What is Snellen Chart?

 A Snellen chart is a common tool used to measure visual acuity, or the clarity of vision. It is named after the Dutch ophthalmologist Hermann Snellen, who developed the chart in the 1860s. The Snellen chart is a series of letters or symbols arranged in a specific pattern and placed at a standardized distance from the viewer. The letters or symbols on the chart are progressively smaller from top to bottom, and the viewer is asked to read the letters or symbols aloud from top to bottom as accurately as possible. 

Visual acuity is typically measured using a Snellen chart that is placed 20 feet (6 meters) away from the viewer. This distance is used because it is the average distance at which people with normal vision can see small details, such as the individual letters on a chart. If the viewer is able to read the letters or symbols on the chart accurately, it indicates that their visual acuity is normal or near normal. If the viewer has difficulty reading the letters or symbols, it indicates that their visual acuity is impaired. 

The Snellen chart is usually presented in a standard format, with the largest letter or symbol at the top and the smallest letter or symbol at the bottom. Each line on the chart contains a different size of letters or symbols, and the size of the letters or symbols decreases as you move down the chart. The size of the letters or symbols is expressed in terms of a fraction, with the distance from the viewer (20 feet or 6 meters) as the numerator and the size of the letter or symbol as the denominator. For example, the top line on a Snellen chart may contain letters or symbols that are 8/8 or 10/10 in size, while the bottom line may contain letters or symbols that are 1/10 or 2/10 in size.



To use a Snellen chart, the viewer stands 20 feet (6 meters) away from the chart and covers one eye. The uncovered eye is then used to read the letters or symbols on the chart, starting with the largest letters or symbols at the top and working down the chart until the viewer is no longer able to accurately read the letters or symbols. The smallest line that the viewer is able to read accurately is recorded as their visual acuity. For example, if the viewer is able to read the 8/8 line at the top of the chart but is only able to read the 4/10 line on the third line down, their visual acuity would be recorded as 4/10. 

The visual acuity measurement obtained using a Snellen chart is expressed in terms of a fraction, with the distance from the viewer as the numerator and the size of the smallest letters or symbols that can be read accurately as the denominator. This fraction is then converted to a decimal value and expressed as a percentage. For example, a visual acuity of 4/10 would be converted to a decimal value of 0.4, which would be expressed as 40%. A visual acuity of 20/20, which is considered normal, would be expressed as 100%. 

There are several variations of the Snellen chart, including charts that use numbers instead of letters or symbols, charts that use pictures instead of letters or symbols, and charts that are designed for use with children. There are also variations in the size and arrangement of the letters or symbols on the chart, and in the distance at which the chart is placed from the viewer. 

In addition to the traditional Snellen chart, there are several other tools and techniques that are used to measure visual acuity. These include the logMAR chart, the Landolt C chart, the Tumbling E chart, and the Bailey-Lovie chart. Each of these charts.


Comments

Post a Comment

Popular posts from this blog

What is myopia?

Image
Myopia is nearsightedness and this condition of eye occurs when the power of eye is increased from the normal powers of eye. The normal powers of eye is 58DS and the eye with >58DS eye power will be marked as myopic eye.  Definition   When parallel light rays coming from the infinity focus Infront of the retina while the accommodation should be at rest this condition of eye is known as myopia.
Read more

Scleritis

Scleritis 👌is a painful inflammation (swelling) the sclera. The tough, fibrous tissues of the sclera form a protective outer layer for the eye and make up 83 percent of the eye’s surface. In almost half of all cases, scleritis is associated with an underlying autoimmune disorder such as rheumatoid arthritis. There are two main types of scleritis: anterior and posterior. Anterior scleritis, the most common type, affects the front portion of the sclera. There are three types of anterior scleritis: Diffuse scleritis is the most common type and fortunately the most treatable. This type displays widespread redness and inflammation throughout the whole or a portion of the front portion of the sclera.     Nodular scleritis, is characterized by the presence of nodules , often tender to the touch, on the surface of the eye.     Necrotizing scleritis is the most severe form of anterior scleritis. It has the ability to destroy scleral tissues an...
Read more

ABETALIPOPROTEINEMIA (BASSEN-KORNZWEIG SYNDROME) Retinopathy

Image
Abetalipoproteinemia, also known as Bassen-Kornzweig syndrome, is a rare genetic disorder that affects the body's ability to absorb and transport fats and fat-soluble vitamins. One of the ways that abetalipoproteinemia can affect the eyes is by causing retinitis pigmentosa, a condition that causes vision loss. Retinitis pigmentosa is a degenerative eye disease that affects the retina (the light-sensitive layer at the back of the eye). It is characterized by the gradual loss of photoreceptor cells (cells that detect light and send signals to the brain) and the accumulation of pigment in the retina. This can cause vision loss and may lead to blindness in severe cases. Abetalipoproteinemia can also cause optic atrophy, a condition in which the optic nerve (the nerve that carries visual information from the eye to the brain) becomes thin and weak. This can cause vision loss and other visual problems. In conclusion, abetalipoproteinemia can affect the eyes by causing reti...
Read more