W6: Contrast sensitivity Flashcards
What is contrast sensitivity and why is it important?
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.
Why is it important to test contrast sensitivity?
To determine ocular problems such as diseases, media opacities, neural issues ect.
What is contrast (luminance contrast)?
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.
What is Simple Contrast?
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
What is Peak-to-Peak Constrast (Michelson Contrast, Modulation)?
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 to calculate Contrast sensitivity?
1/threshold
e.g 1 diveded by 0.1 known as10%
Spatial period:
width of one cycle = wavelength
Spatial frequency
cycles/degree (high = fine detail)
Mean luminance
average of maximum and minimum luminance (Lmax + Lmin / 2)
Modulation amplitude
Difference between maximum and mean luminance ((L max - L mean ) = contrast
What’s the high-frequency cut-off of a patient with 6/12 acuity?
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
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What does it mean to have low or high modulation
Low modulation means low contrast - wavelength is shorter vertically
High modulation means high contrast- wavelenght is higher verticallty
The contrast sensitivity curve
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
Fixed contrast / variable size
◦ Bailey-Lovie low contrast chart
Variable size / variable contrast
◦ Vistech Chart
◦ Functional Acuity Contrast Test (FACT)
Fixed size / variable contrast
◦ Melbourne Edge Test
Type of glares and how is glare sensitivity measured?
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
Causes of sensitivity to glare
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
What is a Brightness Acuity Tester (BAT)
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
What is a Macula photostress test
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
Berkeley Glare Acuity Tester (BGAT)
- 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
