Quote:
Originally Posted by casioqv
I'm not colorblind, but I don't understand why this is. Even without the ability to distinguish colors, the intensity of light from a green object will be lower through a red filter, and vice-versa.
You mention that some colorblind people are totally lacking a type of receptor, and can't detect that wavelength of light at all, so I could understand that a green buoy wouldn't look dimmer through a red filter to these people, but the red buoy would still look dimmer through a green filter (if they happened to lack green receptors).
Therefore, if they can positively identify a buoy or light as not red, then they can assume that it's green, and still distinguish it.
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Hiya Casio
Ok, with respect to your first point. In an ideal world, a pure red filter will filter out all wavelengths but red, so a pure green object will appear black. It is not an ideal world, so most filters let through a range of wavelengths or a number of wavelengths. A neutral density filter (ndf) will theoretically filter out all wavelengths (colours) equally, so that will reduce the overall intensity of light coming through the filter. - eg expensive sunnies like serengetis/maui jims etc are nd filters - before they add other filters. Look through a pair of Brown or rose coloured serengeti or maui jim sunnies - and you will see the change in contrast.
Your 2nd pt: If someone only has a set of blue cone receptors and one other type - either green or red cone receptors - [quite rare], then the same tone red and green light will be indeterminable. However most cvd people are to some extent partially cvd, meaning they have a mix of the 3 colour vision receptor cells, but not at the desired numbers/levels. Therefore they have an ability to determine some of the reds and greens, but not always - they need other cues such as location (eg traffic lights), contrast, etc. Your last sentence is using logic to determine colours which is another 'cue' people use.
I saw one person today who tried to look at the isihara plates from the side, turning them to get the right reflection. On the repeat, he tried to remember what he said the first time. (this usually = a fail) For marine tickets, in Aust, we use what is termed the lantern test [which is a good approx to beacons at night] There are other test to determine the actual level and type of cvd. A isihara test is a pass or fail. The trouble is that all beaurocratic agencies see the type of test in print in their book of regs and cannot look at the candidate. [a little rant on the side]
For partially cvd assistance, a set of glasses are often made with a different set of filters in front of each eye. This is done to enhance the differential contrast and help the person 'see' a difference. It's success depends on the severity of the cvd and the situation - object being fixated on.
Anyway, Bubb said that in the USA, there are other factors taken into consideration for those going for a CG ticket. This is a good approach I believe.
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