- 1 Visual Acuity Definition
- 2 Physiology of Visual Acuity
- 3 Visual Acuity Test
- 4 Visual Acuity Measurement
- 5 Visual Acuity Chart – Snellen chart
- 6 Visual Acuity Testing in Children
Visual Acuity Definition
Visual Acuity refers to the acuteness or clearness of vision. It measures how one can see well. It is simply defined as the smallest visible feature the eyes can distinguish.
In 1861 the term visual acuity was thought of by Franciscus Donders describing the sharpness of vision defining it at a ratio between the visual acuity of the subject and the standard visual acuity.
Dr. Hermann Snellen, a Dutch Ophthalmologist, published his famous letter chart using what he called optotypes in 1862. The design was based on 5×5 grid and Snellen defined standard vision as the ability to recognize one of his optotypes. The chart devised by Dr. Snellen defined the relationship of sizes of certain letters viewed at certain distances.
Picture 1 – Physiology of Normal vision, nearsightedness, farsightedness
Physiology of Visual Acuity
Fovea is a region inside the macula that has the highest density of cone photoreceptor cells and it is where eye optical system has to focus an image. Visual acuity is the property of cone. Fovea is where the light is transmitted from fixation objects through visual axis. The structures and tissues within the visual axis affect the quality of image being transmitted.
Visual acuity is also affected by the action of the pupils whether it is at its maximum or diffraction limit. Normal acuity is at its best when the diameter of the pupil is around 3 or 4 mm.
Visual Acuity Test
Visual acuity test is done in determining an individual’s visual acuity based on a standardized chart measuring the smallest letter an individual can read.
The acuity test can be done anywhere such as in the healthcare clinic, offices, schools and anywhere else that requires routine eye examination or general physical examination. The test does not require any special preparation prior to the test and does not make feel one uncomfortable.
The patient will be asked to remove eyeglasses or contact lenses if they are wearing one on the day of the test. The patient will also be asked to sit or stand in front of the chart placed 20 feet away. It is done on both eyes reading the smallest letter one eye at a time.
Picture 2 – Visual acuity testing procedure
Visual Acuity Test Results
- The result of the test is expressed in fraction with the numerator being the standard distance while the denominator is the distance on how the individual could correctly read the same line normal eyesight can read.
- 20/20 vision is considered normal.
- 20/40 vision indicates that an individual can read line normal eyesight can read from 20 feet away.
- Abnormal results of visual acuity test may need further evaluation as it may indicate other certain condition that a health care professional needs to establish.
Visual Acuity Measurement
Visual acuity measurement does not rely solely on the optotypes of the chart but also on the cooperation and condition of the patient and how well the patient understood the eye examination he is to go through. It is a subjective test that needs the patient’s understanding, cooperation and communication with the health care professional to be able to come up with real visual acuity results.
In visual acuity measurement, if measure of reducing the distance and the patient is still unable to read the chart, another measure will be tried to test the visual acuity of the patient and these are done through gestures such as:
- Counting Fingers
- Hand Motion
- Light Perception
- No Light Perception
Visual Acuity Chart – Snellen chart
Visual acuity chart is an eye chart also called Snellen chart which was devised and named after a Dutch ophthalmologist Dr. Hermann Snellen. It is use to measure the visual acuity of an individual and used mostly by eye professionals. The chart was designed in 1862 and is now use in variation designed by Ian Bailey and Jan Lovie.
The Snellen chart has optotypes or symbols of block letters intended to be seen and read as letters. The chart has series of letters and numbers with largest letter on top. The letters become gradually small as it is read down.
The optotypes are not ordinary fonts and they are designed in simple geometry where the thickness of the lines are equal the thickness of white spaces between the lines and thickness of gap of letter C. The line and width of optotypes is five times the thickness of the line. The traditional Snellen chart uses only ten Sloan letters.
A variation of Snellen chart is designed for those who cannot read alphabet. Tumbling E is used for those who cannot read the alphabet. Capital letter E is designed facing different directions in which the person tested must determine which position the letter E is facing in either up, down or left and right position.
Picture 4 – Visual Acuity testing through Snellen Chart
Snellen Chart Interpretation
Snellen chart provides a standardized visual acuity measurement. It is placed at 6m – 20 feet from the subject. The visual acuity of which is expressed in fraction wherein the distance from the chart of 6 meters is defined as the numerator while the denominator describes the last line a patient can read based on the normal eye can read.
20/20 vision expressed in foot unit or measurement or 6/6 vision expressed in meter unit of measurement is the standardized visual acuity based on optotypes of Snellen chart. A person with normal visual acuity can read the smallest line of the chart at a distance of 20 feet.
Visual Acuity Testing in Children
Snellen chart is not the choice of chart used to test visual acuity of infant, pre-verbal children and pre-school children. A preferential looking technique using Teller acuity cards is a more formal preference. It measures the child’s visual attentiveness through a random presentation of horizontal or vertical bars on one side in comparison with a blank page on the other side.
Visual evoked potentials or VEP is another technique use similar to preferential looking technique. The difference is that visual evoked potentials record brain waves created by presentation of patterns.