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Computer refractometry is usually ordered as part of a routine eye exam. This study tells the eye doctor exactly what prescription a patient needs for glasses or contact lenses.
What is eye refractometry is an interesting question, but it does not have a very simple answer. This requires some understanding of the principles of optics. A computerized vision testing machine is called an autorefractometer. The actual technology in each machine varies greatly, but basically all computer refractometers have the following three properties: infrared light, fixation target, lens.
Infrared light is used in these machines because it has the lowest scattering potential, so it can travel long distances without scattering (for context, a laser pointer). This is ideal for such a machine (which measures the difference in polarization).
The fixation target is used to relax the room of the eye (focusing power of the eye). It uses the fog method to perform this action. It blurs the image of the target before it takes objective measurements.
The lens is taking measurements. The relative distance between the lens and the eye has a linear relationship with the refractive power of the eye.
An autorefractor is a machine that measures the eye’s ability to focus and gives a rough estimate of the state of refraction.
Physical basis of computer refractometry
The actual measurement process involves sending a thin beam of infrared light into the eye, which will be reflected back off the retina. The light sensor in the car detects the polarization of the reflected light and compares it with the incident light. The lens is now shifted to equalize the polarization. The analog movement of the defective lens is captured by the computer and converted into a digital readout.
Of course, modern machines use more complex designs and principles to be more precise, and explaining that is beyond the scope of this answer.
It is also necessary to understand that an autorefractometer gives an objective measurement of refractive error, but glasses and contact lenses are given after subjective refraction – this is the power that the patient feels comfortable, checked by an ophthalmologist with lenses. Thus, human involvement is also required to assign points. Automated refraction simply speeds up the process.
What is a refractive error?
Refractometry is a process an ophthalmologist uses to measure refractive errors, or vision problems.
A refractive error is an optical defect that prevents light from focusing on the retina, resulting in blurry or distorted vision.
Refractive error occurs because the shape of the eye does not bend light correctly, resulting in a blurry image. Examples of refractive errors are nearsightedness, farsightedness, and astigmatism. Refractive error can lead to symptoms such as blurred vision, difficulty reading, and crossed eyes in children (esotropia).
Indications for the procedure
This test tells the doctor if the patient needs glasses or prescription lenses, and which prescription lenses the patient will see best in.
The results of computer refractometry are used to diagnose the following conditions:
- astigmatism, a refractive problem with the eye associated with the shape of the lens that causes blurry vision
- farsightedness, which is also known as hypermetropia;
- nearsightedness, also known as myopia;
- presbyopia, a condition associated with aging that causes the lens of the eye to have trouble focusing.
Test results can help diagnose the following conditions:
- macular degeneration, an aging-related condition that affects sharp central vision;
- retinal vessel occlusion, a condition that causes small blood vessels near the retina to become blocked;
- retinitis pigmentosa, a rare genetic condition that damages the retina;
- retinal detachment, when the retina separates from the rest of the eye.
Who needs research?
Healthy adults under the age of 60 who do not have vision problems should have a refractive test every two years. Children should have a refractive test every one to two years, starting no later than 3 years of age.
If the patient currently wears glasses or contact lenses, they should be examined every one to two years. This will allow the doctor to figure out which prescription is needed as the eyes change. If there are problems with vision between examinations, then you need to see a doctor for another refractive examination.
If the patient has diabetes, the examination should be carried out every year. A number of eye conditions are associated with diabetes, such as diabetic retinopathy and glaucoma. According to the American Diabetes Association, people with diabetes are at greater risk of blindness than other people.
If the patient is over 60 years of age or has a family history of glaucoma, then they should also have a CT scan every year. Glaucoma occurs when pressure builds up in the eye, damaging the retina and optic nerve. Regular examinations will help the ophthalmologist detect diseases such as glaucoma and other eye conditions associated with aging and, if possible, treat them early.
What happens during research?
The patient will be asked to look at a car through two lenses in a picture, such as a hot air balloon at the end of a long straight road, and focus on the picture. The machine zooms in and out of the balloon. In this case, the machine calculates a vision correction needs estimate with the results of how well the eyes focus on the image.
The doctor will first evaluate how light rays of light travel through the cornea and lens of the eye. This will help the eye doctor determine if the patient needs corrective lenses, and if so, what type of prescription is needed. The doctor may use a computerized refractor for this part of the test, or they may simply shine a light into the eye.
In the computerized method, the patient looks into a machine that measures the amount of light reflected by the retina.
The doctor can also perform this test without the aid of a machine. In this case, the doctor will shine a beam of light into each of the eyes and look at the amount of light that is bouncing off the retina to measure the refractive index.
Complex refractometry
During a comprehensive eye exam, the doctor uses refractometry to determine how much energy is needed to give the eyes normal, perfectly focused vision. The doctor will decide which glasses, contact lenses, or laser vision correction will give the patient the clearest vision. Refractometry consists of two parts, objective and subjective.
Objective Refractometry
Objective refractometry is a refraction obtained without receiving feedback from the patient. Objective refraction is obtained by using different tools. The doctor will use a retinoscope or autorefractor to measure refraction without asking the patient for subjective answers.
Cyclopeplegia
Sometimes ophthalmologists prefer to use this method, especially when trying to get accurate refraction in young children who can skew refraction results by adjusting their eyes. Cycloplegic eye drops are applied to the eye to temporarily paralyze the ciliary muscle of the eye.
There are three main types of patients for whom doctors like to perform cycloplegic refraction:
- Children: Children tend to focus at close range and cannot control focus when asked to look at far distances during an exam. The doctor can be sure that they are measuring the vision correctly when performing this method.
- Adults with Presbyopia: Presbyopia is a condition that affects the ability to focus on close objects, mostly in people over 40 years of age.
- Candidates for laser vision correction or other refractive surgery must undergo cycloplegic refraction so that the surgeon knows exactly how much of the vision problem needs to be corrected with the laser.
Retinoscopy
One of the most common instruments used for objective refraction is the retinoscope. Using a retinoscope, the doctor projects light from the pupil. The doctor can examine the light reflex of the pupil. Based on the movement and orientation of this retinal reflection, the refractive state of the eye is measured.
Autorefractometry
Another tool used for objective refraction is the autorefractor. An autorefractor is a computer tool that shines into the eye. Light travels through the front of the eye to the back of the eye and then back. The information bounces back to the instrument, giving an objective measure of refractive power. Autorefractors are quick and easy to use and require no feedback from the patient.
Subjective refraction
Subjective refraction requires responses from the patient. A physician may use a phoropter to measure subjective refractive error to determine an eyeglass prescription. Typically, the patient will sit at the phoropter and look at an eye chart. The doctor will change lenses and other settings, giving feedback to determine which settings give the patient the best vision.
Online refractometry
Many websites now offer online refractometry that purports to determine which eyeglass prescription is needed. The companies claim to provide quick and easy eye analyzes that will lead to a complete and accurate description of the eye’s vision needs. While these online tests can lead to an accurate prescription, online refractometry cannot replace a professional eye health exam.
Relevance of the method
Sometimes an autorefractor may be used for initial vision assessment. Autorefractometry can save time and is useful when examining young children or people who have problems with eye exams, but are not necessarily accurate enough to complete a prescription.
There is a subjective refractometry – the optometrist will always discuss the patient’s responses and their findings with him, based on the results, they can determine the stage of myopia, hyperopia or astigmatism, if any.
Sometimes, when patients are unable to respond verbally or to determine more complex prescriptions, eye drops are used. These drops temporarily prevent the eyes from changing focus when refraction is performed. Drops are not usually required for refractometry because the ophthalmologist is interested in how the eyes react under normal visual conditions.
After computer refractometry, the doctor will determine the prescription and prescribe glasses, contact lenses, based on the specific needs of the patient. Refractometry can also detect visual impairment and tells the optometrist about eye health, an important aspect of the examination. The optometrist may also perform tests to check focus and how well the eyes work together, which is important for daily tasks.
Regular eye exams are critical to maintaining healthy vision. They are a normal part of an eye doctor visit and do not require any preparation on the part of the patient. They can help your doctor diagnose and treat conditions such as glaucoma and determine if corrective lenses are needed, among other things. Healthy adults should have a refractive test every two years, while children need them every one or two years starting at age 3.