Visual Acuity Flashcards
What does VA refer to
The spatial limit of visual discrimination
Image of the iris formed by the cornea refers to
Entrance pupil
Where is the entrance pupil situated
about 3 mm behind the corneal vertex
What are some contributing factors to light spreading and interference
- Diffraction
- Abberations
- Scatter
- Absorption
- focus factors
What does diffraction cause
limitation of the aperture causes a spread of light
Small pupil diameter
actual image spread = the diffraction image
What does aberration refer to
Rays entering the periphery of the pupil may not converge on the geometric image point. This is more prominent as the pupil widen
What happens with pupil diameters larger than 5 mm
the spread is usually increased because the peripheral regions of the cornea and lens are often afflicted with optical aberrations
Why does scatter happen
Ocular media has some microscopic structure so light is scattered in passing from the cornea to the retina.
What does absorption refer to
Media is not uniformly transparent to incoming light
The ______the wavelength of the entering light the _____the proportion that reaches the retinal receptors
shorter, the smaller
Defocus refers to
a person having active accomodation
One example of focus not being correct
Night myopia…No target to look at, dont know where to focus. Your accomodation system focuses more than needed
Fogging refers to
Getting more minus refraction than needed
In the fovea, how are cones packed
Two to the linear minute of arc
In the retinal periphery, where do rods converge
on a ganglion cell
How would you detect that a double star is composed of two separate stimuli
The trough between the two light peaks must be deep enough that an intensity discrimination can be made . delta i/i
T/F As resolution is detiorating, peaks have to be brought closer together
false! As resolution deteriorates, peaks have to be separated further to create a trough large enough to be detected
What is minimum visible VA
The criterion of the presence of a single feature
What is the threshold value of minimum visible VA
1 second of arc
What is minimum resolvable/ordinary VA
The criterion of the presence or internal arrangement, of identifying features in a visible target. You’re dealing with a resolution task
What is the MAR for ordinary va
between 30 seconds to 1 minute of arc
What is minimum discriminable/hyperacuity
The criterion of the relative location of visible features. Threshold is much lower than ordinary VA
What is the best known acuity for hyperacuity
Alignment or vernier acuity
