Six basic eye tests are done when you go for an eye examination for vision acuity and any other vision problems. Apart from reading the eye chart mounted on the wall or reflecting on the mirror, visual examination is complete only when the eye specialist checks for various other aspects of vision in the eyes. The following are the six basic tests done during routine visual examination.
The test requires the patient to read letters from a screen or placed at a distance of 20 feet from them. Some clinics may not have the space and adopt a mirror placed 10 feet away from a screen that displays mirror images, so that the person being tested reads the right way. Each eye is tested separately for the ability to read the letters or numbers or any shapes according to the literate level of the patient. Finally, ability to see using both eyes is tested. Based on the responses, the optometrist compares it to the standard vision of 20/20 vision.
An instrument called phoopter is used by the optometrist to look at the retina inside the eye. While the patient is told to gaze at a particular picture inside the instrument, various lenses are changed. A light is shined into the eyes and the responses of the eyes are recorded.
Refraction test is done along with retinoscopy and reveals the need for the right measure of prescriptive glasses. It gives an idea about the kind of refractive error in the eyes and may be recorded as farsightedness, near-sightedness or astigmatism. After testing a series of lens pairs, the optometrist finally repeats 2-3 pairs for the patient to choose on the basis of most clear vision.
The shape of the cornea is measured using a special computerized machine to determine how the eye perceives and reflects light. Elongated cornea with steep curves results in astigmatism and can be detected through this test. The patient looks into the machine and aligned with the face and eyes. Based on the responses of the eye, the cornea’s shape is measured by the optometrist. This can help in determining whether corneal transplant or contact lens can help the patient.
Although the priority of our visual field is limited to what we are looking at directly, we can also see things on the sides of our visual field. This is called peripheral vision. Various types of peripheral vision are done to identify any gaps in the peripheral vision and determine the size of the visual field.
- Automated periphery – the patient is asked to focus on one spot at the center and when he sees a light flash on the side, he indicates by pressing a button.
- Tangent screen exam – the patient focuses at the center spot and the doctor moves objects on the sides, in and out of peripheral visual field and checks the spot till where the patient is able to see the object.
- Confrontation visual field exam – the optometrist sides opposite to the patient and moves an object like a pen, in and out of the peripheral vision of the patient.
This test measures the force or the pressure created by the fluid that fills the eyes. A quick puff of air is blown into the open eye and the eyes react by closing. The machine checks the eye’s reaction and resistance to the pressure from the air puff and based on this determines the pressure in the eye. It also helps in identifying glaucoma, based on the pressure of the eye.
Manual intra-ocular pressure may be measured by the doctor if required. This requires using an instrument that gently touches the surface of the open eye to measure the internal pressure. Local anesthesia may be used for this test so that the eye does not close as a reaction to the instrument touching the surface.
To perform some of these tests, pupils are dilated using special eye drops. This helps the doctor to see inside the eye more clearly. However, the patient may have slightly blurred vision for about 3-4 hours of using the eye-drops. It is better to arrange for transport while returning from the eye test as driving with dilated pupils is not safe.