[Foundation-l] Re: Colourblindness

Sun Jun 26 00:29:17 UTC 2005

Dear all,

I thought I could help cast some light on this for those interested,
both from a medical point of view and from a colour blind person's
view.

There are many types of colour blindness. The eye as you probably know
have two different light perceiving cells, the rod cells and the cone
cells. The rods are very light sensitive but can only detect light in
a certain frequency range (peak at 505 nm), and it is interpreted in
our brain as shades of grey. The less light sensitive cone cells are
what we use for our daily colour perception. There are normally three
types of cone cells, with different sensitivities to different
wavelengths, viz. 445 nm (blue, greek prefix trit-), 535 nm (~green,
deuter-), and 570 nm (~red, prot-). Combined, these can produce an
image interpreted by our brain as the different colours we know.

In colour blindness, people have either a defect or a
disproportionally low number of one of the cones (-anomaly) or they
have a complete (functional or real) absence of one of the cones
(-anopia). In order of frequency the types of colour blindness are
rougly deuteranomaly (~6% of males), protanomaly, deuteranopia and
protanopia (each ~1%) and tritanomaly and tritanopia (both very rare).
Very few people have no colour vision at all. The common types of
colour blindness are inherited on the X-chromosomes which means that
predominantly men are affected, but about 1% of colour blind are still
women. This happens when women inherit two affected X-chromosomes.

Because the green and red colour blind (deuter- and
protanomalous/-anopic) have defects in a similar frequency range (535
vs 570 nm), the clinical effects are similar, although they may be
leaning towards problems in either green or red perception,
respectively. In addition, this group will in some circumstances have
difficulties discerning the two colours when mixed.

Myself, I am deuteranomalous. When people ask, I usually describe the
green that I see as "less green" or "a greyer shade of green", but I
wouldn't really know as I've never known any different. Driving at
night, I can easily tell a red light, but I may sometimes confuse a
green light with a white light, thinking it is an approaching car or a
street light. So I take extra care when driving in the city, looking
at the shape of the light when in doubt and the context. The
difficulty in perceiving colour applies especially if we're talking
about thin lines or writing. I've been known to be writing with a
green pen thinking it was a weak black pen.

I also have problems distinguishing red and green when mixed, such as
green writing on red background or vice versa, or, as a kid, picking
strawberries :)

In conclusion, the links as they are now on Wikipedia are as good as
they can be for me personally as a deuteranomalous person. I can, like
most people, easily tell what's blue, and personally I perceive red
quite well, and have no problem seeing what's purple. However, there
may be people who have a less intense perception of the red. They
would still, however, be able to distinguish it from blue, and
probably even from purple.

On 26/06/05, Anthere <anthere9 at yahoo.com> wrote:
> The links are not blue and red.
> 
> The non visited links are blue.
> The visited links are purple blue.
> 
> The stubs are purple red.
> The empty are red.

Stubs are purple red? I haven't noticed :D Can you give me a link to a
page which contain examples of all these four coloured links?

> Brion Vibber a écrit:
> > Our links are blue and red. Can you confirm that these really are
> > indistinguishable to someone with red-green colorblindness?
> >
> > I would really like to know if this is based on actual experience (eg,
> > you are colorblind or a colorblind person has reported to you that they
> > are in fact not distinguishable) or if it's speculation. Without hard
> > data it's going to be very difficult to just come up with a change.
> >

So based on what I said above, I think the current colour coding is
actually if anything optimised for the majority of colour blind
people, as long as we don't also throw in a green link somewhere.

Cheers,

Bjarte Sørensen
http://meta.wikimedia.org/wiki/User:BjarteSorensen