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Block 6 > Optics: Visual Acuity > Flashcards

Optics: Visual Acuity Flashcards

1
Q

JND stands for what

A

Just notable difference

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2
Q

Two types of interference patterns

A

Vertical slit and circular aperture

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3
Q

What is the airy disk?

A

The bright center in a circular aperture

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4
Q

If angle is greater than theta minimum then

A

Resolved image

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5
Q

If angle is less than theta minimum then you see

A

Unresolved image

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6
Q

If angle is equal to theta minimum then you see

A

Rayleigh’s criterion

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7
Q

When both central maxima overlap, the image is

A

Unresolved

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8
Q

When the central maxima of one coincides with first minima of other the image is

A

Just resolves (Rayleigh’s criterion)

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9
Q

When the central maxima of both are separate/far apart, the image is

A

Well resolved

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10
Q

The equation for theta minimum in Rayleigh’s criterion is

A

(1.22* wavelength)/Diameter

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11
Q

What are the four types of acuity?

A

-Minimum detectable
-minimum separable
-minimum cognizable
-minimum discriminable

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12
Q

Minimum detectable acuity

A

Ability to detect if an object is present on an empty space

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13
Q

minimum detectable only requires

A

The perception of the presence or absence of an aspect of the stimuli, not the discrimination of target

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14
Q

The limit for minimum detectable is

A

About 1 arc seconds

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15
Q

Minimum separable acuity is

A

(Resolution acuity)
-ability to discriminate two spatially separated targets

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16
Q

For minimum separable acuity threshold for discrimination is

A

30-60 seconds of arc

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17
Q

Minimum angle of resolution is

A

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

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18
Q

Minimum cognizable acuity is

A

(Recognition)
Ability to recognize or name a target, such as with snellen letters

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19
Q

In minimum cognizable you quantify the

A

Smallest letter recognizable

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20
Q

Minimum discriminable acuity is

A

(Hyperacuity)
Discrimination abilities in which the human observer surpasses the acuity limit, often by a large factor

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21
Q

Minimum discriminable acuity involves discriminating differences in …

A

spatial position of segments of a test object, such as a break or discontinuity in contour

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22
Q

The visual acuity test used for minimum discriminable acuity is called

A

Vernier acuity

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23
Q

Visual acuity is

A

The ability to resolve fine detail in a pattern

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24
Q

Visual acuity is usually determined by

A

Reducing the size of a test pattern until the smaller detail in the pattern can just be resolved

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25
Visual acuity can be expressed numerically in terms of
The reciprocal of the size of the smaller resolvable detail
26
Each stroke for a letter is
1 min of arc
27
The numerator and denominator of the visual acuity fraction represents
Viewing distance/ size of letter
28
The MAR value corresponds to
One stroke of the letter, not the whole letter
29
The total angular size of the letter is
Five times the MAR
30
Donders proposed a formula defining the
Sharpness of vision
31
Size seen by patient / size seen by “standard eye” =
Magnification needed by the patient
32
Snellen’s original chart had
A single large letter at the top with each row getting progressively smaller
33
Snellen’s original optotypes were
Serif letters designed with the framework of 5 units high and 5 or 6 units wide
34
Green’s charts important features:
1. Used geometric progression 2. His letter and line spacing was not fixed, but proportional to the letter size 3. He used non-serif letters
35
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
36
The current standard for charts is ETDRS, what does this mean?
-proportional spacing -geometric preogression -non-serif letters -5 letters each line International standard
37
What is the logarithmic(geometric) progression of the optotypes on the chart?
Each row is 10^0.1 times smaller than the previous row
38
Minimum angle of resolution(MAR) expresses
The angular size of detail within the optotypes at threshold
39
Foot-size is defined as
-The distance at which an optotypes subtends 5 minutes of arcs - used at the denominator in the snellen fraction
40
LogMAR gives a constant _____ log unit difference between each successive row on the chart
0.10
41
For better visual acuities log MAR values become
Negative
42
Snellen acuity is equal to the ______ of the minimum resolvable angular detail
Reciprocal
43
Rayleigh’s criteria can be used to estimate
Snellen acuity using the smallest resolvable angle,w wavelength, and size of pupil
44
How do you convert from radians to minutes
1 radian = 3437.75 arc minute
45
Grating acuity can be used..
As another method of measuring visual acuity fraction
46
Visual acuity in snellen notation can be expressed in ____ frequency
Spatial
47
What is the maximum resolution of humans?
60 cycles per degree
48
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
49
IN grating acuity, a 20/20 letter completes ____ min of arc in one cycle
2 min
50
How many min are in one degree?
60 mins
51
How many cycles does one degree contain?
30 cycles
52
The bottom letter in the VA test denotes the _______ ________ but does NOT correspond to the actual_______ ________
Letter SIZE, letter HEIGHT
53
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
54
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
55
Normal testing distance for countries that use the metric system
6 meters
56
MAR acuity is based on the ____-___________ number of minutes of arc a person can see
Just-distinguishable
57
MAR =
1/snellen VA
58
LogMAR is the…
Log of the MAR acuity
59
20/20 is what in MAR? And LogMAR?
MAR = 1 LogMAR = 0
60
How to calculate the acuity when you have to adjust testing distance? (equation)
New test distance / tested acuity letter “size” = 20 feet / x

Block 6 (32 decks)

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