Science Questions

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

Sat, 23rd Oct 2010

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Lesley, Leicester asked:

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...



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

To answer the question about the pens on the whiteboard, basically, this doesnít 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.  Itís 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 doesnít 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 thatís 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 thatís why it doesnít sound like itís just a simple case of red, green colour blindness.

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Diana -   So distinguishing coloured marks from their background shouldnít 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 itís 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 thatís 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, youíll 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 wasnít wearing my glasses or if I had some other reason in my eye, that I wasnít so sensitive to contrast or to acuity.  So thatís why maybe a colour blindness associated with something else could mean that you wouldnít be able to see these pens on a whiteboard.  The reason a colour blind person wouldnít be able to see a laser pointer on a whiteboard is because it isnít darker than the whiteboard.  A laser pointer is adding light to the whiteboard of course, and so, it isnít darker than the whiteboard.  It only differs in colour and thatís 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 arenít present, but the person in the question sounds as if he had some cones.  Perhaps itís 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 ...

If the person has difficulty seeing text on a white board, (regardless of its colour), maybe they have problems seeing in bright (white) conditions ...

Or are simply short sighted ... RD, Mon, 18th Oct 2010

Maybe it was at night and the lights were off? Geezer, Mon, 18th Oct 2010

Colour blindness, apparently there's a lot of it about ...    RD, Mon, 18th Oct 2010

Thanks RD! Now I know what to put on for Halloween. Geezer, Tue, 19th Oct 2010

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>, Tue, 26th Oct 2010

That reminds me of Irlen filters, (some debate as to whether they are legitimate).

A cheap pair of sunglasses would be worth a try
(e.g. those clip-on ones 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.

RD, Wed, 27th Oct 2010

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. gavin1069, Sat, 30th Oct 2010

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. deepfromindia, Sat, 7th Sep 2013

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? Tonya , Wed, 2nd Sep 2015

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