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yes, it's quite rare.. somewhere between 1 in 30000 to 1 in 50000. vision is more like different shades of gray and they differentiate those shades based on brightness.

 4 years ago  

I see. Thanks. By the way, any connection between the topic and your username? ;)

Haha ,, naah , I am a doctor but not Eye-doctor :D

 4 years ago  

Cool! We are happy to have you around (I think it is the first time I comment one of your posts).

Thank you @lemouth. Yes it is the very first time indeed and I hope for more :)

 4 years ago  

I hope too. I cannot read all the supported posts (there are just too many), but I usually read everything that is quite strongly supported.