analysis
- PAPI lights
- Runway lights
- Parking lights
What does PAPI lights stand for
Precision approach path indicator
How do PAPI lights work
- Located on left or right of runway
- 2 colour system code - red and white
- If on correct light path on landing you will see 2 whites and reds
- If too low - see red
- If too high - see more whites
Laboratory simulation for the PAPI test
- Four stimulus channels derived from one light source - split light into 4 channels, create PAPI stimulator, looked exactly like PAPI lights
-Random luminance variation - Viewing distance 4m
- Visual angle ~1.4 min arc
Corresponds to approach
distance ~5.54km - landing plane - Did it 60 times - how many times correct - compared %scores against CAD threshold
Results – functional test vs CAD
- Normals - most got 100%
- Some made mistakes - bored - lose concentration
