Visual Acuity Flashcards
When a point source of light passes through a lens (a circular aperture) it does not result in a point image. What does it result in, and what causes this?
Results in a degraded image that forms an airy disk pattern. Due to diffraction.
What is a point spread function?
The image that an optical system forms of a point source.
What 2 factors affect an airys disc?
- Wavelength is directly related
2. Pupil diameter is inversely related
Why can a perceived size of a point source depend on the pmts detection threshold for contrast and luminance?
Because a small bright point may be perceived the same as a larger, but dimmer point.
For a 2.5 mm pupil, the cut off frequency for a MTF is approx 60cpd. The spot diameter corresponding is approx ___ sec of arc.
Any spot smaller than 30 sec of arc can be treated as a point.
Any spot larger than 30 seconds will have different retinal light distribution.
Any spot smaller than 30 sec of arc can be treated as a ___.
Any spot larger than 30 seconds will have ____
Point
Different retinal light distribution
4 Types of visual acuities and examples
- Resolution acuity. Tumbling E, landolt C
- Detection acuity. 2 forced choice test.
- Orientation/localization/hyperacuity. Vernier.
- Recognition. Snellen, LogMar, Bailey Lovie
Resolution acuity
- Definition
- Requires good
- Normal threshold
- Examples
- Minimum separable acuity. The minimum angle between two objects for them to be reliably discriminated.
- Requires good localization acuity and luminous discrimination.
- 1 arc min
- Landolt C, Tumbling E
Rayleigh Resolution limit
Related to resolution acuity.
When the diffraction minimum of one function is at the same location in space of the max of the other function, that is just enough separation for you to be able to recognize 2 separated images.
Detection acuity
- Definition
- Requires good
- Normal threshold
- Examples
- Minimum visible acuity. The angular subtense of the smallest object that can be detected visually at very high contrast.
- Acuity depends on contrast of the target. Will be high, above threshold.
- 1 sec of arc
- 2 forced choice test. “Which interval has the stimulus?”
Localization/Orientation/Hyperacuity
- Definition
- Resistant to
- Normal threshold
- Examples
- Threshold in visual angle for reliably discriminating small differences in the position of one object relative to another.
- Resistant to optical blur
- 4-10 arc seconds
- Vernier acuity
Recognition Acuity
- Definition
- Requires good
- Normal threshold
- Examples
- Smallest visual angle of critical detail needed to accurately identify an object.
- Good luminance discrimination, localization acuity, and previous knowledge about the object.
- 1 arc min.
- Snellen, log mar, bailey lovie
Snellen acuity vs log mar. Does a low or high decimal represent good Visual acuity?
Snellen: 1 (high dec) represents good acuity.
Log mar: 0 (low dec) represents good acuity.
Psychometric function for visual acuity
Number of correct responses vs letter size.
Steep: Means errors are small.
Flatter: Maybe person has amblyopia due to more errors. Amblyopic pts are more susceptible to crowding.
Bailey and Lovie chart characteristics
5 letters per line.
1 letter width of space between each letter on a row
Between row spacing= height of letters in smaller row
Size progression between lows is 0.1 log unit.
Viewing at non-standard distances is easy
Credit given per letter
Represents equal functional changes in acuity, no matter where person began.
Factors contributing to spatial resolution and recognition acuity
- Pupil size.
- Less than 2.5 causes too much diffraction.
- Between 2.5-5 is optimal
- Greater than 5 causes aberrations - Illumination/adaptation. Rod and cone system working properly.
- Orientation effects.
- Vertical is optimal
- Horizontal is basically optimal
- Oblique is worst - Retinal eccentricity
- Slightly better acuity nasal retina/temporal field - Limited by the density of retinal ganglion cells. not the cones.
- Optical blur.
- Crowding. Worse for amblyopic pts.
- Motion effects: Dynamic acuity.
- Retinal image velocities lower than 3-4 degrees/sec can be tolerated.
- Object motion lower than 60-80 degrees/sec can be tolerated if patient can move head and body. If not, only 3-4 sec. - Age/development.
- Rapid development within first couple months.
- Vision declines in mid 20s. - Reverse contrast acuity is better. What we have. Black letter on white.
Spatial resolution and recognition acuity is limited by the density of _____
GC!! Not the cones!!
Nyquist sampling theorem
- Photoreceptor sample= spatial frequency
- Photoreceptor samples greater than the spatial frequency.
- PR samples less than frequency cycles
The highest spatial frequency that can be detected and reliably transmitted to the retina is equal to half of the sampling frequency.
- Transmission would be flat. PR would sample the frequency at the same part in every cycle. No change over time.
- The photoreceptors sample the frequency multiple times during the same cycle. Will have 100% transmission.
- Not transmitted completely. The PR samples the frequency at a different point during each cycle. The nature of how light is changing across space is not being relayed to the rest of the visual system.
