Dark Mode vs. Light Mode: Which One Is Superior?

For individuals with normal or corrected vision, visual performance is generally better in light mode. However, those with cataracts or similar conditions may find dark mode more effective. Conversely, prolonged reading in light mode might contribute to the development of myopia.

Dark and light mode mobile screen

But first, let's ensure we understand some key terms.

Definition: Contrast polarity refers to the difference between the text and its background:

• Positive contrast polarity (light mode) features dark text on a light background.

• Negative contrast polarity (dark mode) features light text (e.g., white) on a dark background (e.g., black).

Dark mode displays emit less light than light mode displays, which can potentially extend battery life. However, the ambient light level affects not only power consumption but also our perception. To understand this better, let’s briefly review some basics about the eye pupil and its reaction to different light levels.

The Human Pupil Is Sensitive to the Amount of Light

The human pupil acts as a gateway to the retina, allowing light to enter the eye. It automatically adjusts its size based on the amount of ambient light: it contracts and narrows in bright conditions and dilates in the dark to let in more light. When the pupil is smaller, the eyes are less prone to spherical aberrations (which cause blurred images) and benefit from a greater depth of field. This makes it easier to focus on text and reduces eye strain. This mechanism is similar to how camera apertures work; a photo taken at f/2.8 will have a narrower depth of field and more blur compared to one taken at f/16.

As we age, the pupil naturally becomes smaller. Extremely small pupils can limit the amount of light entering the eye, making it harder to read or see text in low-light conditions, such as at night. Additionally, older individuals are more sensitive to glare, which is especially problematic in bright light conditions.

Normal-Vision Users

Early studies from the 1980s suggested that contrast polarity didn't impact visual performance for individuals with normal or corrected vision. However, more recent research challenges this view. Two key studies highlight this: one from 2013 in Ergonomics focused on visual acuity and reading performance, while a 2017 study in Applied Ergonomics examined glanceable-reading tasks, such as quickly reading 1–2 words on a mobile phone, smartwatch, or car dashboard.

Impact of Contrast Polarity on Visual Acuity and Proofreading Performance 

Cosima Piepenbrock and her team at the Institut für Experimentelle Psychologie in Düsseldorf, Germany, conducted a study with two groups of adults with normal or corrected vision: young adults (18-33 years old) and older adults (60-85 years old). None had eye diseases like cataracts.

Participants performed two tasks:

• A visual acuity task, which involved identifying the gap in a Landolt C optotype (a "C" shape oriented in various directions).

• A proofreading task, which involved reading a short passage and spotting errors.

Tasks were presented in either dark mode or light mode, with each participant only seeing one mode. Researchers also measured fatigue indicators (eyestrain, headache, muscle strain, back pain, and well-being) before and after the experiment.

Results showed that light mode was superior for both tasks across all ages. However, the visual acuity task's light mode advantage was smaller for older adults compared to younger ones. Fatigue metrics showed no significant difference between contrast polarities.

In a related study published in Human Factors, the same group found that the positive-polarity (light mode) advantage in proofreading increased as text size decreased. Despite better performance in light mode, participants did not perceive any difference in text readability between light and dark modes, underscoring the principle that user perceptions may not always align with actual usability data.

Impact of Contrast Polarity on Quick Glance Reading

Jonathan Dobres and his team at MIT’s Agelab investigated how ambient lighting conditions affect the advantage of positive contrast polarity in a lexical-decision task. This task resembles glanceable reading, such as when driving or using a mobile phone, where users quickly extract relevant information from displayed text. Participants with normal or corrected vision were presented character strings in light mode or dark mode, under simulated daytime or nighttime lighting, and at various font sizes.

Results showed that lighting, polarity, and text size influenced performance as expected: daytime lighting led to faster judgments than nighttime, light mode outperformed dark mode, and larger font sizes were processed more quickly. However, the significant finding was the interaction between ambient lighting and contrast polarity: during daytime, contrast polarity had no significant effect, but during nighttime, light mode significantly improved performance compared to dark mode. Additionally, reading small-font text in

dark mode was notably more challenging during nighttime compared to light mode.

The absence of polarity effect in simulated daytime settings was unexpected, contrasting with a previous study by Buchner and Baumgartner. However, the latter study used lower ambient light levels (e.g., office lighting) compared to the brighter conditions in the Agelab research (akin to outdoor daylight). Dobres and team suggest that ambient light intensity may influence the positive-polarity advantage, with bright light negating the difference but typical office lighting still allowing for differentiation.

People were faster during daytime than during nighttime, in light mode than in dark mode, and with larger font sizes (4mm) than with smaller font sizes (3mm). During nighttime, light mode was significantly better than dark mode and small font was a lot easier to read when in light mode. (Shorter bars are better than taller ones. Note that the time measure reported in the experiment is the presentation-time threshold; while technically more complicated than the judgement time, it essentially represents the time needed by the participants to make a correct assessment of the presented string.)

Long-Term Effects

While immediate benefits of using light mode may include better performance, particularly in tasks like reading, a study published in Nature Research's Scientific Reports reveals potential long-term consequences. Conducted by Andrea Aleman and her team at the University of Tübingen, the study investigated the effects of sustained exposure to light mode on eye health. They found that participants who read text in light mode for an hour showed significant thinning of the choroid, a vascular membrane behind the retina. Thinning of the choroid is associated with myopia, or nearsightedness, a condition where distant objects appear blurred. Interestingly, participants who already had myopia showed more pronounced thinning of the choroid after reading in light mode. This suggests that while light mode may offer short-term advantages in performance, such as enhanced readability, there could be a long-term cost to eye health, particularly for those predisposed to myopia.

Consideration for Users with Impaired Vision

Interestingly, while research on users with impaired vision is relatively limited compared to those with normal vision, there's a consensus that dark mode might be advantageous for some individuals with visual impairments. Gordon Legge and colleagues at the University of Minnesota categorized low vision into two groups: those with central-vision impairments and those with cloudy ocular media, often caused by conditions like cataracts.

Studies dating back to 1977, such as Sloan's research, have shown a preference for dark mode among some people with low vision. Legge and his team further hypothesized in 1985 that this preference could be linked to reduced light scatter due to cloudy ocular media. Consequently, dark mode, emitting less light, might provide clearer visibility for these individuals.

Legge's studies, notably conducted on CRT displays, demonstrated improved reading rates with dark mode among participants with cloudy ocular media. This research laid the groundwork for recommendations supporting the availability of dark mode in modern user interfaces, as proposed by Papadopoulos and Goudiras in 2005. However, it's worth noting that Legge's studies using CRT displays might not fully align with the performance on modern LED displays, as CRTs were more prone to flicker in light mode, potentially biasing results against it.

Implications

Should you join the dark-mode trend? While dark mode may benefit certain low-vision users, particularly those with conditions like cataracts, research consistently indicates a performance advantage for normal-vision users with positive contrast polarity, i.e., light mode.

This preference for light mode among normal-vision users can be attributed to several factors. Positive contrast polarity provides more overall light, leading to pupil constriction, reduced spherical aberrations, increased depth of field, and better focus on details without eye strain.

Despite this, we don't recommend defaulting to dark mode for the general population. However, designers should offer users the option to switch to dark mode for several reasons: potential long-term effects associated with light mode, improved usability for some with visual impairments, and personal preference. Users should have the freedom to choose their preferred mode.

While it's unlikely users will change display modes frequently for random websites, frequently used applications like book readers or news sites should offer a dark-mode feature. Additionally, if an operating system provides a dark-mode API, it's beneficial for designers to leverage it, enhancing the user experience for those who opt for dark mode.

Example

The Tata-owned Croma retail store application now offers a dark mode option on TATA Neu, aligning with the trend seen in many other applications. This dual-mode feature aims to enhance user experience by allowing users to switch between dark and light modes based on their preference and lighting conditions. With dark mode becoming increasingly popular for its reduced eye strain and improved readability, more applications, including those in the retail sector like Croma, are adopting this feature to cater to user preferences and provide a more comfortable browsing experience.

Reference: https://www.nngroup.com/articles/dark-mode/