The Measurement of Vision and Visual Acuity
The traditional way to measure vision and visual acuity is to use an
Optometrist’s 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
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)
Problem?
The scale is not
linear (i.e. not
equal interval)
H
60
VF
36
ETP
24
NUFCOK
12
UTFEVNP
6
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
=> log 1 =
6/12 = 2 minutes arc => log 2 =
6/60 = 10 minutes arc => log 10 =
0
0.3
1
The Bailey-Lovie chart has the same number (5) of letters on
each line, and it does provide an equal-interval scale.
The Measurement of Vision and Visual Acuity
- The use of different targets