Transcript 99HUB103 Vision - Individual Web Pages | IT Services

The Physical Stimulus: Spatial pattern
(This is a poorly-generated approximation to a sine wave)
The Physical Stimulus: Spatial pattern
Why are we interested in the visual system’s response to sine waves?
When you know the response to individual sine waves you can, if the
system is ‘linear’, determine the response to the complete pattern.
Remember the Fourier analysis tutorial. We can think of the limbs of
the letter E as being a ‘square wave’ - and, as you know, this can be
generated by adding together a series of sine waves.
The Physical Stimulus: Spatial pattern
photometer
(contrast)
Position
N.B. The (spatial) frequency = 1/period
For vision, this is measured in cycles per degree
Spatial (Threshold [foveal] Performance)
Pattern not seen
1
Modulation
(i.e.reciprocal
contrast)
Visual
Acuity.
Pattern seen
(100%)
Spatial Frequency
The following slide allows you to view your own
contrast sensitivity function.
Try viewing it at different distances, and you’ll see
the appearance change.
Also, you may see inconsistencies at the extreme
right of the figure. This is termed ‘aliasing’ and
occurs because the pixel nature of the screen does
not allow perfect reproduction of the picture.
Seeing your own CSF
Contrast
Spatial Frequency
The Measurement of Spatial (foveal) Visual
Performance
Contrast sensitivity is a relatively new way of assessing
vision
The more traditional way is to use an Optometrist’s test
chart, which contains letters of different sizes.
H
60
VF
36
ETP
24
NUFCOK
12
UTFEVNP
6
Letter Size
(explained on
the next slide)
The Measurement of Spatial (foveal) Visual
Performance
The letter size is specified by the distance at which the
complete letter subtends 5 mins of arc.
The detail subtends 1 min arc.
.
6 metres
12 metres
The Measurement of Spatial (foveal) Visual
Performance
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
The Measurement of Spatial (foveal) Visual
Performance
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
Measured this way, the logMAR scale is, arguably, at an
interval level - hence one can perform parametric statistical
analysis on data obtained using this test.
The following are
letter charts which
have decreasing
contrast.
H
1
VF
2
ETP
3
NUFCOK
4
F V U P L N P
5
N
P
U
L
B
E
V
6
E
1
H F
2
ETP
3
NUFCOK
4
P L N F V U L
5
L
B
E
V
N
P
U
6
P
1
H E V
2
ETP
3
NUFCOK
4
E VU T F N P
5
E
V
N
L
B
P
U
6
P
This is a
high-contrast
version of the
previous chart.
1
H E V
2
ETP
3
NUFCOK
4
E VU T F N P
5
E
V
N
L
B
P
U
6
Compare:
E
smaller
higher
E
Letter Size
Spatial Frequency
The Optometrists chart will generally
only contain high-contrast letters.
These provide maximum acuity.
As you will see from the following graph,
one would predict that lower contrast
letters would produce a lesser visual
performance.
0.1%
Contrast Sensitivity
Pattern not seen
1%
Contrast
Pattern seen
10%
100%
Spatial Frequency
Cut-off