The Measurement of Vision and Visual Acuity

1
The Measurement of Vision and Visual Acuity
The traditional way to measure vision and visual
acuity is to use an Optometrists test chart,
which contains letters of different sizes.
On these charts, the letter size is specified not
by its height, but by the distance at which the
complete letter subtends 5 mins of arc. (If
the letter subtends 5 mins arc, then the detail
subtends 1 min arc.)
.
6 metres
12 metres
2
The Measurement of Vision and Visual Acuity
6 6 Vision - the ability to read the 6 metre
letter at a distance of 6
metres (USA - 20 feet) hence 20
20 Minimum angle of
resolution (MAR) here is 1 minute of arc A
recent development is to use as a metric the
logarithm of this angle - logMAR scale
(Bailey and Lovie)
3
H VF ETP NUFCOK UTFEVNP
60 36 24 12 6
Problem? The scale is not linear (i.e. not equal
interval)
4
The Measurement of Vision and Visual Acuity
Minimum angle of resolution (MAR) here is 1
minute of arc A recent development is to use as
a metric the logarithm of this angle - logMAR
scale (Bailey and Lovie) 6/6 1
minute arc gt log 1 0 6/12
2 minutes arc gt log 2
0.3 6/60 10 minutes arc gt log 10
1 The Bailey-Lovie chart has the same
number (5) of letters on each line, and it does
provide an equal-interval scale.
5
The Measurement of Vision and Visual Acuity - The
use of different targets