W6: Contrast sensitivity Flashcards

1
Q

What is contrast sensitivity and why is it important?

A

The ability to detect differences in luminance between regions that are not separated by physical borders.
Contrast sensitivty provides useful info about functional or real-world vision as most things are not high contrast.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Why is it important to test contrast sensitivity?

A

To determine ocular problems such as diseases, media opacities, neural issues ect.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

What is contrast (luminance contrast)?

A

Is the relationship between the luminance of a brighter area of interest and that of an adjacent darker area.
C = (Lmax - Lmin ) / Lmin
- Weber Contrast, and is the most commonly useful one in the
context of lighting.

For non-repetitive backgrounds or background that remains constant e.g.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What is Simple Contrast?

A

Values are often used in photography, to specify the difference between
bright and dark parts of the picture.
- This definition is not useful for real-world luminances, because of their much higher dynamic range and the logarithmic response characteristics of the human eye.

Csimple = Lmax / Lmin

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What is Peak-to-Peak Constrast (Michelson Contrast, Modulation)?

A

measures the relation between the spread and the sum of the two luminances.

  • Modulation = (Lmax - Lmin ) / (Lmax + Lmin )

Used in signal processing theory, to determine the quality of a signal relative to its noise level. In the context of vision, such noise could be caused by scattered light introduced into the view path by a translucent element partly obscuring the scene behind it

  • For repetitive/changing patterns: no large area of luminance to provide Lmax
  • Michelson contrast: Value between 0 and 1 (0 to 100%)
  • Signal to noise: 0 = blank pattern, higher number = easier to see
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

How to calculate Contrast sensitivity?

A

1/threshold
e.g 1 diveded by 0.1 known as10%

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

Spatial period:

A

width of one cycle = wavelength

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

Spatial frequency

A

cycles/degree (high = fine detail)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

Mean luminance

A

average of maximum and minimum luminance (Lmax + Lmin / 2)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

Modulation amplitude

A

Difference between maximum and mean luminance ((L max - L mean ) = contrast

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What’s the high-frequency cut-off of a patient with 6/12 acuity?

A

If there are 30 cycles per degree and one degree of 60 mins then each dark strip has a width of 1min per arc.
6/6 = 30 cycles per degree, if we double stripe width, that will be half the number of stripes per degree i.e 15cpd = 6/12 = 2min per arc

page 11

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What does it mean to have low or high modulation

A

Low modulation means low contrast - wavelength is shorter vertically
High modulation means high contrast- wavelenght is higher verticallty

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

The contrast sensitivity curve

A

Points under the curve can be
seen, while points above curve
cannot be seen
- Bottom of area under curve
indicates familiar high contrast
letter chart, can see that this is
always seen whereas at lower
contrast: changes with acuity

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Fixed contrast / variable size

A

◦ Bailey-Lovie low contrast chart

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Variable size / variable contrast

A

◦ Vistech Chart
◦ Functional Acuity Contrast Test (FACT)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

Fixed size / variable contrast

A

◦ Melbourne Edge Test

16
Q

Type of glares and how is glare sensitivity measured?

A

Measurement of effect of glare on patients’ visual
function using CS chart and brightness acuity tester
(BAT)

Types of glare:
◦ Disability glare: causing reduction of visual performance
◦ Discomfort glare: causing discomfort without any measurable reduction in visual function

17
Q

Causes of sensitivity to glare

A

Partly depends on clarity of media

Increased sensitivity in:
* Patients with cataracts
* Intraocular lenses
* Contact lenses
* Older age (media changes in lens and cornea)
* Ocular surface disorders
* Binocular dysfunction
* Ocular disease

18
Q

What is a Brightness Acuity Tester (BAT)

A

Hand-held instrument with two
functions
◦ Brightness acuity test
◦ Macular photostress test
BAT can simulate three conditions:
high-direct overhead sunlight,
medium/partly cloudy day, low-
bright overhead commercial lighting

19
Q

What is a Macula photostress test

A

NOT a measure of glare sensitivity
* BAT provides hemispherical light source which
stresses entire macular region
* Sensitive test for detecting patients with macular disease e.g. ARMD, macular oedema, CSR

20
Q

Berkeley Glare Acuity Tester (BGAT)

A
  • Reduced high and low contrast BL letter charts
    mounted on opaque panel in the centre of a
    plexiglass screen
  • Chart is front illuminated and glare source
    provided by transillumination of plexiglass
    panel
  • High and low contrast VA measured at 1 m
    without and without glare source