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)