Dark Mode. Some people love it, some hate it with a passion and most everybody prefers it circumstantially. It is one of the most widespread User Interface trends of the past years that continues to dominate the online space, but what is the science behind it? As much as it is often overlooked other than looking sleek and snazzy, negative contrast polarity - or as it is much more commonly referred to as dark mode - serves the purpose of allowing elderly people and people that have compromised vision consume content easier and see better.
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The Human Factor
This all comes down to biology. The human pupil works exactly the same way as aperture on a camera, the narrower the gap, less light reaches the retina - and vice versa. This gap is adjusted by the eye based on the amount of light in the environment. To understand how this all relates to User Experience we have to talk about spherical aberration, or when an image looks unfocused. This phenomenon is mostly mitigated by the eye by contracting the pupil, therefore increasing the depth of field allowing for easier viewing and comprehension of text.
The problem arises when we factor in that with age the pupil decreases in size, often beyond the threshold, making it hard for light to reach the retina which impairs text detection in ambient lighting scenarios. To make matters worse, simultaneously with age the eye tends to develop a glare that is particularly likely to impact vision under bright conditions. These make it difficult for the elderly to continue consuming content in light mode, negative contrast polarity offers help with legibility.
For those that have normal or connected to normal vision, the effects of either polarity weren’t at all adverse. It mostly comes down to the individual’s personal preference. This is also observable in people with impaired vision - especially those with cloudy ocular media - that even though there is no concrete evidence, that the usage of dark mode indeed helps with legibility, people reported their preference of negative contrast polarity over light mode (based on a study by Gordon Legge of the University of Minnesota in 1985).
It is important to also mention that at the time of the study they tested the patients’ reactions on a CRT monitor, that was much more susceptible to flicker in positive contrast polarity than the negative - possibly skewing the results of the survey.
Although there may not be substantial advantages to making all of your designs in dark mode when your target audience is the general population, we strongly recommend that you enable your users to make the decision on how they enjoy your content. As much as it is a trend (and rightly so!), we want to emphasize the positive effects on the engagement with users suffering from vision impairments by enabling dark mode and therefore enabling your content to be seen.