Lecture 5 and 6 The Optics and Physiology of VA, visual angle of optotype

what is visual acuity

the spatial resolving capacity of the visual system

what does spatial resolving capacity mean?

ability to resolve details and separate black and white bar

who first documented the relationship between vision and distance?

Daca de Valdes

who first defined visual acuity as the ratio between a subject’s performance and the standard performance?

F. Cornelius Donders

who first created a chart with standardized letters?

Herman Snellen

what is clinical visual acuity?

measurement of the patient’s ability to resolve fine detail on a standardized chart

how do we test clinical VA?

ask patients to identify optotypes

how we measure distance visual acuity (DVA) and what is the distance?

set chart at optical infinity at 20 ft or 6 cm

how do we measure near visual acuity (NVA)?

measure at 40 cm

what is habitual visual acuity?

indicates whether a patient has corrections (wear glasses or contacts) at a particular viewing distance

what is entering visual acuity?

indicates how well the patient can see with their current habitual correction

what is the best corrected visual acuity

indicates the patient’s visual acuity after refraction

where is the best corrected visual acuity usually measured?

at a distance

how we do test pinhole visual acuity?

at optical infinity and is indicated when entering habitual VA

when do we measure the pinhole visual acuity?

if vision is 20/30 or worse

what is VA required for a full driving license in MA, and under what condition?

at least 20/40 BCVA in the better seeing eye

what is VA required for a day-light only driving license in MA, and under what condition?

if VA is 20/50 to 20/70 in the better seeing eye

what is the VA for legal blindness?

BCVA of 20/200 or less in the better eye, or visual field no more than 20 degrees in diameter

how do we refer to the grating light when talking about visual acuity?

spatial resolution

how do we refer to the flickering light when talking about visual acuity?

temporal resolution

How did people in 500 BC define having good vision?

if you could distinguish the star Alcor from Mizar

which people first discussed having good vision based on stars?

Ancient Persia in 500 BC

what is the unit for angular measurement?

arc minute

what defines an arc minute?

used to describe how far apart objects need to be for a patient to distinguish them as two separate objects

1 min of arc = ? degree

1/60 of one degree

does smaller arc minute value correlates to better or worse? visual acuity

better

does larger arc minute value correlates to better or worse? visual acuity

worse

what is the minimum angle of resolution (MAR)?

the smallest angular separation a human eye can distinctively perceive as two separate points

as the number of cycle per degree increases, the cycles become easier or harder to distinguish as separate lines?

harder

as the number of cycle per degree decreases, the cycles become easier or harder to distinguish as separate lines?

easier

if MAR is increasing, is visual acuity better or worse?

worse

if MAR is decreasing, is visual acuity better or worse?

better

the MAR required to construct a full optotype is how many times larger than the MAR of its components parts?

five times

why is the MAR of a full optotype so much larger than the MAR of its individual components?

because each detail of the letter needs one MAR

what is the MAR of a 20/50 optotype?

2.5 min arc

how many MAR does an entire 20/50 optotype subtend on a VA chart?

12.5 min of arc

what is the definition of a spatial frequency in cycles per degree?

how often sinusoidal components of an image repeat per unit of disance

what is the MAR for snellen acuity of 20/20?

1

what is the cycles per degree for snellen acuity of 20/20?

30 cpd

what is the logMAR for snellen acuity of 20/20?

0

what is the M units for snellen acuity of 20/20 at 4 meters?

4M

what are the two main types of limitations of VA?

optics and anatomy and physiology

what are the two types of optical limits of VA?

optical aberrations and diffraction

what are the two types of anatomy and physiology limits of VA?

photoreceptor density and light/dark adaptation

what is an aberration?

light from one point in an object does not converge into a single point on the retina

what is the results of aberration?

decreased retinal image quality

what aberrations can be fixed with glasses?

low order aberrations like defocus and astigmatism

what aberrations cannot be fixed with glasses?

high order aberrations like coma and trefoil

what is diffraction?

deviation of light rays from rectilinear paths which cannot be interpreted as reflection or refraction

when does diffraction transform planar light beams into waves?

when transmitted through a finite aperture like a pupil

when planar light is transmitted through the pupil, what phenomenon happens?

diffraction

what does the shape of diffraction depend on?

pupil size

what is a large pupil limited by?

aberrations

what is a small pupil limited by?

diffraction

what defines a large pupil?

more light ot retina and less diffraction

what defines a small pupil?

less light to retina and less aberrations

how does anatomy and physiology limit our VA?

physical size of retinal cones limits the potential threshold of our visual acuity

what is the theoretical resolution limit of the cones at fovea?

30 second arc

what is the minimum resolvable letter stroke width that correspond to Snellen VA 20/10?

0.5 min arc

how does the photoreceptor density work?

density and size of photoreceptors varies with retinal eccentricity

Decreased VA in periphery corresponds to increased or decreasd photoreceptor density?

decreased

Increased VA in periphery corresponds to increased or decreasd photoreceptor density?

increased

define the light/dark adaption in the neural limits of VA

the ability of the visual system to adjust to changes in lgiht intensity

what are the mechanisms involved in light/dark adaption?

pupil size, photoreceptor utilization switch, bleaching or regeneration of photopigment, feedback from horizontal cells to photoreceptors

the difference in light threshold of light detection between light and dark adaptation differ by a factor of what?

10^8 times

area of the opening changes by a factor of how many when our eyes are in the dark?

16 times

why does shining light into a patient’s eyes adversely affect VA results?

because it takes time to regenerate photopigments after shining light on eye

what are the three types of photoreceptor utilization?

photopic vision, scotopic vision, and mesopic vision

what are three characteristics of the photopic vision?

bright illumination, high resolution, cone utilization

what are three characteristics of the scotopic vision?

low illumination, poor resolution, rod utilization

what are three characteristics of the mesopic vision?

bright moon light, both rods and cones utilization

the dimensions of the detail of an optotype is defined as?

MAR

MAR is inversely related to what

Snellen Fraction

for Snellen decimal, does a higher number means better or worse vision?

better

for Snellen decimal, does a lower number means better or worse vision?

worse

define the angular size of MAR

size of an optotype’s detail

when talking about angular size of MAR, what width are we talking about?

width of line used to draw the optotype

formula for angular size of MAR?

reciprocal of Snellen fraction

define the angular size of optotype

size of the entire size of a letter

when talking about angular size of optotype, what width are we talking about?

width of space used to draw the entire letter

formula for angular size of optotype?

MAR x 5

how to convert CPD to MAR

CPD = 30/MAR