Visual Acuity: Snellen Test, 2 Line Discrimination, and Astigmatism

What is visual acuity?

The SHARPNESS of vision, the ability to resolve objects

Refraction of Cornea and Lens

The cornea (fixed) and the lens refract light rays to focus them on the retina. The refractive power of the cornea is fixed but the lens adjusts its shape to fine tune the focusing of an image.

What does the cornea and the lens do to the image?

the cornea and lens invert the image on the retina

What is the refractive power of the lens inversely related to?

The refractive power of the lens is inversely related to its focal length, the distance from the lens where the light rays from an object are brought into focus

What is Emmetropia?

normal vision; able to focus the light on the retina just using the lens

What is myopia? Can it be corrected?

Near-sightedness - people move closer to focus

The lens shape is normal, but the eye is elongated, so the lens focuses light before the retina.

Can red-adjust with a thinner corrective lens that helps to focus the light on the retina

What is hyperopia? Can it be corrected?

far sighted - move farther away to see

The lens shape is normal, but the eye is short, so the lens focuses light behind the retina

Can be readjusted with a thicker corrective lens

How can visual acuity be tested?

Snellen chart and 2 line discrimination test

What is the Snellen Chart?

The Snellen chart tests for myopia

The subject stands 20 feet away and reads lines of letters that decrease in size

Snellen eye chart procedure

stand 20 feet away from the chart

cover eye (repeated) and read smallest lines of letters possible

record visual acuity number (20/20,20/50, etc)

Snellen eye chart: How to read 20/20? 20/15? 20/50?

20/20 - At 20 feet you see what a person with normal vision sees as 20 feet

20/15 - At 20 feet you see what a person with normal vision sees at 15 feet; even better than 20/20

20/50- at 20 feet you see what a person with normal vision sees at 50 feet

Significance of 20-40? 20-200?

at a vision of 20-40 or worse you need to wear glasses when driving

at a vision of 20-200 you are legally blind

What is the two line discrimination test?

determining the distance between two cone receptors (density) and the resolving power of the visual cortex?

Two line discrimination: procedure

paper with 2 lines, 1 mm thick and 1 mm apart, taped at eye level

subject closes one eye and gradually move back from the wall

subject stops when two lines fuse into one

distance from the wall to the subject (D3) is recorded

Cones in visual acuity

person with emmetropia focuses directly on the object and the incoming light is focused on the fovea centralis. Where the highest density of cones (and only place) is located. Cones have high visual acuity and sense color P

Rods activity

peripheral areas of the retina contain relatively more rods, which have lower acuity and cannot detect color but are more sensitive to low light levels

2LD: The closer you are when the 2 lines look like one?

the farther?

A smaller distance (closer) means cones have a higher density and higher resolution, which is better vision.

A longer distance (farther) means they are further apart, and you have lower resolution, which is bad vision.

2LDT: How to calculate?

1) plug in

2) cross multiply

3) divide by D3

4) determine X

5) divide D2 by 2

2LDT: What is the normal range?

0.002-0.007 mm

What is astigmatism?

imperfection in the field of view prevents light from passing equally through are parts of the eye

Astigmatism procedure

stand 20 feet away

cover one eye (repeated)

if all radiating lines appear equal, there is no astigmatism. if lines appear darker and more in focus it indicates astigmatism

vertify by rotating chart 90 degrees to see if same lines appear darker

Anst: What is the pathway of light?

cornea → aqueous humor → lens → vitreous chamber

What may cause astigmatism?

any imperfection in the pathway results in astigmatism because it will alter light on the way to the retina

• because the lens and cornea are not equally curved