Contrast Sensitivity

Question 1

what is the snellen acuity equivalent of a high frequency cut off of 60 cpd?

Answer 1

20/10

600/60=10

Question 2

what is the snellen equivalent to a reduced snellen fraction of 0.2?

Answer 2

20/100

20/x=0.2
x=100

Question 3

contrast

Answer 3

defined as the ratio between 1. the difference between the peak luminance and the average luminance, and 2. the average luminance. If is most often represented by a number between 0 and 1; the higher the number the better the contrast. Snellen acuity chart is almost 100% contrast

Question 4

How can contrast be measured clinically

Answer 4

sinusoidal gratings

• the patient is presented with vertically oriented gratings at various contrast levels and various spatial frequencies (diff spacing between the gratings)
• avaialble sinusoidal charts include the Arden Plate test and the Vis-tech chart
• the contrast sensitivity function can be plotted with the spatial frequency on the x-axis (cpd) and contrast sensitivity on the y-axis (in dB). The plot will have a single maximum peak around 3-6 cpd

Question 5

How can contrast be measured clinically

Answer 5

sinusoidal gratings

• the patient is presented with vertically oriented gratings at various contrast levels and various spatial frequencies (diff spacing between the gratings)
• avaialble sinusoidal charts include the Arden Plate test and the Vis-tech chart
• the contrast sensitivity function can be plotted with the spatial frequency on the x-axis (cpd) and contrast sensitivity on the y-axis (in dB). The plot will have a single maximum peak around 3-6 cpd

Question 6

the high frequency cut off on the contrast sensitivity function

Answer 6

the clinically measured visual acuity. In a healthy eye, it is limited by uncorrected reactive error, optical aberrations, and the density of the retinal PRs. This cut offs is located where the CSF curve crosses the x-axis. Remember that the low frequency cut off is due to lateral inhibition in the retina

Question 7

letter charts that test contrast sensitivity

Answer 7

pelli Robson and Bailey-Lovie

Question 8

contrast and age

Answer 8

contrast sensitivity at higher frequencies decreases with increased age, even in healthy, disease free eyes. As a result, older patients have more complaints in low lighting and more problems with glare, even if their snellen visual acuity of 20/20

Question 9

what filters are good or contrast

Answer 9

yellow and orange

Question 10

filters

Answer 10

allow some light to pass through and other light to be absorbed, depending on the wavelength of light. They can improve contrast, reduce glare, and improve the patients overall comfort. Note that increasing the light levels will also be an appropriate treatment option for the patient.

Question 11

narrow band filters

Answer 11

only allow a small range of wavelength to pass through; the remaining wavelengths are absorbed

Question 12

narrow band filters

Answer 12

only allow a small range of wavelength to pass through; the remaining wavelengths are absorbed

Question 13

interference filters

Answer 13

highly selective filters that essentially transmit only a single wavelength

Question 14

borad band filters

Answer 14

transmit a large range of wavelengths

Question 15

long pass filters

Answer 15

transmit only long wavelengths, resulting in altered color perception. They may be used as sunglasses because they block the transmission of short harmful UV rays

Question 16

Neutral density filters

Answer 16

transmit all wavelengths equally, but reduces the amount of light entering the eye; they do not alter color perception. they may be used as sunglasses, and can also be used to measure the severity of an APD. Neutral density are added over the good eye until the APD is no longer visibly in the bad eye

Question 17

blue blockers

Answer 17

amber tinted lenses that block the transmission of short wavelengths of light in order to reduce discomfort glare. Because they only block the transmission of short wavelengths (acting as long pass filters), they alter color perception, resulting in chromatopsia. Blue blockers are often prescribed for patients with low vision and glare issues

Question 18

transmission ofa fitler

Answer 18

specified as a percentage which indicates the amount of light that is allowed to pass through the lens

Question 19

spatial vision

Answer 19

the ability of the eye to detect changes in brightness and the detect fine details (related to perception of space). It is measured clinically with visual acuity and contrast sensitivity

Question 20

temporal vision

Answer 20

used to detect motion (related to perception of time). The CFF is measured by showing a patient a light that continues to flicker at an increased rate. eventually, the patient will perceive that the light is constant rather than flickering. This is the patient’s high temporal resolution limit and is measured in hertz. 1 hertz is equal to 1 cpd