Glare is a common visual sensation caused by excessive and uncontrolled brightness within the field of view. This phenomenon occurs when a light source or a reflection is significantly brighter than the surrounding environment to which the eyes are adapted. Glare can interfere with visual performance or cause discomfort, ranging from distraction to a severe reduction in the ability to see details. This excessive brightness is frequent, such as when driving toward the setting sun or viewing a bright screen in a dark room.
Visual Manifestations of Glare
What a glare looks like is a collection of distinct visual effects that interfere with perception. One primary manifestation is veiling haze, which appears as a sheet of bright, obscuring light spread across the visual field. This haze significantly reduces the contrast between objects and their background, making it difficult to distinguish fine details. For example, a driver facing a low sun might see the road ahead washed out in a bright, formless layer.
Another common appearance is starbursts, where fine spikes or rays of light radiate outward from a bright source. These dazzling streaks turn a point of light, such as a streetlamp or an oncoming headlight, into an overwhelmingly bright, star-like shape. Starbursts are often more pronounced at night and can be caused by the eye’s internal optics scattering the light.
A third visual effect is the halo, a distinct ring of light surrounding the light source itself. Unlike starbursts, halos are typically softer and more circular, forming a glow around the perimeter of the light. Both halos and starbursts result from light being scattered or diffracted within the eye, often due to irregularities in the cornea or lens.
The Physics Behind the Glare
The underlying mechanism of glare involves the physics of light scattering and reflection, which ultimately leads to a reduction in visual contrast. When light encounters particles, either in the environment or within the eye itself, its path deviates from a straight line. Environmental factors like fog, dust, or moisture in the air cause light to scatter, creating an external veil that reaches the eye from multiple directions.
When this stray light enters the eye, it scatters off microscopic imperfections within the ocular media, such as the cornea and the lens. This internal scattering creates a phenomenon known as “veiling luminance” across the retina. Veiling luminance is an unformed, bright light that is superimposed over the image the eye is trying to focus.
This luminous veil raises the brightness of the entire visual scene, including the object of interest and its background. The result is a reduced contrast ratio, meaning the difference in brightness between the object and its surroundings decreases. The visual system relies on contrast to define edges and perceive detail, so this physical process makes objects harder to discern, especially in low-light conditions.
Understanding Glare Categories
Glare is categorized based on the effect it has on the person experiencing it, which are broadly divided into two main types. Discomfort glare is the more common form, causing a sensation of pain, annoyance, or irritation without necessarily impairing the ability to see. This type is generally a psychological reaction to excessive brightness that prompts a person to look away or shield their eyes.
An example of discomfort glare is a poorly positioned office light or a stadium floodlight that is simply too bright. While annoying, the person can usually still perform the task, though they may experience visual fatigue over time. The reaction to discomfort glare is highly subjective and varies among individuals.
The more serious category is disability glare, which actively reduces visual performance by lowering the achievable contrast on the retina. This type of glare makes it difficult or impossible to see details, posing a safety hazard in environments like driving. Oncoming high-beam headlights at night are a classic example, as the intense light scatters within the driver’s eye, severely obscuring the road.
Disability glare is directly linked to the veiling luminance effect and the scattering of light within the eye. The bright source overwhelms the visual system, making it harder to discern objects against a dark background. The scattered light can temporarily blind the observer, requiring a period of recovery before normal vision is restored.
Practical Ways to Minimize Glare
Several practical techniques and specialized products exist to mitigate the effects of glare in various environments. One of the most effective solutions is the use of polarized lenses, primarily in sunglasses. Polarization works by incorporating a chemical filter that selectively blocks horizontal light waves, which are the main cause of reflected glare from surfaces like water, snow, and flat roads.
By filtering out this horizontally oriented light, polarized lenses eliminate the blinding reflections, enhancing visual clarity and comfort. This is particularly beneficial for activities like fishing or driving, where reflective surfaces are common. Polarized lenses do not significantly reduce the scattering of light from direct sources, such as the sun itself.
Another widely used solution is the application of anti-reflective (AR) coatings on eyeglasses. These coatings consist of multiple thin layers of metallic oxides that reduce the amount of light reflected off the lens surface, both from the front and the back. An AR coating helps to minimize the ghost images and halos that can occur from light sources like computer screens or headlights at night.
Beyond optical aids, simple environmental adjustments are often the first line of defense against glare. Positioning light sources so they do not shine directly into the eyes, or adjusting monitor brightness to match the surrounding ambient light, can significantly reduce eye strain. For drivers, using visors and ensuring the windshield is clean helps to prevent external light scattering and direct reflections from reducing visibility.