I was reading in very dim light last night and found that I could not read black text on a white background (the character size was about 8 point) but could read white on a black background. I’m 48 and wear reading glasses, which I did not have with me at the time. I expected not to be able to read black-on-white text, but was surprised I could read the opposite. Why is this? It seems counter-intuitive.
• There are two phenomena which conflate to allow this to happen, by altering the perceived text size and word shape.
Your questioner has presbyopia, an age-related diminished ability to focus on near objects created by a lack of elasticity in the lenses of the eyes. Without reading glasses, images are blurred. This is exacerbated in low light as the pupil enlarges to let more light into the eye and thus shortens the depth of field of clear vision. This is familiar to photographers, who alter the depth of field by changing a camera’s aperture size to blur the background or make it sharp; the smaller the aperture, the clearer the background.
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Lenses focus light, not the lack of it. So as your questioner’s eyes focus light poorly, the white light from the background will impinge on the edges of black text, making it appear smaller and more blurred. Conversely, when viewing white text on a black background the diffuse light from the text impinges on the black background, producing blurred but perceptually larger text. This makes it easier to read than the black text.
The shape of the “larger” white words is also more discernible than the shape of the smaller black text. The importance of the overall shape of words to how a skilled reader perceives them has been debated by cognitive psychologists and was discussed by John Bradshaw in 91av (6 February 1975, p 321).
An analogy is to imagine the difficulty of perceiving black specks in a sunlit sky. Compare that with perceiving white specks of the same size on a black background, for example, stars on a clear, moonless night.
“An analogy is to compare the difficulty of perceiving black specks in a sunlit sky with stars on a clear night”
Even without glasses, a myopic (or short-sighted) person like me can see the diffuse light of the stars as blurry blotches, and can recognise constellations because I am already familiar with their shape, just as your correspondent will be familiar with the shapes of the words on the page.
The latest cellphone models frequently use white text on a dark background. I assume that this is a means of saving battery charge, rather than any specific benefit to the hyperopic (long-sighted) or presbyopic, and is nothing more than serendipitous.
David Muir, Science department, Portobello High School