Optics: Visual Acuity Flashcards
JND stands for what
Just notable difference
Two types of interference patterns
Vertical slit and circular aperture
What is the airy disk?
The bright center in a circular aperture
If angle is greater than theta minimum then
Resolved image
If angle is less than theta minimum then you see
Unresolved image
If angle is equal to theta minimum then you see
Rayleigh’s criterion
When both central maxima overlap, the image is
Unresolved
When the central maxima of one coincides with first minima of other the image is
Just resolves (Rayleigh’s criterion)
When the central maxima of both are separate/far apart, the image is
Well resolved
The equation for theta minimum in Rayleigh’s criterion is
(1.22* wavelength)/Diameter
What are the four types of acuity?
-Minimum detectable
-minimum separable
-minimum cognizable
-minimum discriminable
Minimum detectable acuity
Ability to detect if an object is present on an empty space
minimum detectable only requires
The perception of the presence or absence of an aspect of the stimuli, not the discrimination of target
The limit for minimum detectable is
About 1 arc seconds
Minimum separable acuity is
(Resolution acuity)
-ability to discriminate two spatially separated targets
For minimum separable acuity threshold for discrimination is
30-60 seconds of arc
Minimum angle of resolution is
The smallest angular size at which subjects can discriminate the separation between critical elements of a stimuli patterns such as a pair of dots, a grating or a checkerboard
Minimum cognizable acuity is
(Recognition)
Ability to recognize or name a target, such as with snellen letters
In minimum cognizable you quantify the
Smallest letter recognizable
Minimum discriminable acuity is
(Hyperacuity)
Discrimination abilities in which the human observer surpasses the acuity limit, often by a large factor
Minimum discriminable acuity involves discriminating differences in …
spatial position of segments of a test object, such as a break or discontinuity in contour
The visual acuity test used for minimum discriminable acuity is called
Vernier acuity
Visual acuity is
The ability to resolve fine detail in a pattern
Visual acuity is usually determined by
Reducing the size of a test pattern until the smaller detail in the pattern can just be resolved
Visual acuity can be expressed numerically in terms of
The reciprocal of the size of the smaller resolvable detail
Each stroke for a letter is
1 min of arc
The numerator and denominator of the visual acuity fraction represents
Viewing distance/ size of letter
The MAR value corresponds to
One stroke of the letter, not the whole letter
The total angular size of the letter is
Five times the MAR
Donders proposed a formula defining the
Sharpness of vision
Size seen by patient / size seen by “standard eye” =
Magnification needed by the patient
Snellen’s original chart had
A single large letter at the top with each row getting progressively smaller
Snellen’s original optotypes were
Serif letters designed with the framework of 5 units high and 5 or 6 units wide
Green’s charts important features:
- Used geometric progression
- His letter and line spacing was not fixed, but proportional to the letter size
- He used non-serif letters
Why did Green get rid of the serif letters?
It can cause visual clutter for making it harder to identify the letters
-visual clutter is experienced by amblyopes
The current standard for charts is ETDRS, what does this mean?
-proportional spacing
-geometric preogression
-non-serif letters
-5 letters each line
International standard
What is the logarithmic(geometric) progression of the optotypes on the chart?
Each row is 10^0.1 times smaller than the previous row
Minimum angle of resolution(MAR) expresses
The angular size of detail within the optotypes at threshold
Foot-size is defined as
-The distance at which an optotypes subtends 5 minutes of arcs
- used at the denominator in the snellen fraction
LogMAR gives a constant _____ log unit difference between each successive row on the chart
0.10
For better visual acuities log MAR values become
Negative
Snellen acuity is equal to the ______ of the minimum resolvable angular detail
Reciprocal
Rayleigh’s criteria can be used to estimate
Snellen acuity using the smallest resolvable angle,w wavelength, and size of pupil
How do you convert from radians to minutes
1 radian = 3437.75 arc minute
Grating acuity can be used..
As another method of measuring visual acuity fraction
Visual acuity in snellen notation can be expressed in ____ frequency
Spatial
What is the maximum resolution of humans?
60 cycles per degree
What is Nyquist limit?
Maximum spatial frequency that can be accurately resolved based on the spacing of photoreceptor cells in the retina
-which is roughly equivalent to 60 cycles per degree for the human eye
IN grating acuity, a 20/20 letter completes ____ min of arc in one cycle
2 min
How many min are in one degree?
60 mins
How many cycles does one degree contain?
30 cycles
The bottom letter in the VA test denotes the _______ ________ but does NOT correspond to the actual_______ ________
Letter SIZE, letter HEIGHT
What does “size” really mean in regards to VA?
A number that corresponds to the distance where a given letter will subtend the standard 5’ at the eye
What does “size” really mean in regards to VA?
A number that corresponds to the distance where a given letter will subtend the standard 5’ at the eye
Normal testing distance for countries that use the metric system
6 meters
MAR acuity is based on the ____-___________ number of minutes of arc a person can see
Just-distinguishable
MAR =
1/snellen VA
LogMAR is the…
Log of the MAR acuity
20/20 is what in MAR? And LogMAR?
MAR = 1
LogMAR = 0
How to calculate the acuity when you have to adjust testing distance? (equation)
New test distance / tested acuity letter “size” = 20 feet / x
