- Email: [email protected]
Reading acuity—an important parameter of reading performance
International Congress Series 1282 (2005) 487 – 491
www.ics-elsevier.com
Reading acuity—an important parameter of reading performance August Colenbrander* Smith-Kettlewell Eye Research Institute, San Francisco, CA, USA
Abstract. Visual acuity is the most widely used visual parameter. Reading is a widely expressed goal of patients with vision loss. Yet, reading acuity is often not properly understood and not properly calculated. Reading distances are often estimated and the letter size indicators used do not relate to those used on letter charts. Visual acuity is the reciprocal of the need for angular magnification. Subjects who need letters that are twice as close or twice as large are said to have a visual acuity of 1/2 (0.5, 6/12, 20/40); visual acuity for those that need 5 magnification is said to be 1/5 (0.2, 6/30, 20/ 100), etc. Snellen’s formula: V = distance / letter size is awkward for reading vision, since the numerator becomes a fraction-within-a-fraction. When the formula is inverted to calculate 1 / V (the magnification need), and when 1 / distance is replaced by bdistance in diopters,Q which indicates the required reading add, the result is easier to use. The Modified Snellen Formula becomes: 1 / V = letter size (in M-units) distance (in diopters). This modified formula reveals relationships that are otherwise hidden. This includes the difference between functional reading acuity (magnification required for prolonged reading) and threshold reading acuity. D 2005 Elsevier B.V. All rights reserved. Keywords: Visual functions; Functional vision; Visual acuity; Reading; Rehabilitation
1. Visual acuity Visual acuity is, by far, the most commonly used vision test. It is the most prevalent yardstick for success or failure in clinical trials. Yet, its functional implications and interpretation are not always properly understood [1]. It is often regarded as indicating the overall quality of vision, even though it measures only foveal function, which represents less than 1% of the retinal area. The b20/20Q reference standard is widely interpreted as bperfectQ vision, although Snellen deliberately placed it at the lower limit of normal. For diagnostic purposes, it is important to measure each eye separately and with the best correction. Functional vision, however, is binocular vision. In a population survey * 664 Atherton Avenue, Novato, CA 94945, USA. Tel.: +1 415 209 9529. E-mail address: [email protected]. URL: http://www.ski.org/Colenbrander. 0531-5131/ D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ics.2005.05.003
488
A. Colenbrander / International Congress Series 1282 (2005) 487–491
aimed at finding eye diseases, it is advantageous to measure the best-corrected visual acuity for each eye separately. For a survey aimed at the consequences of vision loss, binocular acuity with the actual correction worn is more relevant. Accordingly, one of the recommendations of a recent meeting at the WHO was to base future health surveys on binocular, presenting acuity [2]. A distinction is commonly made between distance and near acuity. Comparing distance and near acuity can give us information about the accommodative amplitude. For functional vision, the distinction between letter chart and reading acuity is more relevant. It may be said that letter-chart acuity is the most common visual function test, while reading is the most common functional vision test [3]. Letter-chart acuity is commonly measured at a distance, to relax accommodation for refractive measurement. However, most activities of daily living, particularly reading, are performed at closer distances. For a survey aimed at the consequences of vision loss, a tabulation of distance acuity alone would include many un(der)corrected myopes, who do not necessarily experience ADL difficulties. Although this is not customary, it may be argued that if a single number is to be used, tabulating the 1-m acuity might give a better picture of potential functional vision problems. Visual acuity is an excellent screening test since it is easy to administer and few significant disorders leave visual acuity unaffected. In this context, the difference between 20/20 (1.0) and 20/40 (0.5) is important, while the difference between 20/100, 20/200, and 20/400 (0.2, 0.1, and 0.05) does not provide much help in the differential diagnosis. This explains why most clinicians feel little need to specify poor vision beyond vague statements such as bcount fingersQ or bhand motions.Q Many also ignore measurements that are better than 20/20. The situation is very different for low vision rehabilitation, where visual acuity predicts the magnification need. A person with 20/100 needs 5 magnification, 20/200 needs 10 magnification; and 20/400 needs 20 magnification. Vague descriptions, such as count fingers, are meaningless. Visual acuity is expressed as a fraction. Many clinicians are not aware that this fraction has a distinct mathematical meaning: 20/40 is interpreted as bdriving visionQ and 20/200 as beligibility for benefits,Q rather than as statements about the magnification need of that individual. Most practitioners are not comfortable converting measurements from distance to near vision and for different near-viewing distances. This paper will address this issue. 2. Visual acuity calculations Snellen defined standard acuity (1.0, 20/20) as the ability to recognize a standard letter (1 M-unit) at a standard distance (1 m), or, more generally, a letter of n M-units at n meters (e.g., 6 M at 6 m = 6/6). For a subject who needs letters that are 2 larger or 2 closer, visual acuity is said to be 1/2 of standard acuity (0.5, 20/40). If 5 magnification is required, visual acuity is said to be 1/5 (0.2, 20/100), etc. The increase in angular letter size (MAR) indicates the visual threshold, while visual acuity is the reciprocal of that magnification threshold. MAR and logMAR (the logarithm of MAR) are measures of vision loss, not of visual acuity, since higher values indicate worse acuity. When dealing with underlying conditions and physiological optics, MAR is interpreted as bminimal angle of resolutionQ [4]. When talking about the resulting reading ability, we
A. Colenbrander / International Congress Series 1282 (2005) 487–491
489
think in terms of print size and reading distance. In that context, the same acronym MAR is more meaningfully understood as indicating the magnification requirement. Recognizing that visual acuity is simply the reciprocal of the magnification threshold avoids the common misconception that visual acuity would measure the overall quality of vision or that it would denote a percentage of visual ability. Applying calculations to near-vision measurements is complicated by the fact that the letter size numbers used often have no exact numerical meaning. Jaeger numbers (used in the United States) refer to catalogue numbers in Vienna in 1856. Subsequent printers used print from different catalogues, so the use of bJaeger numbersQ is rather inconsistent. bPrinter’s pointsQ refer to line height, rather than letter height, and may vary for different typestyles (e.g., 8.5 pt Arial = 10 pt Times Roman). Furthermore, they do not allow a comparison between lower case letters in text samples and upper case letters on charts. Snellen defined the reference standard as a letter subtending 5 min of arc. Louise Sloan coined the name bM-unitQ for this standard [4]. M-units are consistent with the metric system and are the only units that apply to both reading samples and letter charts. The numerical implications of Snellen’s formula are thus easiest to understand when using M-units for letter size and meters for viewing distance. The usual form of the Snellen fraction is: V = m / M (where V = visual acuity, m = viewing distance in meters, and M = letter size in M-units; 1 M-unit subtends 1 min of arc at 1 m, approximately 1.5 mm or 1/16 in.). This form is useful for distance tests, where m is a whole number, but is awkward for near vision, where the numerator becomes a fraction-within-a-fraction. Inverting the formula [5] and substituting D = diopters for the reciprocal of the metric viewing distance changes the fraction to a multiplication: MAR ¼ 1=V ¼ M =m ¼ M 1=m ¼ M D
or 1=V ¼ M D:
where V = visual acuity, M = letter size in M-units, and D = viewing distance in diopters. The D value can be measured directly with an appropriate ruler and indicates the required accommodation or reading add. Not only is this Modified Snellen formula easier to use, since it is a multiplication rather than a fraction, but measuring the reading distance in diopters is clinically more meaningful than expressing it in centimeters or inches, since D refers directly to the reading add in presbyopes and/or the required accommodation in younger persons. Table 1 applies the modified formula to various letter sizes and viewing distances. Note that the resulting acuity values appear in diagonal bands. This means that the same acuity value can result from different combinations of letter size and viewing distance; e.g., a 4M letter seen at 3D (30 cm, 12 in.) yields a magnification (relative to 1M at 1m) of 4 3 = 12 and a visual acuity of 1/12 (20/250, 0.08). At half the distance (double the reading add), the patient should be able to read print of half the size (2M at 6D). The same acuity value will also result from 1M print (newsprint) at 12D. This patient will thus need at least a 12D lens to read newsprint, a statement also known as bKestenbaum’s rule,Q which states that the required minimum add is the reciprocal of the visual acuity value. Note that this rule does not describe a curious side effect of Snellen’s formula, but is a direct effect of its definition. The statement bat leastQ 12D points to another important difference between visual function and functional vision measurements. Visual function measurements provide
490
A. Colenbrander / International Congress Series 1282 (2005) 487–491
Table 1 Print size and viewing distance
40 cm 30 cm 25 cm 20 cm 16 cm 12 cm 10 cm 8 cm 16" 12" 10" 8" 6" 5" 4" 3" 2.5 D 3 D 4D 5D 6D 8 D 10 D 12 D 5 M 1/12 1/16 1/20 1/25 1/30 1/40 1/50 1/60 4M 3M 2.5 M 2M 1.6 M 1.25 M 1M 0.8 M
6 cm 2.5" 16 D 1/80
5 cm 2" 20 D 1/100
20/1000 20/1200 20/1600
20/2000
20/250
20/300
20/400
20/500
20/600
20/800
1/10
1/12
1/16
1/20
1/25
1/30
1/40
20/200
20/250
20/300
20/400
20/500
20/600
20/800
1/50
1/60
1/80
20/1000 20/1200
20/1600
1/8
1/10
1/12
1/16
1/20
1/25
1/30
1/40
1/50
1/60
20/160
20/200
20/250
20/300
20/400
20/500
20/600
20/800
20/1000
20/1200
1/6
1/8
1/10
1/12
1/16
1/20
1/25
1/30
1/40
1/50
20/120
20/160
20/200
20/250
20/300
20/400
20/500
20/600
20/800
20/1000
1/5
1/6
1/8
1/10
1/12
1/16
1/20
1/25
1/30
1/40
20/100
20/120
20/160
20/200
20/250
20/300
20/400
20/500
20/600
20/800
1/4
1/5
1/6
1/8
1/10
1/12
1/16
1/20
1/25
1/30
20/80
20/100
20/120
20/160
20/200
20/250
20/300
20/400
20/500
20/600
1/3
1/4
1/5
1/6
1/8
1/10
1/12
1/16
1/20
1/25
20/60
20/80
20/100
20/120
20/160
20/200
20/250
20/300
20/400
20/500
1/2.5
1/3
1/4
1/5
1/6
1/8
1/10
1/12
1/16
1/20
20/50
20/60
20/80
20/100
20/120
20/160
20/200
20/250
20/300
20/400
1/2
1/2.5
1/3
1/4
1/5
1/6
1/8
1/10
1/12
1/16
20/40
20/50
20/60
20/80
20/100
20/120
20/160
20/200
20/250
20/300
threshold values because threshold measurements are most reproducible. ADL performance, however, is rarely done at the performance threshold (except for Olympic athletes). We do not like to read a paper with marginal print size and marginal contrast under marginal illumination. We prefer to perform at a comfortable suprathreshold level. The difference is known as the magnification reserve [6]. Legge has defined the functional reading level as the point where reading speed starts to drop off with smaller print size. Using a single standardized reading distance, Legge calls this the bcritical print sizeQ [7]. Speaking of functional reading acuity and threshold reading acuity provides clearer terminology and allows us to use different reading distances as often needed in low vision. The difference between functional and threshold acuity is the functional magnification reserve, schematically represented in Scheme 1. There are similar reserves for contrast and illumination. Functional reading acuity Reading speed
Threshold reading acuity
Critical print size Decreasing print size Magnification reserve Scheme 1. Threshold acuity and functional acuity for reading.
A. Colenbrander / International Congress Series 1282 (2005) 487–491
491
Magnification Need for LETTER CHART (1/V)
Table 2 Letter chart vs. reading acuity
30 25 20 15 12 10 8 6 5 4 3
2
2.5 2 1
2 2
2
1 1 1 2 4
2 6 6 3
1 1 1 1
2
1
6
6 1
2 2. 5 3
4
5
6
3 4
2 1 2 1 1
2 2 2 1 1 1
1 1
2 2
4 4 2 2
3 3
1 1 3 1 1
1 4 1
2 2 2 2
2 1
1
1
1 2
2
20/600 20/500 1 > 20/400 1 > 20/300 20/250 20/200 20/150 20/120 20/100 20/80 20/60
1
20/50 20/40
8 10 12 15 20 25 30 40 50 60 80
Magnification Need for CONTINUOUS TEXT (M × D)
To test these assumptions on actual patient data, Table 2 plots the magnification need found with letter-chart testing against the magnification values patients preferred for actual reading. It is based, retrospectively, on a sample of 150 consecutive patients from the author’s low vision service. If there were perfect consistency between reading acuity and letter-chart acuity, all values would fall in the gray band—most do. There is also some spread to the right (i.e., towards more magnification need for reading). This reflects the bmagnification reserveQ needed for reading fluency. The amount of this magnification reserve is not the same for all patients. A few highlighted cases stray further from the diagonal band. Such cases should not be discarded as measurement errors, but should be explored. In each case, we found special conditions that explain the unusual behavior; usually, we found that a small island was used for letter recognition and a larger, more eccentric retinal area for reading. If we had not known how to calculate reading acuity, we would not have noticed these cases. References [1] A. Colenbrander, Measuring vision and vision loss, in: Jaeger Tasman (Ed.), Vol. 5, Chap. 51 in Duane’s Clinical Ophthalmology, Tasman, 2002 edition, Lippincot, Williams and Wilkins, 2002, See: www.ski.org/ Colenbrander/. [2] Consultation on Development of Standards for Characterization of Vision Loss and Visual Functioning, WHO, Geneva, 2003, See: www.who.int/pbd/blindness/operational_research/en/. [3] A. Colenbrander, Visual functions and functional vision—these conference proceedings. [4] L.L. Sloan, New test charts for the measurement of visual acuity at far and near distances, American Journal of Ophthalmology 48 (1959) 807 – 813. [5] A. Colenbrander, D.C. Fletcher, Visual acuity measurements in low vision patients, Journal of Vision Rehabilitation 4 (1) (1990) 1 – 9. [6] S.G. Whittaker, J. Lovie-Kitchin, Visual requirements for reading, Optometry and Vision Science 70 (1) (1993) 54 – 65. [7] G.E. Legge, et al. MNREAD Acuity Charts, manual. Lighthouse Low Vision Products, 1994.
www.ics-elsevier.com
Reading acuity—an important parameter of reading performance August Colenbrander* Smith-Kettlewell Eye Research Institute, San Francisco, CA, USA
Abstract. Visual acuity is the most widely used visual parameter. Reading is a widely expressed goal of patients with vision loss. Yet, reading acuity is often not properly understood and not properly calculated. Reading distances are often estimated and the letter size indicators used do not relate to those used on letter charts. Visual acuity is the reciprocal of the need for angular magnification. Subjects who need letters that are twice as close or twice as large are said to have a visual acuity of 1/2 (0.5, 6/12, 20/40); visual acuity for those that need 5 magnification is said to be 1/5 (0.2, 6/30, 20/ 100), etc. Snellen’s formula: V = distance / letter size is awkward for reading vision, since the numerator becomes a fraction-within-a-fraction. When the formula is inverted to calculate 1 / V (the magnification need), and when 1 / distance is replaced by bdistance in diopters,Q which indicates the required reading add, the result is easier to use. The Modified Snellen Formula becomes: 1 / V = letter size (in M-units) distance (in diopters). This modified formula reveals relationships that are otherwise hidden. This includes the difference between functional reading acuity (magnification required for prolonged reading) and threshold reading acuity. D 2005 Elsevier B.V. All rights reserved. Keywords: Visual functions; Functional vision; Visual acuity; Reading; Rehabilitation
1. Visual acuity Visual acuity is, by far, the most commonly used vision test. It is the most prevalent yardstick for success or failure in clinical trials. Yet, its functional implications and interpretation are not always properly understood [1]. It is often regarded as indicating the overall quality of vision, even though it measures only foveal function, which represents less than 1% of the retinal area. The b20/20Q reference standard is widely interpreted as bperfectQ vision, although Snellen deliberately placed it at the lower limit of normal. For diagnostic purposes, it is important to measure each eye separately and with the best correction. Functional vision, however, is binocular vision. In a population survey * 664 Atherton Avenue, Novato, CA 94945, USA. Tel.: +1 415 209 9529. E-mail address: [email protected]. URL: http://www.ski.org/Colenbrander. 0531-5131/ D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ics.2005.05.003
488
A. Colenbrander / International Congress Series 1282 (2005) 487–491
aimed at finding eye diseases, it is advantageous to measure the best-corrected visual acuity for each eye separately. For a survey aimed at the consequences of vision loss, binocular acuity with the actual correction worn is more relevant. Accordingly, one of the recommendations of a recent meeting at the WHO was to base future health surveys on binocular, presenting acuity [2]. A distinction is commonly made between distance and near acuity. Comparing distance and near acuity can give us information about the accommodative amplitude. For functional vision, the distinction between letter chart and reading acuity is more relevant. It may be said that letter-chart acuity is the most common visual function test, while reading is the most common functional vision test [3]. Letter-chart acuity is commonly measured at a distance, to relax accommodation for refractive measurement. However, most activities of daily living, particularly reading, are performed at closer distances. For a survey aimed at the consequences of vision loss, a tabulation of distance acuity alone would include many un(der)corrected myopes, who do not necessarily experience ADL difficulties. Although this is not customary, it may be argued that if a single number is to be used, tabulating the 1-m acuity might give a better picture of potential functional vision problems. Visual acuity is an excellent screening test since it is easy to administer and few significant disorders leave visual acuity unaffected. In this context, the difference between 20/20 (1.0) and 20/40 (0.5) is important, while the difference between 20/100, 20/200, and 20/400 (0.2, 0.1, and 0.05) does not provide much help in the differential diagnosis. This explains why most clinicians feel little need to specify poor vision beyond vague statements such as bcount fingersQ or bhand motions.Q Many also ignore measurements that are better than 20/20. The situation is very different for low vision rehabilitation, where visual acuity predicts the magnification need. A person with 20/100 needs 5 magnification, 20/200 needs 10 magnification; and 20/400 needs 20 magnification. Vague descriptions, such as count fingers, are meaningless. Visual acuity is expressed as a fraction. Many clinicians are not aware that this fraction has a distinct mathematical meaning: 20/40 is interpreted as bdriving visionQ and 20/200 as beligibility for benefits,Q rather than as statements about the magnification need of that individual. Most practitioners are not comfortable converting measurements from distance to near vision and for different near-viewing distances. This paper will address this issue. 2. Visual acuity calculations Snellen defined standard acuity (1.0, 20/20) as the ability to recognize a standard letter (1 M-unit) at a standard distance (1 m), or, more generally, a letter of n M-units at n meters (e.g., 6 M at 6 m = 6/6). For a subject who needs letters that are 2 larger or 2 closer, visual acuity is said to be 1/2 of standard acuity (0.5, 20/40). If 5 magnification is required, visual acuity is said to be 1/5 (0.2, 20/100), etc. The increase in angular letter size (MAR) indicates the visual threshold, while visual acuity is the reciprocal of that magnification threshold. MAR and logMAR (the logarithm of MAR) are measures of vision loss, not of visual acuity, since higher values indicate worse acuity. When dealing with underlying conditions and physiological optics, MAR is interpreted as bminimal angle of resolutionQ [4]. When talking about the resulting reading ability, we
A. Colenbrander / International Congress Series 1282 (2005) 487–491
489
think in terms of print size and reading distance. In that context, the same acronym MAR is more meaningfully understood as indicating the magnification requirement. Recognizing that visual acuity is simply the reciprocal of the magnification threshold avoids the common misconception that visual acuity would measure the overall quality of vision or that it would denote a percentage of visual ability. Applying calculations to near-vision measurements is complicated by the fact that the letter size numbers used often have no exact numerical meaning. Jaeger numbers (used in the United States) refer to catalogue numbers in Vienna in 1856. Subsequent printers used print from different catalogues, so the use of bJaeger numbersQ is rather inconsistent. bPrinter’s pointsQ refer to line height, rather than letter height, and may vary for different typestyles (e.g., 8.5 pt Arial = 10 pt Times Roman). Furthermore, they do not allow a comparison between lower case letters in text samples and upper case letters on charts. Snellen defined the reference standard as a letter subtending 5 min of arc. Louise Sloan coined the name bM-unitQ for this standard [4]. M-units are consistent with the metric system and are the only units that apply to both reading samples and letter charts. The numerical implications of Snellen’s formula are thus easiest to understand when using M-units for letter size and meters for viewing distance. The usual form of the Snellen fraction is: V = m / M (where V = visual acuity, m = viewing distance in meters, and M = letter size in M-units; 1 M-unit subtends 1 min of arc at 1 m, approximately 1.5 mm or 1/16 in.). This form is useful for distance tests, where m is a whole number, but is awkward for near vision, where the numerator becomes a fraction-within-a-fraction. Inverting the formula [5] and substituting D = diopters for the reciprocal of the metric viewing distance changes the fraction to a multiplication: MAR ¼ 1=V ¼ M =m ¼ M 1=m ¼ M D
or 1=V ¼ M D:
where V = visual acuity, M = letter size in M-units, and D = viewing distance in diopters. The D value can be measured directly with an appropriate ruler and indicates the required accommodation or reading add. Not only is this Modified Snellen formula easier to use, since it is a multiplication rather than a fraction, but measuring the reading distance in diopters is clinically more meaningful than expressing it in centimeters or inches, since D refers directly to the reading add in presbyopes and/or the required accommodation in younger persons. Table 1 applies the modified formula to various letter sizes and viewing distances. Note that the resulting acuity values appear in diagonal bands. This means that the same acuity value can result from different combinations of letter size and viewing distance; e.g., a 4M letter seen at 3D (30 cm, 12 in.) yields a magnification (relative to 1M at 1m) of 4 3 = 12 and a visual acuity of 1/12 (20/250, 0.08). At half the distance (double the reading add), the patient should be able to read print of half the size (2M at 6D). The same acuity value will also result from 1M print (newsprint) at 12D. This patient will thus need at least a 12D lens to read newsprint, a statement also known as bKestenbaum’s rule,Q which states that the required minimum add is the reciprocal of the visual acuity value. Note that this rule does not describe a curious side effect of Snellen’s formula, but is a direct effect of its definition. The statement bat leastQ 12D points to another important difference between visual function and functional vision measurements. Visual function measurements provide
490
A. Colenbrander / International Congress Series 1282 (2005) 487–491
Table 1 Print size and viewing distance
40 cm 30 cm 25 cm 20 cm 16 cm 12 cm 10 cm 8 cm 16" 12" 10" 8" 6" 5" 4" 3" 2.5 D 3 D 4D 5D 6D 8 D 10 D 12 D 5 M 1/12 1/16 1/20 1/25 1/30 1/40 1/50 1/60 4M 3M 2.5 M 2M 1.6 M 1.25 M 1M 0.8 M
6 cm 2.5" 16 D 1/80
5 cm 2" 20 D 1/100
20/1000 20/1200 20/1600
20/2000
20/250
20/300
20/400
20/500
20/600
20/800
1/10
1/12
1/16
1/20
1/25
1/30
1/40
20/200
20/250
20/300
20/400
20/500
20/600
20/800
1/50
1/60
1/80
20/1000 20/1200
20/1600
1/8
1/10
1/12
1/16
1/20
1/25
1/30
1/40
1/50
1/60
20/160
20/200
20/250
20/300
20/400
20/500
20/600
20/800
20/1000
20/1200
1/6
1/8
1/10
1/12
1/16
1/20
1/25
1/30
1/40
1/50
20/120
20/160
20/200
20/250
20/300
20/400
20/500
20/600
20/800
20/1000
1/5
1/6
1/8
1/10
1/12
1/16
1/20
1/25
1/30
1/40
20/100
20/120
20/160
20/200
20/250
20/300
20/400
20/500
20/600
20/800
1/4
1/5
1/6
1/8
1/10
1/12
1/16
1/20
1/25
1/30
20/80
20/100
20/120
20/160
20/200
20/250
20/300
20/400
20/500
20/600
1/3
1/4
1/5
1/6
1/8
1/10
1/12
1/16
1/20
1/25
20/60
20/80
20/100
20/120
20/160
20/200
20/250
20/300
20/400
20/500
1/2.5
1/3
1/4
1/5
1/6
1/8
1/10
1/12
1/16
1/20
20/50
20/60
20/80
20/100
20/120
20/160
20/200
20/250
20/300
20/400
1/2
1/2.5
1/3
1/4
1/5
1/6
1/8
1/10
1/12
1/16
20/40
20/50
20/60
20/80
20/100
20/120
20/160
20/200
20/250
20/300
threshold values because threshold measurements are most reproducible. ADL performance, however, is rarely done at the performance threshold (except for Olympic athletes). We do not like to read a paper with marginal print size and marginal contrast under marginal illumination. We prefer to perform at a comfortable suprathreshold level. The difference is known as the magnification reserve [6]. Legge has defined the functional reading level as the point where reading speed starts to drop off with smaller print size. Using a single standardized reading distance, Legge calls this the bcritical print sizeQ [7]. Speaking of functional reading acuity and threshold reading acuity provides clearer terminology and allows us to use different reading distances as often needed in low vision. The difference between functional and threshold acuity is the functional magnification reserve, schematically represented in Scheme 1. There are similar reserves for contrast and illumination. Functional reading acuity Reading speed
Threshold reading acuity
Critical print size Decreasing print size Magnification reserve Scheme 1. Threshold acuity and functional acuity for reading.
A. Colenbrander / International Congress Series 1282 (2005) 487–491
491
Magnification Need for LETTER CHART (1/V)
Table 2 Letter chart vs. reading acuity
30 25 20 15 12 10 8 6 5 4 3
2
2.5 2 1
2 2
2
1 1 1 2 4
2 6 6 3
1 1 1 1
2
1
6
6 1
2 2. 5 3
4
5
6
3 4
2 1 2 1 1
2 2 2 1 1 1
1 1
2 2
4 4 2 2
3 3
1 1 3 1 1
1 4 1
2 2 2 2
2 1
1
1
1 2
2
20/600 20/500 1 > 20/400 1 > 20/300 20/250 20/200 20/150 20/120 20/100 20/80 20/60
1
20/50 20/40
8 10 12 15 20 25 30 40 50 60 80
Magnification Need for CONTINUOUS TEXT (M × D)
To test these assumptions on actual patient data, Table 2 plots the magnification need found with letter-chart testing against the magnification values patients preferred for actual reading. It is based, retrospectively, on a sample of 150 consecutive patients from the author’s low vision service. If there were perfect consistency between reading acuity and letter-chart acuity, all values would fall in the gray band—most do. There is also some spread to the right (i.e., towards more magnification need for reading). This reflects the bmagnification reserveQ needed for reading fluency. The amount of this magnification reserve is not the same for all patients. A few highlighted cases stray further from the diagonal band. Such cases should not be discarded as measurement errors, but should be explored. In each case, we found special conditions that explain the unusual behavior; usually, we found that a small island was used for letter recognition and a larger, more eccentric retinal area for reading. If we had not known how to calculate reading acuity, we would not have noticed these cases. References [1] A. Colenbrander, Measuring vision and vision loss, in: Jaeger Tasman (Ed.), Vol. 5, Chap. 51 in Duane’s Clinical Ophthalmology, Tasman, 2002 edition, Lippincot, Williams and Wilkins, 2002, See: www.ski.org/ Colenbrander/. [2] Consultation on Development of Standards for Characterization of Vision Loss and Visual Functioning, WHO, Geneva, 2003, See: www.who.int/pbd/blindness/operational_research/en/. [3] A. Colenbrander, Visual functions and functional vision—these conference proceedings. [4] L.L. Sloan, New test charts for the measurement of visual acuity at far and near distances, American Journal of Ophthalmology 48 (1959) 807 – 813. [5] A. Colenbrander, D.C. Fletcher, Visual acuity measurements in low vision patients, Journal of Vision Rehabilitation 4 (1) (1990) 1 – 9. [6] S.G. Whittaker, J. Lovie-Kitchin, Visual requirements for reading, Optometry and Vision Science 70 (1) (1993) 54 – 65. [7] G.E. Legge, et al. MNREAD Acuity Charts, manual. Lighthouse Low Vision Products, 1994.