The COLENBRANDER Low Vision Measurement System · moderate, severe, and profound, between normal...

Colenbrander – Visual Acuity measurement tools.

The COLENBRANDER Low Vision Measurement System August Colenbrander, MD – San Francisco

See also: www.ski.org/Colenbrander

This paper describes the various modes of acuity measurement (letter acuity / reading acuity / low contrast acuity) and a coordinated set of measurement tools to extend accurate measurement to the Low Vision range. The tools include: A letter chart for acuities down to 20/1000 Reading cards with standardized paragraphs Mixed contrast cards for Contrast Sensitivity screening.

The paper also provides suggestions for the use of these tools.

Introduction Measuring visual acuity is such a basic skill that we rarely stop to think about its numerical meaning. Often, we just interpret 20/40 as driving vision and 20/200 as eligibility for benefits.

The widespread use of the term “(Legal) Blindness” is a serious impediment to proper interpretation, because it suggests a black-and-white dichotomy between seeing and not-seeing. In reality, there is a continuous gray scale with various ranges of visual acuity loss: moderate, severe, and profound, between normal vision and blindness. Visual acuity numbers are only markers along this road (Table 1). Calling a person with severe vision loss “Legally Blind” is as preposterous as calling a person with a severe heart ailment “Legally Dead”.

The process of visual perception includes several distinct phases: Optical factors that affect Image formation Retinal problems that affect Image detection Brain damage that affects Image interpretation and response

Historically, the emphasis in ophthalmology has often been on optical factors: refractive error and opacities (cataracts). For this purpose, visual acuity measurement is useful as a comprehensive measure, since optical deterioration of the foveal image predicts similar deterioration in the rest of the field. We measure distance and near acuity, because the difference, if any, tells us something about the accommodative amplitude.

When we extend our interest to retinal problems, and to Activities of Daily Living, visual acuity no longer tells the whole story. Letter chart acuity can test only the very small retinal area onto which the letter is projected. For reading and other ADLs, the condition of the surrounding area is equally important. Reading tests can give us an impression of the function of this area. From a functional point of view, therefore, the distinction between letter chart acuity and reading acuity is more relevant than that between distance and near acuity.

Cognitive problems of interpretation are independent of visual acuity and magnification.

Principles of Letter Chart Acuity Since Snellen introduced it in 1862, letter chart acuity has remained the dominant clinical measurement. It is measured by determining how much larger a letter (or symbol) needs to be for the subject to reach a standard performance level. If the subject needs letters that are twice as large or twice as close, we say that the visual acuity is 1/2 (20/40). If the letters need to be 5x larger, visual acuity is 1/5 (20/100), etc.

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Clinical visual acuity is measured with high contrast symbols, because in that mode the measurements are minimally affected by changes in contrast and illumination. The accuracy of the measurement further depends on the steps between optotype sizes. Snellen (1862) used a pragmatic sequence with small steps in the normal range and larger steps elsewhere. In 1868 John Green (after a fellowship with Snellen) proposed a logarithmic (geometric) progression, but this was not adopted at the time. A century later this changed: Sloan used a logarithmic progression in the 1960’s, Bailey and Lovie in the 1970’s and in the 1980’s the ETDRS protocol of the National Eye Institute promoted it as a standard. The ETDRS protocol also adopted the Sloan optotypes and the proportional spacing with five letters on each line, as proposed by Bailey and Lovie.

A logarithmic progression conforms to Weber-Fechner’s law, which states that geometric stimulus increases result in linear increases in perception. It also has the advantage that it can cover a far wider range of values than any linear progression.

A wide range is important since the spectrum of visual acuity values extends far beyond the range of our usual measurements. Snellen designed his letter chart as a screening test and placed 20/20 at the “lower limit of normal”. 90% of normal eyes have better than 20/20 vision, usually 20/16 or 20/12.5, occasionally 20/10 or even 20/8. The idea that 20/20 would represent “normal”, let alone “perfect” vision is a persistent urban myth and lacks any scientific base.

At the other end of the chart, the same holds true. It is estimated that 90% of those who are “20/200 or less” do indeed have useful and measurable residual vision. How can we measure this vision, without resorting to vague terms like “hand movements” and “count fingers”?

The COLENBRANDER 1-meter chart Extending the measurement range means either using larger symbols, which is impractical, or bringing the chart closer. Some Low Vision practitioners bring the chart from 20 ft to 10 ft, which gives a 2x extension. Bringing it from 20 ft (6 m) to 1 meter (40 in) provides a factor 6x. At this relatively short distance it is important that the distance is measured accurately. The 1-meter chart comes equipped with a cord and occluder to facilitate this.

The basic advantages of 1-meter testing are:

• The simplest possible Snellen fraction: 1/… , which is easy to convert to other notations.

• The ability to cover the entire Low Vision range, down to 1/50 (20/1000) (See Table 1).

• The chart is exactly 1 D away from infinity.

Additional advantages of our chart include: • The chart follows the ETDRS layout: Sloan letters,

proportional spacing and a logarithmic progression. • The chart folds to fit in a drawer or in an attaché

case for use outside the office. • It comes with a cord to measure 1 meter accurately, rather than estimating 10 ft. • Both patient and chart are within arm’s reach.

• The chart covers acuities from 1/50 (20/1000) to 1/5 (20/100) on the main chart and from 1/6 (1/125) to 1/1 (20/20) on the insert chart. This is a wider range than any other chart.

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• For occasional use in the field, the chart can also be used at longer distances. E.g. at 5 m (200 in, 16½ ft) the main chart covers 5/50 (20/200) to 5/5 (20/20).

Table 1 shows the increased coverage compared to traditional charts. This increased measurement range removes the need for vague statements, such as “count fingers” and “hand movements. It also has a significant psychological advantage, because it allows patients to read several lines, where they previously could only say “I cannot see the big E”. This simple fact allows them to shift their attention from what they cannot see to what they CAN see, so that the rest of the examination can build on their strength instead of on their failure.

Notations for Distance and Near acuity Traditionally, letter chart acuity is measured at a distance, because this relaxes the accommodation, which is an advantage when determining refractive error.

Near acuity is often measured with a miniature letter chart. This, however, gives little additional information, except about the accommodative amplitude. In either case, we measure only the function of the small foveal (or PRL) area onto which each letter in succession is projected.

Reading is a different task. It requires a larger functional area to recognize an entire word rather than a single letter; it also requires some vision farther from fixation to guide appropriate scanning movements along the line. Thus, reading not only requires proper (optical) image formation, but also (retinal) image detection, which brings the topography of the retinal loss into play.

An urban myth is that distance-acuity and near-acuity are often unrelated. This is wrong. The origin of this myth can be traced to the fact that until now reading acuity could not be easily calculated because of the widespread use of Jaeger numbers and inches. Since visual acuity depends on the letter size and the viewing distance, both need to be measured accurately.

The commonly used Jaeger numbers refer to catalogue numbers in a printers’ catalogue in Vienna in 1856; they have no numerical meaning. When others tried to duplicate Jaeger’s samples with locally available fonts, the application of Jaeger numbers became notoriously variable. Another printers’ reference is printers’ points. This notation has a numerical connotation, but since it refers to line height, rather than to letter size it may vary from font to font (E.g. 8-point Arial = 9-point Times Roman). Furthermore, it does not allow a comparison of the lower case letters used in reading tests to the upper case letters used in letter charts.

A third alternative is to designate the letter size with visual acuity numbers. This only adds to the confusion, since the visual acuity associated with a given letter size varies with the viewing distance. A “20/40” letter at 10 cm (4 in) is 5x larger than a “20/40” letter at 50 cm (20 in). This is obvious when viewing Table 2.

The only unit that is well defined and offers comparability with letter chart acuity is the unit defined by Snellen. He defined it as “the distance at which the letter subtends 5 min. of arc”. Louise Sloan simplified this description by introducing the term “M-unit”, where 1 M-unit subtends 5 min. of arc at 1 meter; it follows that a letter of n M-units will subtend 5 min of arc at n m. There is no equivalent for the ft-system. M-units are commonly used on metric letter charts, but are not yet common for reading cards. 1 M-unit is about 1.5 mm (actually 1.454, error 3%) or about 1/16 in (actually 0.92 /16, error 8%).

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Calculating near-acuity values Expressing the letter size in M-units is not enough to make near vision calculations easy. For letter chart testing where the distance is a whole number, the traditional Snellen formula is useful as: V = test distance (in meters) / letter size (in M-units)

For reading tests the formula becomes awkward, since the numerator becomes a fraction-within-a-fraction. Inverting the formula avoids this:

1/ V = letter size / distance = letter size x 1/ distance

The reciprocal of a metric distance is the diopter (1/2 m = 2 D, 1/5 m = 5 D, etc.). We are familiar with this notation from the optical literature, where it converts focal distance to lens power. It can be used in any situation where the reciprocal of a metric distance is used.

Expressing the letter size in M-units and the viewing distance in diopters, the formula is further simplified, so that one can easily calculate it by heart:

1/ V = M x D.

Each of the components of this modified Snellen formula is directly clinically relevant.

1/V indicates the magnification requirement, familiar from Kestenbaum’s formula; it also is the denominator of the 1-m acuity value.

M indicates the letter size. 1 M equals average newsprint.

D indicates the reading distance expressed in diopters and relates directly to the reading add or accommodation needed.

When this modified Snellen formula is used, it is found that distance acuity and near acuity are usually very similar (See Table 3). However, the magnification required for fluent or functional reading is usually somewhat larger than the threshold magnification found from letter chart measurement. The difference between functional reading acuity and threshold reading acuity is known as the magnification reserve. Similar differences between functional and threshold values can be found for other parameters, such as contrast and illumination.

The COLENBRANDER Reading cards From a functional point of view, testing with continuous text is more informative than testing with a miniature letter chart. (a) Because reading is the endpoint the patient wants to reach, and (b) because reading involves a larger retinal area and provides topographic information.

The COLENBRANDER reading cards were designed with the following features: • All sentences have the same length, so that timing of reading speeds is easy. • The card follows the proportional spacing and logarithmic progression of the ETDRS charts. • Print sizes are expressed in M-units, to allow a comparison with letter charts acuity. • The cards are available in multiple languages (Currently; English, Spanish, French,

Portuguese, German, Swedish, Dutch, Finnish).

• The cards are available in several formats:

o (a) For occasional use the reading test is printed on the back of the 1-meter chart.

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o (b) For use with patients with normal or near-normal vision: a small card can be used with a cord for a 40 cm viewing distance.

o (c) For use with Low Vision patients: a larger card with separate sections for the right and left eye can be used. This card comes with a ruler so that even patients who need a very short reading distance can be tested.

Large card with Ruler

Small card with cord

The larger card has two panels (8½ x 11 in) and four sides. Two sides provide letter sizes from 10 M to 3 M; the other two sides each provide letter sizes from 4 M to 0.6 M, so that one side can be used for the right eye, one for the left eye, or one before training and one after training. Since the card can be used at various distances, only the M-unit values are indicated. The card comes with a ruler calibrated in cm, inches and diopters. Using this ruler, the reading distance can be directly compared to the reading add and the visual acuity can be easily calculated using the modified Snellen formula: 1./.V = M x D.

The small card (7 x 9 in) is primarily intended for patients with (near-) normal vision. It comes with a 40 cm (16 in) cord attached and the paragraphs are labeled with the visual acuity for that distance. These acuity values are not valid if the card is used at any other distance. The letter sizes range from 6.3 M to 0.32 M (20/320 to 20/16 at 40 cm).

Contrast sensitivity Thus far, we have discussed letter chart acuity and reading acuity, measured with high contrast black-on-white letters. Many Activities of Daily Living (ADLs), however, involve objects of lesser contrast with details that are not at the threshold of perception. Contrast Sensitivity – i.e. the ability to recognize larger objects of lesser contrast – is generally acknowledged to be an important part of functional vision. It is often measured in the laboratory, but very few practitioners measure it in routine practice. One reason is that busy practitioners do not find time for an extra test where the yield may be relatively low.

Detecting contrast deficits is important, because some patients, especially among the elderly may have reduced contrast sensitivity with normal or near-normal high-contrast acuity. They fall between the cracks if contrast sensitivity is not measured. To often the reality of their complaints is denied and an opportunity to warn them about the consequences (in activities such as night driving) is missed.

The objective for designing a new test was to offer a test that is simple and quick enough to be used on a routine basis, even if the yield of subjects with contrast deficits is low. Once a contrast deficit is detected, additional tests are in order to define it more fully.

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The COLENBRANDER Mixed Contrast Card This card was designed to serve as a simple screening test. It is a variant of the regular reading card; each two-line paragraph contains one line in high contrast and one line in low contrast.

The usual question for a reading test is: “What is the smallest (black) line you can read?” On the Mixed Contrast card one adds the question: “What is the smallest gray line you can read?”

The difference between the two measurements will be obvious and can simply be expressed in “lines”. The time required is minimal.

The chart can also serve as an educational tool to demonstrate the effect of contrast deficits to patients and family members. Increasing the light level will often improve low-contrast vision.

Our preliminary findings indicate that a 2 or 3 line difference is normal. We found this difference even among Low Vision patients with poor acuity. On the other hand, differences up to 9 or 10 lines were found in other patients. If such differences are found, further exploration is warranted.

For additional information, visit: www.ski.org/Colenbrander or contact: [email protected]

SUGGESTIONS FOR USE Using the 1-meter Letter Chart, for Low Vision patients

For occasional use the card can be kept in a drawer. The chart is brought out when the regularly measured acuity is 20/100 or less on a traditional chart or 20/200 or less on an ETDRS-type chart (i.e. a chart with additional lines at 20/125 and 20/160).

The chart is hand held at 1 meter (using the occluder and cord). Since visual acuities down to 1/50 (20/1000) can be thus recorded, vague statements such as “Count Fingers“ or “Hand Movements” are no longer needed when documenting the progression or recovery of various disorders.

If visual acuity is worse than 1/10 (20/200), adjusting the refraction is often not even necessary, since the blur circle from 1 D of defocus is small compared to the letters seen. For visual acuities better than 1/10 (20/200) adding 1 D to the distance correction is recommended, since this may improve the measured acuity.

For routine use, such as in a Low Vision service, it is recommended to place the chart on a moveable easel. The cord is used to adjust the distance. If the patient moves forward, it may be easier to move the easel accordingly than to push the patient back.

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Do not point to letters and do not push the patient to guess. Notice the letter size that can be read comfortably with the patient’s own fixation ability, since that is the level closest to a functional reading level.

If it is necessary to do a subjective refraction, the fact that there are lines the patient can actually see is a significant help. It is recommended to use a trial frame with full size lenses, so that patients with a central scotoma can search for the best viewing angle. Do not use steps of +/– 0.25 D since those steps are too small to be appreciated. Steps of +/– 1 D or even more may yield better responses; adjust the step size to the visual acuity level.

Subtract 1 D to obtain the best distance correction. For patients who spend most of their time indoors, the best correction may be for the TV distance, rather than for the far distance.

Sometimes patient will read slowly at the largest sizes, then speed up and finally slow down again. This may indicate that only a small island of good vision is available that is too small to catch the largest letters without scanning. One may get an idea of the size of the island by asking the patient to fixate a central letter (e.g. the “C” at the 20 M line) and report the number of letters to the left and right that can be seen without eye movements. On the 20 M line the center-to-center distance between letters is about 3 degrees.

On the smallest lines (provided they can be seen) the patient may speed up again, because several letters will now fit into the central island.

Using the 1-meter Letter Chart, for regular subjects

If a portable chart is needed for use in the field, the chart can also be used at longer distances. If a 5

m (200 in, 16½ ft) is chosen, the main chart covers 5/50 (20/200) to 5/5 (20/20) and the insert card offers values from 5/6 (20/25) on, continuing well beyond the threshold.

When using the chart at 1 m for Low Vision patients, the main chart is the primary chart; the insert chart is used only when they reach the bottom of the main chart. For subjects assumed to have normal acuity, the insert chart is the primary test; the main chart is needed only if they cannot read the insert chart.

Remember that 90% of normal subjects have visual acuities better than 20/20. Continue until the true threshold is reached. Never truncate the measurements at 20/20.

Using the Large Reading Card, for Low Vision patients The primary goal when using this card is to find the best form of optical magnification (glasses, magnifiers) to restore reading vision, which for most patients is their main objective.

If the patient has existing reading glasses or bifocals, give them the reading card and ask to bring it to the distance where the print is clearest. Measure that distance with the diopter ruler. If the diopter value of the reading add and the diopter value of the reading distance are the same, the existing distance correction (to which the reading add was added) is approximately correct.

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Patients with a progressive add may not do so well, since simultaneously searching for the best retinal spot and for the best spot in the progressive add is difficult. Try them with a full reading correction in a trial frame.

Give the person some additional add. This should reduce the reading distance and improve the reading performance. The markers along the diopter scale are spaced using the same steps as used for the reading paragraphs. For each marker that the print is brought closer, the patient should be able to read one additional paragraph. At each step, check that the reading distance and reading add are still in agreement. Many patients initially resist short reading distances. For them, it may be helpful to push the print too close and ask them to move it out to the point of best clarity.

At each step, also calculate M x D (letter size in M-units x reading distance in diopters) to verify that the acuity values are consistent. If they are consistent, this is a confirmation of the original acuity measurement. If they are not consistent, verify that the patient holds the material at the right distance, or find another explanation. Do not simply discard such findings as measurement errors.

Example: The patient could read: M x D = Visual Acuity: 12 M at the 1-m chart 1/12 (20/250) 4 M at 3 D (13 in) 4 x 3 = 12 1/12 (20/250) 3 M at 4 D (10 in) 3 x 4 = 12 1/12 (20/250) 2 M at 6 D ( 7 in) 2 x 6 = 12 1/12 (20/250) 1 M at 12 D (3½ in) 1 x 12 = 12 1/12 (20/250)

Note that the M x D calculation is far simpler than any calculation directly based on inches and/or the 20/… acuity notation could be. Also note that the diopter power needed to read 1 M print (average newsprint) is the reciprocal of the visual acuity value; this has long been known as Kestenbaum’s rule.

Some additional magnification may be needed for best reading fluency (See Table 3).

It is useful to ask the patient to start at the largest print sizes. If these sizes are read easily it takes little extra time and encourages patients by demonstrating that there is print they can read. The common sequence of reading speeds is fast-fast-slow: they read with reasonable speed and slow down only at the smaller sizes. The acuity value on which the choice of magnification level should be based is the smallest size at which a reasonable reading speed can be maintained. We call this the functional reading acuity; for actual, prolonged reading, this level is more important than the threshold reading acuity.

Sometimes the pattern is slow-fast-slow. This may indicate that a small island of optimal vision is used. For large texts, not enough words or letters can fit into the island. For medium text, more words can fit and reading speeds up. For the smallest sizes reading slows down again. (Compare to letter chart diagram on page 7).

Sometimes the pattern is slow-slow-slow. This may indicate multiple scattered scotomata that interfere with proper scanning at any level. If reading of unmagnified large text is difficult, reading of magnified small text will not be any easier. Such patients may be better helped with underlining technique than with magnification.

Another group that may exhibit a slow-slow-slow pattern are patients that have both a retinal problem (e.g. AMD) and a cognitive problem (e.g. after a mild stroke). At large sizes the cognitive problem is likely to dominate; at small sizes the retinal problem becomes decisive.

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When the best spectacle correction has been determined, try magnifiers. Since the eye to magnifier and magnifier to print distances are variable, the effectiveness of magnifiers is generally less than the effectiveness of the corresponding spectacle lens. This means that the patient in the earlier example will need a magnifier of at least 12 D.

Using the Small Reading Card, for regular patients For a regular practice where most patients have normal or near-normal vision, the primary use of a reading card is to verify that a prescribed reading add is adequate. For such patients the use of a variable distance is rarely necessary.

For this purpose the small card with a 40 cm cord is available. On this card the visual acuity values are listed next to each paragraph. Remember that these values are not valid if the card is held at any other distance. Using visual acuity values to indicate a letter size is a common mistake, as shown in Table 2.

Here again, a distinction must be made between threshold acuity, indicated by the smallest print size that can be deciphered, and functional acuity, indicated by the print size that can be read comfortably and for a prolonged period of time. Reading 1 M print (average newsprint) at 40 cm (2.5 D) represents a functional acuity of 1/ 2.5 (20/50), which is about 3x or 5 lines better than normal threshold acuity of 20/16.

Using the Mixed Contrast Card The Mixed Contrast card is meant as a screening test. It is used as an extension of the usual high-contrast only reading test. Instead of asking only “What is the smallest (black) line you can read?” the question is added: “What is the smallest gray line you can read?”

A difference of 2 or 3 lines is normal. If a greater difference is found and the cause is not immediately evident, further exploration is indicated. The test may be used to follow the progression or regression of a disease process. It may be used to demonstrate that cataract surgery improves not only high-contrast acuity, but also low-contrast vision.

When a contrast deficit is found, the card may be used as an educational tool to convince the patient and family members of the reality of the deficit. In many cases increased illumination is helpful; the card easily demonstrates this. Patients with contrast deficits should be warned about low-contrast situations, such as night driving and about low-contrast environmental hazards (curbs, going down stairs); they should be advised about ways to enhance contrast and illumination in their home environment.

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TABLE 1 Visual Acuity Ranges – Chart Ranges – Reading Ability Ranges

Classification ( ICD-9-CM )

Tradit. Chart. 1-m

Chart 1 M at

… cm Visual Reading Ability Comments

Range of Normal Vision

20/ 10 / 12.5 / 16 20/ 20 / 25

20/ 10 / 15 20/ 20 / 25

1/ 1 / 1.25

200 160 125 100 80

Normal reading speed Normal reading distance

Reserve capacity for

small print

Since newsprint is generally read at around 40 cm, this range has an ample reserve.

(Near) Normal Vision

Near- Normal Vision

20/ 32 / 40 / 50 / 63

20/ 32 / 40 / 50 / 63

1/ 1.6 / 2.0 / 2.5 / 3.2

63 50 40 32

Normal reading speed Reduced reading distance

No reserve for small print

Individuals in this range have lost their reserve, but have no or only minimal vision rehabilitation needs.

Moderate Low Vision

20/ 80 / 100 / 125 / 160

20/ 80 / 100

1/ 4.0 / 5.0 / 6.3 / 8.0

25 20 16

12.5

Near-normal with appropriate reading aids Low power magnifiers and large print books

Reading at 25...12.5 cm requires strong reading glasses (4D to 8D) or moderate power magnifiers.

20/ 200 Severe Low Vision

20/ 200 / 250 / 320 / 400

1/ 10 / 12.5 / 16 / 20

10 8

6.3 5

Slower than normal with reading aids

High power magnifiers (restricted field)

Reading at < 10 cm precludes binocular vision. The small field of strong magnifiers slows reading.

Low Vision

Profound Low Vision

20/ 500 / 630 / 800 / 1000

1/ 25 / 32 / 40 / 50

4 3.2 2.5 2

Visual reading is limited Uses magnifiers for spot reading, but may prefer talking books for leisure

Use of non-visual skills increases as rehabilitation needs shift gradually from vision enhancement aids to vision substitution aids.

Near- Blind- ness

20/ 1250 / 1600 20/ 2000

1.6 1.25

1

(Near) Blind- ness

Blind NLP

No visual reading

Must rely on talking books, Braille or other non-visual

sources

In this range individuals must rely primarily on vision substitution skills. Any residual vision becomes an adjunct to the use of blind skills.

The left hand section of Table 1 lists the classification of vision loss as found in ICD-9-CM. Note that they are based on a logarithmic progression of acuity values. Also note the column that shows at what distance (in cm) 1 M print (average newsprint) can be seen. As visual acuity gets poorer, reading distances gets shorter, reading adds stronger.

The center section compares the measurement range of a traditional chart and a 1-m chart. Note that the 1-m chart can cover the entire Low Vision range.

The right hand section provides functional descriptions. These descriptions are based on clinical experience and statistical averages. They are provided as a general reference; individual performance may be better or may be worse. If individual performance is far better or far worse, the possibility of a measurement error must be explored.

The lines between the ranges are markers along a continuous scale; they do NOT indicate stepwise increases in ability. They may be compared to mileposts along a road. They are useful as reference points, but the landscape does not change suddenly when we pass a milepost.

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TABLE 2 The Modified Snellen Formula: 1/V = M x D

Viewing Distance (glasses to text , not val id for magnif iers)

5 cm 6.3 cm 8 cm 10 cm 12.5 16 cm 20 cm 25 cm 32 cm 40 cm 1 m 2” 2.5” 3.2” 4” 5” 6.3” 8” 10” 12.5” 16” 40”

Letter Size

20 D 16 D 12.5D 10 D 8 D 6.3 D 5 D 4 D 3.2 D 2.5 D

1 D

0.32 M 1/0.8 20/16

0.4 M 1/1 20/20

0.5 M 1/1.25 20/25

Rang

es in

ICD

-9-C

M

0.63M 1/12.5 20/250

1/10 20/200

1/8 20/160

1/6.3 20/125

1/5 20/100

1/4 20/80

1/3.2 20/63

1/2.5 20/50

1/2 20/40

1/1.6 20/32

1/0.63 20/12

0.8 M 1/16 20/320

1/12.5 20/250

1/10 20/200

1/8 20/160

1/6.3 20/125

1/5 20/100

1/4 20/80

1/3.2 20/63

1/2.5 20/50

1/2 20/40

1/0.8 20/16

1 M 1/20 20/400

1/16 20/320

1/12.5 20/250

1/10 20/200

1/8 20/160

1/6.3 20/125

1/5 20/100

1/4 20/80

1/3.2 20/63

1/2.5 20/50

1/1 20/20

1.25M 1/25 20/500

1/20 20/400

1/16 20/320

1/12.5 20/250

1/10 20/200

1/8 20/160

1/6.3 20/125

1/5 20/100

1/4 20/80

1/3.2 20/63

1/1.25 20/25

Norm

al ra

nge

1.6 M 1/32 20/630

1/25 20/500

1/20 20/400

1/16 20/320

1/12.5 20/250

1/10 20/200

1/8 20/160

1/6.3 20/125

1/5 20/100

1/4 20/80

1/1.6 20/32

2 M 1/40 20/800

1/32 20/630

1/25 20/500

1/20 20/400

1/16 20/320

1/12.5 20/250

1/10 20/200

1/8 20/160

1/6.3 20/125

1/5 20/100

1/2 20/40

2.5 M 1/50 20/1000

1/40 20/800

1/32 20/630

1/25 20/500

1/20 20/400

1/16 20/320

1/12.5 20/250

1/10 20/200

1/8 20/160

1/6.3 20/125

1/2.5 20/50

3.2 M 1/63 20/1250

1/50 20/1000

1/40 20/800

1/32 20/630

1/25 20/500

1/20 20/400

1/16 20/320

1/12.5 20/250

1/10 20/200

1/8 20/160

1/3.2 20/63

Near

-nor

mal

4 M 1/80 20/1600

1/63 20/1250

1/50 20/1000

1/40 20/800

1/32 20/630

1/25 20/500

1/20 20/400

1/16 20/320

1/12.5 20/250

1/10 20/200

1/4 20/80

5 M 1/100 20/2000

1/80 20/1600

1/63 20/1250

1/50 20/1000

1/40 20/800

1/32 20/630

1/25 20/500

1/20 20/400

1/16 20/320

1/12.5 20/250

1/5 20/100

6.3 M 1/125 20/2500

1/100 20/2000

1/80 20/1600

1/63 20/1250

1/50 20/1000

1/40 20/800

1/32 20/630

1/25 20/500

1/20 20/400

1/16 20/320

1/6.3 20/125

8 M 1/160 20/3200

1/125 20/2500

1/100 20/2000

1/80 20/1600

1/63 20/1250

1/50 20/1000

1/40 20/800

1/32 20/630

1/25 20/500

1/20 20/400

1/8 20/160 Mo

dera

te Lo

w Vi

sion

10 M 1/200 20/4000

1/160 20/3200

1/125 20/2500

1/100 20/2000

1/80 20/1600

1/63 20/1250

1/50 20/1000

1/40 20/800

1/32 20/630

1/25 20/500

1/10 20/200

Near-total Visual Acuity Loss Profound Low Vision Severe Low Visio

n

Columns indicate reading distances, indicated in cm, inches and diopters. Rows indicate letter sizes in M-units. The resulting reading acuity values are found at the intersections. The bold number indicates 1/V = M x D, below is the 20/… equivalent. Note that the visual acuity values are arranged in diagonal bands. Many different combinations of viewing distance and letter size can represent the same visual acuity value. Using visual acuity values to indicate a letter size therefore is meaningless. For each diagonal band, the outer edge of the table indicates the ranges of vision loss in ICD-9-CM.

The heavily outlined column indicates values that can be measured with the small card at 40 cm; all other values can be measured with the large card. The right-most column indicates values for the 1-m chart.

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TABLE 3 Magnification Need (1/V) for Letter Chart Acuity and Reading Acuity

80 20/1600

60 20/1200

50 1 > 20/1000

40 1 3 20/800

30 1 2 2 1 20/600

25 1 1 2 1 20/500

20 2 1 2 3 4 2 1 > 20/400

15 1 2 2 1 1 > 20/300

12 4 3 1 1 1 20/250

10 1 2 2 4 3 1 2 20/200

8 1 2 2 20/150

6 1 2 1 2 2 2 2 20/120

5 1 3 6 1 1 1 20/100

4 2 4 6 1 1 20/80

3 2 4 3 1 1 1 20/60

2.5 2 6 2 1 1 20/50

2 1 2 6 1 20/40

1.5 1 1 20/30

1.2 1 1 1 20/25

Mag

nific

atio

n N

eed

– LE

TTER

CH

AR

T (

1 /

V )

1 20/20

1 1.2 1.5 2 2.5 3 4 5 6 8 10 12 15 20 25 30 40 50 60 80 Magnification Need – CONTINUOUS TEXT ( M x D )

For 150 consecutive patients in the author’s Low Vision service, the diagram plots the measured letter chart acuity (1-m chart, expressed as magnification need = 1/V) against the functional reading acuity (as chosen by the patient, expressed as M x D = 1/V). If both were identical, all cases should fall along the diagonal – most do.

A number of patients require some extra magnification for reading (spread to the right of the diagonal). This is the magnification reserve, needed for reading fluency.

A small number requires much more magnification. When these cases were analyzed, they all involved patients with special conditions. Most used a small island for letter acuity and a larger, more eccentric area for reading.

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The COLENBRANDER Low Vision Measurement System · moderate, severe, and profound, between normal...

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