Would you think that it’s easier to read black text on a white background or white text on a black background?
My training as a print journalist taught me that when using white on black you had to use a bolder typeface. White on black is harder to see. The reason for this is that ink spreads, especially on the uncoated paper used for newsprint. This is known as dot gain and makes pictures look fuzzy.
However with sharp printing and with screens the rules are different, and it wasn’t something I realised until I read a piece in New Scientist’s “Last Word” column.
A reader had asked why he found it easier to read white on black in poor light, and David Muir from the science department at Portobello High School in Edinburgh said
“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.”
So light has the opposite effect of dot-gain. The edges are still blurred but it makes the letters bigger, he goes on to say
“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.”
What’s interesting about the sky analogy is that it’s transmissive light, like that from a computer or mobile phone screen, and not reflective as you get from reading a printed page. The diffusion around the edges will be brighter with transmissive displays and so the effect will be greater. In his response David Muir says “cellphone models frequently use white text on a dark background. I assume that this is a means of saving battery charge”, which I don’t think is the case as the light comes from the backlight and is just masked by a black display. The colours are generated by red, green and blue pixels letting the light through. The backlight doesn’t use any more or less power with different pixels on or off. Phones from LG and Sony are now using a new idea which LG calls IPS and Sony “Magic White” where in addition to the red, green and blue there is a fourth pixel which is either black or clear, letting the backlight straight through. This gives a much whiter white than having the three colours on. The manufacturers claim their screens are twice as bright, and this does save power.
What this means for making phones easy to see is that white on black, for both the display and the keypad works better than black on white. The phones that springs to mind which do this well is the Emporia RL1 and the Panasonic KX-TU301.
It’s something to bear in mind when looking at phones for people with restricted vision and focus.