Night glare: what your visual acuity doesn’t tell you

A patient can have 10/10 vision and still be genuinely troubled by headlights at night: visual acuity and the scattering of light inside the eye are two distinct properties. The first describes the ability to separate fine details in high contrast, the second the veil of light the eye itself creates around every bright source, and nothing requires these two parameters to change together.

UNDERSTANDING

Why can you see 10/10 and still be bothered by headlights?

Because visual acuity and straylight describe two different properties of the same eye. When both are measured in the same subjects, the link is weak: in 217 patients due to undergo cataract surgery, acuity accounted for only about 8% of the variation in intraocular straylight, a coefficient of determination (R²) of 0.08 (PMID 22520308); and in a series of 2,422 drivers, acuity and straylight varied largely independently of each other (PMID 17651678). Knowing a patient’s acuity therefore tells you almost nothing about their night-time discomfort.

The acuity test is a resolution test. It asks the eye to separate two very close details in high contrast, under deliberately favourable conditions: black optotypes on a white background, constant lighting, a fixed distance and, crucially, no bright light source in the field of view. These are laboratory conditions, not those of a country road at 9pm.

Straylight measures something else: the portion of light that, as it passes through the eye, plays no part in forming the image. Instead of being focused on a single point of the retina, some of this light is deflected forward, from a few degrees to a few dozen degrees, and settles on areas that should have stayed dark. It is a veil, not a blur — and it is not what a slit lamp shows either, since that sees light reflected backward.

A sharp image can sit on a veiled background

This distinction explains the complaint. An oncoming headlight at eighty metres is still a headlight: its image is sharp. But the light it sends into the eye doesn’t stop at that image — it also illuminates the rest of the retina. The scene’s local contrast collapses, and the pedestrian at the roadside disappears into this haze. Acuity, measured with no headlight in the field of view, cannot by definition detect any of this.

The windscreen analogy is imperfect, but it holds up. A windscreen finely scratched by years of wiper blades causes no trouble in broad daylight: the light is diffuse and the scratches reflect nothing dramatic. At night, every oncoming headlight turns those same scratches into a starburst that covers half the field of view. The windscreen hasn’t changed between noon and midnight; the scene has.

MEASURING

How is straylight measured in the eye?

By a dedicated test that has nothing to do with reading letters. The method used in clinical practice is psychophysical, the “compensation comparison”, whose reliability has itself been studied (PMID 16822076, 17461676): the patient is shown a bright light source placed outside their line of sight and asked to compare the flicker of a small central area. By adjusting a compensation light, the amount of stray light their own eye is depositing on that area can be calculated.

The result is expressed on a logarithmic scale. The higher the value, the more veiling light the eye produces for the same light source. This figure doesn’t replace visual acuity — it complements it: the two describe separate dimensions of optical function, and neither can be inferred from the other. Another quantity is even more often confused with straylight: optical aberrations. These distort the image — ring-shaped halos, starbursts, coma — and are measured by aberrometry, which maps the wavefront. Straylight, by contrast, doesn’t distort the image: it overlays a uniform veil on the scene. Two quantities, two instruments, and often two causes that coexist in the same patient.

The main determinant of this value is the age of the lens. Straylight increases as the fourth power of age: in the cataract-free eyes of a series of 2,422 drivers, it doubles by around age 65 and triples by around age 77 (PMID 17651678); a series of 129 volunteers, all with 10/10 acuity, found the same fourth-power law, with a later doubling than in the previous series, around age 70 (PMID 2378063). The model has been revalidated in healthy eyes, where straylight also increases with axial length, and therefore with myopia (PMID 19875647). A lens still judged transparent on slit-lamp examination may already scatter noticeably more light than it did at twenty-five.

Straylight and contrast sensitivity are not the same thing

The two are often confused. Contrast sensitivity describes the smallest difference in luminance an eye can detect, for a given detail size and light level, in a scene with no bright source. Straylight describes what happens to that scene once a bright source is added. The two often decline in parallel, but a patient can have normal contrast sensitivity and still experience major discomfort when passing an oncoming vehicle.

One practical question remains: why is this measurement requested so rarely? It requires dedicated equipment, a few minutes of patient cooperation, and above all a reason to order it. As long as a patient with 10/10 acuity is assumed to be fine, the question never arises. This is exactly where the weak correlation between acuity and straylight matters most: screening based on acuity alone misses some genuine cases of discomfort.

MYTH

Is it really all about large pupils?

Far less than people think. The intuition is appealing: the pupil dilates in the dark, letting in more light through the periphery of the ocular media, so it should produce more veiling. Measurements in normal eyes show a dependence considerably more modest than this logic would suggest (PMID 17460305).

The same question was pressed hard in refractive surgery, where night-time pupil diameter was long held to be the main predictor of halos and postoperative discomfort. A review of 19 studies led to that idea being abandoned: beyond the first three months, none found a persistent link between pupil diameter and night-time complaints once LASIK was performed with an optical zone of at least 6mm (PMID 24203804).

That doesn’t mean the pupil has no effect at all. It affects depth of field and the total amount of light admitted. But it has become the default explanation for a symptom whose main cause lies elsewhere, in the transparency of the media the light passes through. Blaming the pupil for the discomfort carries a rarely measured cost: it closes the investigation.

In practice, a young patient with a large pupil and perfectly clear ocular media rarely complains of disabling glare. A sixty-year-old patient with an average pupil and a slightly opalescent lens often does. This is the opposite of what pupil mechanics alone would predict.

Two other sources are consistently underestimated. The tear film, first: between blinks, an irregular corneal surface scatters light, and dry eye often affects the same patients. The cornea, next, as soon as it carries a scar, a dystrophy or even mild oedema. All these structures lie in the light’s path before it reaches the retina, and each adds its own share of veiling.

FILTERS

Do yellow lenses improve night vision?

No benefit has been demonstrated to date, and the data remain thin. A night-driving simulator trial, measuring reaction time to a pedestrian’s appearance in 22 drivers, with and without glare from oncoming headlights, found no improvement attributable to yellow lenses (PMID 31369054).

Yet the reasoning behind selling them seems coherent enough: filtering out short wavelengths, said to scatter the most, should reduce the veiling. The premise is in fact false: straylight depends only very slightly on wavelength (PMID 8302533). And the filter, in any case, can’t discriminate. It also removes useful light, in a scene that is already short of it. What is gained on one side is lost on the other.

There is a lesser-known side effect, one that complicates assessment: a yellow filter increases the measured straylight value — slightly but significantly — in the psychophysical test used to quantify it, so that the comfort felt while wearing yellow lenses corresponds to no actual reduction in straylight (PMID 28917410). A patient who turns up to such an examination wearing tinted lenses is therefore not just reporting a subjective impression — they may also be skewing the very figure meant to make it objective.

What actually makes a difference to the night-time scene

  • A windscreen that’s genuinely clean, inside as well as outside: the greasy film that builds up on the inner surface scatters as much light as a scratch does.
  • Clean, unscratched glasses, with an anti-reflective coating in good condition.
  • An up-to-date prescription, including for mild astigmatism: it weighs more heavily at night than during the day.
  • Correct adjustment of the vehicle’s headlight height.
  • Looking towards the edge of the road furthest from the oncoming headlights, for the duration of the crossing.

None of these measures corrects the straylight produced by the eye itself. They simply remove the additional sources of veiling glare that add to it, and that share is far from negligible: on the same road, a driver may owe a good part of what they blame on their eyes to a dirty windscreen.

DRIVING

Should you stop driving at night?

This question isn’t settled by an acuity score, but it deserves to be asked seriously. The association between cataract itself and crashes remains uncertain in the largest meta-analysis: a relative risk of 1.15, 95% confidence interval 0.97 to 1.36 (PMID 37567751). Surgery, on the other hand, is associated with fewer crashes in several observational studies: half as many crashes among operated patients as among unoperated cataract patients in a cohort of 277 patients aged 55 to 84 (rate ratio 0.47; 95% CI 0.23 to 0.94; PMID 12186601), and 12.7% fewer reported crashes in the year following first-eye surgery among 27,827 drivers aged 60 and over (PMID 22273356). A much smaller cohort of 2,849 drivers found larger reductions still: 61% fewer crashes after first-eye surgery and 23% after second-eye surgery, both measured against the same reference year — in other words the benefit is mostly gained with the first eye, and the second does not add 23% to the first (PMID 30265273). A series of this size gives a less stable estimate: these orders of magnitude do not carry over from one population to another.

These figures deserve to be read for what they say, and no more. They describe populations, not individuals, and they say nothing about what will happen to any particular driver. What they do show is a consistent direction: after cataract surgery, driving-related difficulties fall markedly in a meta-analysis of seven studies (odds ratio 0.12; 95% CI 0.10 to 0.16; PMID 18836050), and loss of contrast sensitivity weighs more heavily on crash risk than the drop in acuity alone (relative risk 1.40 versus 1.21; PMID 37567751).

The questions that really inform the decision

  • Have you started avoiding night driving, or choosing better-lit routes?
  • Do you slow down sharply when passing an oncoming vehicle, or look away?
  • How long does it take you to see normally again after passing an oncoming vehicle?
  • Have you given up an evening journey you were still making last year?
  • Have those close to you commented on your night driving?

In everyday practice, a patient who answers yes to several of these questions is describing functional impairment, whatever figure is written on their prescription — no numerical threshold has been validated for this list. Conversely, a patient who has changed nothing about their habits may well have a high measured straylight value without this justifying anything beyond monitoring.

In regulatory terms, fitness to drive is assessed by an approved doctor, on the basis of criteria set out in an official text. Intraocular straylight is not among the numerical criteria it lists. This is precisely where the blind spot lies: a driver can meet the criteria and still be genuinely troubled once night has fallen.

HISTORY

Are today’s headlights really to blame?

Only partly, and the complaint is already more than twenty years old. In 2003, a paper titled “Why HID headlights bother older drivers” described precisely today’s complaint, in relation to the xenon headlights that were then replacing halogen bulbs (PMID 12488274). The grievances were the same: glare, a cold-looking tint, a sense of assault.

History repeated itself with LED headlights, then with matrix-beam optics. Each generation follows the same sequence: a brighter, whiter technology reaches the market, complaints rise, and people conclude that headlights have become unbearable. The cyclical nature of this complaint doesn’t make it illegitimate. It simply invites the question of what, in the equation, has actually changed.

Yet two things are changing at once. Light sources have indeed gained in luminance and shifted towards blue. It is luminance that matters: the 2003 analysis concludes that headlight glare comes down to brightness rather than blue tint (PMID 12488274), and intraocular straylight depends very little on wavelength (PMID 8302533). And the population that drives is ageing, with lenses that scatter more light. An identical headlight does not produce the same veiling in a twenty-five-year-old eye as in a sixty-five-year-old one.

So there is little point in pitting the two explanations against each other. The headlight is an external cause over which a patient has no control. The eye is an internal cause over which, sometimes, something can be done. Sending a patient away with nothing but their exasperation at headlights amounts to denying them the examination that could give them back their evening journeys.

TREATMENT

What can be done when straylight is the cause?

When straylight stems from a lens that has lost its transparency, it becomes treatable, since the structure responsible is replaced. In a series of 160 eyes whose preoperative corrected acuity already exceeded 8/10, straylight improved on average by 0.10 log(s) after surgery, and by more than 0.20 log(s) in 44 of these eyes — a gain the study’s authors compare to two lines on an acuity chart, at a time when acuity itself had almost no room left to improve (PMID 24631202). A more recent series shows that this value stabilises as early as the first postoperative week (PMID 37882099).

This average improvement, observed in the cited studies, does not prejudge any individual outcome. It depends on the type of opacity, and on the condition of the cornea, the retina and the tear film. It is also why a decision to operate should not be based on the acuity figure alone: a patient with 10/10 acuity whose night-time life has shrunk raises a genuine surgical question, one that deserves to be examined rather than dismissed.

Cataract surgery is carried out at the Clinique Sainte-Geneviève, in the 14th arrondissement of Paris. It is preceded by a pre-operative assessment that measures a good deal more than acuity, and which serves precisely to establish whether the lens is the real culprit.

When the lens isn’t the cause

Several other situations produce the same symptom and call for a different approach. Posterior capsule opacification, a few months or years after cataract surgery, brings back exactly the same complaint and is treated with laser in an outpatient appointment. Dry eye, a corneal scar, early oedema or endothelial dystrophy each have their own management. The role of the examination is to disentangle these possibilities before proposing anything at all.

Frequently asked questions

Why do I see 10/10 and still feel bothered at night?

Because visual acuity and straylight in the eye are two largely independent measurements. Acuity is tested in high contrast, with no light source in the field of view — the exact opposite of conditions on a road at night. Straylight, on the other hand, describes the veil your own eye creates around every headlight. An eye can perfectly well resolve small letters and still scatter a great deal of light.

Is this night-time glare serious?

It does not damage the eye by itself, but it can be disabling behind the wheel, and it is often the first sign of a change in the eye’s transparent media. The real challenge is identifying the cause: the lens, the cornea, the tear film or the posterior capsule. Discomfort that develops over a few months deserves a faster examination than discomfort that has been stable for years. Halos that appear suddenly, however, together with pain, a red eye or a rapid loss of vision, fall outside the scope of this article: they suggest an acute angle-closure glaucoma attack and require seeing an ophthalmologist that same day or, failing that, going to an ophthalmic emergency department.

Should I stop driving at night?

This decision isn’t made on a single figure. The most useful signal in practice is your own behaviour: if you’re already avoiding certain journeys, if you slow down sharply when passing an oncoming vehicle, or if it takes you several seconds to see normally again, the discomfort is functional. In that case, it’s reasonable to limit night driving while awaiting an assessment. Some of these situations improve once the underlying cause is treated.

Are yellow lenses any use?

The available evidence shows no benefit. A night-driving simulator trial, with and without glare from oncoming headlights, found no better pedestrian detection with yellow lenses. They also remove useful light from a scene that is already dark, and they increase the measured straylight value if the examination is carried out through them.

Will cataract surgery fix this?

When the lens really is the source of the veiling, studies that measured straylight before and after surgery report an average improvement of 0.10 log(s), including in patients who could already see better than 8/10. This does not prejudge an individual outcome, which depends on the cornea, the retina, the tear film and the type of implant. If the discomfort comes from another structure, the surgery will not have the hoped-for effect — hence the importance of the pre-operative assessment.

Is it simply age, which nothing can be done about?

Ageing of the lens is indeed the main determinant of the increase in straylight over a lifetime. But age is not a functional inevitability: it is precisely because the cause lies in the lens that the situation becomes accessible to surgery. It still has to be confirmed, though, that nothing else is contributing to the symptom.

Can this scattered light be measured?

Yes, using a dedicated psychophysical test that has nothing to do with reading letters. A bright light source is presented outside the line of sight, and the stray light it deposits at the centre of the field of view is quantified. The result is expressed on a logarithmic scale. This measurement doesn’t replace acuity: it documents a dimension that acuity simply doesn’t capture.

Cachan practice · Tel. +33 1 45 47 08 11

Scientific sources

  • van der Meulen IJ, Gjertsen J, Kruijt B, et al. Straylight measurements as an indication for cataract surgery. J Cataract Refract Surg. 2012;38(5):840-8. PMID 22520308
  • Van Den Berg TJ, Van Rijn LJ, Michael R, et al. Straylight effects with aging and lens extraction. Am J Ophthalmol. 2007;144(3):358-363. PMID 17651678
  • IJspeert JK, de Waard PW, van den Berg TJ, de Jong PT. The intraocular straylight function in 129 healthy volunteers; dependence on angle, age and pigmentation. Vision Res. 1990;30(5):699-707. PMID 2378063
  • Rozema JJ, Van den Berg TJ, Tassignon MJ. Retinal straylight as a function of age and ocular biometry in healthy eyes. Invest Ophthalmol Vis Sci. 2010;51(5):2795-9. PMID 19875647
  • Coppens JE, Franssen L, van Rijn LJ, van den Berg TJ. Reliability of the compensation comparison stray-light measurement method. J Biomed Opt. 2006;11(3):34027. PMID 16822076
  • Franssen L, Coppens JE, van den Berg TJ. Modulation depth threshold in the Compensation Comparison approach. J Vis. 2007;7(1):8. PMID 17461676
  • Franssen L, Tabernero J, Coppens JE, van den Berg TJ. Pupil size and retinal straylight in the normal eye. Invest Ophthalmol Vis Sci. 2007;48(5):2375-82. PMID 17460305
  • Myung D, Schallhorn S, Manche EE. Pupil size and LASIK: a review. J Refract Surg. 2013;29(11):734-41. PMID 24203804
  • Hwang AD, Tuccar-Burak M, Peli E. Comparison of Pedestrian Detection With and Without Yellow-Lens Glasses During Simulated Night Driving With and Without Headlight Glare. JAMA Ophthalmol. 2019;137(10):1147-1153. PMID 31369054
  • van Os A, Stassen MJC, Tassignon MJ, Rozema JJ. Influence of yellow filters on straylight measurements. J Cataract Refract Surg. 2017;43(8):1077-1080. PMID 28917410
  • Mainster MA, Timberlake GT. Why HID headlights bother older drivers. Br J Ophthalmol. 2003;87(1):113-7. PMID 12488274
  • Whitaker D, Steen R, Elliott DB. Light scatter in the normal young, elderly, and cataractous eye demonstrates little wavelength dependency. Optom Vis Sci. 1993;70(11):963-8. PMID 8302533
  • Reus NJ, van den Berg TJTP. Changes in straylight after cataract surgery. J Cataract Refract Surg. 2024;50(3):244-249. PMID 37882099
  • Lapid-Gortzak R, van der Meulen IJ, van der Linden JW, Mourits MP, van den Berg TJ. Straylight before and after phacoemulsification in eyes with preoperative corrected distance visual acuity better than 0.1 logMAR. J Cataract Refract Surg. 2014;40(5):748-55. PMID 24631202
  • Owsley C, McGwin G Jr, Sloane M, Wells J, Stalvey BT, Gauthreaux S. Impact of cataract surgery on motor vehicle crash involvement by older adults. JAMA. 2002;288(7):841-9. PMID 12186601
  • Meuleners LB, Hendrie D, Lee AH, Ng JQ, Morlet N. The effectiveness of cataract surgery in reducing motor vehicle crashes: a whole population study using linked data. Ophthalmic Epidemiol. 2012;19(1):23-8. PMID 22273356
  • Meuleners LB, Brameld K, Fraser ML, Chow K. The impact of first- and second-eye cataract surgery on motor vehicle crashes and associated costs. Age Ageing. 2019;48(1):128-133. PMID 30265273
  • Subzwari S, Desapriya E, Scime G, Babul S, Jivani K, Pike I. Effectiveness of cataract surgery in reducing driving-related difficulties: a systematic review and meta-analysis. Inj Prev. 2008;14(5):324-8. PMID 18836050
  • Nguyen H, Di Tanna GL, Coxon K, et al. Associations between vision impairment and vision-related interventions on crash risk and driving cessation: systematic review and meta-analysis. BMJ Open. 2023;13(8):e065210. PMID 37567751

This article is intended for information purposes and does not replace a medical consultation. Indications, expected benefits and risks depend on each individual situation. For any question concerning your vision, consult an ophthalmologist.

Related articles

Written and reviewed by Dr Moïse Tourabaly, ophthalmic refractive surgeon — former chef de clinique (Quinze-Vingts National Eye Hospital).

Last updated: September 20, 2026

Similar Posts