Why can't he see red or green ink on a whiteboard?

24 October 2010

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Question

I have a friend who can't see anything written in red or green on a whiteboard. Being the curious person I am, I've tried to get him to explain why this is - but unfortunately he can't remember much of the diagnosis. I've also tried to search on the internet but cannot find one similar reference - to do with pens and whiteboards, only to do with colour blind individuals not being able to see red laser points on a whiteboard. However - he swears he's not even colour blind...

Answer

We put this question to Petroc Sumner of Cardiff University...

To answer the question about the pens on the whiteboard, basically, this doesn't sound to me like a simple case of red, green colour blindness, but it might be a case of red, green colour blindness combined with something else. Basically, in the eye, we have two types of receptor called rods and cones. The rods do night time vision when things are relatively dark and the cones do daytime vision. It's often a misnomer that cones only do colour vision, but they also do light and dark vision during the day when the rods are not active. If you are red/green colour blind, it means that you don't have one type of cone. But that doesn't mean that you can't distinguish any colours and it also means that you should still be able to distinguish things at different lightness. So for example, yellow and brown or grey and white would still be pretty clearly distinguished for you. And because white on a whiteboard is basically the lightest colour, anything that's coloured is normally darker than white which means that even though you might not be able to tell the difference between the red and the green pens, you should be able to tell the difference between the white board and the red and the green pens, because both of the pens will be darker than the white board. So that's why it doesn't sound like it's just a simple case of red, green colour blindness.

Diana - So distinguishing coloured marks from their background shouldn't be a problem. What else could be going on?

Petroc - Having said that, the green and red pens will be less dark than say, a black or a blue pen would be. You can show this actually by doing a little experiment which I just did myself. I wrote red and green writing on my whiteboard, I shut the curtains, made it as dark as possible in the room, turned the lights out, waited until I was accustomed, and then it's clear to me that even though I've now lost my colour vision because I'm seeing with my rods, and not my cones anymore, I can still see both the red and the green writing. But you should also see that the green is less distinct than the red and the red is less distinct than black writing, and that's because both the coloured pens won't be as dark as the black pen. You can then imagine that if you combined this, not having colour vision with say, not being fully able to see the thin lines. So if you squint at these lines for example, you'll probably see that they disappear or I can get the green one to disappear with some kind of squint at it, and that might be sort of simulating if I wasn't wearing my glasses or if I had some other reason in my eye, that I wasn't so sensitive to contrast or to acuity. So that's why maybe a colour blindness associated with something else could mean that you wouldn't be able to see these pens on a whiteboard. The reason a colour blind person wouldn't be able to see a laser pointer on a whiteboard is because it isn't darker than the whiteboard. A laser pointer is adding light to the whiteboard of course, and so, it isn't darker than the whiteboard. It only differs in colour and that's why a colour blind person would have difficulty seeing the red laser pointer on a whiteboard.

Diana - Colour blindness occurs when certain types of cones in the eyes aren't present, but the person in the question sounds as if he had some cones. Perhaps it's more of an issue with detecting differences in luminosity or the brightness of surfaces.

Comments

I don't think it has anything to do with colour blindness, I can see the colour perfectly, but I can't see what's written

I can’t read red on white or white on red no matter the circumstances, idk why. My whole spectrum of reds used to be very vibrant but now I only see a couple reds in the “red family” which is base red, dark red and pink. Everything that used to be an offset color of red is now only one of those. Idk why this is and I haven’t seeked medical attention because it really doesn’t affect me that much because I know what things are now that I have seen them before but like my eyesight in general has deteriorated.

So basically, I am 12 years old and I do wear glasses myself. But the problem is that when the teachers write something with red or blue markers, I can't see anything and I always need to look at my friend book or ask my friend (that is seating next or in front me) what is the word or the alphabet inside the passage. So can someone tell me is that normal or do i need to ask the person checking my eyes?

i dont think im color blind and i dont wear glasses. when im in school, i can see the diference between red and green in a whiteboard, but i can not read it. i can read things writen in blue or black, but i cant read whats writen in green or red, but i can defferentiate the colors. any sugestions why?

I have the same exact issue as you, I can see what colour it is in, but I can never read it usually no matter where I am in the room. I actually get in quite a bit of trouble because of it. I like sitting in the back too because I have an issue with someone being right behind me. I feel like I am doomed. My teachers only ever write in red and green, I do not understand how other students read this as I can not. It is harder at the end of the year when their ink is running out too!

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