When a bright light source, such as a distant streetlight or car headlight, appears surrounded by radiating spikes, a luminous glow, or a circular mist, you are experiencing a common visual phenomenon known as starbursts, glare, or halos. This effect transforms a simple point of light into a dazzling, star-like shape, often becoming more noticeable in low-light environments. This experience is a consequence of how light physically interacts with the optics of the human eye. The appearance of these light distortions results from natural physical laws and the biological structure of your eye working together.
The Optics of Light Diffraction
The fundamental reason any light source appears to spread out is a principle of physics called diffraction. Diffraction occurs when a light wave encounters an obstacle or a small opening and spreads out as it passes through. Instead of projecting a perfectly focused point of light onto the back of the eye, the edges of the pupil and tiny imperfections within the eye’s optical pathway cause the light to scatter.
This scattering prevents the light from converging to a single, sharp focus on the retina. While the effect is always present, it becomes more prominent in low-light conditions when the eye’s internal structures change.
The Role of Pupil Size in Night Vision
The pupil acts as the aperture of the eye, controlling the amount of light that reaches the retina. In low-light conditions, the pupil naturally dilates (mydriasis) to maximize light intake for better night vision. This dilation is the main physiological factor that makes light distortion worse at night.
When the pupil is constricted in bright daylight, light travels only through the central, most optically precise part of the cornea and lens. As the pupil widens in the dark, it exposes the peripheral edges of the lens and cornea. These edges are not optically uniform and possess more irregularities, which introduce higher-order aberrations. One significant aberration is spherical aberration, where light rays passing through the periphery focus at a different point than rays passing through the center. This differential focusing causes the light to scatter and streak, turning a focused point into the radial pattern of a starburst or a halo.
Structural Causes of Light Distortion
Beyond the natural effect of pupil size, specific structural irregularities within the eye can significantly exacerbate the appearance of starbursts and halos.
Tear Film and Corneal Irregularities
The quality of the tear film, the thin layer of fluid covering the cornea, plays a large role. If the eyes are dry, the tear film surface becomes uneven, creating microscopic ridges that scatter incoming light rays. An irregular corneal shape, even a mild degree of astigmatism, is another common culprit for light distortion.
Astigmatism means the cornea or lens is curved more like a football than a perfectly round basketball, causing light to focus on two points rather than one. This improper focusing often manifests as elongated streaks or a distorted starburst pattern around lights at night.
Lens Changes and Surgery
Changes within the eye’s natural lens can also contribute to light scattering. As the lens ages, microscopic changes in its density and protein structure occur, which can scatter light before a full cataract develops. Even minor clouding of the lens can increase light scatter, leading to pronounced glare and halos.
Individuals who have undergone refractive surgeries like LASIK or PRK may also experience increased night glare. While modern techniques have reduced this side effect, the reshaping of the central cornea can sometimes create a sharp boundary between the treated and untreated tissue. If the pupil dilates past the treated optical zone, light passing through the edge of the altered cornea can cause significant scattering and starbursts.
When to Seek Professional Advice
While seeing starbursts around bright lights is common and often linked to normal nighttime pupil dilation, certain symptoms warrant a professional eye examination. If the visual distortions appear suddenly, become significantly more severe, or interfere with daily activities like driving, consult an optometrist or ophthalmologist.
A specific warning sign is the sudden appearance of colored halos, which can indicate acute angle-closure glaucoma. This condition involves a rapid increase in intraocular pressure and is considered a medical emergency requiring immediate attention. Other concerning symptoms include light distortion accompanied by severe eye pain, nausea, vomiting, or a noticeable loss of vision.
Regular, comprehensive eye exams are the best way to monitor your eye health. If the distortion is persistent and not simply a mild nighttime effect, a professional can determine if the cause is a correctable refractive error, dry eyes, or a developing condition like cataracts.