W6: Contrast sensitivity

Question 1

What is contrast sensitivity and why is it important?

Answer 1

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.

Question 2

Why is it important to test contrast sensitivity?

Answer 2

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

Question 3

What is contrast (luminance contrast)?

Answer 3

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.

Question 4

What is Simple Contrast?

Answer 4

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

Question 5

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

Answer 5

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

Question 6

How to calculate Contrast sensitivity?

Answer 6

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

Question 7

Spatial period:

Answer 7

width of one cycle = wavelength

Question 8

Spatial frequency

Answer 8

cycles/degree (high = fine detail)

Question 9

Mean luminance

Answer 9

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

Question 10

Modulation amplitude

Answer 10

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

Question 11

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

Answer 11

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

Question 12

What does it mean to have low or high modulation

Answer 12

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

Question 13

The contrast sensitivity curve

Answer 13

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

Question 14

Fixed contrast / variable size

Answer 14

◦ Bailey-Lovie low contrast chart

Question 14

Variable size / variable contrast

Answer 14

◦ Vistech Chart
◦ Functional Acuity Contrast Test (FACT)

Question 15

Fixed size / variable contrast

Answer 15

◦ Melbourne Edge Test

Question 16

Type of glares and how is glare sensitivity measured?

Answer 16

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

Question 17

Causes of sensitivity to glare

Answer 17

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

Question 18

What is a Brightness Acuity Tester (BAT)

Answer 18

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

Question 19

What is a Macula photostress test

Answer 19

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

Question 20

Berkeley Glare Acuity Tester (BGAT)

Answer 20

• 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