From Wikipedia, the free encyclopedia
Visual acuity (VA) commonly refers to the clarity of vision,
but technically rates a person's ability to recognize small details
with precision. Visual acuity depends on optical and neural factors.
Optical factors of the eye influence the sharpness of an image on its retina.
Neural factors include the health and functioning of the retina, of the
neural pathways to the brain, and of the interpretative faculty of the
brain.
The most commonly referred-to visual acuity is distance acuity or far acuity
(e.g., "20/20 vision"), which describes someone's ability to recognize
small details at a far distance. This ability is compromised in people
with myopia, also known as short-sightedness or near-sightedness. Another visual acuity is near acuity, which describes someone's ability to recognize small details at a near distance. This ability is compromised in people with hyperopia, also known as long-sightedness or far-sightedness.
A common optical cause of low visual acuity is refractive error
(ametropia): errors in how the light is refracted in the eyeball.
Causes of refractive errors include aberrations in the shape of the eyeball or the cornea, and reduced ability of the lens
to focus light. When the combined refractive power of the cornea and
lens is too high for the length of the eyeball, the retinal image will
be in focus in front of the retina and out of focus on the retina,
yielding myopia. A similar poorly focussed retinal image happens when
the combined refractive power of the cornea and lens is too low for the
length of the eyeball except that the focused image is behind the
retina, yielding hyperopia. Normal refractive power is referred to as emmetropia. Other optical causes of low visual acuity include astigmatism, in which contours of a particular orientation are blurred, and more complex corneal irregularities.
Refractive errors can mostly be corrected by optical means (such as eyeglasses, contact lenses, and refractive surgery). For example, in the case of myopia, the correction is to reduce the power of the eye's refraction by a so-called minus lens.
Neural factors that limit acuity are located in the retina, in
the pathways to the brain, or in the brain. Examples of conditions
affecting the retina include detached retina and macular degeneration. Examples of conditions affecting the brain include amblyopia (caused by the visual brain not having developed properly in early childhood) and by brain damage, such as from traumatic brain injury or stroke. When optical factors are corrected for, acuity can be considered a measure of neural functioning.
Visual acuity is typically measured while fixating, i.e. as a measure of central (or foveal) vision, for the reason that it is highest in the very center). However, acuity in peripheral vision
can be of equal importance in everyday life. Acuity declines towards
the periphery first steeply and then more gradually, in an
inverse-linear fashion (i.e. the decline follows approximately a hyperbola). The decline is according to E2/(E2+E), where E is eccentricity in degrees visual angle, and E2 is a constant of approximately 2 deg. At 2 deg eccentricity, for example, acuity is half the foveal value.
Visual acuity is a measure of how well small details are resolved
in the very center of the visual field; it therefore does not indicate
how larger patterns are recognized. Visual acuity alone thus cannot
determine the overall quality of visual function.
Definition
Eye examination for visual acuity
Visual acuity
is a measure of the spatial resolution of the visual processing system.
VA, as it is sometimes referred to by optical professionals, is tested
by requiring the person whose vision is being tested to identify
so-called optotypes – stylized letters, Landolt rings, pediatric symbols, symbols for the illiterate, standardized Cyrillic letters in the Golovin–Sivtsev table,
or other patterns – on a printed chart (or some other means) from a set
viewing distance. Optotypes are represented as black symbols against a
white background (i.e. at maximum contrast). The distance between the person's eyes and the testing chart is set so as to approximate "optical infinity" in the way the lens attempts to focus (far acuity), or at a defined reading distance (near acuity).
A reference value above which visual acuity is considered normal is called 6/6 vision, the USC
equivalent of which is 20/20 vision: At 6 metres or 20 feet, a human
eye with that performance is able to separate contours that are
approximately 1.75 mm apart.
Vision of 6/12 corresponds to lower performance, while vision of 6/3 to
better performance. Normal individuals have an acuity of 6/4 or better
(depending on age and other factors).
In the expression 6/x vision, the numerator (6) is the distance in metres between the subject and the chart and the denominator
(x) the distance at which a person with 6/6 acuity would discern the
same optotype. Thus, 6/12 means that 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.
A simple and efficient way to state acuity is by converting the
fraction to a decimal: 6/6 then corresponds to an acuity (or a Visus) of
1.0 (see Expression below), while 6/3 corresponds to 2.0, which is often attained by well-corrected healthy young subjects with binocular vision. Stating acuity as a decimal number is the standard in European countries, as required by the European norm (EN ISO 8596, previously DIN 58220).
The precise distance at which acuity is measured is not important
as long as it is sufficiently far away and the size of the optotype on
the retina is the same. That size is specified as a visual angle, which is the angle, at the eye, under which the optotype appears. For 6/6 = 1.0 acuity, the size of a letter on the Snellen chart or Landolt C chart is a visual angle of 5 arc minutes (1 arc min = 1/60 of a degree). By the design of a typical optotype (like a Snellen E
or a Landolt C), the critical gap that needs to be resolved is 1/5 this
value, i.e., 1 arc min. The latter is the value used in the international definition of visual acuity:
- acuity = 1/gap size [arc min].
Acuity is a measure of visual performance and does not relate to the
eyeglass prescription required to correct vision. Instead, an eye exam
seeks to find the prescription that will provide the best corrected
visual performance achievable. The resulting acuity may be greater or
less than 6/6 = 1.0. Indeed, a subject diagnosed as having 6/6 vision
will often actually have higher visual acuity because, once this
standard is attained, the subject is considered to have normal (in the
sense of undisturbed) vision and smaller optotypes are not tested.
Subjects with 6/6 vision or "better" (20/15, 20/10, etc.) may still
benefit from an eyeglass correction for other problems related to the
visual system, such as hyperopia, ocular injuries, or presbyopia.
Measurement
Manual hand eye test in Ghana (2018).
Visual acuity is measured by a psychophysical procedure and as such relates the physical characteristics of a stimulus to a subject's percept and their resulting responses. Measurement can be by using an eye chart invented by Ferdinand Monoyer, by optical instruments, or by computerized tests like the FrACT.
Care must be taken that viewing conditions correspond to the standard,
such as correct illumination of the room and the eye chart, correct
viewing distance, enough time for responding, error allowance, and so
forth. In European countries, these conditions are standardized by the European norm (EN ISO 8596, previously DIN 58220).
History
| Year
|
Event
|
| 1843
|
Vision test types are invented in 1843 by the German ophthalmologist Heinrich Kuechler (1811–1873), in Darmstadt, Germany. He argues for need to standardize vision tests and produces three reading charts to avoid memorization.
|
| 1854
|
Eduard Jäger von Jaxtthal, a Vienna
oculist, makes improvements to eye chart test types that were developed
by Heinrich Kuechler. He publishes, in German, French, English and
other languages, a set of reading samples to document functional vision.
He uses fonts that were available in the State Printing House in Vienna
in 1854 and labels them with the numbers from that printing house
catalogue, currently known as Jaeger numbers.
|
| 1862
|
Herman Snellen, a Dutch ophthalmologist, publishes in Utrecht
his "Optotypi ad visum determinandum" ("Probebuchstaben zur Bestimmung
der Sehschärfe"), the first visual chart based on "Optotypes",
advocating the need for standardized vision tests. Snellen's Optotypes
are not identical to the test letters used today. They were printed in
an "Egyptian Paragon" font (i.e. using serifs).
|
| 1888
|
Edmund Landolt introduces the broken ring, now known as the Landolt ring, which later becomes an international standard.
|
| 1894
|
Theodor Wertheim in Berlin presents detailed measurements of acuity in peripheral vision.
|
| 1978
|
Hugh Taylor uses these design principles for a "Tumbling E Chart" for illiterates, later used to study the visual acuity of Australian Aboriginals.
|
| 1982
|
Rick Ferris et al. of the National Eye Institute chooses the LogMAR chart layout, implemented with Sloan letters, to establish a standardized method of visual acuity measurement for the Early Treatment of Diabetic Retinopathy Study
(ETDRS).
These charts are used in all subsequent clinical studies, and did much
to familiarize the profession with the new layout and progression. Data
from the ETDRS were used to select letter combinations that give each
line the same average difficulty, without using all letters on each
line.
|
| 1984
|
The International Council of Ophthalmology approves a new "Visual
Acuity Measurement Standard", also incorporating the above features.
|
| 1988
|
Antonio Medina and Bradford Howland of the Massachusetts Institute of
Technology develop a novel eye testing chart using letters that become
invisible with decreasing acuity, rather than blurred as in standard
charts. They demonstrate the arbitrary nature of the Snellen fraction
and warn about the accuracy of visual acuity determined by using charts
of different letter types, calibrated by Snellen's system.
|
Physiology
Daylight vision (i.e. photopic vision) is subserved by cone receptor cells which have high spatial density (in the central fovea) and allow high acuity of 6/6 or better. In low light (i.e., scotopic vision), cones do not have sufficient sensitivity and vision is subserved by rods. Spatial resolution is then much lower. This is due to spatial summation of rods, i.e. a number of rods merge into a bipolar cell, in turn connecting to a ganglion cell, and the resulting unit for resolution is large, and acuity small. There are no rods in the very center of the visual field (the foveola), and highest performance in low light is achieved in near peripheral vision.
The maximum angular resolution of the human eye is 28 arc seconds or 0.47 arc minutes,
this gives an angular resolution of 0.008 degrees, and at a distance of
1 km corresponds to 136 mm. This is equal to 0.94 arc minutes per line
pair (one white and one black line), or 0.016 degrees. For a pixel pair
(one white and one black pixel) this gives a pixel density of 128 pixels
per degree (PPD).
6/6 vision is defined as the ability to resolve two points of
light separated by a visual angle of one minute of arc, corresponding to
60 PPD, or about 290–350 pixels per inch for a display on a device held
250 to 300 mm from the eye.
Thus, visual acuity, or resolving power (in daylight, central vision), is the property of cones.
To resolve detail, the eye's optical system has to project a focused image on the fovea, a region inside the macula having the highest density of cone photoreceptor cells
(the only kind of photoreceptors existing in the fovea's very center of
300 μm diameter), thus having the highest resolution and best color
vision. Acuity and color vision, despite being mediated by the same
cells, are different physiologic functions that do not interrelate
except by position. Acuity and color vision can be affected
independently.
The diagram shows the relative acuity of the human eye on the horizontal meridian. The
blind spot is at about 15.5° in the outside direction (e.g. in the left visual field for the left eye).
The grain of a photographic mosaic has just as limited resolving power as the "grain" of the retinal mosaic.
To see detail, two sets of receptors must be intervened by a middle
set. The maximum resolution is that 30 seconds of arc, corresponding to
the foveal cone diameter or the angle subtended at the nodal point of
the eye. To get reception from each cone, as it would be if vision was
on a mosaic basis, the "local sign" must be obtained from a single cone
via a chain of one bipolar, ganglion, and lateral geniculate cell each. A
key factor of obtaining detailed vision, however, is inhibition. This
is mediated by neurons such as the amacrine
and horizontal cells, which functionally render the spread or
convergence of signals inactive. This tendency to one-to-one shuttle of
signals is powered by brightening of the center and its surroundings,
which triggers the inhibition leading to a one-to-one wiring. This
scenario, however, is rare, as cones may connect to both midget and flat
(diffuse) bipolars, and amacrine and horizontal cells can merge
messages just as easily as inhibit them.
Light travels from the fixation object to the fovea through an
imaginary path called the visual axis. The eye's tissues and structures
that are in the visual axis (and also the tissues adjacent to it) affect
the quality of the image. These structures are: tear film, cornea,
anterior chamber, pupil, lens, vitreous, and finally the retina. The
posterior part of the retina, called the retinal pigment epithelium
(RPE) is responsible for, among many other things, absorbing light that
crosses the retina so it cannot bounce to other parts of the retina. In
many vertebrates, such as cats, where high visual acuity is not a
priority, there is a reflecting tapetum
layer that gives the photoreceptors a "second chance" to absorb the
light, thus improving the ability to see in the dark. This is what
causes an animal's eyes to seemingly glow in the dark when a light is
shone on them. The RPE also has a vital function of recycling the
chemicals used by the rods and cones in photon detection. If the RPE is
damaged and does not clean up this "shed" blindness can result.
As in a photographic lens,
visual acuity is affected by the size of the pupil. Optical aberrations
of the eye that decrease visual acuity are at a maximum when the pupil
is largest (about 8 mm), which occurs in low-light conditions. When the
pupil is small (1–2 mm), image sharpness may be limited by diffraction of light by the pupil (see diffraction limit).
Between these extremes is the pupil diameter that is generally best for
visual acuity in normal, healthy eyes; this tends to be around 3 or
4 mm.
If the optics of the eye were otherwise perfect, theoretically,
acuity would be limited by pupil diffraction, which would be a
diffraction-limited acuity of 0.4 minutes of arc (minarc) or 6/2.6
acuity. The smallest cone cells in the fovea have sizes corresponding to
0.4 minarc of the visual field, which also places a lower limit on
acuity. The optimal acuity of 0.4 minarc or 6/2.6 can be demonstrated
using a laser interferometer
that bypasses any defects in the eye's optics and projects a pattern of
dark and light bands directly on the retina. Laser interferometers are
now used routinely in patients with optical problems, such as cataracts, to assess the health of the retina before subjecting them to surgery.
The visual cortex is the part of the cerebral cortex in the posterior part of the brain responsible for processing visual stimuli, called the occipital lobe. The central 10° of field (approximately the extension of the macula)
is represented by at least 60% of the visual cortex. Many of these
neurons are believed to be involved directly in visual acuity
processing.
Proper development of normal visual acuity depends on a human or
an animal having normal visual input when it is very young. Any visual
deprivation, that is, anything interfering with such input over a
prolonged period of time, such as a cataract, severe eye turn or strabismus, anisometropia
(unequal refractive error between the two eyes), or covering or
patching the eye during medical treatment, will usually result in a
severe and permanent decrease in visual acuity and pattern recognition
in the affected eye if not treated early in life, a condition known as amblyopia.
The decreased acuity is reflected in various abnormalities in cell
properties in the visual cortex. These changes include a marked decrease
in the number of cells connected to the affected eye as well as cells
connected to both eyes in cortical area V1, resulting in a loss of stereopsis, i.e. depth perception by binocular vision
(colloquially: "3D vision"). The period of time over which an animal is
highly sensitive to such visual deprivation is referred to as the critical period.
The eye is connected to the visual cortex by the optic nerve coming out of the back of the eye. The two optic nerves come together behind the eyes at the optic chiasm,
where about half of the fibers from each eye cross over to the opposite
side and join fibers from the other eye representing the corresponding
visual field, the combined nerve fibers from both eyes forming the optic tract. This ultimately forms the physiological basis of binocular vision. The tracts project to a relay station in the midbrain called the lateral geniculate nucleus, part of the thalamus, and then to the visual cortex along a collection of nerve fibers called the optic radiation.
Any pathological process in the visual system, even in older
humans beyond the critical period, will often cause decreases in visual
acuity. Thus measuring visual acuity is a simple test in accessing the
health of the eyes, the visual brain, or pathway to the brain. Any
relatively sudden decrease in visual acuity is always a cause for
concern. Common causes of decreases in visual acuity are cataracts and scarred corneas, which affect the optical path, diseases that affect the retina, such as macular degeneration and diabetes, diseases affecting the optic pathway to the brain such as tumors and multiple sclerosis, and diseases affecting the visual cortex such as tumors and strokes.
Though the resolving power depends on the size and packing
density of the photoreceptors, the neural system must interpret the
receptors' information. As determined from single-cell experiments on
the cat and primate, different ganglion cells in the retina are tuned to
different spatial frequencies,
so some ganglion cells at each location have better acuity than others.
Ultimately, however, it appears that the size of a patch of cortical
tissue in visual area V1 that processes a given location in the visual field (a concept known as cortical magnification)
is equally important in determining visual acuity. In particular, that
size is largest in the fovea's center, and decreases with increasing
distance from there.
Optical aspects
Besides
the neural connections of the receptors, the optical system is an
equally key player in retinal resolution. In the ideal eye, the image of
a diffraction grating can subtend 0.5 micrometre on the retina. This is certainly not the case, however, and furthermore the pupil can cause diffraction
of the light. Thus, black lines on a grating will be mixed with the
intervening white lines to make a gray appearance. Defective optical
issues (such as uncorrected myopia) can render it worse, but suitable
lenses can help. Images (such as gratings) can be sharpened by lateral
inhibition, i.e., more highly excited cells inhibiting the less excited
cells. A similar reaction is in the case of chromatic aberrations, in
which the color fringes around black-and-white objects are inhibited
similarly.
Expression
Visual acuity scales
| 20 ft
|
10 ft
|
6 m
|
3 m
|
Decimal
|
MAR
|
LogMAR
|
| 20/1000
|
10/500
|
6/300
|
3/150
|
0.02
|
50
|
1.70
|
| 20/800
|
10/400
|
6/240
|
3/120
|
0.025
|
40
|
1.60
|
| 20/600
|
10/300
|
6/180
|
3/90
|
0.033
|
30
|
1.48
|
| 20/500
|
10/250
|
6/150
|
3/75
|
0.04
|
25
|
1.40
|
| 20/400
|
10/200
|
6/120
|
3/60
|
0.05
|
20
|
1.30
|
| 20/300
|
10/150
|
6/90
|
3/45
|
0.067
|
15
|
1.18
|
| 20/250
|
10/125
|
6/75
|
3/37
|
0.08
|
12.5
|
1.10
|
| 20/200
|
10/100
|
6/60
|
3/30
|
0.10
|
10
|
1.00
|
| 20/160
|
10/80
|
6/48
|
3/24
|
0.125
|
8
|
0.90
|
| 20/125
|
10/62
|
6/38
|
3/19
|
0.16
|
6.25
|
0.80
|
| 20/100
|
10/50
|
6/30
|
3/15
|
0.20
|
5
|
0.70
|
| 20/80
|
10/40
|
6/24
|
3/12
|
0.25
|
4
|
0.60
|
| 20/60
|
10/30
|
6/18
|
3/9
|
0.33
|
3
|
0.48
|
| 20/50
|
10/25
|
6/15
|
3/7.5
|
0.40
|
2.5
|
0.40
|
| 20/40
|
10/20
|
6/12
|
3/6
|
0.50
|
2
|
0.30
|
| 20/30
|
10/15
|
6/9
|
3/4.5
|
0.67
|
1.5
|
0.18
|
| 20/25
|
10/12
|
6/7.5
|
3/4
|
0.80
|
1.25
|
0.10
|
| 20/20
|
10/10
|
6/6
|
3/3
|
1.00
|
1
|
0.00
|
| 20/16
|
10/8
|
6/4.8
|
3/2.4
|
1.25
|
0.8
|
−0.10
|
| 20/12.5
|
10/6
|
6/3.8
|
3/2
|
1.60
|
0.625
|
−0.20
|
| 20/10
|
10/5
|
6/3
|
3/1.5
|
2.00
|
0.5
|
−0.30
|
| 20/8
|
10/4
|
6/2.4
|
3/1.2
|
2.50
|
0.4
|
−0.40
|
| 20/6.6
|
10/3.3
|
6/2
|
3/1
|
3.00
|
0.333
|
−0.48
|
Visual acuity is often measured according to the size of letters viewed on a Snellen chart or the size of other symbols, such as Landolt Cs or the E Chart.
In some countries, acuity is expressed as a vulgar fraction, and in some as a decimal
number. Using the metre as a unit of measurement, (fractional) visual
acuity is expressed relative to 6/6. Otherwise, using the foot, visual
acuity is expressed relative to 20/20. For all practical purposes, 20/20
vision is equivalent to 6/6. In the decimal system, acuity is defined
as the reciprocal value of the size of the gap (measured in arc minutes)
of the smallest Landolt C, the orientation of which can be reliably identified. A value of 1.0 is equal to 6/6.
LogMAR is another commonly used scale, expressed as the (decadic) logarithm of the minimum angle of resolution (MAR), which is the reciprocal
of the acuity number. The LogMAR scale converts the geometric sequence
of a traditional chart to a linear scale. It measures visual acuity
loss: positive values indicate vision loss, while negative values denote
normal or better visual acuity. This scale is commonly used clinically
and in research because the lines are of equal length and so it forms a
continuous scale with equally spaced intervals between points, unlike
Snellen charts, which have different numbers of letters on each line.
A visual acuity of 6/6 is frequently described as meaning that a
person can see detail from 6 metres (20 ft) away the same as a person
with "normal" eyesight would see from 6 metres. If a person has a visual
acuity of 6/12, he is said to see detail from 6 metres (20 ft) away the
same as a person with "normal" eyesight would see it from 12 metres
(39 ft) away.
The definition of 6/6 is somewhat arbitrary, since human eyes
typically have higher acuity, as Tscherning writes, "We have found also
that the best eyes have a visual acuity which approaches 2, and we can
be almost certain that if, with a good illumination, the acuity is only
equal to 1, the eye presents defects sufficiently pronounced to be
easily established."
Most observers may have a binocular acuity superior to 6/6; the limit
of acuity in the unaided human eye is around 6/3–6/2.4 (20/10–20/8),
although 6/3 was the highest score recorded in a study of some US
professional athletes. Some birds of prey, such as hawks, are believed to have an acuity of around 20/2; in this respect, their vision is much better than human eyesight.
When visual acuity is below the largest optotype on the chart,
the reading distance is reduced until the patient can read it. Once the
patient is able to read the chart, the letter size and test distance are
noted. If the patient is unable to read the chart at any distance, they
are tested as follows:
| Name
|
Abbreviation
|
Definition
|
| Counting Fingers
|
CF
|
Ability to count fingers at a given distance. This test method is
only used after it has been determined that the patient is not able to
make out any of the letters, rings, or images on the acuity chart. The
letters CF, and the testing distance, would represent the patient's
acuity.
For example, the recording CF 5' would mean the patient was
able to count the examiner's fingers from a maximum distance of 5 feet
directly in front of the examiner.
(The results of this test, on the same patient, may vary from
examiner to examiner. This is due more so to the size differences of the
various examiner's hands and fingers, than fluctuating vision.)
|
| Hand Motion
|
HM
|
Ability to distinguish whether or not there is movement of the
examiner's hand directly in front of the patient's eyes. This test
method is only used after a patient shows little or no success with the
Counting Fingers test. The letters HM, and the testing distance, would
represent the patient's acuity.
For example, the recording HM 2' would mean that the
patient was able to distinguish movement of the examiner's hand from a
maximum distance of 2 feet directly in front of the examiner.
(The results of the Hand Motion test are often recorded without
the testing distance. This is due to the fact that this test is
performed after the patient cannot "pass" the Counting Fingers test. At
this point, the examiner is usually directly in front of the patient,
and it is assumed that the Hand Motion test is performed at a testing
distance of 1 foot or less.)
|
| Light Perception
|
LP
|
Ability to perceive any light. This test method is used only after a
patient shows little or no success with the Hand Motion test. In this
test, an examiner shines a pen light
at the patient's pupil and asks the patient to either, point to the
light source, or, describe the direction that the light is coming from
(up, out, straight ahead, down and out, etc.). If the patient is able to
perceive light, the letters LP are recorded to represent the patient's
acuity. If the patient is unable to perceive any light, the letters NLP (No Light Perception)
are recorded. A patient with no light perception in one eye is
considered blind in the respective eye. If NLP is recorded in both eyes,
the patient is described as having total blindness.
|
Legal definitions
Various
countries have defined statutory limits for poor visual acuity that
qualifies as a disability. For example, in Australia, the Social
Security Act defines blindness as:
A person meets the criteria for
permanent blindness under section 95 of the Social Security Act if the
corrected visual acuity is less than 6/60 on the Snellen Scale in both
eyes or there is a combination of visual defects resulting in the same
degree of permanent visual loss.
In the US, the relevant federal statute defines blindness as follows:
[T]he term "blindness" means
central visual acuity of 20/200 or less in the better eye with the use
of a correcting lens. An eye that is accompanied by a limitation in the
fields of vision such that the widest diameter of the visual field
subtends an angle no greater than 20 degrees shall be considered for
purposes in this paragraph as having a central visual acuity of 20/200
or less.
A person's visual acuity is registered documenting the following:
whether the test was for distant or near vision, the eye(s) evaluated
and whether corrective lenses (i.e. glasses or contact lenses) were used:
- Distance from the chart
- D (distant) for the evaluation done at 20 feet (6 m).
- N (near) for the evaluation done at 15.7 inches (400 mm).
- Eye evaluated
- OD (Latin oculus dexter) for the right eye.
- OS (Latin oculus sinister) for the left eye.
- OU (Latin oculi uterque) for both eyes.
- Usage of spectacles during the test
- cc (Latin cum correctore) with correctors.
- sc: (Latin sine correctore) without correctors.
- Pinhole occluder
- The abbreviation PH is followed by the visual acuity as measured with a pinhole occluder, which temporarily corrects for refractive errors such as myopia or astigmatism.
- PHNI means No Improvement of visual acuity using a pinhole occluder.
So, distant visual acuity of 6/10 and 6/8 with pinhole in the right
eye will be: DscOD 6/10 PH 6/8. Distant visual acuity of count fingers
and 6/17 with pinhole in the left eye will be: DscOS CF PH 6/17. Near
visual acuity of 6/8 with pinhole remaining at 6/8 in both eyes with
spectacles will be: NccOU 6/8 PH 6/8.
"Dynamic visual acuity" defines the ability of the eye to visually discern fine detail in a moving object.
Measurement considerations
Visual
acuity measurement involves more than being able to see the optotypes.
The patient should be cooperative, understand the optotypes, be able to
communicate with the physician, and many more factors. If any of these
factors is missing, then the measurement will not represent the
patient's real visual acuity.
Visual acuity is a subjective test meaning that if the patient is
unwilling or unable to cooperate, the test cannot be done. A patient
who is sleepy, intoxicated, or has any disease that can alter their
consciousness or mental status, may not achieve their maximum possible
acuity.
Patients who are illiterate in the language whose letters and/or
numbers appear on the chart will be registered as having very low visual
acuity if this is not known. Some patients will not tell the examiner
that they do not know the optotypes, unless asked directly about it.
Brain damage can result in a patient not being able to recognize printed
letters, or being unable to spell them.
A motor inability can make a person respond incorrectly to the
optotype shown and negatively affect the visual acuity measurement.
Variables such as pupil size, background adaptation luminance,
duration of presentation, type of optotype used, interaction effects
from adjacent visual contours (or "crowding") can all affect visual
acuity measurement.
Testing in children
The newborn's
visual acuity is approximately 6/133, developing to 6/6 well after the
age of six months in most children, according to a study published in
2009.
The measurement of visual acuity in infants, pre-verbal children
and special populations (for instance, disabled individuals) is not
always possible with a letter chart. For these populations, specialised
testing is necessary. As a basic examination step, one must check
whether visual stimuli can be fixated, centered and followed.
More formal testing using preferential looking techniques use Teller acuity
cards (presented by a technician from behind a window in the wall) to
check whether the child is more visually attentive to a random
presentation of vertical or horizontal gratings
on one side compared with a blank page on the other side – the bars
become progressively finer or closer together, and the endpoint is noted
when the child in its adult carer's lap equally prefers the two sides.
Another popular technique is electro-physiologic testing using visual evoked (cortical) potentials (VEPs or VECPs), which can be used to estimate visual acuity in doubtful cases and expected severe vision loss cases like Leber's congenital amaurosis.
VEP testing of acuity is somewhat similar to preferential looking in using a series of black and white stripes (sine wave gratings)
or checkerboard patterns (which produce larger responses than stripes).
Behavioral responses are not required and brain waves created by the
presentation of the patterns are recorded instead. The patterns become
finer and finer until the evoked brain wave just disappears, which is
considered to be the endpoint measure of visual acuity. In adults and
older, verbal children capable of paying attention and following
instructions, the endpoint provided by the VEP corresponds very well to
the psychophysical measure in the standard measurement (i.e. the
perceptual endpoint determined by asking the subject when they can no
longer see the pattern). There is an assumption that this correspondence
also applies to much younger children and infants, though this does not
necessarily have to be the case. Studies do show the evoked brain
waves, as well as derived acuities, are very adult-like by one year of
age.
For reasons not totally understood, until a child is several
years old, visual acuities from behavioral preferential looking
techniques typically lag behind those determined using the VEP, a direct
physiological measure of early visual processing in the brain. Possibly
it takes longer for more complex behavioral and attentional responses,
involving brain areas not directly involved in processing vision, to
mature. Thus the visual brain may detect the presence of a finer pattern
(reflected in the evoked brain wave), but the "behavioral brain" of a
small child may not find it salient enough to pay special attention to.
A simple but less-used technique is checking oculomotor responses with an optokinetic nystagmus
drum, where the subject is placed inside the drum and surrounded by
rotating black and white stripes. This creates involuntary abrupt eye
movements (nystagmus)
as the brain attempts to track the moving stripes. There is a good
correspondence between the optokinetic and usual eye-chart acuities in
adults. A potentially serious problem with this technique is that the
process is reflexive and mediated in the low-level brain stem, not in the visual cortex. Thus someone can have a normal optokinetic response and yet be cortically blind with no conscious visual sensation.
"Normal" visual acuity
Visual
acuity depends upon how accurately light is focused on the retina, the
integrity of the eye's neural elements, and the interpretative faculty
of the brain. "Normal" visual acuity (in central, i.e. foveal vision) is frequently considered to be what was defined by Herman Snellen as the ability to recognize an optotype when it subtended 5 minutes of arc,
that is Snellen's chart 6/6-metre, 20/20 feet, 1.00 decimal or 0.0
logMAR. In young humans, the average visual acuity of a healthy, emmetropic eye (or ametropic eye with correction) is approximately 6/5 to 6/4, so it is inaccurate to refer to 6/6 visual acuity as "perfect" vision.
On the contrary, Tscherning writes, "We have found also that the best
eyes have a visual acuity which approaches 2, and we can be almost
certain that if, with a good illumination, the acuity is only equal to
1, the eye presents defects sufficiently pronounced to be easily
established."
6/6 is the visual acuity needed to discriminate two contours
separated by 1 arc minute – 1.75 mm at 6 metres. This is because a 6/6
letter, E for example, has three limbs and two spaces in between them,
giving 5 different detailed areas. The ability to resolve this therefore
requires 1/5 of the letter's total size, which in this case would be 1
minute of arc (visual angle). The significance of the 6/6 standard can
best be thought of as the lower limit of normal, or as a screening
cutoff. When used as a screening test, subjects that reach this level
need no further investigation, even though the average visual acuity
with a healthy visual system is typically better.
Some people may have other visual problems, such as severe visual field defects, color blindness, reduced contrast, mild amblyopia,
cerebral visual impairments, inability to track fast-moving objects, or
one of many other visual impairments and still have "normal" visual
acuity. Thus, "normal" visual acuity by no means implies normal vision.
The reason visual acuity is very widely used is that it is easily
measured, its reduction (after correction) often indicates some
disturbance, and that it often corresponds with the normal daily
activities a person can handle, and evaluates their impairment to do
them (even though there is heavy debate over that relationship).
Other measures
Normally,
visual acuity refers to the ability to resolve two separated points or
lines, but there are other measures of the ability of the visual system
to discern spatial differences.
Vernier acuity measures the ability to align two line segments. Humans can do this with remarkable accuracy. This success is regarded as hyperacuity. Under optimal conditions of good illumination, high contrast, and long line segments, the limit to vernier
acuity is about 8 arc seconds or 0.13 arc minutes, compared to about
0.6 arc minutes (6/4) for normal visual acuity or the 0.4 arc minute
diameter of a foveal cone.
Because the limit of vernier acuity is well below that imposed on
regular visual acuity by the "retinal grain" or size of the foveal
cones, it is thought to be a process of the visual cortex
rather than the retina. Supporting this idea, vernier acuity seems to
correspond very closely (and may have the same underlying mechanism)
enabling one to discern very slight differences in the orientations of
two lines, where orientation is known to be processed in the visual
cortex.
The smallest detectable visual angle produced by a single fine
dark line against a uniformly illuminated background is also much less
than foveal cone size or regular visual acuity. In this case, under
optimal conditions, the limit is about 0.5 arc seconds or only about 2%
of the diameter of a foveal cone. This produces a contrast of about 1%
with the illumination
of surrounding cones. The mechanism of detection is the ability to
detect such small differences in contrast or illumination, and does not
depend on the angular width of the bar, which cannot be discerned. Thus
as the line gets finer, it appears to get fainter but not thinner.
Stereoscopic acuity is the ability to detect differences in depth
with the two eyes. For more complex targets, stereoacuity is similar to
normal monocular visual acuity, or around 0.6–1.0 arc minutes, but for
much simpler targets, such as vertical rods, may be as low as only 2 arc
seconds. Although stereoacuity normally corresponds very well with
monocular acuity, it may be very poor, or absent, even in subjects with
normal monocular acuities. Such individuals typically have abnormal
visual development when they are very young, such as an alternating strabismus, or eye turn, where both eyes rarely, or never, point in the same direction and therefore do not function together.
Motion acuity
The eye has acuity limits for detecting motion. Forward motion is limited by the subtended angular velocity detection threshold
(SAVT), and horizontal and vertical motion acuity are limited by
lateral motion thresholds. The lateral motion limit is generally below
the looming motion limit, and for an object of a given size, lateral
motion becomes the more insightful of the two, once the observer moves
sufficiently far away from the path of travel. Below these thresholds subjective constancy is experienced in accordance with the Stevens' power law and Weber–Fechner law.
Subtended angular velocity detection threshold (SAVT)
There is a specific acuity limit in detecting an approaching object's looming motion. This is regarded as the subtended angular velocity detection threshold (SAVT) limit of visual acuity. It has a practical value of 0.0275 rad/s. For a person with SAVT limit of 
, the looming motion of a directly approaching object of size S, moving at velocity v, is not detectable until its distance D is
where the S2/4 term is omitted for small objects relative to great distances by small-angle approximation.
To exceed the SAVT, an object of size S moving as velocity v must be closer than D; beyond that distance, subjective constancy is experienced. The SAVT can be measured from the distance at which a looming object is first detected:
where the S2 term is omitted for small objects relative to great distances by small-angle approximation.
The SAVT has the same kind of importance to driving safety and
sports as the static limit. The formula is derived from taking the derivative of the visual angle with respect to distance, and then multiplying by velocity to obtain the time rate of visual expansion (dθ/dt = dθ/dx · dx/dt).
Lateral motion
There are acuity limits () of horizontal and vertical motion as well. They can be measured and defined by the threshold detection of movement of an object traveling at distance D and velocity v orthogonal to the direction of view, from a set-back distance B with the formula
Because the tangent of the subtended angle is the ratio of the orthogonal distance to the set-back distance, the angular time rate (rad/s) of lateral motion is simply the derivative of the inverse tangent multiplied by the velocity (dθ/dt = dθ/dx · dx/dt).
In application this means that an orthogonally traveling object will
not be discernible as moving until it has reached the distance
where for lateral motion is generally ≥ 0.0087 rad/s with probable dependence on deviation from the fovia and movement orientation,
velocity is in terms of the distance units, and zero distance is
straight ahead. Far object distances, close set-backs, and low
velocities generally lower the salience of lateral motion. Detection
with close or null set-back can be accomplished through the pure scale
changes of looming motion.
Radial motion
The motion acuity limit affects radial motion in accordance to its definition, hence the ratio of the velocity v to the radius R must exceed :
Radial motion is encountered in clinical and research environments, in dome theaters, and in virtual-reality headsets.
