Frequently Asked Questions:
Beyond the step sizes discussed earlier, another factor that affects the measurement accuracy of letter charts is the number of letters on each line. Traditional charts have a rectangular format. This allows many small letters, but only one or two big letters on a line. In 1976 Bailey and Lovie (then in Melbourne) proposed a new layout, combining the logarithmic progression with 5 letters on every line and spacing equal to the letter size. Their design produced a much wider chart with a triangular array of letters that did not fit traditional letter chart boxes or projector charts.
With 5 letters per line the common criterion of counting a line as read when “more than half” of the line is read correctly, becomes 3/5 or 60% correct, about halfway between guessing and 100% correct.
In 1982 the National Eye Institute (NEI) needed new charts for its Early Treatment Diabetic Retinopathy Study (ETDRS). They chose the Bailey-Lovie layout and combined it with the letter set designed by Louise Sloan in 1959. These charts became known as ETDRS charts. They have since become a world-wide accepted, de-facto standard.
Since the letter spacing, the number of letters per line and the steps between lines are all standardized, the letter size is the only variable on ETDRS and similar charts.
The MassVat layout and the effects of crowding will be discussed in the pediatric section.
Since visual acuity is defined by the angle under which letters are viewed, its measurement can be done at any distance, provided that the scale is adjusted for the distance used.
When a letter chart is used as a target for subjective refraction, the viewing distance is important, because the longer the viewing distance, the more accommodation will be relaxed. Snellen’s original charts were calibrated for a viewing distance of 20 Parisian feet (in 1862 more than 20 different feet were in use across Europe). As soon as the Treaty of the Meter was signed (1875) he converted to metric distances and made charts for 5 meter (more convenient with the decimal system) and 6 meter (closer to 20 feet). These distances have the advantage that they relax accommodation and that small forward movements of the patient have a negligible influence.
For testing of children a 3 meter distance (10 ft) is often used, since it is easier to keep their attention at that distance. For refractive use, this distance is not recommended.
For testing of Low Vision patients a 1 meter distance is advantageous, since it can cover a much wider range of visual acuity values. The Colenbrander 1-meter chart covers visual acuities from 20/1000 to 20/20. It requires 1 D over the distance correction.
For testing of reading acuity, various distances have been used. The distance of 40 cm is a commonly recommended standard.
When the testing distance is short, maintaining the accurate viewing distance becomes more important. At a 20 ft distance, a 1 ft variation represents 5%, equivalent to one letter on an ETDRS chart. At 1 m, 5 % is only 5 cm (2”); at 40 cm it is 2 cm (0.8”).
Visibility of objects is determined by their size, their illumination and their contrast. In visual acuity measurement we want to determine a size threshold. We therefore have to make sure that illumination and contrast are at levels where a ceiling effect is reached, so that small variations in illumination and contrast do not influence the measurement. This leaves a fairly broad range of acceptable values.
For routine clinical use of front illuminated charts, a rule of thumb can be that contrast should be maximal and that the charts should be illuminated well enough so that extra illumination will not improve the visual acuity readings.
Before Snellen vision was usually assessed with existing printer’s fonts, which could vary enormously in recognizability. Snellen’s innovation was to design specific characters to be used only for the measurement of visual acuity. He called these characters optotypes and designed them all on Snellen's 5x5 grid.
Although Snellen also experimented with other symbols, he chose letters for his chart since they have most face validity for patients, whose main desire is to be able to read. As explained above for contrast and illumination, we also need to make sure that letter recognition and letter naming are tasks of trivial difficulty that do not confound the size recognition task. If this is not the case, as for young children and illiterate adults, other symbols should be used.
Letters 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 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.
Tumbling Es can be used for young children. They are the optotype of choice for many studies in the developing world.
Landolt Cs are often used in research studies, but have found limited application in clinical practice in the US. Tumbling Es and Landolt Cs offer four alternative directions; a prerequisite is that the subject can duplicate this direction, which may be a problem for children with a young developmental age.
Children who are too shy to respond verbally, can be asked to match the letter or optotype to one of four flash cards in front of them. For some this is easier than indicating a direction.
HOTV charts also offer only four choices. The four letters H, O, T and V have been chosen because they are R/L symmetrical.
Numerous picture cards have been designed. A problem is that not all children are equally familiar with all pictures and that many picture sets have uneven difficulty. Most pictures cannot be designed on a 5x5 grid.
Patti Pics are stylized pictures designed by Precision Vision on the basis of the 5x5 Snellen grid and calibrated for equal recognizability against Sloan letters. Visual acuity readings should not change appreciably when children are advanced from Patti Picks to a regular letter chart.
The opaque printed chart is the oldest format. It has the advantage of easy transportability. It is the preferred format in offices where visual acuity is only measured occasionally or for school screenings where the venue often changes. For each setup, care must be taken that the viewing distance conforms to the distance for which the card has been labeled.
Translucent charts need a special illuminator cabinet. When that cabinet is available, they have the advantage of a standardized light level that is largely independent of the room illumination.
Projector charts are the choice of most eye care professionals for use in the office. Since most rooms are smaller than they were in Snellen’s days, the fixed installation usually involves a mirror to increase the viewing distance. Since the projector lenses are adjustable, care must be taken that the letter size on the screen is adjusted for the actual viewing distance (patient to mirror + mirror to screen). Since most examination rooms are only semi-dark, care must also be taken that there is no stray light reaching the screen, since this could seriously reduce the contrast.
In recent years computer displays have replaced many projectors. Such displays have several advantages: Optotypes and a wide variety of display modes can be changed with the click of a button on a remote control. The screen is less sensitive to stray light. Presentations can be switched from a fixed letter sequence (easy for the doctor who knows it by heart) to a random sequence for patients who have been tested quite often. As with projector charts, the letter size on the screen must be coordinated with the actual viewing distance.