Measurement of vision and visual acuity Flashcards
what is vision?
.the eyes ability to discriminate or resolve spatially organised detail.
.measure of smallest angle can be resolved
what are the two types of vision?
peripheral and central
what is the standard observation of vision?
6/6
why do we measure vision?
.allows us to quantify what people can see and cant
.early detection and diagnosis of disease
.assess benefits of optical devices
.set base line to monitor changes
what are the reasons for reduced vision/visual acuity?
.un corrected refractive error
.eye disease
what is the difference between vision/visual acuity?
vision- is what we see without our spectacles or contact lens
visual acuity- ability to distinguish between two different points with spectacles or contact lens
what commonly used charts do we use to measure vision/visual acuity?
.snellen chart
.LOGMAR chart
what are the snellen numbers?
numbers relate to the distance that this letter would create an angle of 5 at the eye
what does 6/12 mean in the snellen chart?
6- is how many m away the chart is
12- refers to the line read on snellen chart
what is MAR?
minimum angle of resolution
which is the angle created by a limb of a letter that can be seen
how do you write down MAR?
6/12
MAR is 2
what are the advantages of snellen chart?
.cheap and readily available
.can be used to obtain baseline visual acuity measurement
what are the disadvantages of snellen chart?
.insufficient acuity levels for accurately testing low vision patients
.non-uniformity in letter size progression
.inability to easily score letter by letter acuity
how to measure vision/visual acuity for child under 5?
- place snellen chart at 6m
- room lighting ON
- for VA : ask patient to wear their distance spectacle
- occlude left eye
- ask what is the smallest line that you can read
- repeat for left eye by covering right eye
how many letters are there in log MAR chart?
there are five letter per line
we can score 0.02 logMAR per letter
acuity levels on this chart progress in step of 0.1
how to record vision/visual acuity in logMAR chart?
.for each error that is made add 0.02 to the vision score
e.g. if a patient reads all of line 0.30 except one letter their score is 0.32
.for each letter that is correct subtract 0.02 to vision score
e.g. if patient reads 0.00 line and 2 letters of next line their score is -0.04
e.g. if patient reads the 0.30 line and 2 letters on next score is 0.26
what are the advantages of logMAR chart?
.equal number of letters in every line
.more popular in research setting
what are the disadvantages of logMAR chart?
.large size
.cost
when is it important to use pinhole?
if visual acuity is less than 6/9 or logMAR 0.20
assess through pinhole
how does pinhole work?
pinhole helps your eyes focus by shielding your vision from indirect rays of light
what to do if vision is less than 6/60?
.test at 2m then 1m
.if patient cant see at 1m ask how many fingers your holding
.if patient still cant see try hand movement then check if they can determine light
how to measure near vision?
.use specifically designed reading card .each size is numbered .test each eye separately make sure the lighting is good .record smallest print read e.g. R.N8 at 25cm
what does the N notation mean?
indicates measurement is near
N is the point size of the print
what other test can be used?
illiterate E - where the E is facing
landolt c
