Thursday, May 28, 2009

Text color polarity

For the last month, when I had some dead time waiting until batch jobs run or the IDE rebuilds our GPU RIP system, I have been reworking the manuscript on understanding readability of colored text by crowd-sourcing on the Web I am trying to publish with Silvia Zuffi and Carla Brambilla, who did the actual work.

The work is about the threshold for readability efficiency, i.e., we are asking where the color contrast threshold is, at which reading performance in speed decreases or the number of errors increases.

In the introduction, we summarize the previous results. Here is what we learned about the effect of text polarity and background luminance:

The sign of the difference between background and foreground luminance defines polarity. Positive polarity, or negative contrast, is for example black-on-white, while an example of negative polarity, or reversed contrast, is white-on-black. Normal vision acuity is slightly better for reversed contrast, but the majority of studies on readability found that positive polarity is more suitable for text.

Previous studies performed on old displays produced different results about the effect of polarity on reading performance. This was due to the characteristics of the equipment; for the rapid refresh rate of modern displays, dark characters on light background seem to be better, but for the common refresh rate of old displays a dark background was preferable. Reading performance was also influenced by lighting conditions that could affect the perceptibility of flicker. Another review of early studies found an advantage of black-on-white for reading and characters recognition, while Legge did not find any difference in normal vision, but found that people with low vision read faster with reversed-contrast text. These were usually people with abnormal light scatter in the eyes. Pastoor found no evidence for an influence of luminance polarity on reading and search performance. According to Shiel polarity has a significant effect on visual performance in the sense that subjects perform better and have greater preference for dark targets on lighter backgrounds (positive polarity).

Sanders and McCormick suggested that a light background might be advantageous under situations with glare or reflection problems because it may reduce the visibility of reflected light. Scharff and Ahumada investigated whether the effect of text polarity is due to different sensitivities in the “on” and “off” retinal pathways, or the result of more experience with dark text on light background. They observed that light backgrounds yield better performance, with a predominant effect on the polarity of the text. As for the effect of luminance background, Lin found that in the case of positive polarity, better performance is obtained for a lighter background at the same luminance contrast but at different background luminance. The lessons learned from previous studies that are relevant to our experiment can be summarized as follows:

  1. Luminance contrast is the dominant factor to address readability
  2. The luminance polarity affects readability (negative polarity is more difficult)
  3. The luminance of the background affects readability (lighter backgrounds are preferable)
  4. An additional color contrast does not facilitate reading when a sufficient luminance contrast exists
  5. On achromatic background, wavelengths at the extremes of the spectrum are more difficult for text.

As seems to be the annual May ritual, our blogging system was updated. As you are noticing, the design now matches the overall look with its small white font. I spent a day fixing up old posts that became unreadable due to transparency for a light background, or because the font color was that of the color being discussed, such as International Klein Blue or Blue Iris.

I am very busy, as we are still fighting to get the performance up to the 1600 ppm target, so I took the liberty to take shortcuts, like just dropping transparency. In some places the text might be fuzzy because the anti-aliasing was done on a white background, and I do not have time to redo it on a black background. My apologies, but blog posts are ephemeral.

À propos 1600 ppm, although our blog has been moved from research to technology at home in the HP blog directory, Nathan and I are still totally focused on commercial printing (like the 7000 below, which is our current test device), so do not expect to see our technology on your kitchen table. And our target audience are still our fellow color scientists.

HP Indigo 7000 Digital Press, glow