Naked Science Forum
General Science => Question of the Week => Topic started by: thedoc on 26/10/2010 16:34:15
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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...
Asked by Lesley, Leicester
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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.
[img float=right]/forum/copies/RTEmagicC_Ishihara_9.png.png[/img]
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.
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Even if the person had total colour blindness, seeing red or green text on a white background would not be a problem: they would appear as grey on white ...
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[only red on green background, and vice versa, would be difficult]
If the person has difficulty seeing text on a white board, (regardless of its colour), maybe they have problems seeing in bright (white) conditions ... http://en.wikipedia.org/wiki/Hemeralopia
Or are simply short sighted ... http://en.wikipedia.org/wiki/Myopia
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Maybe it was at night and the lights were off?
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Colour blindness, apparently there's a lot of it about ... [:)]
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http://www.museumofbadart.org/collection/portraiture-3.html
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Colour blindness, apparently there's a lot of it about ... [:)]
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http://www.museumofbadart.org/collection/portraiture-3.html
Thanks RD! Now I know what to put on for Halloween.
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Interesting Q. As a person who once taught in a specialist school for dyslexic pupils, I was trained to write each line on the board a different colour, as this facilitated pupils' 'scanning'. Then I moved to a college with a SMART Board. There were at least 2 dyslexic students in the class, so I continued (for a while) with my use of different colours on the board. However, I had since modified it so that I only colour-coded specialist/key words, thus performing a dual function. However, this was thwarted by one student who claimed that he was 'colour blind' and couldn't see the red and green at all. I wondered if he might've been 'having a laugh' as, to my knowledge, people who're colour blind can still see the characters; they just can't distinguish one colour from another.
Having read the above, I'm now wondering whether be was being genuine... <erm>
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Interesting Q. As a person who once taught in a specialist school for dyslexic pupils, I was trained to write each line on the board a different colour, as this facilitated pupils' 'scanning'. Then I moved to a college with a SMART Board. There were at least 2 dyslexic students in the class ... one student who claimed that he was 'colour blind' and couldn't see the red and green at all.
That reminds me of Irlen filters (http://en.wikipedia.org/wiki/Irlen_filters), (some debate as to whether they are legitimate) (http://www.ldrc.ca/contents/view_article/207/?print=false).
A cheap pair of sunglasses would be worth a try (http://en.wikipedia.org/wiki/Hemeralopia#Management)
(e.g. those clip-on ones (http://www.sunglasses-shop.co.uk/Cocoons-Sunglasses-Cocoons-Rectangle-Clip-on-Sunglasses-Grey-LF401G/7027) if the person already wears glasses).
If the problem only occurs with the distant whiteboard, not close up white paper on desk, then a visit to the optician is in order to correct the refractive error.
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I am colour blind and I have that problem, when I am close to the white board (< 1 meter) I see all colours, the problem comes in when I move away from it. I do not see yellow at all, I see it as green so that is a problem, with lights. Green disappears after 2-3 meters. All dark colours become black after about 3.5-4 meters, so colour charting does not work. Too light a colout disappears at about 3.5-4 meters.
And this is not only on whiteboards, it also happens with overheads and projectors. If you switch the lights off, it is better, I can see the colours from further away but still not green/yellow. It makes interesting reading when you go to seminars and can not see the entire presentation.
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I also have the problem.when the teacher writes by green or yellow I do get a lot of problem.but the problem is not just for me all of the students who sit at a good distance face it.I thought about it like, I can see when the teacher writes in blue or green then why do the problem arises for the other 2 colors.I think the mathematical reasoning is this----
V I B G Y O R ( the different colors of the spectrum)
wavelength increases from left to right
we know from planck' s law that. E= hc/wavelength .so we can say Energy, that blue or violet carries is more than red as energy is inversly proportional to wavelength.the photo receptors in our eyes are thus get more stimulated by blue or green in comparison to red or yellow( wavelength higher E LOWER).this is the only reason I think is the culprit for the mess.
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I have trouble reading anything written on a whiteboard with colored pens. It looks blurry to me unless it's written in black. Why is this?