3. Contrast sensitivity Flashcards
Define spatial frequency
separation in space (fine or course gratings)
Define temporal frequency
separation in time (fast or slow reversals)
Define contrast
the peak to peak amplitude of the luminance profile
Define spatial acuity
ability to resolve 2 points in space
What is spatial acuity dependant on?
Illumination and retinal location
How to convert VA to cycles per degree (spatial frequency)
Multiply the snellen decimal by 30.
What are the 5 parameters?
- Contrast
- Spatial frequency
- Mean luminance
- Orientation
- Phase
Draw contrast sensitivity function
CS, Contrast, Spatial frequency
Band pass shape
What is the resolution limit?
At max contrast, grating can’t be detected. This is due to optical aberrations and the packing of retinal cone mosaic.
What is the typical value of resolution limit?
40 cycles/degree
How are edges and contrasts detected?
Due to circular receptive fields of ganglion cells, lateral inhibition is induced. This makes the visual system less sensitive to low spatial frequencies. This enhances the ability to detect edges and contrasts for detail visual information.
What is the difference between periodic and non periodic?
Periodic - gratings
Non periodic - a single bar
How does luminance affect Contrast sensitivity?
Higher luminance improves resolution limit
Draw graph for luminance on contrast sensitivity
Scotopic =
poor high SP
Good low SP - this is important for detection in the dark.
How does adaptation affect contrast sensitivity? What does this imply?
After adapting to a high contrast grating for a few minutes, sensitivity was reduced.
This implies that there are a series of channels which specialise in detecting different spatial frequencies. At least 8 channels.
