The Eye Chart Explained

The eye chart is synonymous with a trip to the eye doctor. Many of you will have the shared experience of duly covering one eye as you read the letters as best you can from across the room, but how much do we actually know about the eye chart? And what exactly is being measured?

The Eye Chart Explained

The eye chart as we know it today was first developed by Dutch ophthalmologist, Herman Snellen, in 1862. In its earliest forms, Snellen experimented with abstract symbols across a 5 x 5 grid. A year later he started using the alphanumeric capitals as we know them today.

In 1976 the National Vision Research Institute of Australia developed the LogMAR eye chart to enable a more accurate estimate of acuity as compared to other charts. When using the LogMAR chart, visual acuity is scored with reference to the Logarithm of the Minimum Angle of Resolution, as the chart’s name suggests. In the words of its designers, Ian Bailey and Jan E Lovie-Kitchin:  “We have designed a series of near vision charts in which the typeface, size progression, size range, number of words per row and spacings were chosen in an endeavour to achieve a standardization of the test task.”

So, Snellen and LogMAR eye charts measure Visual Acuity. But what precisely is that?

What is Visual Acuity?

Visual acuity: this refers to clarity of vision which is dependent on optical and neural factors, the sharpness of the retinal focus within the eye, the health and functioning of the retina and the sensitivity of the interpretive faculty of the brain. Visual acuity is a static measurement, i.e. measured whilst seated and is often measured under high contrast conditions, such as the contrast of the monochrome eye chart.

Visual perception/Eyesight: this is the ability to interpret the surrounding environment by processing information that is contained in visible light. The resulting perception is also known as eyesight, sight, or vision.

Vision: In addition to sharpness of sight, the broader term “vision” usually includes a wider range of visual abilities and skills. These include contrast sensitivity, the ability to track moving objects with smooth and accurate eye movements, colour vision, depth perception, focussing speed and accuracy, and more.

So we hoped you learned something new about the eye chart today. If the eye chart is a hazy memory for you then we’d suggest it’s most definitely time you booked in for a visit. Just click here to book your appointment in no time.