the rules accommodate that possibility. The Isihara plates are really for screening.
Sure but it is always the detail which really matters:
If you are in a country which CV tests annually, and say you fail Isihara at each medical, then at each medical have to go to a consultant ophthalmologist for a Lantern Test (or one of the others) which is a) a big hassle because you won’t get your medical until then and b) it will cost you a few hundred on top of the 200-300 (the current UK rate, chucking in the occassional ECG etc; yes I know we will now get loads of posts saying x gets it for €50 
) for the medical itself.
And it’s stupid because CV rarely varies over a lifetime. But then so much in aviation medicine is stupid, in terms of diagnostic value for pilot incapacitation.
And if Isihara is expected to be normally/often failed (“for screening” for what; this isn’t like screening for colorectal cancer etc) then this is just an abuse of the system, resulting in an extra income for the industry, and a decimation of the GA community which is already very ready to give up flying, upon any medical issue. Why don’t then require an annual angiogram – that would be even better for screening And nice money, too.
but tritan color blindness is very physical, and directly linked to damage receptors
What I was getting at is more subtle: you cannot establish what colour I see when I am looking at say red Maybe you can but I can’t see how. The only thing AFAICT which could be established in monotonicity of brightness.
Peter wrote:
you cannot establish what colour I see when I am looking at say red
Which red? For any “red” colour it be reproduced in a theoretically unlimited different ways. Your screen relies on this principle…
All that you can really prove is that under sets of conditions a certain person can distinguish a difference and when you can’t.
Peter wrote:
And if Isihara is expected to be normally/often failed
This assumption is not right – while it is possible that people fail Ishihara but pass a lantern test, that is quite a rare exception. Most people pass Ishihara in the first place and out of those who don’t only few pass a lantern test.
Peter wrote:
because CV rarely varies over a lifetime.
For almost anybody CV varies over lifetime – it’s just not relevant for most as there is still enough differentiation left. There is lots of research on this (e.g. here ).
LeSving wrote:
It’s also called blue-yellow colorblindness (for some odd reason)
No odd reason at all: People with this form of color blindness have difficulties differentiating blue and yellow – not blue and green.
Color blindness medically is not at all associated with the inability to “correctly” (in the sense of majority consensus) name a color when looking at it. It is always of if you can tell two colors apart if you se them next to each other (in the sense of saying “these are two different colors”)
people fail Ishihara but pass a lantern test, that is quite a rare exception
Not according to my information
The CV test here is not about identifying a colour, as Malibuflyer wrote, it is a book containing colours with numbers in a different colour. You don’t say the colour you say the number.
The B+W test with three dimensional shapes hidden in the background and you have to identify the shape is much more difficult. I haven’t a clue what they are testing for.🙂
Peter wrote:
“for screening” for what; this isn’t like screening for colorectal cancer etc
“Screening” means making a quick and simple check that may have false positives. If you get a positive result, then you make a more reliable (and complicated) check.
Peter wrote:
Not according to my information
Not sure where you get your information from – but there is much published on that. E.g. here
The results in brief: Out of 200 people that took a 14 plate Ishihara test (which is close to what is used by EASA rules), 166 passed and 33 failed (one being borderline and therefore excluded). Out of the 33 that failed Ishihara 30 also failed the (Farnsworth) lantern test – and only 3 passed it.
So it is 3 out of 200 that fail in 14 plate Ishihara but pass the lantern – I would call this a rare event.
How about the Wright-Holmes lantern test?
this site has all the tests simulated to run on your screen, including the lantern test. The colours are simulated by your screen, so they will only strictly “correct” for someone with “normal” vision, but then again it the same technology that is used in modern EFIS 
Isihara (my score is near zero) is really a colour pattern recognition test. I get 100% on the W-H Lantern test.
But you could easily memorise the Isihara plates. It’s like memorising the bottom line of the eye test, or doing the computer QB for your exams
Aviation medicine is full of weird stuff. It’s like, here in the UK, once you need glasses to pass your medical, you cannot get that limitation removed. It is apparently possible via a special procedure the CAA has to agree to, but few people bother. Whereas on the FAA medical, it is done per-medical. I passed my last 5-10 medicals including the last one without glasses, but the limitation remains
