Vision and visual acuity are the traditional ways of measuring visual performance. Visual acuity measurement provides important information about the resolution ability for distance and intermediate distances.1 Obtaining an accurate visual acuity evaluation will establish a baseline from which to monitor ocular pathology and can be used as a measure of success in clinical intervention in all patients, particularly the partially sighted. Hopefully, the outgoing acuity is better that the incoming.2
Optimal visual acuity requires optimal performance of the whole visual system; this is from the cornea to the visual cortex. A decrease in visual acuity indicates a problem in this pathway. Hence, visual acuity measurement is sensitive to a wide variety of disorders yet is not specific to any single disorder.1
Letter charts commonly used for assessing visual acuity have various features that may influence the accuracy of the acuity result.
Variation of Letter Styles and Legibility
One of the main differences between letter styles used in test charts is the difference between serif and non-serif letters. Snellen originally used serif style (letter forms with short lines at the end of the main strokes of a character, as printed in the body type of this article). However, there has been a move to non-serif (sans serif) letters with bold typefaces which appear less cluttered as well as being generally easier to read as single characters.3 Figure 1 shows the two letter styles.
Of the many studies done, it is argued that all the letters used should be of similar legibility, the ideal being that the subject should be able to read all or none of the letters on any line. On the other hand, it has been argued that the test becomes more reliable if every line contains one of the more difficult letters such as C, G, F, P, H, and N. Under both arguments, the letters F, P, H and N were found to be of medium legibility.3
Sloan et al recommended the following letters in the 5x5 non-serif format: C, D, H, K, N, O, R, S, V and Z. These were reported to be of equal legibility and are comparable to the legibility of the Llandolt rings of the same size.
'Test charts for clinically determining distance visual acuity'3 in the 1968 version of the British Standard BS 4274 stipulated that the letters shall be of 5x4 non-serif design. The selection of letters is also limited to D, E, F, H, N, P, R, U, V and Z, which are all found to be of equal legibility. These are the letters Bailey-Lovie initially chose for the original Bailey-Lovie chart.5
Finally, in addition to or instead of letters, numbers or shapes can be used; these charts are for specific groups of patients who may be illiterate, or have communication difficulties.
Progression of sizes
There is a strong consensus of opinion in favour of a geometric progression of optotype sizes. A geometric progression is a mathematical series each number bears a constant ratio to the previous one.3
However, there are differing views on which ratio of progression to adopt. Two views in particular have found some distinguished advocates. The first states that there should be an exact doubling of the optotype every second line; this ratio would be the square root of 2 (1.414). However, this ratio is found not to produce a close approximation to the 60m line.
The second opinion,6 generally adopted now, is the cube root of 2 (1.260). Hence there is a doubling in size of optotypes every third line, no matter what line a person starts from. This fortunately produces a size close enough to the 60m line. The main objection to this is that the intervals are held to be a little too small for clinical use.3
The crowding phenomenon
The crowding phenomenon is the difficulty found in separating the letters on a line of type or of a test chart. It particularly affects patients with strabismic amblyopia and macular degeneration (low vision patients). The ends of letters of the line may be read but those in the centre appear jumbled and the order may be confused. The acuity may be higher if the letters are shown singly.7
The Snellen System
Snellen in 1862, introduced the concept of visual acuity as a ratio comparing the patient's ability to recognise fine detail to that of a standard eye. Snellen suggested that a series of letters of geometric size, such as the letter E in Figure 2, could just be seen by the average corrected eye if the thickness of the limbs and the spaces between them each subtend one minute of arc at the eye. The angular subtense therefore is 5' vertically, and 4-6' (depending on the particular letter of the alphabet and style type) at varying distances.3,8
Visual acuity fraction as described by Snellen is as follows:
VA = Testing distance in metres
Distance in metres at which the best line subtends at 5'
'Best line' is the line of print of such size that it subtends 5' for the distance specified for a person with normal vision.8
For a person with 'normal' sight, a letter of size 6.73mm (6 x tan 5') can just be discerned. Therefore the VA is designated 6/6. This is illustrated in Figure 3.
The Snellen system receives various amendments in different countries across the world, although the same letter sizes, angular subtenses and distances are used. The fraction obtained can be expressed in decimal notation, percentage notation and visual angle notation (Table 1).
However, the loss of testing distance in the various notations, other than the fraction, is a disadvantage from the clinical standpoint.1
The Snellen chart (Figure 4) is most frequently used to determine VA. However, it does carry various limitations, which will have great effect on the measured VA. The traditional Snellen chart no longer meets today's standards. The chart is unscientific and inaccurate with many flaws.1,3,8,10
Snellen's original chart was designed to be tested at 20 Paris feet, which equates to approximately 6.5m. The metric progression here is 6, 9, 12, 15, 21, 30 and 60m. This range was selected intuitively and is fairly close to a regular geometric progression. The progression of letter sizes is uneven, such that the larger letters are more widely spaced than the smaller letters. For example, there are no letters between 6/36 and 6/60, therefore a person whose VA is 6/48 has a VA automatically recorded as 6/60, which is the next largest size from 6/36.
This presents potentially serious situation in which the VA could decrease from 6/48 to 6/60, yet the practitioner measures 6/60 in both cases. This may be an important clue to missed active pathology. Hence, the acuity is underestimated with the Snellen chart. It is therefore seen that these 'gaps' lead to insufficient detail in the range commonly needed for low vision patients.11
There are different numbers of letters on each row such that rows with the smaller letters have more letters per row, whereas the larger optotype rows have one or two letters per row. Hence, the task is not equally difficult for all acuity sizes, affecting the accuracy of acuity measured. Having one or two letters for the larger acuity sizes leads to a measure of uncrowded acuity. Hence, the acuity is not affected by the presence of contours next to the letters. This can lead to an overestimation of acuity as objects/letters in the real world are rarely presented in isolation.11
There is a strong consensus in favour of geometric progression of letter size1 since in certain optometric assessment such as low vision assessment, testing distances are changed and consistent results are only achieved through the use of charts with geometric progression.1 The Snellen chart is non-linear and does not have geometric progression of letter sizes. The chart has irregular spacing between different letters on different lines; hence the letters are not equally legible. Here, the difference in VA from 6/60 to 6/36 is not the same as that of 6/6 to 6/5.
Some letters are more difficult to recognise than others, and awkward notation arises when some letters are missed on one line and others seen on the line below, leading to, for example, 6/9-3.
The Snellen chart commonly used in optometric practice will be a projector chart, where the chart is projected on a screen or a mirror creating a testing distance of 6m. The luminance level of this chart is generally fixed and not easily varied during examination.12 This can prove disadvantageous to groups of patients such as low vision patients.
With the Snellen chart, due to the need for 6m for accurate scoring methods, it is difficult to measure acuity at various distances because, in most cases, the patient blocks the projected image as he or she walks up to the screen.
Since the traditional Snellen chart does not meet today's standards, a modern chart should have equally legible symbols arranged in rows of decreasing size, that are in a geometric progression of sizes with an equal number of symbols per line. Acuity charts that fulfil these requirements are based on the logMAR system.
THE LogMAR SYSTEM
In 1976, two Australian optometrists, Bailey and Lovie, expressed visual acuity in terms of the logarithm of the angular limb width (in minutes of arc) of the smallest letters recognised at 6m.7 This notation is termed logMAR. Where the 'MAR' represents minimum angle of resolution. Thus, the 6m line with its limb subtense of 1 minute of arc is denoted by logMAR 0 and the 60m line of limb subtense at 10 minutes of arc by logMAR 1.
The logMAR scoring system for acuity can be complicated since logMAR values decrease with letter sizes hence logMAR values become negative. The simplest scoring method is to score 0.02 logMAR unit for every letter correctly identified, beginning with the logMAR 1 (6/60) line. Thus if every letter on the Bailey-Lovie chart were read correctly the score would be 1.40.
In contrast to the Snellen chart, logMAR charts have several advantages. The main design features of most 'logMAR' charts are as follows:
Letters are of 'almost equal legibility'
There are five letters on each of the rows
The letter spacing on each row is equal to one letter width (ie equal to four stroke widths)
The row spacing is equal to the height of the letters on the smaller row.
These features result in smaller spacing in the higher acuity levels, giving the charts the characteristic triangular configuration.
The progression of letter sizes follows a geometric progression whose ratio or multiplier is equal to 0.1 log units (or 1.26). Hence, letters on each line are 25 per cent larger than the preceding line.5
The chart is designed for standard testing distance of 4m. The largest letters have stroke widths subtending 10 minutes of arc, (1.0logMAR and 6/60 Snellen equivalent notation) and the smallest letters have stroke widths subtending 0.5 minutes of arc (-0.3logMAR and 6/3 Snellen equivalent notation) at 4m.
The rows of the chart are labelled on the left hand side with the equivalent standard Snellen notation (for convenience some of the Snellen fractions have to be rounded). On the right hand side of each row of letters, the VA ratings are labelled with the logarithm of the minimum angle of resolution (logMAR), which is the logarithm to the base 10 of the angular subtense (in minutes of arc) of the stroke widths at 4m. Thus 0.0logMAR is equivalent to 6/6 (ie one minute of arc).
Bailey-Lovie chart
The Bailey-Lovie chart (Figure 5) is used commonly in research, universities and general use. It is considered to be the gold standard for VA evaluation in low vision with its scientific accuracy.
The chart has a geometric progression of 1.2589, chosen by the Australian optometrists Bailey and Lovie in 1976.5 This ratio is virtually indistinguishable from the 3Ã2 progression ratio discussed earlier. The letters chosen for the chart are the 5Ã4 non-serif design and the letters chosen are selected from the 10 specified by the British Standard BS 4724.3
As the example of the Bailey-Lovie chart shows, there are five letters on every line, even for the biggest letters. Having five letters per line, which are of similar legibility, eliminates one of the inconsistencies associated with the Snellen chart where the top letter measures single letter acuity while the rest of the chart records linear acuity.2 The inner-letter spacing on each line is equal to the letter width, and the inter-row spacing is equal to the letter height of the lower row. This therefore equalises the possible effects of the crowding phenomenon.
The tasks on each line are therefore of equal difficulty, that is the same number of letters on each line with the same relative spacing and it has a constant relative change in optotype size between all lines.
The chart can be used without a mirror at distances between 1-6m, therefore, some people with severely impaired vision usually see some of the letters.2
Having a logarithmic progression and proportional spacing of optotypes allows consistent and accurate evaluation of visual acuity levels.12 Hence, there is better interpretation of the significance of noted visual acuity changes. This is because a three-line change in the higher acuity ranges would represent the same degree of change as a three-line difference in the lower acuity ranges.
Figure 6 indicates the cumulative number of letters, which will be presented to a patient and read correctly, achieving a given level on a standard Snellen or Bailey-Lovie letter chart. The fixed geometric decrease (0.1 log unit, 0.8x) in letter size on adjacent rows reading down the chart for the Bailey-Lovie chart is shown.4
The current draft for revision of BS 4274 Part 1 entitled 'Specification for test charts for clinical determination of distance visual acuity' proposes the Bailey-Lovie chart size to be reduced from the 12m size. The letter selection also suggested is C, D, E, F, H, K, P, R, U, V and Z in a 5x5 format.3
Although the Snellen chart is widely used in optometric practice, it has many flaws that are overlooked Ð many features of the chart make visual acuity assessment unreliable. Visual acuity assessment is vital in rehabilitation of many patients, for example low vision patients, and such a chart is not satisfactory. Hence, the logMAR charts are now more commonly used, their accuracy in visual acuity assessment gives a much truer and more reliable reading of a patient's acuity. Therefore, subsequent rehabilitation for patients is kept to an optimum standard.
References
1 Donald C. Fletcher. Low Vision Rehabilitation; Caring for the Whole Person, American Academy Of Ophthalmology, 1999.
2 Functional Assessment in Low Vision Practice Part 2, optician, Feb 2001; 221:28-30.
3 Bennett and Rabbetts. Clinical Visual Optics. In: Butterworth Heinemann, third edition. (1998).
4 Catherine Dickinson. Low Vision Principles and Practice. In: Butterworth Heinemann, (1998).
5 Bailey and Lovie. New Design Principles for Visual Acuity Letter Charts. Am J Optom & Phys Sci, Nov 1976;53:11;740-745.
6 Green, J. On a new series of test letters for determining the acuities of vision. Trans Am Ophthal Soc, 1986; 1:3:68.
7 Dr David Thompson. Part II lecture notes, Visual Perception. Dec 2002; City University London.
8 Albert T. Dowie. Management and practice of low visual acuity. The Eastern Press Ltd (1988).
9 Gerald E. Fonda. Management of Low Vision. Thieme-Stratton Inc. (1981).
10 Rodney W. Nowakowski. Primary Low Vision Care. Appleton & Lang (1994).
11 Scott Mackie and Roisin Mackie. Low-Vision assessment in practice; Part 3 Ð Dispensing low vision aids. optician, March 9 2001; 5791:222:18-24.
12 Hartridge H and Owen HB. Test types. Br J Ophthl, 1922; 6:543-549.
Rabiah Narband is a pre-reg student at Specsavers, Tottenham Court Road, LondonVision and visual acuity are the traditional ways of measuring visual performance. Visual acuity measurement provides important information about the resolution ability for distance and intermediate distances.1 Obtaining an accurate visual acuity evaluation will establish a baseline from which to monitor ocular pathology and can be used as a measure of success in clinical intervention in all patients, particularly the partially sighted. Hopefully, the outgoing acuity is better that the incoming.2
Optimal visual acuity requires optimal performance of the whole visual system; this is from the cornea to the visual cortex. A decrease in visual acuity indicates a problem in this pathway. Hence, visual acuity measurement is sensitive to a wide variety of disorders yet is not specific to any single disorder.1
Letter charts commonly used for assessing visual acuity have various features that may influence the accuracy of the acuity result.
Variation of Letter Styles and Legibility
One of the main differences between letter styles used in test charts is the difference between serif and non-serif letters. Snellen originally used serif style (letter forms with short lines at the end of the main strokes of a character, as printed in the body type of this article). However, there has been a move to non-serif (sans serif) letters with bold typefaces which appear less cluttered as well as being generally easier to read as single characters.3 Figure 1 shows the two letter styles.
Of the many studies done, it is argued that all the letters used should be of similar legibility, the ideal being that the subject should be able to read all or none of the letters on any line. On the other hand, it has been argued that the test becomes more reliable if every line contains one of the more difficult letters such as C, G, F, P, H, and N. Under both arguments, the letters F, P, H and N were found to be of medium legibility.3
Sloan et al recommended the following letters in the 5x5 non-serif format: C, D, H, K, N, O, R, S, V and Z. These were reported to be of equal legibility and are comparable to the legibility of the Llandolt rings of the same size.
'Test charts for clinically determining distance visual acuity'3 in the 1968 version of the British Standard BS 4274 stipulated that the letters shall be of 5x4 non-serif design. The selection of letters is also limited to D, E, F, H, N, P, R, U, V and Z, which are all found to be of equal legibility. These are the letters Bailey-Lovie initially chose for the original Bailey-Lovie chart.5
Finally, in addition to or instead of letters, numbers or shapes can be used; these charts are for specific groups of patients who may be illiterate, or have communication difficulties.
Progression of sizes
There is a strong consensus of opinion in favour of a geometric progression of optotype sizes. A geometric progression is a mathematical series each number bears a constant ratio to the previous one.3
However, there are differing views on which ratio of progression to adopt. Two views in particular have found some distinguished advocates. The first states that there should be an exact doubling of the optotype every second line; this ratio would be the square root of 2 (1.414). However, this ratio is found not to produce a close approximation to the 60m line.
The second opinion,6 generally adopted now, is the cube root of 2 (1.260). Hence there is a doubling in size of optotypes every third line, no matter what line a person starts from. This fortunately produces a size close enough to the 60m line. The main objection to this is that the intervals are held to be a little too small for clinical use.3
The crowding phenomenon
The crowding phenomenon is the difficulty found in separating the letters on a line of type or of a test chart. It particularly affects patients with strabismic amblyopia and macular degeneration (low vision patients). The ends of letters of the line may be read but those in the centre appear jumbled and the order may be confused. The acuity may be higher if the letters are shown singly.7
The Snellen System
Snellen in 1862, introduced the concept of visual acuity as a ratio comparing the patient's ability to recognise fine detail to that of a standard eye. Snellen suggested that a series of letters of geometric size, such as the letter E in Figure 2, could just be seen by the average corrected eye if the thickness of the limbs and the spaces between them each subtend one minute of arc at the eye. The angular subtense therefore is 5' vertically, and 4-6' (depending on the particular letter of the alphabet and style type) at varying distances.3,8
Visual acuity fraction as described by Snellen is as follows:
VA = Testing distance in metres
Distance in metres at which the best line subtends at 5'
'Best line' is the line of print of such size that it subtends 5' for the distance specified for a person with normal vision.8
For a person with 'normal' sight, a letter of size 6.73mm (6 x tan 5') can just be discerned. Therefore the VA is designated 6/6. This is illustrated in Figure 3.
The Snellen system receives various amendments in different countries across the world, although the same letter sizes, angular subtenses and distances are used. The fraction obtained can be expressed in decimal notation, percentage notation and visual angle notation (Table 1).
However, the loss of testing distance in the various notations, other than the fraction, is a disadvantage from the clinical standpoint.1
The Snellen chart (Figure 4) is most frequently used to determine VA. However, it does carry various limitations, which will have great effect on the measured VA. The traditional Snellen chart no longer meets today's standards. The chart is unscientific and inaccurate with many flaws.1,3,8,10
Snellen's original chart was designed to be tested at 20 Paris feet, which equates to approximately 6.5m. The metric progression here is 6, 9, 12, 15, 21, 30 and 60m. This range was selected intuitively and is fairly close to a regular geometric progression. The progression of letter sizes is uneven, such that the larger letters are more widely spaced than the smaller letters. For example, there are no letters between 6/36 and 6/60, therefore a person whose VA is 6/48 has a VA automatically recorded as 6/60, which is the next largest size from 6/36.
This presents potentially serious situation in which the VA could decrease from 6/48 to 6/60, yet the practitioner measures 6/60 in both cases. This may be an important clue to missed active pathology. Hence, the acuity is underestimated with the Snellen chart. It is therefore seen that these 'gaps' lead to insufficient detail in the range commonly needed for low vision patients.11
There are different numbers of letters on each row such that rows with the smaller letters have more letters per row, whereas the larger optotype rows have one or two letters per row. Hence, the task is not equally difficult for all acuity sizes, affecting the accuracy of acuity measured. Having one or two letters for the larger acuity sizes leads to a measure of uncrowded acuity. Hence, the acuity is not affected by the presence of contours next to the letters. This can lead to an overestimation of acuity as objects/letters in the real world are rarely presented in isolation.11
There is a strong consensus in favour of geometric progression of letter size1 since in certain optometric assessment such as low vision assessment, testing distances are changed and consistent results are only achieved through the use of charts with geometric progression.1 The Snellen chart is non-linear and does not have geometric progression of letter sizes. The chart has irregular spacing between different letters on different lines; hence the letters are not equally legible. Here, the difference in VA from 6/60 to 6/36 is not the same as that of 6/6 to 6/5.
Some letters are more difficult to recognise than others, and awkward notation arises when some letters are missed on one line and others seen on the line below, leading to, for example, 6/9-3.
The Snellen chart commonly used in optometric practice will be a projector chart, where the chart is projected on a screen or a mirror creating a testing distance of 6m. The luminance level of this chart is generally fixed and not easily varied during examination.12 This can prove disadvantageous to groups of patients such as low vision patients.
With the Snellen chart, due to the need for 6m for accurate scoring methods, it is difficult to measure acuity at various distances because, in most cases, the patient blocks the projected image as he or she walks up to the screen.
Since the traditional Snellen chart does not meet today's standards, a modern chart should have equally legible symbols arranged in rows of decreasing size, that are in a geometric progression of sizes with an equal number of symbols per line. Acuity charts that fulfil these requirements are based on the logMAR system.
THE LogMAR SYSTEM
In 1976, two Australian optometrists, Bailey and Lovie, expressed visual acuity in terms of the logarithm of the angular limb width (in minutes of arc) of the smallest letters recognised at 6m.7 This notation is termed logMAR. Where the 'MAR' represents minimum angle of resolution. Thus, the 6m line with its limb subtense of 1 minute of arc is denoted by logMAR 0 and the 60m line of limb subtense at 10 minutes of arc by logMAR 1.
The logMAR scoring system for acuity can be complicated since logMAR values decrease with letter sizes hence logMAR values become negative. The simplest scoring method is to score 0.02 logMAR unit for every letter correctly identified, beginning with the logMAR 1 (6/60) line. Thus if every letter on the Bailey-Lovie chart were read correctly the score would be 1.40.
In contrast to the Snellen chart, logMAR charts have several advantages. The main design features of most 'logMAR' charts are as follows:
Letters are of 'almost equal legibility'
There are five letters on each of the rows
The letter spacing on each row is equal to one letter width (ie equal to four stroke widths)
The row spacing is equal to the height of the letters on the smaller row.
These features result in smaller spacing in the higher acuity levels, giving the charts the characteristic triangular configuration.
The progression of letter sizes follows a geometric progression whose ratio or multiplier is equal to 0.1 log units (or 1.26). Hence, letters on each line are 25 per cent larger than the preceding line.5
The chart is designed for standard testing distance of 4m. The largest letters have stroke widths subtending 10 minutes of arc, (1.0logMAR and 6/60 Snellen equivalent notation) and the smallest letters have stroke widths subtending 0.5 minutes of arc (-0.3logMAR and 6/3 Snellen equivalent notation) at 4m.
The rows of the chart are labelled on the left hand side with the equivalent standard Snellen notation (for convenience some of the Snellen fractions have to be rounded). On the right hand side of each row of letters, the VA ratings are labelled with the logarithm of the minimum angle of resolution (logMAR), which is the logarithm to the base 10 of the angular subtense (in minutes of arc) of the stroke widths at 4m. Thus 0.0logMAR is equivalent to 6/6 (ie one minute of arc).
Bailey-Lovie chart
The Bailey-Lovie chart (Figure 5) is used commonly in research, universities and general use. It is considered to be the gold standard for VA evaluation in low vision with its scientific accuracy.
The chart has a geometric progression of 1.2589, chosen by the Australian optometrists Bailey and Lovie in 1976.5 This ratio is virtually indistinguishable from the 3Ã2 progression ratio discussed earlier. The letters chosen for the chart are the 5Ã4 non-serif design and the letters chosen are selected from the 10 specified by the British Standard BS 4724.3
As the example of the Bailey-Lovie chart shows, there are five letters on every line, even for the biggest letters. Having five letters per line, which are of similar legibility, eliminates one of the inconsistencies associated with the Snellen chart where the top letter measures single letter acuity while the rest of the chart records linear acuity.2 The inner-letter spacing on each line is equal to the letter width, and the inter-row spacing is equal to the letter height of the lower row. This therefore equalises the possible effects of the crowding phenomenon.
The tasks on each line are therefore of equal difficulty, that is the same number of letters on each line with the same relative spacing and it has a constant relative change in optotype size between all lines.
The chart can be used without a mirror at distances between 1-6m, therefore, some people with severely impaired vision usually see some of the letters.2
Having a logarithmic progression and proportional spacing of optotypes allows consistent and accurate evaluation of visual acuity levels.12 Hence, there is better interpretation of the significance of noted visual acuity changes. This is because a three-line change in the higher acuity ranges would represent the same degree of change as a three-line difference in the lower acuity ranges.
Figure 6 indicates the cumulative number of letters, which will be presented to a patient and read correctly, achieving a given level on a standard Snellen or Bailey-Lovie letter chart. The fixed geometric decrease (0.1 log unit, 0.8x) in letter size on adjacent rows reading down the chart for the Bailey-Lovie chart is shown.4
The current draft for revision of BS 4274 Part 1 entitled 'Specification for test charts for clinical determination of distance visual acuity' proposes the Bailey-Lovie chart size to be reduced from the 12m size. The letter selection also suggested is C, D, E, F, H, K, P, R, U, V and Z in a 5x5 format.3
Although the Snellen chart is widely used in optometric practice, it has many flaws that are overlooked Ð many features of the chart make visual acuity assessment unreliable. Visual acuity assessment is vital in rehabilitation of many patients, for example low vision patients, and such a chart is not satisfactory. Hence, the logMAR charts are now more commonly used, their accuracy in visual acuity assessment gives a much truer and more reliable reading of a patient's acuity. Therefore, subsequent rehabilitation for patients is kept to an optimum standard.
References
1 Donald C. Fletcher. Low Vision Rehabilitation; Caring for the Whole Person, American Academy Of Ophthalmology, 1999.
2 Functional Assessment in Low Vision Practice Part 2, optician, Feb 2001; 221:28-30.
3 Bennett and Rabbetts. Clinical Visual Optics. In: Butterworth Heinemann, third edition. (1998).
4 Catherine Dickinson. Low Vision Principles and Practice. In: Butterworth Heinemann, (1998).
5 Bailey and Lovie. New Design Principles for Visual Acuity Letter Charts. Am J Optom & Phys Sci, Nov 1976;53:11;740-745.
6 Green, J. On a new series of test letters for determining the acuities of vision. Trans Am Ophthal Soc, 1986; 1:3:68.
7 Dr David Thompson. Part II lecture notes, Visual Perception. Dec 2002; City University London.
8 Albert T. Dowie. Management and practice of low visual acuity. The Eastern Press Ltd (1988).
9 Gerald E. Fonda. Management of Low Vision. Thieme-Stratton Inc. (1981).
10 Rodney W. Nowakowski. Primary Low Vision Care. Appleton & Lang (1994).
11 Scott Mackie and Roisin Mackie. Low-Vision assessment in practice; Part 3 Ð Dispensing low vision aids. optician, March 9 2001; 5791:222:18-24.
12 Hartridge H and Owen HB. Test types. Br J Ophthl, 1922; 6:543-549.
Rabiah Narband is a pre-reg student at Specsavers, Tottenham Court Road, London
Rabiah Narband argues that a logMAR acuity reading has many advantages over Snellen