The experience of seeing bright, star-like rays radiating from point sources of light at night is often called starbursts or excessive glare. This visual phenomenon is particularly noticeable around streetlights, car headlights, or lamps in low-light environments. While often a minor optical quirk, this light distortion can also indicate an underlying change in the eye’s physical structure. Understanding the cause is the first step toward clearer vision.
The Mechanism of Light Distortion
The appearance of starbursts is primarily caused by light scattering and diffraction within the eye’s optical pathway. Normally, the cornea and lens focus incoming light precisely onto the retina, creating a single, sharp image. When the eye’s structures are irregular, light rays bend unevenly, causing them to spread out. The brain interprets this scattering as rays or streaks around a light source.
This effect is amplified at night because the pupil dilates, allowing more light into the eye. A large pupil exposes the outer edges of the lens and cornea, where natural imperfections and aberrations are pronounced. These imperfections, known as higher-order aberrations, cause the light to form caustics, which the brain perceives as the starburst shape. Reducing the pupil diameter to less than three millimeters can significantly reduce the size of these starbursts.
Common and Benign Causes
The most frequent cause of starbursts is astigmatism, a common refractive error. This condition results from the cornea or lens having an irregular curvature, shaped more like a football than a perfectly round basketball. This uneven shape causes light to focus on multiple points instead of a single one, producing the characteristic stretching or streaking of light rays. Corrective lenses compensate for this uneven bending of light.
Another common and temporary cause is dry eye syndrome, where the tear film covering the corneal surface is unstable. The tear film is a crucial part of the eye’s optics; when it evaporates too quickly, the uneven surface momentarily scatters light. Blinking often refreshes this film, which is why the starburst effect from dry eyes appears intermittently. People who have undergone refractive surgery, such as LASIK, may also experience starbursts if their pupil dilates larger than the reshaped corneal area, though this often improves as the eye heals.
Underlying Health Conditions That Cause Starbursts
When starbursts or halos become persistent and interfere with daily activities like night driving, they may signal a progressive eye disease.
Cataracts
The most frequently encountered condition is a cataract, the clouding of the eye’s natural lens, often due to aging. As lens proteins clump together, the resulting opacity scatters incoming light instead of allowing it to pass straight through. This internal scattering creates significant glare and starbursts that worsen over time, often accompanied by faded colors or hazy vision.
Corneal Disorders
Other conditions relate to the structure of the cornea itself. Keratoconus, a progressive disorder, causes the cornea to thin and bulge into a cone shape, creating severe and irregular astigmatism. This change drastically increases light scattering and produces pronounced starbursts and distorted vision. Glaucoma, associated with increased pressure inside the eye, can cause the cornea to swell slightly (corneal edema). This swelling leads to the perception of halos or starbursts, sometimes appearing suddenly with eye pain or a severe headache.
When to Seek Professional Eye Care
While mild, unchanging starbursts may only require a routine prescription adjustment, certain accompanying symptoms require immediate professional attention. A sudden onset of new starbursts, especially if paired with eye pain, redness, or rapid vision loss, warrants an urgent consultation. These sudden changes can signal acute issues like angle-closure glaucoma or other serious ocular events.
You should also schedule an appointment if the starbursts are progressively worsening or significantly impairing your ability to drive safely at night. During an eye examination, a doctor uses specialized equipment, such as a slit lamp, to examine the lens and cornea for physical evidence of these causes. The appropriate management strategy depends entirely on an accurate diagnosis of the underlying cause, whether it involves prescription changes, eye drops, or surgical options.