What are High Order Aberrations?
High Order Aberrations: It is no surprise that the human eye is not a perfect optical system but suffers from many high order aberrations that degrade the retinal image quality. Light aberration causes blurred vision.
An aberration simply means anything that prevents light from traveling in a straight line. In other words, an aberration causes the ray of light to be misdirected from its desired image point.
Zernike Polynomials are derived by mathematically converting the wavefront error created by a specific aberration and transforming it into a three-dimensional model that mimics the shape of the aberration.
Even though over 65 total aberrations up to the eighth order have been identified, the human eye is able to perceive distortion effects only up to the fifth or sixth order.
Clinically significant High Order Aberrations (HOA) can degrade vision in spite of full correction being used for defocus
There are two main types of light aberrations:
Low Order Aberrations: These aberrations are corrected with prisms, glasses and contact lenses.First-Order Aberrations: These include: Piston (or plano; i.e., no prescription lenses) and Prism correction, which is used if your eyes are misaligned; for example, if you have
Prisms divert light from its original path so that you do not have
Second-Order Aberrations: These include: lenses to correct for Myopia, Hyperopia, Presbyopia, and Astigmatism.High Order Aberrations (HOA): Everything higher than Low Order Aberrations is considered HOA. Your set of HOA is unique to your eye, as your fingerprint is unique to you.
Another way to demonstrate how your vision may be distorted by aberrations is using a tool known as a Point Spread Function (PSF), which uses another mathematical calculation.
Basically, when a point of light passes through the eye, the aberration(s) that eye possesses will disperse or spread the beam, causing the point to blur. The point of light will take on a characteristic appearance when it hits the retina, depending on the aberration(s) present.
See the point spread function chart below.
What causes HOA?
Any condition that affects the optics of the eye will result in changes in HOA.
These include, but are not limited to:Tear film abnormalities; for example, as seen in
Distortion of the cornea as seen in
Corneal Degenerations, Corneal Dystrophies, Corneal Scarring, Corneal Infection or Inflammation,
Distortion of the Crystalline Lens as seen in Cataracts and Lens Implants
Even conditions that result in irregular Retina surfaces may create High Order Aberrations such as: Macular Edema Epiretinal Membranes Macular Degeneration
affecting any of the optical components of the eyeLarge or slowly-responding pupils because the effects of HOA increase with pupil size.
Pupil issues may be seen in: Children Individuals who work in low-light situations Individuals who take medications that dilate the pupils such as Antidepressants, Beta-blockers, Antihistamines, Amphetamines, and Nitroglycerine
What are symptoms of HOA?
Uncorrected HOA may cause specific bothersome symptoms in vision such as: Double images Lack of contrast or crispness Reduced color perception Glare sensitivity
problems Starburst patterns and/or comet’s tails around lights at night. These complaints may be seen in Third-Order Aberrations such as Coma. Halos: This complaint may be seen with Fourth-Order Aberrations such as Spherical Aberration.Decreased distance and near vision: This complaint may be seen with Fourth-Order Aberrations such as Secondary Astigmatism, which can have a negative effect on vision, often described as “central island”-type distortion. The light appears pushed out to the edges of the image.
How are HOA measured?
High Order Aberrations are measured using an aberrometer. There are several commercial brands available which work on the same general concept.
In a nutshell, two or more parallel beams of light are passed through the eye and then a method is used to measure the extent to which those beams cease to travel in a visually optimal path. Complex mathematical formulas then predict aberrations based on the altered angle of those beams.
The most commonly used method today is known as the Shack-Hartmann Method, which is used in several wavefront refractive surgery platforms. In this technique, a bright beam of light is projected into the retina and reflects back out through the eye.
The exiting beam then passes through an array of lenses (called lenslets) where it is split into many tiny beams that project spots of light on a digital camera. The displacement of these spots corresponds to the amount of aberration.
The commercial brands available can give the eye care provider much more information about each patient than he or she ever had previously.
For example, there is a brand of wavefront aberrometer that incorporates auto refraction (estimating the prescription of the patient; i.e. calculating the low-order aberrations), corneal topography (showing a map of the corneal surface), keratometry (measuring corneal curvatures), pupillometry (measuring the distance between the pupils), along with measuring High Order Aberrations.
How can HOA be corrected?
HOA can be corrected with spectacle glasses, for example. Some contact lenses companies are touting that their lenses help correct High Oder Aberrations.
In the area of spectacle correction, there is a commercial brand aberrometer available which is a high resolution system measuring more than 11,300 points within a 6 mm pupil diameter.
Benefits of using this equipment include: You can have your eyeglass prescription calculated in a completely objective way (not relying on your responses of which image is clearer) Your eye doctor can show you a printout that explains to you your eye’s aberrations.The process takes only a few minutes for a full measurementCan provide information to manufacture spectacle lenses to correct many HOABecause your HOA are as unique to you as your fingerprint, this evaluation is often referred to as an “optical fingerprint,” or “iPrint.”
Not everyone needs to have their HOA corrected and this instrument estimates which patients are most suitable for this correction.
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