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My college recently switched to whiteboards. Though writing on a whiteboard with a black ink sounds similar to the conventional way of writing with white chalk on a blackboard, it feels very different. At least for me as a student, blackboards used to be a much better experience.

Physically, are there any benefits associated with writing in white colour on a blackboard as opposed to vice versa?

Consider only the effects it would have on the contrast of writings. Assume the textures of both writing instruments to be same. Please avoid any non-physical or aesthetic factors.

(By contrast, I mean the difference between the intensities of light coming from writings and the surrounding blank space.)


I believe the answer lies in the scattering of light. Learning from the comments, a more insightful scenario would be the same situation during the projection of presentations onto a screen.

Which layout (black text in white background or the reverse) would provide more clarity at a distance?

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    $\begingroup$ I'm voting to close this question as off-topic because it seems to be a question about human visual perception, not a question about physics. $\endgroup$ – Solomon Slow Nov 12 at 14:35
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    $\begingroup$ Whiteboards are often shiny, but blackboards (and chalk) are not. So the display quality of whiteboards is more affected by the light sources in the room. $\endgroup$ – alephzero Nov 12 at 14:43
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    $\begingroup$ @SolomonSlow I think it is clear now after the edit. $\endgroup$ – Krishnanand J Nov 12 at 15:19
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    $\begingroup$ Questions about visual perception can be on topic here. Check the visible-light tag. $\endgroup$ – mmesser314 Nov 12 at 15:45
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    $\begingroup$ As the answers suggest, whiteboards might reflect more light and thus strain the eye more than necessary. This is particularly relevant to think about on screens and powerpoint projections. A projector will display a black pixel as no light, basically as a turned-off pixel. So, the image that you look reflects/emits much less light for your eyes to absorb when the background is black. $\endgroup$ – Steeven Nov 12 at 15:56
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I think that there is somewhat of a biophysics question here but it is perhaps buried.

In theory there is no contrast difference, but in practice...

If you are designing for contrast in particular, there is almost no sense in which light-on-dark and dark-on-light can be properly distinguished in the abstract. Like, if "black" and "white" are consistently defined between the two situations, there is no difference between a smattering of white light inside of a black field of view and a smattering of black space inside of a white field of view: the difference between black and white is exactly the same and the contrast difference is zero.

But in practice with contrast, one needs to look at the deep particulars of the two solutions: blackboards are frequently dark grey rather than black and their chalk is often off-white and only deposited in a thin layer, whereas whiteboards are more often really white and a dry-erase marker that is not running out can be nearly black: the contrast is greater on most whiteboards. (On the other hand, the reason that some of us are apparently plagued with dry-erase markers running out on us, is that we like to write high-up with our pens angled upwards, in which case gravity is pulling the ink away from the writing tip into the back of the pen, which is simply not an issue with chalk: if it touches the surface it can write on it.)

Similarly both chalkboards and dry-erase boards lose contrast due to “ghosts” of previous marks sticking around, but this problem is somewhat more of a problem in practice with chalkboards as the chalk is a much more nonreactive substance than the alcohol-affiliated ink.

Some other features of biology also matter

Interestingly, you speak as if contrast is an absolute good but I am not sure that it is. It may be that humans prefer the lower contrast to the higher contrast and that this is why you preferred the blackboards of your youth. I am not sure. Once we get into visual perception I think most of the physics in the situation is gone; I for example find it really hard at night to read signs that are written in glowing blue letters but I cannot easily explain this as a consequence of the physics of blue light. But there are two physics effects that I think are more universal and still matter for the larger consideration.

The first is hysteresis, the fact that your eyes are not coming to the situation from a completely blank context, but rather they are immersed in that context and constantly switching back to it, so that the surroundings matter a lot. In a bright room with white walls, a whiteboard will have less contrast with its surroundings. Conversely if you are giving a presentation in a room where the lights have been turned out, white-on-black presentation is going to reduce eye strain as people look up from your presentation and then back down to you. Probably this is the most important effect.

The second is depth of field, the fact that a brighter context will cause your pupil to contract. As long as we are not illuminating things with the brightness of direct sunlight, your pupil should be able to adjust painlessly to any particular context; the pupil’s contraction is not directly an issue. However a contracted pupil can keep more in focus than a dilated pupil can, with less work for the lens: photographers say that narrower apertures have a greater depth of field. This is why it is harder to take a sharp photograph at night. In a way this also improves contrast; blurring leads to some muddling of contrast around the edges of a letter or the thinnest strokes and dots. So at the same definitions of black and white, there should exist text at a size and distance which is too small to read in the white-on-black configuration (your pupil is just too dilated to help your lens to fully resolve the details) but quite legible in the black-on-white configuration (where your pupil is contracted).

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  • $\begingroup$ Depth-of-field does not apply here. Depth-of-field tells how far something can be from plane of focus but still be perceived as sharp. When subject is looking at board, their pupils are focused on board, so board is the plane of focus, i.e. smack in the middle of depth-of-field anyway, hence always in focus. Also, a dilated pupil would be able to resolve better than contracted one because of larger aperture due to better diffraction-limit resolution. I don't know if retina has enough cones to make use of extra resolution offered by dilated pupil though. $\endgroup$ – GPS Nov 13 at 6:05
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I am finding it easier to come up with questions than answers.

Given you want textures of both writing instruments to be the same, you might consider a similar problem on your phone. It white text on a black background better than black text on a white background. Pixels make it easier to get a uniform comparison. Which is better? I believe some monochrome displays have gone with white on black for better readability. Color displays tend not to.

As alephzero said, specular reflection can make a shiny whiteboard hard to read if the room lights are bright. But in dim light, more light reflected from the board can make it easier to see. Concrete barriers next to roads are painted white for this reason. But is detail in the whiteboard easier to see?

Off topic, but somewhat relevant: Some whiteboard marker colors become hard to erase when dried. Chalk dust is a nuisance, and perhaps a minor health hazard. Perhaps the solvent in markers is unhealthy.

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Practical considerations are more often a factor than the visuals. Many Colleges are adept at chronically understocking their classrooms with pens. A small stub of chalk can still be used to make a good contrast line, but a nearly-empty pen will be barely visible.

Similarly, if the professor is lazy and doesn't wipe the board properly, or doesn't use water enough, the contrast and legibility of a blackboard can be terrible too.

Whiteboards don't make as much of a mess, and some people get asthma from chalk. These were probably a bigger consideration for the school.

I would encourage you to actually ask your school why they opted for whiteboards. I would be interested. If students were to show an obvious preference, it could be enough to sway their decisions in future.

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