The visual phenomenon of lights appearing to radiate spikes or rays, often described as starbursts, is a common experience when viewing bright point sources of light against a dark background. This effect, sometimes called glare or diffraction spikes, is not created by the light source itself, but rather results from the eye’s anatomy scattering or bending light before it reaches the retina. Understanding the underlying causes explains why the effect is often more noticeable at night and why it varies significantly from person to person.
The Eye’s Anatomy and Normal Light Diffraction
The eye functions as a complex optical system where light must pass through several layers, including the cornea and the lens, before focusing on the retina. The starburst pattern is created inside the eye through a process known as diffraction—the bending of light waves over sharp edges—and the presence of slight optical imperfections.
In a healthy eye, this diffraction happens around the edges of the pupil. The microscopic structure of the eye’s natural lens also contributes, containing tiny fibers called suture lines that scatter incoming light. These slight imperfections in the lens material and the edges of the pupil cause light to fan out into the characteristic spike pattern instead of focusing perfectly to a single point.
The effect becomes most noticeable when the pupil is dilated, a condition known as mydriasis, which typically occurs in low-light environments like driving at night. When the pupil widens, it exposes the more peripheral areas of the lens and cornea, which naturally possess more optical errors, or aberrations, than the center. These peripheral areas scatter light more aggressively, resulting in longer and more prominent starbursts.
Studies have demonstrated that when the pupil constricts to a diameter smaller than about three millimeters, the starburst effect is significantly reduced or even eliminated. This is because only the most optically pure central zone of the lens is used.
Common Factors That Intensify the Starburst Effect
While some degree of light scattering is normal, several temporary factors can increase the severity of starbursts, making them distracting. The size of the pupil remains a primary factor; the wider the pupil opens in the dark, the more the starburst effect is amplified, as more of the imperfect outer optical surfaces are utilized. This explains why streetlights and oncoming headlights can appear particularly dazzling to certain individuals after sunset.
The condition of the eye’s outer surface also plays a significant role, particularly the tear film that covers the cornea. Mild surface irregularities, such as those caused by dry eye syndrome, can temporarily roughen the corneal surface. When the tear film is unstable or breaks up too quickly, it creates an uneven surface that scatters light more aggressively than a smooth, healthy film.
External optics, like glasses or contact lenses, can also intensify the phenomenon. Corrective lenses that are smudged, scratched, or lack anti-reflective coatings introduce additional surfaces for light to scatter or reflect off. Another element is astigmatism, a common refractive error that causes the cornea or lens to have an irregular shape. This irregularity scatters light in different directions, resulting in an elongated or distorted starburst pattern. Managing the tear film and ensuring clean, properly coated lenses can often mitigate these temporary intensifications.
When Starbursts Signal a Health Concern
A sudden increase or worsening of starbursts, particularly if accompanied by other visual changes, may indicate a persistent or pathological change within the eye. The most common age-related condition that significantly worsens light distortion is the development of cataracts. This involves the gradual clouding and yellowing of the eye’s natural lens, which acts like an internal filter scattering light.
As cataracts progress, the lens material becomes increasingly opaque, causing light to scatter severely and creating intense glare, halos, and starbursts, often making night driving challenging. Certain corneal issues also drastically alter the light path, leading to pronounced starbursts. Conditions such as keratoconus (where the cornea thins and bulges into a cone shape) or corneal edema (swelling of the cornea) change the structure needed for accurate light refraction.
Starbursts are also a known side effect following refractive surgeries like LASIK, which reshapes the cornea. The effect can persist if the pupil dilates larger than the area of the cornea that was treated by the laser. Furthermore, pressure changes associated with conditions like glaucoma can affect corneal clarity and contribute to visual disturbances. If starbursts are sudden, rapidly worsening, or occur alongside pain or a significant drop in visual clarity, consulting an eye care professional is important.