Seeing bright, glowing rings, often with a rainbow effect, surrounding light sources is known as seeing halos or glare. This phenomenon is most noticeable at night around streetlights and car headlights, where a bright point of light contrasts sharply with a dark background. This visual symptom can be caused by a variety of factors, ranging from harmless temporary conditions to serious underlying eye diseases. Understanding the physics of how light travels through the eye provides the foundation for knowing why these halos appear.
The Optics Behind Light Halos
The appearance of halos around lights is fundamentally an issue of light being improperly bent or scattered before it reaches the retina. In a healthy eye, the cornea and lens precisely focus incoming light onto a single point. When light encounters an obstacle or irregularity, it undergoes a process called diffraction or scattering, causing it to spread out.
When light scatters, the different wavelengths that make up white light are separated, similar to how a prism works. This separation results in the distinctive colored rings or rainbow effect that characterizes a halo. The irregularities causing this light distortion can be microscopic and may occur in the tear film, the cornea, or the lens inside the eye.
In low-light conditions, the pupil naturally enlarges, allowing more light to enter the eye. This larger opening exposes the edges and periphery of the cornea and lens, areas that may have slight imperfections. This exposure increases the amount of scattered light, making the halos significantly more visible during the night.
Common and Temporary Causes
Many instances of seeing halos are not signs of disease but are related to temporary changes or external factors affecting the eye’s surface. One frequent cause is dry eye syndrome, which occurs when the tear film lacks quality or quantity. When the eye’s surface is not adequately lubricated, the tear layer becomes irregular, leading to light scattering as it passes through.
Wearing contact lenses can also contribute to halos, especially if the lenses are dirty, scratched, or improperly fitted. The lens material itself can cause light diffraction, and deposits on the lens surface can further scatter light. Recent eye surgeries, such as LASIK or cataract surgery, may cause temporary halos due to inflammation or slight swelling of the cornea during the healing period.
Halos related to recent surgery often diminish as the eye fully recovers and the swelling subsides over weeks or months. The natural aging process can also introduce slight changes in the structure of the lens, which can begin to scatter light. In these common, non-pathological cases, the visual disturbance is often mild and manageable.
Underlying Medical Conditions
When halos are persistent, severe, or accompanied by other symptoms, they can indicate a specific, chronic eye condition affecting the clarity of internal structures. Cataracts are a leading cause, where the natural lens inside the eye becomes cloudy or opaque. This clouding is caused by protein clumping and disrupts the uniform passage of light, causing it to scatter and resulting in halos and increased glare sensitivity.
The opaque lens material diffuses incoming light, preventing a sharp focus on the retina. This effect often progresses slowly, and the halos become more pronounced as the cataract thickens over time. Cataracts are common, particularly in older adults, and can significantly impair night vision and driving ability.
Another group of conditions that cause halos involves corneal swelling, a state known as corneal edema. This swelling can be a symptom of chronic eye conditions like Fuchs’ dystrophy, where the cells responsible for pumping fluid out of the cornea fail. The fluid buildup distorts the cornea’s shape, causing light to bend irregularly and creating the halo effect.
Chronic forms of glaucoma can also lead to halos, related to the pressure’s effect on the cornea. Chronically high intraocular pressure can cause slight corneal swelling, which then scatters light and produces the colored rings. However, the most concerning presentation of halos in the context of glaucoma is the acute form, which requires immediate attention.
When to Seek Immediate Medical Attention
While many causes of halos are benign or manageable, the sudden onset of halos coupled with severe symptoms may signal an acute medical emergency. This constellation of symptoms is a hallmark of acute angle-closure glaucoma, a condition where the eye’s internal drainage system becomes rapidly blocked. This blockage causes intraocular pressure to spike suddenly and dramatically.
The extremely high pressure causes the cornea to swell quickly, which is the direct cause of the suddenly perceived halos. Seek emergency medical care immediately if you experience halos accompanied by severe eye pain, a sudden and significant blurring of vision, or systemic symptoms like nausea and vomiting.
This rapid rise in pressure can cause permanent vision loss within hours if not treated promptly. The pain associated with this condition is often described as intense, sometimes radiating to the head, and it is a distinct warning sign that requires urgent consultation with an eye care professional or a visit to the emergency room.