Seeing bright, colorful rings around light sources at night, often referred to as visual halos or glare, is a common experience. This phenomenon makes lights appear to be surrounded by a rainbow-like circle, indicating that light is not focusing cleanly within the eye. The appearance of these halos signals an issue with the eye’s optical system, primarily the cornea or the lens. Understanding the cause depends on whether the halos are a temporary annoyance or a symptom of an underlying health condition.
The Science of Light Splitting
The perception of a halo around a light source occurs due to a process called light scattering or diffraction. Normally, the clear front surface of the eye, the cornea, and the lens work together to refract incoming light to a single, sharp focal point on the retina. This precise focusing is what creates a clear image.
When there are irregularities or imperfections within the eye’s optical pathway, light is scattered instead of being cleanly focused. This scattering causes the light rays to spread out, creating the luminous, circular pattern perceived as a halo. The rainbow effect specifically arises because the scattered light rays are separated by the imperfection into their component wavelengths, much like light passing through a prism.
The size of the halo and its intensity are directly related to the extent of this light scattering within the eye’s media. In low-light conditions, the pupil dilates to let in more light, which also allows scattered light from the periphery of the lens or cornea to enter, making halos and glare more noticeable at night. This contrasts with daytime vision, where a smaller pupil naturally blocks many of the scattered rays.
Common Temporary Causes of Halos
Many causes of visual halos are temporary and do not indicate a serious eye disease. The most frequent temporary cause is an irregular tear film on the eye’s surface, such as from rubbing the eyes or having moisture on the cornea. This thin layer of tears needs to be smooth for clear vision, and any disruption can immediately scatter light.
Wearing dirty eyeglasses or improperly cleaned contact lenses can also create a scattering effect on the light before it enters the eye. Scratches, dust, or residue on the lens surface act as physical imperfections that diffract light, causing halos that disappear once the corrective lens is cleaned or removed.
Certain types of refractive surgery, such as LASIK, can temporarily induce halos because the procedure reshapes the cornea. Halos are a common side effect of corneal swelling and healing in the weeks to months following surgery. The artificial lenses used in cataract surgery (IOLs), especially multifocal designs, can also cause some degree of light diffraction and halos. These post-surgical halos often diminish over time as the eye and the brain adapt to the new optical system.
Underlying Eye Conditions That Cause Halos
When halos are persistent, do not fluctuate, or are accompanied by other symptoms, they may signal a progressive eye disease. The most commonly known cause is the development of cataracts, where the eye’s natural lens becomes progressively cloudy. This clouding disrupts the normal transmission of light, causing it to scatter wildly as it passes through the opaque lens material before reaching the retina.
Glaucoma, particularly the acute angle-closure form, is a serious medical condition where halos are a classic symptom. A rapid and severe increase in the pressure inside the eye, known as intraocular pressure, causes the cornea to swell. This corneal swelling, or edema, disrupts the clear, ordered layers of the cornea, which then acts as a diffraction grating, splitting light into rainbow-colored rings.
Corneal edema can also be caused by inherited conditions like Fuchs’ dystrophy. Cells that pump fluid out of the cornea slowly fail over time, leading to fluid buildup and swelling on the cornea’s surface. This results in persistent light scattering and halos. Similarly, severe dry eye syndrome creates an unstable and irregular corneal surface, which can cause light to scatter and produce halos, especially in low-light environments.
When to Consult an Eye Doctor
The presence of visual halos warrants a professional eye examination to determine the underlying cause, whether it is a simple refractive error or a serious condition. If halos appear suddenly, or if they are accompanied by other symptoms, immediate medical attention is required. These concerning symptoms include severe eye pain, intense headache, nausea, or vomiting.
These specific symptoms, combined with halos, are the hallmarks of an acute angle-closure glaucoma attack, which can cause rapid and irreversible vision loss if not treated promptly.
For persistent but non-acute halos, a comprehensive eye exam can identify the problem, such as early-stage cataracts or chronic dry eye, and guide appropriate management. Regular check-ups are the most effective way to monitor eye health and address any changes in vision, including the perception of light halos.