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.