Visual Acuity Flashcards
Visual Acuity
Ability of the eye to see fine detail
A measure of the spatial resolving capability of the visual processing system.
What factors can affect Visual Acuity?
1) Diffraction
2) Aberrations: the failure of rays to converge at one focus because of a defect in a lens or mirror
3) Photoreceptor density
4) Refractive Error
5) Illumination
6) Contrast
7) Location of object in visual field
Unaided Vision:
Determined from the size of the smallest line of letters in a visual acuity chart that can be read by a person without the use of an optical device such as: spectacles or CLs.
Habitual Vision:
Determined from the size of the smallest line of letters in the test chart that can be read by the patient with their present spectacles or contact lens correction.
Visual Acuity:
Determined from the size of the smallest line of letters in a visual acuity chart that can be read with the aid of optimal, optical correction.
What is Visual Acuity used for?
- Adequacy of spectacle lens corrections
- Key indicator of ocular health and to monitor eye health conditions
- Fitness to drive
- Entry into professional registration (RAF, Police)
What is 3 principle measures of VA?
- Unaided VA (Visions)
- Habitual VA
- Optimal VA (Visual Acuity?
When should you record Unaided VA?
- When a patient does not wear spectacles
- Has lost or broken their spectacles so you cannot measure habitual VA
- Do not wear spectacles for the distance
- When a profession requires it i.e. a report is filled out
- When a practitioner suspects that a patient does not need to wear their spectacles for the distance.
Testing Distance
Reference Value –
6m in UK equates to 20FT in USA
In tests of Distance Vision, the testing distance should be…
large enough to not stimulate accommodation.
The distance between the person’s eyes and the testing chart is set to “optical infinity” to mimic the lens focusing ability.
Letter size -
The letter size is specified not by its height, but by the distance at which the complete letter subtends 5 minutes of arc.
Letter Size Info:
360’ in a circle
60 minutes of arc in 1 degree
60 seconds of arc in 1 minute
Snellen Chart
11 lines of letters in a progression of sizes
Each line designated by the distance at which the overall height of the letter subtends 5 minutes of arc.
High contrast - black letters on white background
Non-serif letters – sans serif typography
No extending features
Overall height of the 12m letter subtends 5 minutes at 12m
Overall height of the 6m letter subtends 5 minutes at 6m
Snellen Fraction
d/D
Testing distance ‘d’ in m
/
Distance at which the letter would subtend 5 minutes of arc ‘D’
so 6/60
Explain 6/6 Vision?
The ability to read the 6m letter at a distance of 6m.
Explain 6/9 Vision?
The ability to read the 9m letter at a distance of 6m
What is 20/20?
USA metric unit
Minimum Angle of Resolution, MAR -
For VA 6/6, one of the strokes of the letter subtends 1 minute of arc at the eye.
The angle at which two points of a grating are just perceived as separate.
Its logarithmic form (logMAR) is commonly used as a metric for visual acuity.
MAR can be used to specify VA
Taking the reciprocal of the Snellen fraction.
Decimal Acuity
Calculate the decimal of the Snellen Fraction e.g. 6/36 = 0.17
Advantages of Snellen -
- Widley available
- Snellen notation is universally understood
- Can be produced in a smaller format, on a projector, and as an addition to other targets
- The lower part of the chart has a similar number of letters to other charts
- Easily reproduced
- Quick and easy to use
Disavantages of Snellen -
- The scale is not linear i.e. not equal interval between the lines
- Some letters are easier to see than others especially when small
- The task is easier if you have poorer visual acuity
- Relative legibility of letters will depend on the magnitude and axis of any uncorrected astigmatism
- Many charts fail to adhere to the recommendations and standards relating to the selection of letters
What are the task differences with Snellen?
- Most charts have only one 6/60 letter and an increasing number of letters on lower lines.
- Patients with poor Acuity are required to read less letters than those with good Acuity.
- Letters on lower lines are more crowded than those towards the top of the chart.
- Crowding increases the task difficulty.
- There is no systematic relationshop between the spacing of each letter to its width or height; therefore, the visual demand changes down the chart.
- VA measured at a distance of less than 6m cannot easily be equated to a 6m equivalent.
- Small number of large letters means that it is not useful when meausring VA in low Vision Patients.
Name Alternative Optotypes
- Numerals
- Landolt Rings
- Illiterate E or Tumbling E
What is LogMAR?
Bailey and Lovie expressed VA in terms of the logarithm of the angular limb width in minutes of arc at 6m.
Gold Standard method for assessing Visual Acuity
What is the Minimum Angle of Resolution (MAR) for the 6/6 letter?
1 minute of Arc
Express 6/12 Snellen (Metric)
in
Snellen ( Imperial )
Minutes of Arc
MAR
LogMAR
Decimal
20/40
6/12
2 minutes Arc
=
log 2 = 0.3
=
0.50
6/12 letter subtends 5 minutes of arc, which equates to a MAR of 2 minutes of Arc and a logMAR of 0.3.
The LogMAR Acuity Chart overcomes many of the shortcomings of the Snellen Chart, specify..
5 letters on each line
This ensures that the task is equivalent for each row and helps to ensure equal contour interaction.
Spacing between each letter and each row is related to the width and height of the letters respectively
Constant progression of letter sizes ratio 10√10 VA is logarithm of the angular limb width of smallest letters recognised at 6m
Logarithmic Progression in 0.1 LogMAR intervals so equal scale.
After LogMAR 0 there are 3 lines of smaller size, having negative LogMAR values (-0.1, -0.2 and -0.3) – the angular sub tense is less than 1 minute of arc
The MAR is taken as the stroke width of the letters, which is 1/5th of their vertical angular subtense.
Provides more letters for patients with poor VA.
What is LogMAR an acronym for?
Log10 of the Minimum Angle of Resolution
What are the Illumination standards that charts have to adhere to?
Test charts illuminated at a level where Acuity does not alter greatly with change in illumination.
To avoid glare, surrounding illumination should be at a similar level.
Room lights on.
VA affected by poor contrast and poor illumination.
Contrast on charts should be a minimum of 90%
International standard ISO 8596 – standard test type, Landolt C in a logMar progression
Full illumination in the room (chart luminance between 80 to 320 cd/m2)
What is the prodecure for testing Visual Acuity?
1) Ensure the chart is at the appropriate distance and is calibrated correctly.
2) Leave room lights on
3) Full illumination in the room (chart luminance between 80 to 320 cd/m2)
4) Seat the patient comfortably with an unobstructed view of the test chart.
5) Sit in front (and off to the side) of the patient in order to monitor facial expressions and reactions.
6) If you are going to measure both vision and habitual VA, measure vision first to avoid memorisation.
7) Measure the VA of the poorer eye first.
8) Occlude left eye with occluder (measure RE VA first).
9) Explain what measurement you are about to take. This can be as simple as “Now we shall find out what you can see in the distance.”
10) Instruct patient:
“Please read the smallest line of letters that you can see on the chart.”
11) Note down value and swap occluder
What should you do when testing VAs?
Monitor patients facial expressions.
Encourage patient to read a little more.
What should you NOT do when testing VAs?
Do not get patient to use their hand to cover their eye.
Do not allow the patient to peep around the occluder.
Do not allow your patient to screw up their eye or squint as this can invalidate results.
What to do if a patient cannot read any letters on the chart at 6m:
1) If less than 6/60, record at 3m.
If less than 3/60, record at 1m
Get the patient to move 1m nearer to the chart, then even closer if letters are still not visible.
2) Count Fingers held at some specified distance.
3) If less than 1/60, record
Hand movements (HM):
Move a hand at measured distance (record distance)
4) Light Projection (Lproj):
Pen light is held at 50cm away from 8 different directions.
Ask the patient to report the direction of light and record the areas of the visual field that a patient has light perception.
5) Light Perception (LP):
- If the pen light is perceived, but not the direction, note down LP.
- If the light is not perceived, then note down NLP.
Define a minute of Arc:
A unit of angular measurement equal to 1 degree.
Since one degree is of a turn, one minute of arc is of a turn.
Measurement System
UK - Metric System
USA - Imperial System
Name some Other types of Acuity:
- Vernier Acuity
- Stereo Acuity
- Children’s Visual Acuity Charts
How would you convert Distance -
Snellen Notation to Decimal Notation?
d / D
Exemplar –
Snellen
6 / 12
=
Decimal
0.5
How would you convert Distance -
Decimal Notation to Snellen Fraction?
d / Decimal Acuity
d = Testing Distance
Exemplar –
Decimal
0.5
= 6 ÷ 0.5 = 12
Snellen = 6/12
How would you convert Distance -
Snellen Notation to LogMAR Notation?
LogMAR = Log10 (12/6)
Log10 (D ÷ d)
If Snellen = 6/12
0.30log units
Recording LogMAR -
Each individual letter is assigned a score value of 0.02log units.
5 letters per line
Total score for a line on the LogMAR chart represents a change of 0.1 log units
How to Calculate LogMAR?
lowest line completed correctly - (number of letters read on line below x 0.02)
Adjusting LogMAR for different distances:
Conversion factor for testing distance that is not 6m
New d = Log10(6/d)
Adjusting Snellen for different distances:
d / D x New distance
Why can children be difficult to measure Visual Acuity on:
1) They might be unable to recognise letters
2) They might be unable to verbalise what they see
3) They might lack concentration
4) They might dislike occluding of one eye
Why choose Kay Picture Test to assess distance Visual Acuity?
For young children who are unfamiliar to letters -
Ages 2 1/2 to 5yrs
What does Kays Pictures consist of?
Optotypes/Shapes which are considered to be easily recognisable by this age group.
Recognition booklet provided to help determine how child interprets shapes and to encourage familiarisation.
What is the testing distance for Kays Picture Test?
6m
Does NOT work well with mirror
Advantages of Kay Pictures:
Single Optoytype test and therefore there are no crowding effects.
Optotype:
Figures or letters of different sizes used in testing the acuity of vision.
Why type of test is Keeler Acuity Test?
Grating preferential looking test
What was Keeler Acuity Chart orginally called?
Sheridan-Gardener Test
Revised Version
What does Keeler Acuity consist of:
The patterns used are Square wave gratings in terms of their Spatial Frequency
What does Keeler Acuity consist of:
The patterns used are Square wave gratings in terms of their Spatial Frequency
Spatial Frequency
Number of black and white pairs in each degree of Visual Angle.
The higher Spatial Frquency, The finer the grating
What testing distance for Keeler Acuity Test?
3m
No mirror required
What does Keeler Acuity test consist of:
- Multi-Optotype Crowded and Single Optotype Uncrowded version
- Portable
- Useful for slightly older children
- External Illumination
What notation is used to record Keeler Acuity Test?
Snellen
Modified LogMAR
Why choose LH Distance Acuity Symbols to assess distance Visual Acuity?
Children aged 18 months +
What does LH Distance Acuity Symbols consist of:
- The symbols are uniform in detail, line width and overall size.
- The test is based on 4 symbols: house, heart, circle and square which blur equally so all symbols are equally sensitive to blur and equally difficult to distinguish. This means when blurred all the symbols appear to the observer to be circles.
- The patient can still perceive answering correctly, while the examiner can easily detect the VA threshold without revealing to the child the failure of not recognizing the symbol.
Why choose Cardiff Cards to assess distance Visual Acuity?
Designed to measure VA in toddlers aged 1-3years.
Can be used in older children and adults with Intellectual Impairments.
What type of target is used in Cardiff Cards?
Vanishing Optotypes
What does the Cardiff Cards consist of?
White band boarded by two black bands, each half the width of the white band on a neutral grey background.
Average luminance of target is equal to that of the grey background.
How does the Cardiff Cards work?
If the target lies beyond the patients Acuity limit, it merges with the grey background and becomes invisible.
Targets are all of the same size but decrease in width of the black and white bands.
The acuity is given by the narrowest white band for which the target is visible (see back of card).
If a non-verbal patient is being tested, the practitioner must observe the fixation.
What is the procedure for Cardiff Cards?
1) The examiner holds the card in front of the child (1 m or 50 cm) and is unaware of the location of the target (bottom or top).
2) The examiner judges where in which direction the child is looking (up or down).
3) If the direction of gaze of the child and the position of the target are the same, another card with finer lines is presented.
What principle does the Cardiff Cards Test use?
Preferential looking
Why is Cardiff Cards a useful test for Infants?
An infant will choose to look towards a target rather than towards a plain stimulus when they are visually attentive.
Snellen can be used to assess…
‘minimum recognisable ability’
Cardiff Cards can be used to assess…
‘minimum resolvable ability’
Why test Near Vision?
As we get older, the crystalline lens within the eye stiffens up so its ability to move and focus at nearer distances becomes harder.
Near vision Adequacy is a more appropriate term than near visual acuity.
Near VA measurements do not measure an acuity threshold.
The measurements can be approximately related to Snellen distance acuity, but this is difficult in reading-test types.
Reading of a near chart involves different perceptual processes than the recognition of single letters.
Advantage of Near Adequacy:
The measurement is quicker than a threshold measurement.
Reading Notation
In the printing industry, the size of a typeface is specified by ‘points’.
One point is 1/72 of an inch
The height of a typeface is known, and hence the angular subtense at a working distance can be calculated.
An approximation to Snellen Acuity can be made.
Times New Roman typeface was originally designed for The Times newspaper.
- These letter sizes were denoted by a number indicating the point size and prefixed by an ‘N’
- Starts at N5 up to N48
- N8 is approximately equal to 6/15, generally the size of a tabloid newspaper - benchmark
Times New Roman typeface was originally designed for The Times newspaper.
- These letter sizes were denoted by a number indicating the point size and prefixed by an ‘N’
- Starts at N5 up to N48
• N8 is approximately equal to 6/15, generally the size of a tabloid newspaper - benchmark
Sloan and Habel in 1956 described a way for expressing the height of a letter using an ‘M’ notation:
• The distance in m at which it subtends an angle of 5 minutes of arc.
• This is the same as the ‘D’ in the Snellen fraction
So if the height of the 6m letters on a standard Snellen chart is 8.73mm, then the notation is 6M
1M is therefore 8.73/6 or 1.45 mm.
• This is very near the N8 size – 1M is therefore approximately equal to N8
Bailey-Lovie Reading test type:
Bailey and Lovie designed a logMAR Near chart that used Times Roman typeface.
There are 17 sizes from 14.5mm (N80) to
0.36mm (N2)
There are unconnected words instead of continuous reading material.
Keeler Near Adequcy Test
- Long paragraphs, N5 to N48
- Do not progress logarithmically
- No recommended distance
Faculty of Opthalmologists Near Adequacy Test
- The most common ‘N’ notation chart
- Book
- Should be held at 35cm
The Maclure Reading Test Type for Children
Designed especially for children at different reading ages.
Each age has been giving a grade, numbered 1 - 7, so that if a child’s reading ability is not equal to the average age, it can still be recorded by the grade number.
Within each grade, specimens of printing sizes N5, N6, N8, N10, N12, N14, N18, N24, N36 and N48 have been used.
Differentiates between children’s ability to see and their ability to read. The test is based upon the Ladybird Keywords reading scheme using school script, which is a simplified type written without serifs.
There are 3 principle measures of near VA:
- Unaided near VA (near vision)
- Habitual near VA (with the patient’s own spectacles)
- Optimal near VA (with best refractive correction)
Measuring Near VA Adequacy:
- At least two of these should be performed at every eye examination for each eye and then binocularly.
- The N notation must be recorded together with a reading distance e.g N8 at 35cm R and L
Procedure for Near Vision Testing:
- Room lights on and explain:
“Now we shall find out what you can see at close distances” - Measure RE first (or ‘poorer’ eye if known)
- Instruct Px:
“please place the chart at your normal reading distance” - Hold the occluder in place and ask Px:
“please read the smallest paragraph that you can see” - If appropriate, prompt Px:
“Try and read some of the words on the smaller paragraph” - Repeat for LE and binocularly
Procedure for Near Vision Testing:
- Room lights on and explain:
“Now we shall find out what you can see at close distances” - Measure RE first (or ‘poorer’ eye if known)
- Instruct Px:
“please place the chart at your normal reading distance” - Hold the occluder in place and ask Px:
“please read the smallest paragraph that you can see” - If appropriate, prompt Px:
“Try and read some of the words on the smaller paragraph” - Repeat for LE and binocularly
N.B. for testing Near Vision:
You are measuring ‘near vision adequacy’ rather than a true near VA.
At 6m or 20FT, a human eye with 6/6 or 20/20 vision is able to…
separate contours that are approximetly 1.75mm apart.
Explain the expression 6/x Vision in laymans term?
The numerator, 6 is the distance in metres between the patient and the testing chart.
The denominator, x is the distance in metres at which a patient with 6/6 Acuity would discern the same optotype.
Explain 6/12?
A person with 6/6 vision would discern the same optotype from 12 metres away (i.e. at twice the distance).
This is equivalent to saying that with 6/12 vision, the person possesses half the spatial resolution and needs twice the size to discern the optotype.
Visual Angle
The angle a viewed object subtends at the eye in degress of arc.
Acuity is a measure of…
Visual Performance
Dynamic Visual Acuity
Defines the ability of the eye to visually discern fine detail in a moving object.
Vernier Acuity
This measures the ability to discern a misalignment among 2 line segments /gratings.
Steroacuity
Is the smallest detectable depth difference that can be seen in BV.
Perceptual Process
- Presence of objects
- Observation
- Selection
- Organisation
- Interpretation
- Response
Acuity in the periphery
Acuity declines towards the periphery in an inverse-linear fashion (i.e. the decline follows approximately a hyperbola).
Cryillic Script
Writing system used for Russian language and various langauges across Eurasia.
How would you convert Near -
Snellen Notation to N (Wcm)
N = [50 × W × Tan(D / 360)] / 0.1875
How would you convert Near -
N to Snellen Fraction:
D = 360 × Arctan[(0.1875 × N) / (50 × W)]
Landolt Ring –
This test involves the letter C positioned in either up, down, right or left and thus the patient has to indicate which one it is.
Why should you use the Landolt Ring in VA assessment?
- Does not demand literacy
- Avoids the difficulties of unsuitable alphabets
When can the Landolt Ring be invalid?
If uncorrected Astigmatism is present, recognition of the gap is easier sometimes.
The illiterate E test –
This test involves the letter E positioned in either up, down, right or left and thus the patient has to indicate which one it is.
Stycar tests – Sheridan tests for Young Children and Retardates
The test has 9 letters: A,C,H,L,O,T,U,V and X; these letters were chosen because they can be easily copied by young children.
The child is given a key cad with the appropriate choice of letters.
Ffooks’ tests –
This test uses 3 symbols free from directional bias: a square, a circle and an equilateral triangle. The presentation is by means of a book with two to four symbols on each page. Cut out symbols are given to the child.
Adaptation of VA measurement with older patients –
A hyperopic shift is common in older patients and some varifocal wearers adapt to this change by habitually raising their chin to improve distance vision by viewing through the additional plus power in the intermediate section of the lens. This can be seen when assessing habitual VA as these varifocal wearers will raise their chin during measurements to improve VA. Vas should be measured through the distance portion of the lens.
State some tasks the eye is involved with?
- Light perception e.g. the threshold of vision in the normal eye
- Discrimination: ability of the visual system to distinguish an object from its background
- Form vision and recognition: ability to identify letters and words
- Resolution: ability to see in detail, the smallest interval measurable
- Localisation: object situation compared to other objects
- Higher tasks: stimulation of other responses e.g. motor response in handling something
How is Vision and Visual Acuity measured?
1) Target Detection: The presence or absence of a stimuli
2) Target Localisation: Discriminating differences in the spatial position of segments of a test object, such as a break or discontinuity in contour.
3) Target Recognition: The recognition or naming of a target
Resolution and Form –
- An image of a letter in focus on the retina has sharp lines
- An image of a letter out of focus forms a blur circle for each point of the object
Pinhole
A trial case accessory
The Pinhole test
Used to determine whether reduced VA is due to refractive error or ocular disease.
Function of Pinhole
The pinhole reduces the effect of ametropic blur by reducing effective pupil size
If the VA improves with the pinhole test….
then the reduced VA is due to uncorrected refractive error
Pinhole makes it better
If the retinal image is out of focus PH will reduce the size of the blur circle.
Stenopaeic slit
A trail case accessory
Function of Stenopaeic Slit
Can be used to locate the principle meridians in the astigmatic eye of a patient with marked lens opacities; provided that the individual blurs composing the retinal image are sufficiently elongated, rotation of the slit will enable a ‘best position’.
- Reduces effective pupil size in the meridian that is perpendicular to the slit.
- Place slit in a correctly centred trial frame and rotate until the patient identifies the orientation that gives rise to maximum clarity on the letter chart.
Scheiner disc
An opaque disc with two pinholes separated by a distance less than the pupil diameter e.g. 2-4mm apart.
Function of Scheiner disc
- The disc is placed close to the eye and the patient looks at a small distant spotlight
- The view that the patient sees i.e. the 2 separated images can indicate refractive error
What results does the Scheiner Disc provide?
- If the image is doubled, the patient sees 2 separate images, then indicates myopia/hyperopia
- If the 2 images seen appear reversed/crossed this indicates hyperopia
- If uncrossed indicates myopia
Airy disk
The central bright circular region of the pattern produced by light diffracted when passing through a small circular aperture. The central disk is surrounded by less intense concentric rings, so light intensity takes local maxima and mimina while it decreases away from the center.
