Visual Acuity Testing Standards
By Dr.Yazan Gammoh, Moh, PhD. Head of Scientific Committee-Jordan General Association of Optometry
What is visual acuity? What are its limitations?
Visual acuity describes the acuteness or "sharpness" of vision; that is the ability to perceive small details. The primary measurement tool is the letter chart introduced in 1862 by Donders and Snellen at the Eye Infirmary at Utrecht in the Netherlands.
Visual acuity measurement is so common that visual acuity measurement is often mistaken as a distinctivegauge for vision in general which is a misapprehension. VA loss can detect many disorders, but not all. An important example is glaucoma, which can cause extensive and irreversible visual field damage before visual acuity is affected. Letter chart acuity tells us something about the very small retinal area onto which the letter seen is projected.
VA as a screening tool
VA measurement is a good screening tool because normal VA requires that all levels of the visual system function properly. the optical system of the eye must project a sharp image of the outside world onto the retina. the retina must then be able to convert this image into neural impulses. Finally, the neural impulses must travel to the brain, where they are analysed and recognised. therefore, a wide range of different visual disorders can affect visual acuity.
Because visual acuity is so easily measured, it is often used as a primary eligibility criterion, such as 6/6 for a pilot's license, 6/12 for a driver's license, 6/60 for certain disability benefits.
Definition and recording of VA
The notation of VA is usually expressed as a fraction, but few understand what these fractions mean. If a subject needs letters that are twice as large or twice as close as those needed by a standard eye, that person's visual acuity is said to be 1/2. If the letters needed are five times larger, the acuity is 1/5, if ten times larger: 1/10, etc.
the value of those fractions can be expressed in different ways. E.g. 1/2 = 20/40 = 6/12 = 0.5 or 1/5 = 20/100 = 6/30 = 0.2. the Magnification requirement is also known as MAR, so that MAR = 1/V and V = 1/MAR.
Snellen expressed this in the well known Snellen formula (here in its metric version):
VA = M/M = viewing distance (in meters)/letter size (in M-units)
In the US the 20/.. notation is routinely used, even if the testing distance is not 20 ft. In Europe the decimal notation is common; in Britain (and former British dominions) the 6/..notation is common.According to the ICO Visual Acuity Measurement Standard (1984) a line is considered read if "more than half" of the characters are identified correctly. For an EtDrS chart with 5 letters per line, this means 3 or more correct.
Not all of Snellen's optotypes are equally recognizable. Landolt in 1888 addressed this problem by proposing an eye chart that had only one symbol, a ring with a break at top, bottom, left or right, and 45 degree positions in between, basically the letter C in various orientations (figure 3). to match Snellen's results, the "standard" size of the C was 0.35" (which subtends 5 arc minutes at 20 feet) with a gap of 0.07" or 1 arc minute.the International Council of Ophthalmology considers the Landolt C the purest research standard and requires all other research approaches to be calibrated against the Landolt C.
What should be the measurement steps?
Since letter charts contain only discreet letter sizes, the measurement accuracy depends on the step size between lines. Most traditional charts have an irregular sequence of letter sizes (as did Snellen's original chart). From 6/5 to 6/6 is a 33% increase, from 6/6 to 6/9 is 25%, from 6/18 to 6/24 is 17%, but from 6/30 to 6/60 is a 100% increase.
What is the Preferred Numbers series?
Many different geometric progressions (progressions with a constant ratio between adjacent terms) are possible. the progression used in the EtDrS protocol is known as the Preferred Numbers series. this series is defined in standard #3 of the International Standards Organization and is used in a wide variety of industrial standards.
The preferred numbers series:
(a) Fits well with the decimal system, since each step represents the same 10 10 (100.1) ratio. thus, 10 steps equal exactly 10x, and after 10 steps the same digits reappear with only a shift in the decimal position.
(b) Preferred numbers are convenient because, with only slight rounding, the series contains mostly whole numbers. Each step represents a 4:5 ratio (rounding error = +0.7%), 3 steps equal a factor 2x (rounding error = -0.2%). the same ratios are used to calculate decibels; 3 decibels = 2x.
(c) Being anchored at 1.0 (10, 100, 1000, etc.) the series is well suited for visual acuity, since the reciprocal of a preferred number and the product or quotient of two preferred numbers is again a preferred number. Letters (figure 4) are the obvious first choice for adults.
Many different letter sets have been used. Since the establishment of the EtDrS protocol, Sloan letters have become the preferred choice. they are designed on the same 5x5 grid on which Snellen designed his letters.
Numbers (figure 5) are the second choice for adults. Even illiterate adults can often recognize numbers. Deaf/ non-speaking adults can indicate the number seen with simple finger signs.