If you are creating or recreating software controls for a medical, industrial, or other device, at some point you may consider whether to design your user interface (UI) with light mode, dark mode – or both. Light mode is characterized by dark foreground elements (such as text) on a light background, dark mode by light foreground elements on a dark background.
Dark mode has been something of a fad in recent years. Surveys show it to be popular on consumer devices. If you go looking for insight on the subject, you will find claims that dark mode reduces eye strain, extends battery life, and even enhances concentration. But scientific research on the subject demonstrates that light mode tends to be more legible for most users in most environments, suggesting the popularity of dark mode rests on aesthetic preferences and fashion. Nonetheless, dark mode may well enhance the usability of your device.
Science Can Help You Determine Whether You Need Dark Mode or Light Mode – or Both
What exactly is happening between the device screen, the eye and the brain? And how can science inform decisions about your product? To answer these questions, we need to explore three factors that affect how we see: illumination, adaptation, and focus.
Illumination refers to the amount of light reaching our eyes. Depending on the amount of that light, our pupils expand or contract to let more or less light reach the retina. But this “pupillary light reflex” plays only a small part in our response to light. The retina itself contains two types of photoreceptor cells, known as rods and cones (named for their shapes), which play a larger part. In bright environments, the cones are most active. They enable perception of color and fine details.
In low-light conditions, the rods – which are vastly more sensitive to light, but less sensitive to color and detail – do most of the work. Within both types of photoreceptors, photons stimulate biochemical events that ultimately generate electrical signals to the brain (a process known as phototransduction).
Some nocturnal animals have only rods in their retinas, and some of our pre-human ancestors were nocturnal, leaving us a legacy of rods. But for most of our evolutionary history, humans have been diurnal animals – hunting, gathering, farming etc. in daylight. This is why we see better in lighter environments.
Consistent with this evolutionary perspective, a widely cited investigation by Cosima Piepenbrock, et al., persuasively demonstrates the greater legibility of light mode, which they label “positive polarity,” leading to a general “positive polarity advantage.” They also measured the total amount of light reaching their subjects’ eyes from the combined sources of controlled ambient light and the TFT-LCD displays they used, recording 30 times more in light mode compared with dark mode. Light mode’s greater illumination meant subjects’ pupils were contracted and their cones were more fully engaged.
The team concluded that, “The positive polarity advantage seems to be primarily due to the typically higher overall luminance of positive polarity displays.” In other words, we see best when more light reaches our eyes, and some of that light comes from our device screens. Other research has demonstrated that both light and dark mode are more legible in brighter light conditions than in darker conditions, which reinforces the hypothesis that overall illumination is a decisive factor.
Another critical factor affecting legibility is adaptation – the process our eyes go through when adjusting between light and dark environments. When illumination is reduced, dark adaptation takes place, and rod activity ramps up while cones ramp down. The reverse occurs during light adaptation. Neither process is instantaneous. Although estimates vary, dark and light adaptation each require at least several minutes. Some sources claim full dark adaptation can take up to an hour.
For most of our evolutionary history, bright light only came from the sun, and changes were gradual. So, our eyes are not made for quick adaptation. Adaptation takes time, during which our vision is not optimized.
We must also consider focus duration and focus shifting. Ambient light varies as much as it did eighty thousand years ago, but now we depend on devices that, unlike a rock or a stick, actually emit light. For many embedded systems use cases, users’ attention shifts back and forth between a device screen and other things--such as the road, the patient, or the physical work task. The interval of these shifts may be measured in minutes or seconds – sometimes milliseconds.
An understanding of illumination, adaptation and focus points toward the value of dark mode. For many devices in the wild, user attention is predictably intermittent and ambient light predictably variable. Each time a user’s attention shifts to or from your device screen discrepancies in illumination will trigger adaptation.
And because adaptation is a slow process during which our eyes work hard but our vision is sub-optimal, we should design device screens that minimize the need for adaptation. In other words, our screens should emit only about as much light as that emitted or reflected in the rest of the work environment.
So, if your device’s use environment will include low-light conditions – especially if it will receive intermittent user attention – your UI should include a dark mode.
If the use environment is always dark, you don’t even need a light mode. But if ambient illumination will vary widely, both modes are necessary for good legibility. If possible, the mode should switch automatically depending on ambient light or time of day, a feature of many auto dashboard touchscreens and smartphones. It is also best to offer a way to override automatic switching – primarily to accommodate users with certain abnormal eye conditions. Of course, actual light conditions can vary infinitely between bright and dim so a convenient brightness control can help users tune a display for maximum legibility.
A Word of Caution
I want to mention a few caveats. Users with the common condition of astigmatism may have trouble with dark mode in any environment, as they often report a “halation effect” or glow around light foreground elements on dark backgrounds. Conversely, other visual impairments may cause some users to see best in dark mode.
Also, sharp readers will have noted that I have ignored the significant variables of screen size, viewing distance and how much of a user’s field of vision a screen may occupy – all of which should be considered for any specific device. (Topic for another blog!) The takeaway is this: device designers need to know as much as possible about users and their environments, and build in as much flexibility as necessary in the UI taking into account how human eyes process light.