Colour vision test: annually, or once per lifetime? (and colour vision discussion)

Are you saying that the fundamentals say that the numbers or symbols shouldn’t stand out? If the number or symbol shouldn’t stand out, what’s the point of the test?
In spite of what you might think to the contrary I believe I do have a good understanding of optics, optoelectronics,colour spectography and light, having spent many years finding ways to fool the human eye into believing what they think they see is real. When their mind is in reality being tricked

gallois wrote:

Are you saying that the fundamentals say that the numbers or symbols shouldn’t stand out?

I think Ted is referring to “whatever the lighting conditions”

1) if there is not enough light, you will have no colour perception
2) it is possible to have specific lightning in which even with perfect colour vision, the numbers or symbols won’t stand out (e.g. if the light does not contain blue at all, you won’t be able to distinguish two colours that differ only in their blue component)

gallois wrote:

Are you saying that the fundamentals say that the numbers or symbols shouldn’t stand out?

If you use a different light, specifically one with Colour Rendering Index less then 90%. (I looked up the standard it is supposedly 6500K and a CRI above 90%).

Then some people might get a better result, or at least different plates wrong and others correct, some of those may have a real deficiency and some may not. Cheap fluorescents can have a CRI less than 60%.

gallois wrote:

In spite of what you might think to the contrary I believe I do have a good understanding of optics,

I am sure you do, I did not mean to cause offence.

esteban wrote:

2) it is possible to have specific lightning in which even with perfect colour vision, the numbers or symbols won’t stand out (e.g. if the light does not contain blue at all, you won’t be able to distinguish two colours that differ only in their blue component)

Yes that is correct, but the light could also still look the same as normal daylight (pick for example CAVOK mid day) because instead on containing “blue” light from 400 -550 nanometres equally distributed like sunlight. It might also have a more intense band say 10nm wide at say 450nm such that to a normal person it looks exactly the same. It can look same because the short wave cones are centred around 450nm. This light might give a different result to people taking the test, depending on there deficiency or ability.

Peter wrote:

So, how do you fail Isihara, but pass a lantern test 100% ?

I think you have to ask a real doctor not a pilot But I can easily see how it is possible.

Like the silver halides used in film, the receptors in your eyes are sensitive to different colours (this is not strictly correct as it has more to do with wavelengths and spectra and electrical impulses. )
The sensitivity of the design and manufacture of piece of film means that a particular halide should produce the same colour when hit by the same light.
The eye or more exactly the brain has to be and is trained (usually from birth) that when a particular (I will call it a charge) travels from the receptor to the brain, the brain translates that into a particular colour, that it matches in the memory bank.
For years, possibly due to colour movies, people were convinced that moonlight was blue. There were even songs about it (blue moon)
Directors of photography would always add blue to any night scene.
Then one day somebody came along and pointed out if you can’t see colour in moonlight, why is it blue. Nowadays, lighting cameramen not longer add blue filters to the lights. When lighting cameramen first started to do this many colour grades in laboratories automatically added a hint of blue because directors and producers thought that the audience would not believe it was really moonlight.
So the difference in a colour pattern test where basically everything under the same light has not been tainted by exterior light is that your brain is translating from the different charges due to the colour wavelengths from the receptors and seeing a number rather than a colour.
If you put a light behind a number as in a lantern your brain is expecting to see a colour and not a symbol or number ie it is not searching for.a pattern.
Now a question. If you were to photograph the Ishihara plates in black and white, would you still see the numbers?
I’ve realised that this is a very clumsy explanation of what I believe to be the case, but some things are easier to demonstrate than to write about.

gallois wrote:

Now a question. If you were to photograph the Ishihara plates in black and white, would you still see the numbers?

It denpends,
a) on lightning
b) on the frequency response curve of your bw film/sensor

a) The lightning might be constructed such that the background and the pattern reflect exactly the same frequency curve (you might need to adjust the lightning per picture)
b) even if the reflected light has not the same frequency curve, the bw sensor might map it to the same shade of gray

Of course, you can always construct an image using many different colours such that this is not the achievable. However, that’s not how ishigara plates are designed (after all, they should fool colour-blind person).

If we fix the lightning to be a natural daylight and the sensor to a standard bw film, it is likely that its frequency response won’t exactly match the frequency response of a colour-blind person (as the goal is to match the frequency response of person with normal colour vision).

Hence, it is likely that you (and a colour blind person) will see a pattern in the b/w image. Bot not necessarily under all light conditions and for all sensors.

gallois wrote:

The eye or more exactly the brain has to be and is trained (usually from birth) that when a particular (I will call it a charge) travels from the receptor to the brain, the brain translates that into a particular colour, that it matches in the memory bank.

Yes that is something call colour constancy, Its my understanding that to this day we still don’t really understand how the brain works regarding vision. There are other theories for example Land’s Retinex theory, which attempt to explain why we can see colour in some patterns when there is none.

gallois wrote:

Now a question. If you were to photograph the Ishihara plates in black and white, would you still see the numbers?

That a good question because black and white film is designed to be colour sensitive. And if you use a digital camera it will include the manufactures simulation of film, if you watch a modern black and white shot digitally, in the post production process all the colour information will be used by using a look up table to simulate the look of popular film stock from yesteryear.

There are a lots you tube videos on how to simulate tri-x, or plus-x on photoshop for example if you wanted to test for yourself.

Peter wrote:

So, how do you fail Isihara, but pass a lantern test 100% ?

I don’t have time to read all of this

This document might be useful if need wish to add a new rating to your certificate

If you search for Falant and read those pages it may be useful. One of the things I gleaned is the lights chosen by Dr Farnsworth, covered spectral ranges that he thought were problematic. Interesting the red chosen is out of spec for the ICAO spec for red signal lights…

I am talking really of old fashioned film black and white rather than digital. Although on digital you can change the colour balance by using to white balance to tell the sensors that eg blue is actually white.
But in B+W film I believe,without trying it,that most films will actually show the number/symbol except when it comes to the red channel. Many B+W films are not red sensitive which is why red lights tend to be used in B+W processing and printing.
I think digital works more like the human brain in terms of how it detects colour so maybe it would show red symbols much better.
These days though, I wonder why colour vision problems are accepted as being so important in GA.
The only time I have seen it to be a problem, was not in GA. It is a well known professional UK snooker player who has a problem differentiating between brown and red and sometimes has to ask the referee which is the brown. AIUI it never stopped him becoming a world champion.
So what circumstances in GA would make Colour Vision problems unsafe?