I have been listening to this advice that white text on black background is not comfortable and shouldn't be used for large volume of text. This aspect is proven through an experiment conducted by Bauer and Cavonius where they found that 26% participants were more accurate in reading dark text on light background. Okay so we know for real that light text on dark backgrounds is a no no but I have been wondering about "why" until I across some these reasons.
1 - Expansion of retina for light and dark backgrounds
Our eye reacts to light and dark colours and ajusts ratina depending upon the intensity and contrast of colours it looks at. As we know light colours reflect more light than dark ones because of which they appear bright. Retina of eye is responsible to allow a specific among of light into the eye as excessive light causes "vision bleed" and hampers eye's ability to perceive and understand. When we look at an object or writing with white background, our retina contracts to reduce the amount of light which enters our eyes - but - when we look at a writing that has dark or black background, retina dilates as black colour reflects less light.
Nearly 30% of world's population is suffering from an optical defect called Astigmatism because of which vision is blurred as eye is unable to focus a point object into a sharp focused image on the retina. In addition to that, as we grow old, our eyes's lenses experiences lense defects like Lense Aberration and Chromatic aberration because of which outer part of the eye's lense doesn't converge light uniformly and results in blurred vision. We know this aspect without learning from physics about it and as we experience weak vision, we tend to squeeze our eyes and look through the central part of eye-lense only and by doing that, we reduces the effect of lense aberration and our vision improves.
NOW - if I was looking at black background with white text on it, my retina would be dilated and I would be using larger area of my eye-lense, and ultimately, the effect of lense aberration would be more "visible" causing a blurred vision and a reading discomfort.
2 - Impact of White on Visual Receptors
Surface of eye is made of up three kinds of photoreceptor cells namely, Rods, Cones and Ganglion cells. White color stimulates all three types of color sensitive visual receptors in the human eye in nearly equal amounts and proving eye a full spectrum of infomration. Thus exposing eyes with "meaningful" text is stressful and tires the eyes more. When we have white as a background, even though white color is in larger quantity but it doesn't contain information for the eye and its presence is largely cancelled and eye only focusses on the dark text which is less stressful to process.
3 - Reflection of light and color bleeding
Light colors reflect more light than dark ones and because of this additional reflection our brain preceives light text as if it was expanding against a dark background. Because of that, white characters in a word appear closer together than they actually are and thus white words tend to cuase a blobbing effect on our preception. Because of this small text loses it crispiness and eyes have to scan the words harder to identify letters and words in a paragraph. This slows down the reading and eyes also feel tired after reading relatively less content than otherwise.
4 - Relative Contrast
When looking at multiple colors together, our eyes preceive colors according to their relative contrast instead of actual contrast. Because of this amaizing ability to compre and comprehend colors, our eye is able to identify colors hues under drastically diofferent enviornments. When foreground and background colors have a low contrast between them, eyes have to work harder to isolate meaningful content (text) from non-meaningful content (background). Same is also true when the contrast value between two colors is unreasonably high where eye is able to distinguish foregrond from the background easily but because of high contrast, it has to exert extra energy to cancel non-meaningful colors values. That explains why a pure white on pure black is uncomfortable to read but reversing and putting pure black on pure white is also relatively stressful.
WCAG are doing a good job in explaining what should be the minimum contrast level between text and its background but they are only covering a half aspect of this discomfort. Howerver Accessibility Guidlines are only trying to make text "readable" instead of "comfortable". This is where you go to pay Visual Designers and get the job done.
What to do if we had to use light text on dark background colors
- Reduce the font-weight and use lighter fonts
- increase tracking (letter-spacing) so that when white bleeds out, letters are still distinguishable
- prefer a larger font
- Prefer sans-serifs fonts