Dark mode and astigmatism: why text glows
Last updated: 20 August 2026 · By the Eclipse team.
The short answer: if light text on a dark background smears, glows, or develops halos for you, the effect is called halation and it is not your imagination or your monitor. Roughly 47% of people have some degree of astigmatism, often undiagnosed, and dark backgrounds make its effects worse because your pupil opens wider in low light. The fix is counter-intuitive: reduce contrast rather than increase it, and add ambient light to the room.
What is happening in your eye
Astigmatism means the cornea or lens is not perfectly symmetrical, so light entering the eye does not converge to a single sharp point. Instead it smears slightly along an axis. In normal daylight reading this is often unnoticeable or fully corrected by glasses.
Two things change on a dark screen:
- Your pupil dilates. Less light entering means the iris opens. A wider pupil uses more of the lens, including the peripheral parts where the irregularity is greatest — so the optical aberration gets worse rather than better.
- Bright shapes on dark fields bloom. A thin bright letter stroke surrounded by darkness spreads its light into the surrounding dark area. The higher the contrast, the more pronounced the bloom.
Combine them and white text on pure black becomes a field of glowing, slightly doubled letters. Reading it is possible but tiring, which is why some people conclude that dark mode “just isn’t for them.” Often the problem is not dark mode — it is one specific, fixable implementation of it.
Why pure black is the worst choice
Maximum contrast feels like it should be maximum readability. For halation it is precisely backwards. #FFFFFF text on #000000 is the highest-contrast combination a screen can produce, and therefore the maximum possible bloom.
A softer black — around #121212 — with off-white rather than pure white text drops the contrast enough to substantially reduce the glow while still reading as a properly dark interface. Most well-designed dark themes land in this range for exactly this reason.
| Combination | Effect |
|---|---|
| Pure white on pure black | Maximum halation. Avoid. |
| Off-white on near-black (#121212) | Comfortable for most people |
| Light grey on dark grey | Gentlest, but can drop below usable contrast |
| Saturated colours on near-black | Poor — bright saturated text blooms badly |
Things you can change today
- Add ambient light. The single most effective change. A lamp behind your monitor keeps your pupil constricted, which directly reduces the aberration. Working in a fully dark room is what makes dark mode hardest to read.
- Lower the contrast, not the brightness alone. If your tool offers a contrast control, come down from maximum.
- Increase font size by a step or two. Halation affects thin strokes worst; larger glyphs have proportionally less edge relative to their body.
- Try a slightly heavier font weight. Thin and light weights are the worst case for bloom.
- Get your prescription checked. Astigmatism is straightforwardly correctable, and many people carrying a mild uncorrected amount have simply never been told.
- Use light mode where it suits. In a bright room, light mode genuinely is easier for most people to read. Using dark mode in the evening and light mode during the day is a reasonable answer, not a failure.
Why most dark mode tools get this wrong
Many dark mode extensions maximise contrast by default, because contrast is easy to measure and looks decisive in a screenshot. The standard accessibility metric encourages this: WCAG asks for a contrast ratio of at least 4.5:1 for normal text, and a naive way to guarantee it everywhere is to push text toward white and backgrounds toward black.
The problem is that the WCAG ratio was derived for dark text on light backgrounds and does not model light-on-dark perception well. A combination can comfortably pass the ratio and still be a halation nightmare. Perceptual models such as APCA exist because of this gap; they explicitly treat dark mode as something other than an inversion of light mode, and they discourage highly saturated or maximally bright text on near-black backgrounds even when the legacy numbers approve — the full comparison is here.
Eclipse uses APCA to solve each text colour against its actual background rather than forcing everything to the extremes, targets a comfortable rather than maximal contrast for body text, and exposes live brightness and contrast sliders so you can tune to your own eyes. If dark mode has always felt slightly wrong to you, adjusting contrast downward is usually the setting that fixes it.
Try it yourself: Eclipse applies a generated dark theme to any site, with per-site overrides and live brightness and contrast controls. Free, no account, no site limits.
Common questions
Does everyone with astigmatism have this problem? No. Severity varies widely, and well-corrected astigmatism may cause no noticeable halation at all.
Will glasses fix it? Correcting the astigmatism reduces the aberration, so usually yes, substantially. It will not eliminate the pure bloom effect of very bright shapes on very dark fields, which is partly a property of the display.
Are blue-light glasses relevant here? No. Blue light filtering addresses a different question about sleep timing, not optical focus.
Is this the same as “dark mode hurts my eyes”? Often, yes — halation is one of the most common concrete reasons behind that complaint. The broader picture is in is dark mode better for your eyes?
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